MAURITIUS

a british crown colony in the in- dian ocean, 500 m. east of madagascar. its area is 720 square miles. the estimated population at the end of 1923 was 381,678. an influenza epidemic swept the island during the monthsof may, june, july 1918 when 11,000 persons died, the death-rate in that year reaching the high figure of 64-9 per thousand. primary ed- ucation is free but not compulsory. the total number of primary schools in 1919 was 148, of which 57 were government schools and gi schools recciving state aid. the number of pupils on the registers was 22,120. of the state-aided schools 67 were roman catholic, ro church of england, two mahommedan and one hindu. in all schools under government inspection the teaching of elementary hygiene and sanitation is compulsory. finance and trade.—the highest figures for the revenue of the colony were reached during the financial year 1920-1 when the total revenue was rupees 40,168,210; expenditure in the same year was rs. 24,729,510. figures for subsequent financial years were :— 1921-2 1922-3 rs. rs. reven ue : 25,599,994 23,547,734 expenditure 26,307,658 22,031,176 840 at the close of the financial year 1923 the public debt of the colony was £1,265,724. sugar production remains the principal industry of the colony, the total average under sugar cultivation being in excess of 175,923 acres. arecord sugar crop was produced during the crop year 1918-9 when the output of the sugar factories was 252,772 metric tons; the output for 1923-4 was 195,356 metric tons. the production of aloe fibre ranks next to sugar in order of impor- tance; rum, copra, coconut oil are other products that have shown an increase. increased attention has been given to tobac- co growing and the cultivation of tea has progressed. lectures and demonstrations in modern agricultural methods are given by the government agricultural instructor. the total trade of the colony reached record figures during 1919 when the com- bined values for the year were rs. 174,843,348—imports rs. 47,037,024, exports rs. 127,806,321. imports for 1923 were val- ued at rs. 76,835,565 and exports rs. 69,841,935. the total mileage of the railways on the islandin 1923 was 144 miles. the mauritius govt. railway main line gauge is 4 ft. 8; inches. the short bois cheri light railway hasa 2 ft. 6in. gauge. the mileage of telegraph lines in 1923 was 415} m., and of telephone lines 169} miles. cable communication is main- tained with zanzibar, australia, reunion, madagascar, durban and other places. see a. macmillan, mauritius illustrated (1914); also colonial office list (annual series, london); mauritius blue book (annual, mauritius). (tec. wa.) maurras, charles (1868- ), french writer and poli- ticlan, was born at martigues, provence, of a royalist family. he first became prominent through his literary criticisms in the revue encyclopedique and the gazette de france. he was at first the only royalist on the staff of the action frangaise, but soon converted almost all his colleagues. after visiting greece and italy he published trets idees politiques (1898); les amants de venise (1902); and l’avenir de l’intelligence (1903). this was his most productive period, during which his royalist prop1- ganda exerted a powerful influence; he assisted leon daudet in transforming the action frangaise into a daily paper, which took place in 1908, and soon became engrossed in journalistic work. in his kiel ef tanger (1910) he criticised french foreign policy, while le difemme de marc sangnier (1906) dealt with religious problems. though an avowed atheist, maurras some- what paradoxically favoured an alliance with the catholic church which he regarded as closely connected with the mon- archy. and indeed the action frangatse lost considerable in- fluence when the french clergy withdrew their support and defeated daudet during his candidature for parliament in 1923 and 1925. | during the war maurras worked for the disintegration of germany and for an overwhelming victory for france. his war articles appeared in eight volumes entitled les conditions de la victoire (1915-20). though his polemical writing injured his literary fame, maurras nevertheless gained the reputation of being one of the most accomplished of modern french authors; and his critical works, such as l’etang de berre (1915); le conseil de dante (1920) and l’allee des philosophes (1924) are written in restrained but powerful prose. his poems, often of great beauty, were collected in la musique interieure (1925). though the writings of maurras produced a profound effect on french thought, his influence on practical politics has been but slight; for his political theories suffer from extremism and exaggeration, both characteristics of his race. his nationalist ideas failed to take deep root in his own country, but they triumphed in italy; and l’idea nazionale with its fascist doctrines owed much to the action francaise. the french movement with which the name of maurras is associated may be regarded as an inchoate fascism; a minor intellectual revolution, which has not developed into any great political upheaval. among maurras’ many works may be mentioned the following: jean moreas (1891); le chemin de paradis (1894); anthinea (1901); l’enquete sur la monarchie (1900-9); la politique religieuse (1912); quand les frangais ne s’aimatent pas (1916). maurras—maxse max, adolphe (1869- ), belgian politician, was born in brussels dec. 31 1869. from 1894 to 1903 he was successively provincial councillor of brabant, councillor of the commune and alderman of brussels, and finally burgomaster (1909). in iq14 when the german troops entered brussels he refused to perform his duties under the authority of the german governor, and demanded complete freedom of action. he protested vigorously against the abuses of the army of occupation, and fought with indomitable energy for the rights of his fellow subjects, and for the reduction: of the heavy taxes and requisitions which were imposed on the town. he further founded a central committee to deal with supplies which, under the name of le comite national, rendered invaluable services to his countrymen. but the german authorities soon took exception to his spirited resistance, and on sept. 26 1914 he was arrested and imprisoned in the fortress of namur, and from there was sent into germany, where he was closely confined. on nov. 13 1918 he succeeded in escaping and returned to belgium, where he was welcomed with delirious enthusiasm. m. max was elected to the chamber of represent- atives in 1919; he was made minister of state on nov. 21 1918, and was also elected member of the belgian academy. maxim, sir hiram stevens (1841-1916), anglo-american engineer and inventor (see 17.918), died in london nov. 24 1916. maximilian (1867- ), prince of baden, born june to 1867 at baden-baden, was a son of prince william of baden. as the nearest agnate to the reigning grand duke, of whom he was a cousin twice removed, he was heir presumptive to the grand ducal throne. from 1907 to 1918 he was president of the first chamber of the baden diet. during the war prince max did_ much to improve conditions for british prisoners in germany, as also for german prisoners, especially in russia. on oct. 3 1918, when the old military and political system in germany was on the verge of collapse, he was appointed imperial chancellor. it fell to his lot to initiate the negotiations for the armistice, and also to carry through in hot haste those alterations in the old constitution which had long been demanded by the liberals and the socialists, but which now came too late to avert the fate of the empire and the prussian monarchy. it also became his duty to put pressure upon the emperor in order to induce him to. abdicate. as the imperial decision was delayed from day to day and the revolution became imminent, he declared on nov. 9 1918 the abdication of william ii. as german emperor and as king of prussia. 