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    "source_key": "britannica_1926",
    "source_title": "Encyclopaedia Britannica (1926)",
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    "chunk_id": "1926:parasitology:0ad965e8376d",
    "title": "PARASITOLOGY",
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    "verified_text": "parasitology deals with organ- isms which dcrive their nourishment from the bodies of otheror- ganisms with which they live in varying degrees of close associa- tion and in so doing prevent them from thriving as they otherwise would do. a parasite is thus definitely harmful to its host, be- cause it directly destroys its tissues, absorbs its fluids or excretes substances which are injurious. a malarial parasite (see mala- ria) lives in a red blood corpuscle from which it abstracts its food. the cell is ultimately destroyed and toxic substances and granules of pigment are liberated from the parasite. these lead to secondary changes in tissues in which the parasite does not actuclly live, which are as dangerous to the host as the damage directly produced. individuals infected with schistosoma hae- matobium (see bilharziasis) are only slightly affected by the worms themselves but suffer very serious damage as a result of the spined eggs which work their way through the tissues toward the bladder. sometimes organisms, though intimately associated with oth- ers, merely feed on waste products of the body, while at other times two live together for mutual benefit. in neither instance is this parasitism. in the one the associated organism is a com- mensal, while in the other the two are symbionts. parasites of man.—varasites belong cither to the vegetable or animal kingdom and may parasitise either plants or animals, but here only the parasites of man which enter into the field of medical parasitology will be considered. the vegetable parasites of man belong to the fungi, bacteria and the allied spirochaetes. ring- worm and madura foot are caused by parasitic fungi; enteric fever, tuberculosis, leprosy (see leprosy, venereal disrases), pneumonia and diphtheria are due to parasitic bacteria, while syphilis, yaws and relapsing fever result from invasion of the body by spirochaetes. it is possible that the structures named rickettsia, which occur in typhus and rocky mountain fever, are bacteria but at present the evidence on this point is incon- clusive. in the animal kingdom the parasites of man are either unicellular protozoa {see prorozootocy) or multicellular metazoa. to the latter belong the groups cestoda, trematoda, nematoda and arthropoda. there is also evidence that beyond the scope of the ordinary microscope there exist ultra-microscopic viruses or parasiles which are responsible for disease. they are so minute that they pass through the pores of filters (sce filter- passing micropes) which will not permit the passage of organ- isms which can be seen with the microscope. the virus of foot- and-mouth disease is of this type, while recent investigations indicate that similar organisms may be responsible for at least certain types of malignant growths. it is only possible to con- jecture to which group these parasites belong. dependence on hosts.—parasites in general are absolutely dependent for their existence on their hosts. except in specially 48 prepared artificial culture media which imitate the natural fluids in which they live, they are unable to survive in an active condi- tion for more than a short time when separated from their hosts, though in many cases encapsuled resting stages are formed in which the parasites can live for long periods and are able to withstand adverse conditions such as desiccation and cold. mode of transference —if therefore any race of parasites is to survive, it or its offspring must find their way to new hosts. the transference takes place in one of two ways. either the parasite passes directly from one host to another of the same kind (direct transmission) or it passes into an intermediate host of another kind from which it is transferred to one of the original kind. sometimes a series of hosts is involved, as in the case of the trematode clonorchis sinensis which lives in the bile ducts of man. the miracidium, hatching from the egg in water, enters a mollusc in which sporocysts and cercariae are produced. the cercariae infect carp, which lead to infection of human beings. similarly the tape worm dibothriocephalus latus requires two hosts apart from man, a copepod crustacean and a fish. the majority of tape worms which live in the human intestine require only one intermediate host. in the case of the tape worm taenia solium it is usually the pig and in taenia saginata the ox. such transmission is indirect, there being an alternation of hosts. with fungoid and bacterial parasites transmission is nearly always direct, a notable excep- tion being bacillus pestis of plague, which may have an alter- native host in the flea, though direct transmission frequently occurs. if rickettsia are bacterial organisms they constitute another exception, as they are transmitted by lice or ticks. spiro- chaetal parasites are transmitted either directly from host to host, as in the case of syphilis, yaws and weil’s discase, or indi- rectly, as