11 was clear that the hohenzollern dynasty was doomed; and prince max handed over the government to the majority socialist leader ebert, who became the president of the german reich. he continued, after the abdication of the grand duke, to reside at karlsruhe. (see germany.) maxse, sir ivor (1862- ), british soldier, was. born dec. 22 1862 and was commissioned in the royal fusiliers in 1882, transferring to the coldstream guards in 1891. he served in the sudan campaigns 1897-9, during which he gained the d.s.0., and in the boer war 1809, as stafi officer to mounted infantry and as commander of the transvaal constabulary after the capture of pretoria. from 1902-10 he was regimental commander of the coldstream guards, and on the outbreak of the world war he went to france in command of the rst guards brigade, which he led during the retreat from mons and in the battles of the marne and the aisne. in oct. 1914 he was pro- moted to the command of the 18th division. he brought this division to france in 1915, and his unique training methods bore fruit in the battle of the somme, where the 18th div. obtained an opening success in brilhant contrast to the fate of other divisions on the tragic july 1 1916. as a consequence his pro- motion to command of the 18th army corps followed in jan. 1917, and this he led with distinction down to, and including, the st. quentin battle in march 1918. in 1917 he was created k.c.b. the british army was so sorely shattered in resisting the german offensive of 1918 that its reconstruction and trans- formation into the irresistible striking force of the autumn appears almost a miracle. in the glory of this achievement sir ivor maxse has a notable share. the need for a unified direction of training was realised and he was chosen to be inspector-general mayo, charles horace—measurements | of training to the british armies in france. by the develop- ment of the new tactics of infiltration and manoeuvre, by the improved co-operation between the different armies, and by the new elasticity of tactical handling, his work is engraved in the astounding success and low casualty list of the army during the advance to victory. after the war, at the hand of the northern command 1919-23, maxse’s influence in recasting the training of infantry and in making tactics intelligible to the novice was as marked as his practical achievement in the reorganisation of the depdt system. dynamic in thought and energy, receptive to ideas, and generous in his support of youthful ability, he had a genius for training which was reminiscent of the influence of sir john moore, a century before. sir ivor contributed to this encyclopedia the article infantry. mayo, charles horace (1865- ), american surgeon, was born at rochester, minn., july ro 1865. after studying at the rochester high school, northwestern university and the chicago medical college (m.d. 1888), he began the practice of surgery at rochester, minn., and with his brother became sur- geon at the mayo clinic. he made a special study of goitre, and as a result succeeded in reducing the death-rate in this class of cases by half. he has published a number of papers on surgical topics in medical journals. he was elected president of the minnesota state medical society in 1905, president of the surgical section of the international tuberculosis congress in 1908, and president of the american medical assn. in 1916. he was appointed mayo foundation professor of surgery in 1915. during 1917-8 he was president of the examining board of applicants for commissions in the medical corps, having super- vision over several states, including minnesota. in 1918 he was appointed colonel in the medical reserve corps, u.s.a., and for a year was associate chief consultant for surgical service. beginning with 1912, graduate courses in medicine were offered at the mayo clinic in rochester. early in 1915 he and his brother incorporated the mayo foundation for medical educa- tion and research at rochester, and to it the brothers gave $1,500,000. in june of the same year, by mutual agreement, the funds and resources of the foundation were placed under the direction of the regent of the university of minnesota for promot- ing ‘‘ graduate work in medicine and research in this field.” on sept. 13 1917 the foundation, with its fully-equipped staff, laboratories and clinics in rochester, became a department of that university. (see surgery.) mayo, william james (1861- ), american surgeon, was born at le sueur, minn., june 29 1861. he was graduated in medicine from the university of michigan in 1883 and began the practice of surgery in rochester, minnesota. a small hospital was organised under the local branch of the sisters of st. francis, which developed into st. mary’s hospital. here he, with his younger brother, charles horace, developed the mayo clinic (arganised 1889), which became famous throughout the world for the number and success of operations performed. the records of operations have been so carefully and accurately made and preserved that they form a valuable asset to medical science. though the clinic has contributed no discovery of new methods or new cures for diseases, all modern methods have been covered and reduced to their ultimate accuracy. accuracy has been the keynote to the mayos’ success. goitre, gall bladder, appendix and gastro-intestinal operations constituted the great majority of the cases, and the proportion of deaths to the total number of patients operated upon was remarkably low. dr. w. j. mayo specialised in the surgery of the stomach, and pub- lished a large number of papers on gastric surgery and kindred topics. he was elected president of the minnesota state medical society in 1895, and in 1907 was appointed a regent of the uni- versity of minnesota. hewas elected president of the society for clinical surgery in tort, and the following year president of the american surgical association. on america’s entrance into the world war he was appointed a colonel in the medical corps, u.s. army, and chief consultant for surgical service, alternating with his brother, c. h. mayo, in this capacity. in 1919 he was awarded the medal for distinguished service. (see surgery.) 841 mayor, john eyton bickersteth (1825-1910), british classical scholar (see 17.937), died at cambridge dec. 31 1910. mead, larkin goldsmith (1835-1910), american sculp- tor (see 17.945), died at florence, italy, oct. 15 1910. | measles: see infectious fevers. measurements (sce 18.134).—the science of measurement is in practice restricted to mean measurement of the three fun- damental quantities, mass, length and time, from which all other quantities, such for example as volume, density, velocity, accel- eration, force and power, are derived. the problem‘of meas- urement of these three fundamental quantities presents itself primarily as a question of providing suitable units of reference against which other quantities whose values it is desired to meas- ure may be compared. importance of mctrology.—on the purely scientific side all other scientific quantities are ultimately definable in terms of the three fundamental quantities with which metrology deals, and every quantitative determination resolves itself in the last resort into a measurement of one or other of these quantities. if “‘sci- ence is measurement ” then without metrology there is no science. even in its most direct application it covers a very wide field (see mensuration [15.134]; surveying; geodesy; earth, fig- ure of the [8.801]; micrometer [15.38]; toot, part 13 (27.43); measuring instruments; time [26.983]; clocks [6.536]; weights and measures [25.477]; weighing macnines [25.468]; metric system [15.299]). the question of the time standard, moreover, is particularly an astronomical one. the specification of any quantity involves two factors. if we speak of three pounds, two metres, or 24 hours, in each case the complete statement consists of firstly a pure numeric, and secondly of a unit of measurement which is either the fun- damental unit of reference, or a definite multiple or submultiple of it. the reference units in themselves are entirely arbitrary. there is no “‘ absolute ’’ standard; all measurement is relative. problems involued.—the problem of metrology is twofold. first to provide and maintain unaltered the standards of refer- ence by which other quantities are compared and measured; and secondly to provide means by which the comparisons may be made with accuracy sufficient for the particular purpose in view. the demands of science and industry at the present day require, for certain purposes, an extremely high degree of accu- racy in these fundamental comparisons, so that what, at first sight, would appear to be a fairly simple problem becomes in fact one of great difficulty, involving the most expensive and elaborate apparatus. no measurement is ever absolutely cor- rect. some degree of experimental error is always necessarily present, and the approximate degrees of accuracy at present attainable in certain of the more fundamental operations in metrology, are as follows:— comparison of two platinum-iridium copies of the [nternational prototype kilogramme: i part in 100,000,0co. comparison of ordinary chemical weights: i part in 1,000,000. comparison of smaller masses by micro-balance: i part in 100,000,000, , comparison of two yard or metre (line or end) standards: i part in 10,000,000. comparison of end standard with dine standard: 1 part in 1,000,000, determination of volume and density for very special work: i part in 1,000,000. determination of volume and density, ordinary: i part in 10,000. calibration of set of end standards (not less than 1 in. in length): fr part tn [,000,000. calibration of subdivisions of graduated yard or metal scale in terms of whole length: 0000005 in. or 0-o001 millimetre. standards of length the history of standards of length is one of varying ascend- ency of three principal competing types. a length may be defined by the distance, under certain specified conditions, either be- tween the two end surfaces of a material standard bar, or between {wo suitable marks engraved upon it. alternatively reference may be made to some “ natural ” standard. the standard yards of henry vii. and elizabeth preserved in the standards de- partment of the board of trade, are end standards, incisions marking subdivisions of the yard being secondary only. the 842 elizabethan yard was superseded by one defined by the distance between two small dots on gold plugs inserted in it. when this bar was legalised in 1824, it was provided that in the event of loss it should be replaced by reference to a “ natural ” standard, the length of the pendulum beating seconds in the latitude of london. it so happened that within a very short period this standard was in fact destroyed by the fire in the houses of par- liament in 1834. the commission charged with its replacement found, however, that it was impossible to reproduce the seconds pendulum with so high an accuracy as the length of the bar itself could be re- produced by means of direct comparison with other bars which had previously been compared with the lost standard. the legal prescription was therefore abandoned, and the new standard yard, which is that legal at the present day, is a line standard constructed by comparison with other bars, without reference to any control provided by a natural standard. the metre was originally intended to be the 10,000,c00th part of the earth’s meridional quadrant. but it was soon found that not only was the determination of this natural standard an extremely labori- ous undertaking, but the accuracy attainable was less than that possible in the comparison of material standards, and the mate- rial metre des archives, a platinum end standard, became the accepted standard of reference for the metric system until super- seded in 1889 by the present international prototype metre, a platinum iridium line standard (see weights and measurfs). wave lengths as natural standard.—at the date of construc- tion of the present imperial standard yard, and the interna- tional prototype metre, sound judgment was exercised in the choice of material line standards as affording the greatest pros- pect of accurate comparison and reproduction and in the legal provision made for their replacement, if necessary, by reference to other similar standards periodically compared with them. recent developments, however, have appreciably changed the situation. in the first place the experiments of michelson, fol- lowed by those of fabry and perot (see interference of lignt, 14.685) have finally established a natural standard (the wave length of the red line in the spectrum of cadmium) which is re- producible with accuracy at least as great as that attainable in the comparisons of material standards, which is definitely free from the suspicion of possible secular variation inevitably at- taching to all material standards, and by means of which the material standards necessarily employed in everyday practice can be verified in any part of the world without the risk of acci- dent or damage involved in the periodical transport of material national reference standards to and from the international cen- tury laboratory for purposes of comparison with the prototype. secondly, such improvement has been effected during the last few years in the production of flat-ended standards that bars with accurately parallel ends of the quality of optical mirrors are now available, whose lengths can be more directly determined by the method of optical interference than is the case with line standards, and which are also more accurately comparable with each other than the latter. the international committee on weights and measures, at its meeting in 1923, decided in principle on the eventual adoption of a wave-length standard of length subject to the determination by experiment in the various national laboratories, of the best method of realising such a standard and the conditions neces- sary to ensure the greatest degree of constancy in its reproduction. such experiments are proceeding and it is to be anticipated that within the period beginning in 1925 or 1926 a very important change may be made in the ultimate basis of reference for all measures of length. use of material standards whatever ultimate basis of con- trol may be selected, for practical purposes, in everyday meas- urement, material standards will always be needed. both line standards and end standards will always be required for differ- ent purposes. so long as material standards are used for the ultimate reference the primary importance attaches to secular stability of the material. there is evidence to suggest that the present imperial standard yard has probably shortened by about measurements 0-0002 inch since it was originally constructed in 1844, but has remained unchanged for the last 40 years. so faras is known, the international prototype metre has remained unchanged since its verification (1875-9). the evidence for this rests partly on subsequent recomparison with various national copies constructed in the same manner at the same time, and partly on two deter- minations, by different methods, at an interval of about 15 years, the first by michelson and benoit, and the second by fabry and perot, against the wave length of the red line of cad- mium. these metre standards are all made of an alloy of 90% platinum and 10°%% iridium, which is now regarded as the most satisfactory alloy for the construction of a material standard for purposes of ultimate reference. its cost, however, would be prohibitive for any other purpose. temperature conditions.—all materials change in size to a greater or less degree with variation of temperature. it is there- fore essential to specify exactly the temperature at which the material standard defines the unit of length, and further either to control the standard exactly to this temperature when making comparisons with it, or at least to ascertain its temperature exactly, and make allowance for its known expansion between that temperature and the standard temperature. the imperial standard yard is correct at 62° f., the international prototype metre at o° centigrade. some confusion of thought frequently arises on this point. each of these bars serves, at its own stand- ard temperature, to define a simple unit of pure length, that is, of absolute extension in space. these units themselves, once deter- mined, are entirely independent of temperature. it is not correct to say that either of the fundamental reference standards is a yard, or a metre, except at its appropriate temperature of defini- tion, 62° f. or o° c., respectively, nor that a length of any other material is a yard or a metre solely because at 62° f. or at 0° c. it agrees with the fundamental standard. a length of any mate- rial is a yard or a metre, at whatever temperature it may be, if at that particular temperature the length is equal to the unit de- fined by the standard bar when the latter is at its own tempera- ture of reference. there is a definite numerical relationship be- tween the two principal units which has been twice accurately de- termined: in 1895 by chaney and benoit, with the result 1 metre= 39.370113 in.;and more recently (1922-5) at the national phy- sical laboratory, with the result 1 metre=39°370137 inches. the two results may be said to be in agreement within the ex- perimental error of the various observations involved, and for all practical purposes the simple ratio 1 in. = 25:4 mm. (equiva- lent to 1 metre=39-°370079 in.) may be taken as sufficiently exact. the use of this ratio implies that the two objects, being com- pared, are both simultaneously at their common temperature of employment. for all everyday purposes of measurement, as for example in measuring a piece of work in an engineering work- shop, it is unnecessary to bring the object to be measured to the standard temperature. if the work and the gauge with which it is being measured are made of materials having the same co- efficient of expansion, and the former has been compared with the reference standard at the standard temperature, it is only necessary, when comparing the work with the gauge, to ensure that both are at the same temperature (not necessarily the stand- ard temperature) to ensure that the work will be correctly measured. use of invar.