in yellow fever (sce yellow fever) and relapsing fever. in the case of spirochaetes of relapsing fever a further modification occurs, for not only can the tick which feeds on an infected human transmit infection to other human beings, but it passes the infection to its own offspring, which are then able to cause the disease. this in many respecis is comparable with the hereditary transmission of the spirochaete of syphilis from parent to child. protozoal parasites, again, are transmitted either directly through the ingestion of encysted forms, as in the transmission of the various parasitic amoebae and flagellates of the intestine and the coccidia, or indirectly, as in malaria, trypanosomiasis and probably kala azar (sce kala azar) and oriental sore. with the metazoan parasites, the majority of the cestoda and trema- toda depend on indirect transmission, though exceptionally, as in the case of the tape worm hvmenolepis nana, direct trans- mission by eggs from man to man is the rule. both methods of transmission occur with the nematoda. the intestinal forms pass directly from man to man by means of the eggs which es- cape from the intestine, while the various filaridae, including the guinca worm, require the intervention of arthropod hosts to en- able them to pass from man to man. the round worm ascaris lumbricoides, the thread worm enterobius vermicularis and the whip worm trichuris trichurus produce eggs which are directly ingested by another human being after they have undergone maturity outside the body in the development of an embryo within the egg shell. in the case of hookworms of the genera ankylostoma (see hookworm) and necator, the eggs themselves are not ingested but the embryos which develop in the eggs after they escape from the body hatch in moist soil and enter the human body by penetrating the skin. infection in the case of strongyloides stercoralis takes place in a similar manner but the embryos usually hatch from the eggs before they leave the intestine. filaridae.—the filaridae referred to above require to develop in either mosquitoes or flies of the genus chrysops. the adult worms living in the tissues of human beings produce embryos which circulate in the blood, whence they are taken by the blood- sucking diptera. in these flies they pass through a metamor- phosis and eventually reach the proboscis. when the fly is feeding on a human being the embryos escape from the proboscis on to parasitology the skin and make their way into it by passing through the open- ings of the sebaceous glands. vhe guinea worm is transmitted in a similar manner, but in this case the intermediate host is a crustacean of the genus cyclops. the embryos do not occur in the blood but escape from the female worm directly into water. they enter the bodies of the crustacea in which a metamorphosis occurs. the human being becomes infected through drinking water in which cyclops harbouring the parasites occur. arthropeda.—parasitic arthropoda in many ways stand apart from other parasites. many of them are purely ectoparasites . which live outside the host. they vary in the degrce of associa- tion. the ticks of the genus ornithodorous, bedbugs of the genus cimex, reduviid bugs of the genus triatoma and allied forms, together with blood-sucking flies such as mosquitoes, sand-flies (see sand-fly fever) and tse-tse flies, visit their hosts only when blood is required. they can hardly be regarded as para- sites. fleas are more intimately associated with thcir hosts in that they spend at least some part of their life in the clothing and do not thrive if separated from them. lice are still more depen- dent and quickly perish if removed from association with their host. the chigger tlea sarcopsylla penctrans at one stage of its existence is a true parasite. males and females live apart from the host and only suck blood when this is needed. when they reach maturity fertilisation takes place and the female fixes itself to the skin by its proboscis and gradually works its way through the epidermis till it is lodged in a pocket. it becomes globular in form and produces a large number of eggs, which escape in the pus which discharges through the opening. the eggs reach the ground, where hatching of larvae, pupation and arrival at the adult stage take place. the itch mite sarcopies scabiei has a similar life-history but the whole development takes place on the surface of the skin till the fertilised female produces the characteristic lesion by bur- rowing through the epidermis before laying cggs. various dip- tera, the larvae of which develop usually in decomposing material, like the common blue bottle fly, may lay their eggs in wounds and give rise to a condition known as myiasis. the larvae hatch and feed on the breaking down tissues till pupation occurs, when they are discharged. these larvae are only secondarily parasitic. they feed on decomposing material whether in a wound or on the ground. but having commenced to feed on decomposing tissues 77 situ they aggravate the iesion and produce an extension of the necrotic area. other flies, particularly the oestridae, actually deposit their eggs on healthy skin, the larvae burrowing beneath the epidermis. when fully grown, the larvae pupate and fall to the