—for many purposes where very precise meas- urement is involved, it is of great advantage to have a material with a very small thermal expansion. two such materials have been discovered. the first, known as “ invar,”’ is a nickel-steel alloy, containing 36 % of nickel, invented by dr. ch. ed. guillaume of the bureau international (sevres). different samples have coefficients ranging from about 1.5x10°® per 1° c., for large bars, down to zero, or even slightly negative values, for smaller bars, pendulum rods, drawn wire or rolled tape. these figures are to be compared with 11 x 1076 per 1° c. for steel, and 18 x 10-6 per 1° c. for brass. invar, unfortunately, has one very serious defect as a standard of length. it grows longer, rapidly at first and subsequently more slowly but continuously, so that after measurements 20 years the length of a bar of invar will still be increasing at a rate of about 1 part in 2,000,000 per annum. more recently dr. guillaume has introduced a slightly different alloy, contain- ing a percentage of chromium in addition to the nickel, which is described as “‘ stable”? invar. this alloy grows at an appre- ciably slower rate than ordinary invar, but still cannot be regarded as constant. invar therefore is principally useful in a laboratory, in work where temporary constancy of length is the primary consideration, or in circumstances, as for example in the case of tapes and wires used for geodetic surveying, where the accurate ascertainment of temperature presents considerable difficulty. in these cases it is necessary to return the tapes or wires to the laboratory periodically for reverification. for the reasons indi- cated above, in everyday workshop measurements, gauges or scales should be made of material having a similar coefficient of expansion to the work to be measured, and invar therefore should not ordinarily be used for the construction of workshop ‘standards. fused silica —the other material which has a very low co- efficient of expansion is fused silica, which expands only o-4 x 10-® per1° centigrade. a metrestandard constructed of fused silica, in the form of a tube, with parallel plates fused in at the ends on the platinised surfaces of which the defining lines are ruled, has been made and kept under observation at the national physical laboratory since 1913. so far as can be detected, no change has occurred in its length. such a standard is extremely fragile, and for this reason would hardly be adopted either as a funda- mental reference standard or for everyday use, except in a metro- logical laboratory. natural crystal ouartz—reference should be made to the recent work of perard, at the bureau international, on end standards of natural crystal quartz. such standards cannot, of course, be of the full length of the yard or metre. but they present several great advantages. firstly, since the material is of great age and the molecules of which it is composed are arranged structurally in definite crystallised array, there seems little pos- sibility of any secular change. secondly, it lends itself to perfect optical finish of the defining end planes, which enables direct determination of length to be made in terms of light waves, with extremely high precision resulting in a proportional accuracy no less than is at present obtained in the comparison of yard or metre line standards, although the largest available specimens of crystal quartz, as well as the method of use, only enable such standards to be made and verified up to a length of about 4 inches. line standards; mode of comparison.—the comparison of two line standards is effected as follows: the bars are placed side by side, each on one of two parallel girders contained in a water bath, which is mounted on a carriage capable of being moved in a direction be aleuia to the length of the bars. the bars are constructed with their graduations on polished surfaces in the neutral plane of their cross section, and each is supported on two rollers spaced at such a distance apart that the distance between its graduation marks has a maximum value. under these conditions any changes in the flexure of the bars due to slight errors in the positions of the supports can have no effect on the measured distances between the graduations. two micrometer microscopes (see micrometer, 18.381) are rigidly held in brackets supported in such a manner as to be unaffected by the motion of the carriage. each girder has independent adjustments by means of which the lines on the bars can be brought into focus in the field of the microscopes. the water in the bath is stirred and the temperature read by means of accurately calibrated thermometers. readings are taken by means of the microscopes alternately on the defining lincs of each bar, and the mean of several independent readings is taken. the comparison is repeated with each of the bars turned end for end in turn in order to overcome any effect of asymmetry in the illumina- tion of the graduation marks, and the bars are then interchanged on the two girders and the whole repeated once more, making eight complete sets of readings in all, from which the difference in length between the two bars is finally computed. in an important deter- mination several bars (say six or eight) may be included, and each will be completely compared in the manner described above, against all the others, the best values for the differences between all the various bars being then computed from the individual observed differences by the method of least squares (see prorability, 22.376), the residual errors serving to indicate the degree of accuracy which has been attained in the work. 843 calibration.—the calibration of the subdivisions of a divided scale is done in a similar way, except that in this case the carriage of the comparator moves in the direction of the length of the bar. the microscopes are fixed successively at a series of suitable inter- vals apart, for example, 1 dm., 2 dm., 3 dm., etc., and each principal interval of 1 dm., 2 dm., etc., is compared with every other interval of the same magnitude throughout the metre. by computation from the results so obtained the value of cach decimetre is determined in relation to the whole length of the scale. in a similar manner each centimetre of one decimetre is compared with every centimetre of another decimetre, and as a result the value of each centimetre is found in relation to the whole of the other decimetre, and so in turn, in relation to the whole length of the metre. millimetres are derived from centimetres in the same way, and so on. the complete calibra- tion of a divided scale will be seen to be a very laborious process involving an enormous number of observations. the process 1s, however, simplified by the fact that the whole of the measurements are made on the one bar, so that, provided reasonable precautions are taken to ensure constancy of temperature, exact temperature measurements are not important. the apparatus used to determine coefficients of expansion 1s similar to the transverse comparator, but has two independent water baths mounted on the carriage. one of these, containing a bar preferably of invar, is kept at a constant temperature, while the other, containing the bar under examination, is brought successively to a scries of suitable temperatures. the two bars are compared under the microscopes at each temperature of the second, and so the variations of its length with temperature are determined. a number of machines of different types, and varying sensitivity, are available for comparing end standards by contact measurement, or, if the end faces are of sufficiently perfect finish, they may be directly compared by the method of optical interference. in the latter case the two bars are brought in turn between two semi-silvered optically flat glass surfaces, and the number of wave lengths in the small spaces at either end between the measuring surface of the bar, and the semi-silvered surfaces of the optical flats are determined by direct measurement of the angular diameters of the interference rings formed by monochromatic light. this enables the fraction of a wave length to be determined with exactness, and if several different wave lengths are used in turn, whose mutual ratios are known, the whole number of waves is easily determined by deduction, as only a particular set of corresponding whole numbers will give fractions agrecing in every case with the series actually observed. in the mechanical contact type of measuring machine the bar is measured between two opposed anvils, one of which may either be fixed, or may be movable by means of a micrometer screw, while the other is capable of a smal! movement operating some form of sensi- tive indicator. the two bars to be compared are inserted in turn between the anvils, and if the whole difference between them is sufficiently small the movement of the indicator over a calibrated scale may suffice to determine it. if the difference is greater, the indicator must be brought to a fixed mark by an adjustment of the moving anvil, the difference in length being then ascertained by the difference in reading of the micrometer wheel. mode of calibration of end siandards.—the calibration of a set of subdivisional end standards is effected by taking them together in pairs of nominally equal added lengths, and comparing their sums, in the manner just described. in the last 20 years there have been developed, first by the firm of c. e. johansson, sweden, and later at the national physical laboratory, england, and by hoke in america, methods of producing short flat ended gauges of such per- fection that any two of them if put together will adhere firmly to each other by “ wringing.’ the process of wringing is not yet fully understood, but appears to depend essentially on the presence of a very minute trace of liquid (grease or moisture) between the surfaces of the gauges. these gauges are usually made in series, ¢.g., i in, 0-9 in., o-8 in, . . . o-1 inch. suppose we wish to calibrate such a series, of which the i in. is supposed to be already known as the result of some previous calibration. for convenience we will assume that we have available a duplicate set of pieces, which we denote by 0-1’, 0:2’,0-9 foot. to determine, for example, the value of the 0-7-in. gauge, we wring up in turn all the various nominally equal com- binations indicated below, and compare them in a suitable measur- ing machine, the small observed differences being indicated by o'7 = 0-7 oo 0-7 o5 = i1°0 02 5§ o7 ot = o8 i 07 o06 = 1:0 03 6 o07 o2 = 0-9 2 0-7 of = 10 o4 7f 0-7 o03 = i1:0 2 07 o8 = 10 o55 8 0-7 o4f = 10 ot 4 07 o9 = i10 o06 q adding up all these equations we see that the sums of the second columns on either side cancel out, and we get 10oxo0-7= 7x1-0 ; west we od cad 7 ion 10 o the sizes of each of the other pieces may be determined in a pre- cisely similar manner. 844 it may be noted that in the determination of sizes in this manner, the length of each gauge is automatically associated with the thick- ness of one wringing film, which, for clearness in conception, may be regarded as representing half a film thickness on either end surface, so that when two gauges are wrung together a whole film is estab- lished between them. as the gauges are normally used in this man- ner, this result is logically what is required. the wringing films, moreover, are in any case exceedingly thin. when initially formed, their thickness depends to some extent on the viscosity of the liquid of which they are composed, but if a sufficient length of time is allowed to elapse they tend to thin down to a limiting thickness which is less than 0-o00001 in. for all liquids. there is one more fundamental operation which, whatever may be the nature of the ultimate standard adopted, will always be required in its practical application to everyday requirements, and that is the determination of the length of an end standard in terms of the corresponding line standard, or vice versa. this is a matter of considerable dithculty, and several methods have been employed for the purpose. probably the best method is that introduced by mr. h. l. p. jolly, formerly of the national physical laboratory, and now of the ordnance survey department, southampton. it involves the use of an intermediate end standard, and of two special parallel faced end blocks, which can be wrung on to the ends of this standard. | each of the end blocks carries graduation marks as shown on its polished upper surface. the composite bar is compared in the ordinary way in a comparator, with the standard line bar. each of the end blocks is turned round in turn, and re-wrung on the end of the bar, and the comparison repeated. we thus obtain the four results:— | ag b, = sy i ll. ai b, = su 2 l ai be = sr. 3 lah = s, 4 or, adding and dividing by 2 2l a: ac bh bh = 2 §$ € 2 3 4 each of the end blocks is then removed in turn and the other wrung centrally on the end of the bar, the new combinations being com- pared in a measuring machine with the end standard end bar, with the results laas = l bi be = se 2 or adding a ai ag b; be = from this equation, and the one previously found, we obtain the result 2s; “7 (1 2 3 4) = @s— 1 2 which gives us the desired value of the line standard s, in terms of the end standard sp or vice versa. to complete our survey of the fundamental operations involved in length measurement, reference must be made finally and very briefly to the process of verifying longer measures, such as are used in surveying. in the first place a long bar, graduated in multiples of the standard length unit, is required. this is compared, yard by yard, or metre by metre, with the standard line bar, in a large com- parator. this bar in turn is compared either with suitable reference marks engraved on metal studs let into a mural base at intervals corresponding to the length of the bar, or else directly with the divisions of a graduated tape. the tapes or wires after verification, either directly, or by comparison with the mural base, are used to determine base lines in the field, by comparison with temporary bench marks set up at intervals apart roughly equal to the length of the tape. 2 se i 2 standards of mass.—no attempt has so far been made to define a unit of mass by means of any natural standard, though a standard of this kind, e.g., a definite multiple, say, of the atomic mass of helium, would not be inconceivable. prior to the dis- covery of the radioactive elements mass was regarded as the essentially constant attribute of matter, and there was no reason to anticipate any change in a material standard of mass except by actual damage due to abrasion, oxidation, hygroscopic ab- sorption or other similar causes. and there still remains a reason- able choice of materials, which, given due care in preservation and handling, may be expected with considerable confidence to exhibit constancy of mass. platinum and platinum-i ridium.—our predecessors, ignorant of radioactive processes, were fortunate in the choice of platinum and platinum-iridium (10%, iridium) as the materials of con- struction for the ultimate reference standards of mass, the im- perial standard pound, and the international prototype kilo- gramme, respectively. the degree of consistency (within 1 part in 10°) with which recomparisons of various national copies of measurements the kilogramme, made after the lapse of many years, have in general repeated the original determinations, speaks convinc- ingly, not only as to the suitability of the standards themselves, but as to the perfection of the balances used in the comparisons. the relation between the two units, according to the best ascer- tained determination, is 1 kg.=2-2046223 pounds. this value received legal sanction. crystal quariz.—another material presenting a high degree of constancy of mass is crystal quartz. this, however, has the dis- advantage of having a comparatively low density. in comparing two standard weights, masses in air, allowance must be made for the upward buoyant effect due to the volumes of air which they respectively displace. the less the density of the mass, the greater will be the buoyancy correction. the accuracy attained in the intercomparison of a series of platinum-iridium standards is no doubt attributable to a considerable degree to the fact that they all have comparatively high, and very closely equal, den- sities, so that the net buoyancy corrections are very small, and a comparatively rough determination of the air density con- sequently suffices to give the correction with negligible error. in comparing a number of masses dilfering appreciably in den- sity, e.g., platinum, quartz and brass, the accurate determination of the buoyancy correction presents much greater difficulty, and several attempts have been made to overcome it by actually conducting the weighings in vacuo. this involves enclosing the whole balance in an air-tight case, and manipulating the weights entirely by mechanical means from outside, without opening the case. leakage at the glands where the operating spindles enter the case has, however, so far proved an almost insuperable ob- stacle to successful weighing 1m vacuo. everyday weighings for commercial purposes are of course necessarily conducted in air, but the differences in buoyancy between the weights used, and the goods weighed, are negligible for this purpose. it is neces- sary, however, to provide a basis for the periodical reverification by inspectors of weights and measures of traders’ weights, which may be of iron, brass or other materials. for this purpose a “commercial ” standard is employed. this standard is of brass (of density 8-143) adjusted to agree tn vacuo with the imperial standard pound of platinum. inspectors’ standards are also of brass, and all verifications of these standards, and thus indi- rectly of traders’ weights, are made by comparison, 17 air, with the commercial brass pound. use of the balance.—even when weighings are not conducted in vacuo the construction and manipulation of a balance for the accurate comparison of primary standards of mass are distinctly elaborate. it is necessary for the greatest care to be taken to preserve constancy of temperature, in order to maintain a steady zero reading of the balance. for this reason the room containing the balance must be thermostatically controlled, and the observer either works entirely from outside the room, or, if he enters it, must remain at a distance from the balance, all the manipu- lation of the weights being effected from outside the balance case by mechanical control operated by means of long rods, and the movement of the balance beam being observed either through a telescope, or by the movement across a scale of a spot of light reflected by a small mirror attached to the beam. consecutive weighings.