ground. certain muscidae are truly parasitic in the larval stage. thus cordwobia anthropophaga lays its eggs on the skin. the larvae which hatch bore their way through the epidermis and attain maturity and pupate in the subcutaneous tissues. the fly dermatobia cyaniventris is of interest in that the fly does not deposit its eggs directly on the skin but on the body of mosquitoes, from which the larvae migrate while the mos- quito is sucking blood. the larvae of auchmeromyia luteola, the congo floor maggot, lives in cracks and crevices but sucks the blood of human beings at night, returning to its hiding place after a feed, like the common bedbug. the adult fly is not a blood feeder. reproduction of parasites—the above brief summary indicates the type of parasitism which occurs in conncction with human disease. ‘transmission is either direct or indirect. when it is direct the stage of the parasite which is outside the body is either completely passive, as in encysted forms of protozoa and the eggs of many helminths, or active, as in the case of larvae of hookworms and many bacteria. during the active stage reproduction may or may not occur. the larvae of hookworms do not reproduce, though those of strongyloides stercoralis are capable of doing so outskle the human intestine. many parasitic bacteria fike those of enteric and cholera (see ciiolera) multiply in water or other situations. when transmission is indirect and two or more hosts are necessary for the completion of the life cycle the course of development may or may not be asso- ciated with reproduction. the embryos of filaridae merely undergo a metamorphosis but do not reproduce. the number of parasites which are capable of infecting man from one mosquito, for instance, is at most the same as the number of embryos originally taken up by the mosquito when it fed on infected parasitology blood. in other cases definite multiplication occurs. a pair of male and female gametocytes of a malarial parasite are able to give rise to hundreds of sporozoites in the salivary glands of an anopheline mosquito. the sporozoites, however, are unable to reproduce, so that when they have all been discharged the mosquito is no longer capable of transmitting the disease. a single miracidium of a trematode, like schistosoma haematobium or fasctolopsis buskt, can in the mollusc give rise to a large but limited number of cercariae, whereas a single egg of the tape worms 7aenia solium and taenta saginata will give rise to a single scolex in the pig and ox respectively. in the case of trypanosoma gambiense and t. rhodesiense, which give rise to sleeping sickness, the flagellates which invade the salivary glands of tse-tse flies are constantly reproducing, so that infectivity is maintained probably for the duration of life of the fly. sumilarly in the case of triatoma megista, the vector of trypanosoma cruzi of south america, the infection of the hind gut, once acquired, is re- tained for the rest of life. thus, in the case of certain parasites, even when multiplication in the intermediate host occurs, the forms taken up by the vector can give rise only to a limited number of infective stages, while in others the number is limited only by the length of life of the host. similarly, in the case of man himself, the behaviour of the parasite varies. the rarious bacteria which attack him reproduce indefinitely in his body, the only check being the development of protective substances which destroy many of the organisms. the same condition obtains in malaria, trypanosomiasis, spirochaetosis and many other diseases. metazoan parasites, however, do not as a rule reproduce in the body of man. if only a single larva of a hookworm penetrates the skin only a single adult could be found in the intestine. if only one ascaris egg were eaten only one adult would be developed from it. a single cercaria of schistosoma haematobium or other trematode will give rise to only a single adult, while a single scolex of a tape worm eaten in fish, pork or beef will give rise to but a single tape worm in the intestine. in all these infections, if a large number of parasites is present a corresponding number of infective forms must have entered the body. it follows therefore that if active reproduction does net occur in the human body the persistence of infection depends on the length of life of the individual parasites. in certain instances this may extend to many years, as in the case of schistosoma haematobium, but in all such cases, when the infection is not a heavy one and reinfec- tion can be avoided, the ill effects may be slight and a cure occur owing to the death of the parasite. the only metazoan parasite which can be said to reproduce in the human body is the tapeworm taenia echinococcus. it differs from other tape worms of man in that man is not infected with the actual tape worm stage. this occurs in the intestine of the dog. the egg, which escapes in the faeces of the dog, is eaten by man. a large hydatid cyst is devel- oped and in it a number of scolices is produced. each scolex will give rise to a single tape worm in the dog. reservoir hosts —another aspect of the parasitology of man which is of great importance is that of reservoir hosts. certain parasites which attack human beings are capable of living in other hosts without disturbing them to any marked extent. the exist- ence of such hosts in close association with man is a serious danger. the bacillus of plague lives in rats, human beings being con- stantly infected by tleas which have fed on these animals. pigs harbour the ciliate bulantidium coli, and the majority of cases of balantidial dysentery in man are in those whose work it js to tend these animals. the wild game of africa act as reservoirs of various trypanosomes and it is highly probable that tse-tse flies may infect themselves from these animals with trypanoso- ma rhodesiense and later on hand the infection to human beings. wild rats very commonly harbour the spirochaete of weil’s disease and it is extremely probable that they are responsible at least in part for maintaining a strain of organism which not infre- quently infects human beings. similarly these animals are res- ervoirs of the spirellum of rat bite fever. development of antibodies —it might be expected that those parasites such as the bacteria, malarial parasites, trypanosomes and spirochaetes which reproduce rapidly in the human body would very soon be present in overwhelming numbers. in cer- tain instances this actually happens when the strain of parasite is said to be a highly virulent one. fortunately in the majority of instances such an unlimited development is checked by the development in the host of antibodies which are harmful to the parasite. the exact nature of these substances and the method of their production are questions for the immunologists (see im- munity) to settle, but the degree of response on the part of the human host as regards the rate of production of antibodies varies considerably. in the case of relapsing fever antibodies are 49 rapidly produced in sufficient quantity quickly to destroy all the organisms. similarly in various bacterial diseases production of antibodies in many cases leads to rapid elimination of the parasites. in syphilis, tuberculosis and many other chronic infections antibody production may be sufficient to check repro- duction to some extent but insufficient to kill all the organisms, which continue to multiply slowly and produce toxins which give rise to the characteristic symptoms of the diseases. as a general rule, antibody production is much more marked in bacterial diseases than in those caused by protozoa and metazoa. meth- ods of vaccination (see vaccine therapy) have been devised for increasing the antibody production, while the introduction of antibodies, previously produced by vaccination of animals, into the human host after infection has occurred is a recognised meth- od of treatment in such diseases as diphtheria, tetanus and pos- sibly yellow fever. idiosyncrasy and immunity.—an important feature of invasion of the body by parasites is the varying effect they have on different individuals. in one individual the parasites introduced multiply so rapidly that the host is quickly destroyed, while in another repro- duction may take place so slowly that any slight derangement which occurs is overlooked. sometimes, this is due to a variation in viru- lence of the parasite itself or, in other words, in its capacity for multi- plication. in most instances, however, it undoubtedly depends on what is termed the susceptibility of the individual. certain in- dividuals are able to produce antibodies more rapidly than others, while some have them present before exposure to infection takes place. if they are present in a sufficient amount the parasites intro- duced are immediately destroyed. such individuals are said to be naturally immune and they resemble in many respects those who are imimune as a result of an actual infection from which recovery has occurred, or a vaccination which has led to the artificial production of antibodies. | carriers.—others, again, are only relatively immune, and though the parasites introduced establish themselves they give rise to few, if any, symptoms. such infections very frequently pass unnoticed. they may rapidly come to an end or may persist for long periods (see epidemiology; cerebro-spinal fever). jn the latter case the individual is known as a carrier, for, though in him the parasite apparently does little harm, it may find its way into another in- dividual who possesses no natural immunity and give rise to very serious or fatal infections. such carriers are actually reservoirs of infections like the reservoir animals mentioned above. carriers of this type occur with entamoeba histolytica, the cause of amoebic dysentery (see dyysentery). individuals have retained their infec- tions for many years without having suffered from any of the symp- toms which the amoeba may produce in others. on the other hand, when an infection takes place the parasite may reproduce rapidly and give rise to definite symptoms, which finally subside. the in- dividual may no longer realise his infection, though parasites can still be demonstrated. tle has passed into the carrier condition and is known as a convalescent carrier, to distinguish him from the contact carrier who has acquired the infection without having suffered in any recognisable way. carriers of both kinds are of even greater danger to the community than actual sufferers from diseases, for they are often unrecognised and may be a constant source of infection to others. outbreaks of enteric fever are repeatedly traced to carriers who quite unknowingly have been disseminating the parasites which cause the disease. importance in therapeutics—a complete understanding of the life histories of parasites, the response of the host to infec- tions and finally the effect of drugs is of the utmost importance in the prevention and treatment of disease. if the life history of a parasite is completely known, an attack uponit may be made at any stage of its life history, but the measures can be more drastic in the case of stages which are outside the human body. amfalarta.