—to eliminate any effect due to slight inequality in the lengths of the two arms of the balance, at the same time to minimise the effect of any small residual drift of zero due to a gradual change in temperature conditions and to obtain a determination of the sensitivity of the balance at the time of weighing, consecutive weighings should be made to the following order:— left-hand right-hand mean pan pan reading a x b pi a b x p2 b a x ps b x a ps b x a ps b a x ps a b x p: a xx b pa measurements here a and b are the two masses being compared, and x a small known weight which serves to determine the sensitivity. each mass is weighed an equal number of times on each pan of the balance, and the mean time of all the weighings of each mass on each pan is the same, so that the effect of any steady temperature drift is eliminated. in weighing, the balance is not brought to rest, but the pointer, or spot of light, ts observed while swinging, and the extreme positions of several successive swings to right and jeft are noted. from these the mean readings or rest points corresponding to each arrangement of weights on the scale pans are calculated. if, for example, four suc- cessive readings are i 2 3 4, then, allowing for the effect of the gradual reduction in the amplitude of the swing, duc to damping, the corresponding rest point, p, is 7 (1 3 2 3 3 4). if p be the rest point when two egual masses of values nominally equal to a and b are on the pans, then we have a-b x=k(pi—p) b-a x=k(ps—p) a-b-x=k(p—p) b-—a—x=k(ps—p) b-—a—x=k(p3—p) a—-b-—x=k(p;—p) b-a x=k(pi—p) a-b x=k(ps—p) 8 (a—b)=k(pi pe—p3—ps—ps—ps pr ps) 8 x =k(pi-ppe—ps ps ps—pe—p7 pa) pe em «| ay pl passa ras pa pas ps ‘pripe—-ps ps ps—ps—p7 pa usually of course several masses will be compared each with each in turn, and the best values for the differences between them finally computed by the method of least squares from all the observed differences obtained in the above manner. the construction of the balance must be such as to ensure that after each successive arrestment of the beam and pans, the knife edges are brought into exactly the same relationship with the planes, and to secure this it is necessary to support the pans by means of a series of crossed knife edges below each terminal knife edge on the bean. if the weighings are not conducted 27 vacuo, the air density must be calculated for each weighing from observations of tempera- ture, pressure and hygroscopic state of the air, and each observed reading corrected for air buoyancy, allowing for the effect of tem- perature on the volumes of the weights. the volumes, and hence the densities, of the weights are deter- mined by weighing them, first in air, and then immersed in distilled water, against other weights always in air. the density of the air must be calculated as usual from its tempcrature, barometric pressure and humidity, and that of the distiled water is known from its temperature. the difference between the two weighings, due allowance being made for the air buoyancy corrections on all the weights involved, is equal to the difference in weight between the quantities of water and air at the observed densities of each, which would fill a volume equal to that of the weight being examined. if for any reason it is considered objectionable to immerse a standard weight in water, it is possible to determine its volume, though some- what less accurately, by means of a volumenometer, which is an apparatus for observing the change of pressure of the air enclosed within a given space, when a definite change is made in its volume, firstly when it is filled with air alone, and secondly when the weight is enclosed within it. the calibration of a set of fractional or multiple weights in terms of the original unit can be effected by weighing together in groups of nominally equal sum by a process preciscly analogous to that de- scribed above for a set of fractional end standards. ordinary brass weights usually exhibit a certain variability of mass, which is probably attributable to variations of surface condi- tion depending on the degree of humidity of the surrounding atmos- phere. care should, of course, be taken to see that the metal of which weights are made is free from porosity, and weights intended for precision work should be made solid, without screwed-in tops covering holes containing adjusting material. the surfaces of high class weights are frequently electro plated with gold or platinum, with the object of minimising the effects of oxidation and hygroscopic action on the surface. of the two, platinising is the preferable process, but weights coated with either. gold or platinum have been found to exhibit instability to a certain degree, though usually less than in the case of unplated weights. the success of gilding or platinising depends upon the care exercised to obtain a firm and hard deposit. to minimise the effects of temperature upon the action of a balance, a beam of invar may be used. but it must be remembered that invar is to a certain extent magnetic, and if results of high pre- cision are required, care must be taken with such a beam as to ensure that it is completely screened from any possible magnetic influence. the balance should preferably be entirely enclosed within a shect iron case. for weighing very small quantities, and in particular for com- paring the densities of small quantities of gases, micro-balances con- structed entirely of fused quartz have been used. such balances have been made both with knife edges, and preferably, with the beam torsionally supported on a thin horizontal quartz fibre at right angles to its length, and the pans supported from its ends by means of other fibres fused directly to it. the weighings are usually whence 845 made by arranging a small bulb or similar contrivance at one end of the beam, counter-balanced by solid quartz at the other end. the two ends of the beam are therefore dilfcrently affected by the buoy- ancy of the air, and weighings are made by adjusting the pressure of the air in the balance case until the buoyancy just restores the heam to balance. the air pressure is read by means of a suitable manom- eter, and serves to measure the weight of the object under examina- tion. with such balances loads as small as sth gramme have been weighed to an accuracy of 1 part in 108), standards of capacity theoretically, the unit of capacity should be the same as the unit of volume—that is, the volume of a cube each of whose sides is equal to the unit of length. in practice, however, it is extremely difficult toconstruct such a cube with accuracy, and still more difficult to measure the internal volumes of vessels of different shapes in terms of the unit of length. practical neces- sity has therefore ordained the use of a unit of capacity based on the unit of mass, rather than on the unit of length. in the metric system the kilogramme was originally intended to be the mass of one cubic decimetre of pure distilled water when at its temperature of maximum density (4° centigrade). although great care was taken in the construction of the origi- nal kilogramme from this definition, and the result attained was closer than might have been anticipated in view of the difficulty of the problem,.it has been found by very careful experiment that the litre, which is now defined as the volume occupied by 1 kilogramme of water at 4° c., actually equals 1-000027 cubic decimetres. the gallon is somewhat similarly defined as the volume occupied by 10 lb. of pure distilled water at 62° f. when weighed in air at a barometric pressure of 30 in. of mercury, against brass weights. as the weighing has to be made in air, and the density of the brass weights is not prescribed by the act (weights and measures act 1878) there is a certain ambiguity about this definition. on reasonable assumptions it has been calculated that 1 gallon=4-5459631 litres. it will be noted that in the case of the litre the definition re- fers to the kilogramme mass—that is to say, the weighings are to be reduced to vacuum, by applying a suitable correction for the difference of the air buovancy on the distilled water and on the weights used for weighing it. this entirely eliminates the difficulty mentioned as regards the exact definition of the gallon. much confusion of thought has, however, arisen from the pref- erence of some chemists for the use of the so-called ‘‘ mohr’s litre,’ which is defined, on the same lines as the gallon, as the volume occupied by 1 kg. of distilled water, when weighed in air, against brass weights, at a temperature which has never been explicitly laid down. roughly, a mohr's litre equals 1-002 true litres. it is very unfortunate that the term litre should have been appropriated