—since human beings never become infected with malaria parasites except from the bites of anopheline mosquitoes, malaria will be exterminated if all anopheline mosquitoes can be destroyed in a district, if individuals can avoid their bites, if the mosquitoes can be prevented from feeding on already in- fected persons, or if parasites can be eliminated from the bodies of infected individuals. any one of these measures properly carried out will lead to the same result. guinea worm.—infection with guinea worm is acquired by swallowing the crustacean cyclops into which the larvae which escape from the human host into water penetrate. the simple procedure of straining water through a fine mesh prevents the crustaceans being ingested. eggs of the hookworm, if deposited in soil, hatch out larvae which penetrate the skin. the institu- 50 tion of latrines to avoid contamination of soil stops the larvae developing in dangerous ground, while the wearing of suitable coverings for the fect prevents the larvae from reaching the skin. bitharziasis.—the cercariae of species of schistosoma, after they emerge from molluscs, survive in water for not more than 48 hours, so that water which has been kept for this length of time is not infective. the avoidance of known contaminated waters for washing and bathing purposes prevents infection. other bacterial diseases.—many bacterial diseases, such as enteric fever and cholera (see cholera), are contracted chiefly from water in which the organisms can survive and multiply. the drinking of boiled water alone and the avoidance of contamina- tion of food or feeding utensils prevents infection. ‘the exter- mination of rats, which act as reservoirs for the bacillus of plague, hinders the spread of the disease. the illustrations indicate the importance of the study of parasitology from the point of view of preventive medicine. when the complete life history of any parasite is known, an attack can be made upon it with confidence. treatment of hosts.—as regards the response of the host to infection, a study of the causes which bring about the natural elimination of parasites from the human body has a direct bear- ing on treatment directed towards the production of active or passive artificial immunity and the period of isolation necessary for complete disinfection. finally, the effect of drugs on parasites is most important, both from the point of view of the individual infected and from that of the spread of infection to others. if by treatment parasites can be got rid of in any individual, he is cured of his infection and ceases to be a source of danger to othen specific remedies.—an enormous amount of work has been done and is still in progress in connection with the search after specific remedies for each infection and the mechanism of action of such as have been discovere. this is one of the most difficult problems which confront the investigator, for both the effect of a drug on the parasite and the influence it may have on the host have to be taken into account (see puarmaccy; pharmacology). recent investiga- tions show that in many cases the view that a drug introduced into the body of a host directly attacks the parasite is erroneous. in many, if not all, cases the actual substance introduced ts not that which affects the parasite. the fluids and tissues of the host react upon it chemically so that some derivative is the actual toxic agent, or the drug itself stimulates the cells of the body to produce second- ary substances which are injurious to the paras‘tes. such specific agents are quinine for malaria, emetin for amoebic dysentery, or- ginic arsenic compounds for syphilis and relapsing fever, antimony salts for kala-azar and schistosomiasis. though the administration of these drugs to infected persons rids them of their parasites, the exact mechanism of the action has not been elucidated. many of these specific remedies have no action whatever on the parasites outside the body of the host. parasites as therapeutic agents——the antagonism between one parasite and another is now being applied to the treatment of disease. in certain stages of infection with the spirochaete of syphilis the artificial infection of the host with the spirochaete of relapsing fever or the parasite of malaria, both of which can be easily eradicated by treatment, so alters the environment of the original parasites that they cease to multiply and are apparently destroyed. the view that the one parasite directly attacks the other is hardly tenable. more probably the superimposed infection changes the host in such a way that he ceases to be suitable for the continued development of the spirochaetes originally present. : undiscovered parasites.—in many cases in which there is evidence that disease is due to parasitic invasion of the body, the causative organism is known and much information regarding its life history has been obtained. in some instances, however, the parasite has still to be discovered. it has not yet been definitely decided whether the bacillus of pfeiffer or a filterable virus is responsible for in- fluenza (see influenza). the parasite which gives rise to the common cold is still unknown, while there is no precise information regarding any organism which can be held to be the cause of smallpox. en- cephalitis lethargica, or sleepy sickness, and several other diseases are ina similar position, but rie advances which are constantly being made in the methods of investigation will undoubtedly solve the many problems which surround these diseases, the etiology of which is still obscure. ; bistiocrapny.—j. j. clarke, protists and disease (1922); h. b. fantham, j. w. w. stephens and f. v. theobald, zhe animal parasites of man (1916). (co ne w.*) paratyphoid fever: see infectious fevers. paravane.—the paravane was developed during the world war, in connection with the anti-submarine and anti- paratyphoid fever—paravane mine campaign, as a means of attacking submerged submarines . for which purpose it was filled with an explosive charge and called an explosive paravane. subsequently it was further devcloped as an anti-mine device, and when used for this pur- pose was called a protector paravane. the exprlosivee paravane a considerable amount of auxiliary apparatus was required in connection with the explosive paravane, the whole installation being called the high speed submarine sweep. it enables a vessel to attack a submerged hostile submarine whose position is but approximately known. it consists essentially of two torpedo- shaped bodies called paravanes, which are towed one on each quarter of the attacking vessel. fach paravane contains an explosive charge, and runs at a definite and pre-arranged depth below the surface of the sea, whilst the speed of towing makes it move outwards a certain distance from the fore and aft line of the ship. the paravanes are towed from steam winches by spec‘a!ly prepared armoured electric cables, led over fairleads in the stern of the attacking vessel. the towing wire takes up a curve of definite shape which is practically independent of the speed. methods of detonating.—the charge may be detonated in one of three ways: (1) by impact; (2) by strain; (3) by hand. when the towing wire fouls a submerged submarine it will slip over her smooth surface until eventually the submarine is struck by the paravane. the striking gear which is fitted to the nose of the paravane is moved inwards by the impact and thus a firing switch is actuated and detonates the charge. an alternative device for detonating the charge is fitted in case the first does not operate. if, as might easily happen, the nose of the paravane does not strike the submarine, the wire will in con- seucnce be held up or will be nipped over some part of the sub- marine’s hull, when an excessive load comes upon the wire. any load above a predetermined one trips a dynamometer switch which will again make the electrical contact requisite to detonate the charge. the third method of detonating the charge is by a hand switch on the attacking vessel, and meets the case when it is desired to get rid of the paravane purposely, either te mect some sudden emergency necessitating full speed, or to llow up the paravane on some marked spot such as a patch of oil escaping from a sup- posed damaged submarine. spread of sweep.— the maximum spread of the sweep is about 30 yd., t.e., 60 yd. from paravane to paravane, and about 120 yd. of towing wire are then required for each paravane. dve to the design of the paravanes, this spread can be obtained inde- pendently of the depth at which the paravanes are being towed. the “ q” type paravanc hercin described consists essentially of a torpedo-shaped body carrying a plane which gives the neces- sary thrust to get the maximum spread, and a rudder actuated by a hydrostatic valve to give the requisite depth of running independently of the amount of towing wire veered. in addition to the hydrostatic valve, the paravane carries within it a charge of t.n.t., depth-recording gear, firing switch, detonators, etc. the advantages of this type of paravane are: (1) it can be towed at high speeds. (2) it leaves unaffected the manoeuvring powers of attacking vessels. (3) being light it is easily handled. (4) it is not dangerous under shell fire when stowed on board. construction —the paravane is divided into four parts, viz.: (1) head; (2) bonnet; (3) body; (4) tail. the [fead.—the head carrics externally the tow anchorage, plane and striking device, and internally the gear for transmitting the effect of the impact to the striker-switch. the bonnet.—yhe bonnet is a steel dished plate carrying the striker-switch and depth-recording gear. the depth-recording gear registers on the attacking vessel the depth at which the paravanc is running, the bonnet, carrying with it the striker-switch, hes wholly within the head, but the depth-recording gear, fastened to the after side of the bonnet, lics within the body of the paravane. both the bonnet and body are made watertight. the body.