to a unit so vaguely defined and which differs from the true htre by an amount that, alchough small, is too great to be negligible. for most purposes the difference between the millilitre and the cubic centimetre can safely be neglected. commercial purposes —for commercial purposes the gallon and the litre are both represented by material standards of capac- itv, constructed as nearly as possible in accordance with their re- spective definitions. these standards are in the form of cvlindri- cal metal vessels with flat brims, and are filled exactly to their brims by the aid of flat glass “ strikes.”” comparisons with other vessels are made by transfer, the vessel under comparison being first filled and emptied to compensate for the amount of water left behind in the standard when the latter is emptied into the vessel being tested. scientific purposes —for scientific apparatus, such as flasks, burettes, pipettes, etc., where higher accuracy is needed, it is found necessary as a rule to base the verification directly on the original definition by actually weighing the quantity of pure dis- tilled water contained in, or delivered from, the vessel, making due allowance for the temperature of the water at the time of weighing and for the buoyancy of the surrounding air. in the case of vessels, such as burettes and pipettes, which are intended to deliver, and not to contain, definite quantities of liquid, it must be remembered that after delivery the walls are left wet to an extent which depends not only on the rate at which delivery is 846 measuring instruments effected and the time allowed for subsequent drainage but also | magnified image of the displacement of the anvil on a scale of about on the viscosity and surface tension of the liquid being measured. the rate of delivery is of more importance than the drainage time, and provided it is sufficiently slow the quantity of liquid delivered will be reasonably constant. suitable delivery times and error allowances are scheduled in a pamphlet on testing vol- umetric glassware issued by the national physical laboratory. bibliography.—the literature of metrology is somewhat scat- tered. the best collected account will be found in a series of articles contained in the dictionary of applied physics (1922-3) (and in particular vol. 3 thereof), edited by sir r. t. glazebrook, k.c.b., f.r.s., and published by macmillan, london, many detailed refer- ences are given in these articles. for full descriptions of the more fundamental operations the various volumes of the travaux et memoires du bureau international des poids et mesures (paris, gautier-villars) should be consulted. in addition, consult the annual reports of the standards department, board of trade (h.m. stationery office, london); miller, phil. trans., 146 (1856), on the construction of the new standard pound; airy, phil. trans., pt. 3, p. 17 (£857), on the construction of the new standard yard; kaye, ‘a silica standard of length,” proc. roy. soc., a85 (1911); michelson, light waves and their uses (chicago university press); c. e. guillaume et benoit, la mesure rapide des bases geodesiques (1908); and guillaume, les applications des aciers au nickel (paris, 1904); and proc. phys. soc., 32, 374; sears, precise length measurements, cantor lectures, roya! society of sar 1923). . be, se.) measuring instruments.—the world war was respon- sible for a considerable development and improvement of appara- tus intended for the measurement of engineers’ gauges and prod- ucts. (sec measurements.) whitworth gauges —improvement in accuracy of measurement necessarily goes hand in hand with improvement in accuracy and perfection of manufacture of the articles to be measured. the original measuring machine of sir joseph whitworth would have been of little value apart from the system of accurate stand- ards (both end gauges and cylindrical gauges) which he produced for use with it. if an object is not regularly formed to a certain degree of accuracy, it cannot be said to possess a definite meas- urement to that degree of accuracy, and it is useless to employ measuring apparatus capable of yielding results of still higher accuracy. on the other hand, while there is no great difficulty in designing highly sensitive indicating devices, it is practically impossible to calibrate them in the absence of standards meas- urable to the degree of accuracy aimed at. johansson’s gauges.—the first substantial advance in accuracy subsequent to whitworth’s work was the introduction by the swedish firm of johansson, in 1908, of flat parallel faced slip and block gauges. of such perfection of workmanship that any two of them, when cleaned, would ‘ wring” together. the gauges were made in series differing by definite small amounts, so chosen that by wringing together a suitable combination of pieces, any desired size, to the nearest o-ooor in., could be pro- duced. to ensure this accuracy, it is necessary that the individ- ual pieces should each have a guaranteed accuracy of (say) 9-ooo01 in., in order that the cumulated error of a group of four or fiveshould not exceed o-oooo05 inch. and in order to assert with confidence that this degree of accuracy is, or is not, in fact at- tained in an individual piece it is necessary to be able to measure with an accuracy of the order of o-ocooo1t inch. when the johansson gauges were first introduced no appliances were available which could be relied on to give measurements of this accuracy. three entirely different methods have since been developed, however, which enable it to be done, and which, with due precautions, give mutually consistent results. these are:— tilting level comparator.—(a) the ‘tilting level’ comparator, due to a. j. c. brookes, in which the two gauges, or groups of gauges, to be compared are stood side by side on a level surface plate, and the difference in their heights is determined by the reading of a highly sensitive level, which rests by point contact upon their upper sur- faces, through two ball feet. millionth comparator.—(b) the ‘ millionth” comparator, due to j. e. sears, jun., wherein measurements are made between two flat parallel anvils, giving a local surface contact, the one anvil being fixed, and the other connected, through a series of spring suspensions designed to eliminate all frictional effects, with a sensitive tilting mirror which causes a spot of light to move across a scale, giving a 30,000 to i. optical interference-——(c) the method of. optical interference developed at the bureau of standards, washington, wherein the whole surfaces of the gauges under comparison may be inspected, being marked out into contours of approximately o-oooor in. differ- ence in height by the alternate light and dark bands forming the interference pattern. . measurement of internal dimensions another matter of fundamental importance which has but lately found a satisfactory solution is the measurement of inter- nal dimensions. the transference from external to internal measurement constituted a considerable difficulty, and the sizes of such objects as ring gauges, either plain or screwed, were usually estimated by the nature of their fit upon corresponding plug gauges. as the fit depends to a very marked extent upon the amount and nature of the lubricant used, considerable un- certainty existed as to the correct interpretation of the observed results. if well finished and liberally lubricated with thick grease, a cylindrical plug gauge will enter and pass through a ring gauge definitely smaller than itself without damage to either. two instruments have now been produced for the measurement of internal diameters of either plain or screwed rings, which give results mutually consistent, if due care is taken, and the work being measured is sufficiently uniform in its dimensions, to the order of o-oooor inch. chord contact type.—in the ‘ chord contact ’’ machine, due to g. a. tomlinson, the diameter of the ring is deduced by calculation from the measured displacement, in a direction per- pendicular to its length, of a double ball-ended distance piece of known length, which is allowed to make contact inside the ring, first on one side, and then on the other side, of the diameter. displacement machine—in the “ displacement ” type of in- ternal measuring machine, due to j. e. sears, the position of the ring is so adjusted that measurements are made exactly across the diameter. the ring is mounted on a carriage, which can be moved bodily in the direction of the diameter being measured by means of suitable micrometers. contact is made first on one side of the ring, and then on the other, with a double-ended stvlus attached to a sensitive indicating mechanism, and the micrometer readings are taken when the indicator reads zero. the same is done in turn with a standard plug substituted for the ring, provision being made for withdrawing the stylus tem- porarily as the plug passes across. if x1 4) x2 yo are the four mi- crometer readings, the displacements d; and dz are equal to x.