—tvhe body is made of steel plate jg in. in thickness, and isacetylene-welded to ensure watertightness. the body carries within it the charge of t.n.t., the primer tin and the detonating gear. the most complex part is the detonating gear, which derives paravane its complexity from the arrangements necessary to prevent the charge from detonating whilst the paravane 1s on board or in close proximity to the ship. as now arranged, the detonators are withdrawn from the primer tin, and the two safety shutters, being simultaneously dropped, form an effective fire screen between the two. this safety device ts in operation whilst the paravane is on board or moving at a slow speed higouh the water. at a speed greater than about 10 knots, a water flap, which projects from the under side of the para- vane, is forced aft by the water pressure upon it. this movement is transmitted by linkwork to a detonator carriage, which moves aft, forcing the safety shutters to one side and placing the detonators in position in the primer. the tail. —the tail consists of a bronze casting and carries the two rudders, the hydrostatic valve whch actuates the rudders, and the horizontal and vertical fins. when the paravane is required to run at a certain depth, a compression corresponding to this depth is put upon the hydrostatic valve spring. this load due to this com- pression forces the rudders ‘“‘ down.” the water pressure acting on the hydrostatic valve, however, tends to force the rudders '‘ up.” when the two balance, the rudders return to the amidships position and so keep the piravane on a course which undulates about the set depth. this apparatus was fitted in the allied navies as well as to the british navy. the official record of the results is as follows, as regards the british navy alone:— submarines known sunk 5; probably sunk 1; probably severely damazed 4; and probably slightly damaged 11. the protector paravane the protector paravane installation is designed to protect ships of all classes from moored mines. on either side of the vessel, by means of a length of special wire, is towed the paravane, a torpedo-shaped buoyant body. these paravanes stand out at a considerable lateral distance from the fore and aft line of the ship, and remainat a definite depth below the surface of the water. the planes and fins with which the paravanes are fitted have therefore a considerable thrust at high speeds, and this load is transmitted to the ship through each towing wire. but the tow- ing wires constitute, indeed, one of the protective elements of the installation. due to the position taken up by the paravanes, the wires lie in a wedge-shaped position on either side of the ship, and sweep through a broad path. if they foul the mooring wire of a mine, the mine is deflected away from the path of the vessel. not only so, but the mooring wire will slip along the paravane wire until it reaches the paravane. there it 1s caught in a cutter fitted on the head of the paravane which severs the mine from its mooring. the buoyant mine springs to the surface, where it can he readily seen and destroyed. the location of the position at which each towing wire leaves the vessel is clearly very important. in prac- tice the towing wire is attached to the vessel at a specially de- signed permanent fitting situated on or near the line of keel and as far forward as practicable. different ty pes.—when the towing wire fouls a mine mooring wire, 2 minimum outward thrust must be imparted to the mine and sinker in order to deflect them from the path of the vessel. it is computed that this thrust is over half a ton, and must be imparted independently of the type of paravane fitted if the pro- tection is to be of any value. difficulties arise on this account when dealing with vessels of widely different speeds. ifa paravane be designed to give this thrust whilst moving at a moderate speed through the water, its total thrust when moving at a high speed would be very great, and an excessive load would be thrown upon the towing wire. two different types of paravanes are therefore constructed, according to the maximum speed of the vessel to which they are to be fitted, viz., type “b,” for battle- ships and large vessels, whose speed is under 22 knots; and type ““c,” for cruisers and vessels, whose speed is over 22, but docs not exceed 28 knots (see fig. 1). in each type of paravane the towing rope can withstand the load produced when the vessel travels at her maximum speed, whilst the paravane itself provides the minimum outward thrust at the lower speeds necessary to deflect the mine. since the out- ward thrust produced by a paravane increases approximately as the square of the speed, the range of speed over which the paravane remains effective is reduced as the maximum speed for which the paravane is designed is increased. broadly speaking, however, a suitable paravane will protect a vessel at all speeds si from about one-third her maximum upwards. the effective speed of a paravane also depends upon the beam of the vessel to which it is attached—the throw-out necessary for a battleship 1s, for example, much greater than that for a mine-sweeper or other similar small vessel. hence a “b” type paravane is effective for the latter class of vessel at a much lower speed than it would be in the case of a battleship. buoyancy.