—x, and yi— y2 respectively, and it is to be noticed that the pressure on the stylus is in the same direction when both x readings are taken, and also when both y reaclings are taken, so that any possible errors due to backlash or flexure in the indicating mech- anism are eliminated. the transition from external to inter- nal measurement is thus directly accomplished, and the final result is given by the equation r— ditd.—p=(r—x1)+(n—y2)-p scientific principles the question of the general geometrical principles underlying the correct design of instruments intended to give the highest accuracy of measurement has received greater attention in recent years. questions of the proper application of the theory of kinematic constraint to ensure definite and repeatable regis- ter of parts, of eliminating backlash and friction effects, of pre- venting components of errors due to inevitable imperfections of workmanship (e.g., in sliding ways) from becoming effec- tive in the actual direction of measurement, of the design and relationing of parts so that elastic deformations due to chang- ing distributions of load do not affect the results, and so on, are all involved here. travelling microscope comparator—these various points have been fully considered in the “ travelling microscope com- parator ”’ designed by j. e. sears, jun., for the metrology de- partment of the national physical laboratory. the long leg of an l-shaped carriage, which bears two microscopes, is sup- meat—medals and decorations ported on two wheels running in a v-guide along the upper side of the machine bed, so that whatever position is taken up by the carriage its form is undisturbed. the focal points of the two microscopes are arranged to lie in the extension of the axis of the micrometer screw, and the object to be measured is supported on an independent carriage which, by means of a small weight attached to a cord passing over a pulley, is held permanently in contact with a stud at the end of the screw abutment. thus errors of straightness in the v-groove are non-effective, and so is any distortion of the base of the machine proper, due to the movement of the microscope carriage. advantage is taken of the simple ratio 1 in.=25-4 mm. (which is correct to within 13 parts in 1,000,000) to obtain simultaneous readings of equal accuracy in either british or the metric system, by means of suitable gearing in the compound micrometer head. the ver- niers read to o-oooor in, and o-ooor mm. respectively. the instru- ment can be used either with one microscope for making direct measurements against its own calibrated micrometer screw, or, using the two microscopes as a comparator, for determining the values of the sub-divisions of short scales. it has been found of considerable value in measuring the spacing intervals of diffraction gratings. screw gauges.—in the course of the war the necessity for rapid measurement of large numbers of screw gauges led to the development of special machines for the measurement of both pitch and effective diameter of screw threads. e. m. eden, then of the national physical laboratory, was largely instrumental in this work and in the development of the optical projection method of examining profiles of gauges, both screws and flat templets. for the latter, he found lens combinations capable of giving an un- distorted magnification of 50 times over a field of initial diameter approaching two inches. these combinations were incorporated in the now well-known “ horizontal projector.” for screw gauges the “‘ vertical projector ”’ was designed, in which the path of the light 1s vertical, and the image-forming rays are reflected back from an optically flat mirror overhead on to a specially prepared thread-form diagram placed on the table of the machine. the screw in this case is mounted in a carriage provided with two horizontal micrometer traverses in directions parallel and per- pendicular to its axis, respectively, so that measurements of pitch and diameter can be made at the same time as the accuracy of the thread form is examined. wilson projection gauge.—mention should also be made of the wilson projection gauge, in which the optical system is dupli- cated, so that the two opposite sides of a screw can be projected simultaneously on to the screen, in such a manner that the two images of the thread intermesh, and if the screw is of direct form and size, and the apparatus correctly adjusted, exactly meet. if the screw is small a space is left between the images of its two sides; if large they overlap. the projection method has naturally proved of great value in a number of other ways. in particular, it has been used in con- nection with the measurement of gear wheels and gear cutting hobs. machines for this purpose have been designed by g. a. tomlinson, and depend for their use on the accurate reproduction of the profiles of the teeth of the gear or hob, in the form of traces made on smoked glass, by a needle point attached to a specially designed pantograph. one of the machines is also fitted with a device for recording, in a similar manner, the relative velocities of rotation of two gear wheels when in mesh. in both cases the smoked glass, bearing the record, is put into the pro- jection apparatus and magnified 50 times at the screen. it is found that the traces are quite sharp in the magnified image, and measurements can be made corresponding to an accuracy of about o-o001 or o-0002 inch on the original. bibliography.—annual reports of the national physical labora- tory, stationery office, london; r. t. glazebrook, dictionary of applied physics, vol. 3. articles on ‘‘ design of scientific instru- ments,” ‘‘ gauges,” “ meters,” ‘' metrology’; e. a. griffiths, engineering instruments and meters (1921); rk. s. whipple, “ design and construction of scientific instruments,” trans. opt. soc., 22, no. 2, 1920-1; c. g, peters and h.s. boyd, “ interference meth- ods for standardizing and testing precision gauge blocks,”' bureau 847 of standards, scientific papers, no. 436; a. f. c. pollard, ‘ the mechanical design of scientific instruments,” cantor lectures, royal society of arts, 1922; j. e. sears, jun., ‘‘ precise length measurements,’’ cantor lectures, royal society of ari pg) (j. e. sr. meat: sec food supply. mecca (sce 17.950), capital of the hejaz, had a resident population of about 70,000 in 1916, including some 12,000 indians. the annual pilgrimage brings normally at least 200,000 visitors to the city, of whom pilgrims from british india and the malay states may number 25,000. pilgrims have been fewer during and since the world war, owing to disturbed con- ditions in the hejaz. mecca was taken by the arabs in 1916, and bombarded by the turks, who damaged the great mosque. the grand sherif of mecca, hussein ibn ali, of the hushimite dynasty, proclaimed the independence of the hejaz in 1916, and assumed the kingly title. in may 1924 he proclaimed him- self caliph, but shortly afterwards was driven from mecca and forced to abdicate in nov. of the same year by ibn saud, sultan of nejd, who invaded the hejaz and entered mecca without opposition. the holy places were not damaged by ibn saud and his wahabis, who performed the customary rites in the great mosque. the general aspect of mecca, as observed during the war, had changed but little, though some modern buildings had sprung up; there were a number of stately residences, including the new royal palace of six storeys, and a considerable display of wealth. the sanitation and water supply, previously very bad, had been improved. there was telegraphic communication between mecca and jidda, and between mecca and taif, the summer residence of the king, 75 m. to the southeast. representatives of foreign powers live at jidda, since non-moslems are not allowed to enter mecca. 7 mechnikov, ilya (1845-1916), russian biologist, was born at ivanovka, in the province of kharkov, may 15 1845. at the age of 17 he entered the kharkov university and two years later went to germany. in 1867 he returned to russia and was a do- zent in zoology both at odessa and st. petersburg, becoming in 1870 professor ordinarius of zoology and comparative anatomy at odessa. in 1882 he went to messina and there began his stud- ies into the nature and habits of microbes. henceforth he de- voted himself to pathological study and in 1888 went to pasteur in paris, who gave him a laboratory in the ecole normale. by 1892 his views on the essential importance of phagocytosis were firmly established. in that year he published the comparative pathology of inflammation, followed in 1901 by his chief work, immunity in infectious diseases, and a more popular treatise, phe nature of man (1903). in later years he made a special study of the bacteria infesting the alimentary canal of man. in 1908 he was awarded the nobel prize for medicine. he died in paris july 16 1916. see life by his wife, olga mechnikov (1920), trans. by mrs. r. l. devonshire (1921).