—at an early stage in the experimental design of the paravane, it was found that buoyant and non-buoyant paravanes were equally effective. the non-buoyant paravane has, however, been discarded for two reasons. firstly, there would be a possi- bility of the paravane being damaged. when the ship slows down or stops in shallow water, the paravane would sink to the bottom. a rocky bottom would probably have a disastrous effect —tr -\" fender bar plan _of_ tail fic. 1.—diagrams illustrating the general arrangement of two types of paravane. upon a paravane when the ship went ahead again. secondly, the horizontal angle of tow would vary with the speed. the excess of the weight of the paravane over its buoyancy being con- stant and acting vertically downwards, would, when compounded with the varying outward thrust of the plane, give a resultant force whose clirection varied constantly. hence the angle from the vertical at which the paravane was towed could not remain at all speeds. the greater the excess of weight over buoyancy of the paravane, the greater would be ihe variation of this angle. this would cause the paravane to run deeper and deeper as the speed decreased. although this is not a drawback from a pro- tection standpoint, it is a considerable disadvantage from a navigation standpoint, seeing that the virtual draught of the ship increases as her speed is reduced. with a buoyant paravane, should the vessel be compelled to stop suddenly, the paravane, being buoyant, will float on the surface and support the towing wire. it will always sink to its set depth when the vessel is under way, as long as the speed is only a little above two knots. length of towing wire and spread.—the protection afforded by the paravanes depends, amongst other things, upon the dis- tance from the fore and aft line of the ship at which the para- vanes are towed. this distance or spread in turn depends upon three things:—(1) the length of the towing wire; (2) the size of the paravane; (3) the size of the towing wire. depths of tow points and paravwines——vhe paravanes must be attached to the ship as far forward and as low down as prac- ticable. if the point of attachment were above the level of the keel, a mine could pass under a towing wire and hit the ship. the point of attachment usually selected is the point of inter- section of the fore perpendicular with the line of keel. this en- ce. tails the design on all warships of a permanent fitting or stem extension. further, the paravanes should not tow at any considerable depth below the keel line of the ship. if they did so, and the ship travelled from deep into shallow water, the paravanes might strike the bottom and be destroyed. as moored mines are usu- ally laid in shallow water, the paravanes are arranged to run at little greater depth than the maximum draught of the vessel using them, and at approximately a constant depth, no matter how the vessel to which they are attached varies in speed. an essential feature of the paravane design is, therefore, a rudder actuated by a hydrostatic valve. this valve can be set to run at a definite depth before the paravane is thrown overboard. when a paravane is fitted with this gear, the depth at which it runs does not vary more than 4 ft. on either side of the set depth. the paravane is usually set to run at a depth which is about s ft. in excess of the deepest draught of the vessel. the para- vane towing wire and the tow point are thus practically in a horizontal plane in which also lies the keel of the vessel. any mine within the sweep of the paravanes, which lies between the line of keel and the surface of the sea, must therefore be deflected by the towing wire. if a mine be moorcd so deeply that it passes under the towing wire, it cannot, under ordinary circumstances, strike the ship in calm water. the paravane attachment does not claim to be an absolute protection to a pitching vessel. oscillator.—the depth-keeping mechanism in the paravane is composed of two distinct portions, the hydrostatic valve and the mercury oscillator, the hydrostatic valve depends, for its action, on a change of depth above or below the normal depth. it therefore causes the paravane to follow a sinusoidal path through the water. at high speeds this motion becomes excessive and a device is re- quired which will return the paravane to its correct position as soon as it is inclined to move from its set depth. this function is per- formed by the oscillator. it is the same in ‘‘b” and “‘c” para- vanes, and is composed of a triple valve, comprising two mercury valves and one hydrostatic valve, and a compression spring. the cutter.—the cutter requires no setting, and can take the two parts of a mine mooring in rapid succession. the cutter supplied is very effective, and will sever a 1} in. wire with a pull of about seven cwt. further experiments show that this cutter will sever a j to 4 in. chain at a speed of 20 knots. since it is impossible to moor any existing design of mine with such a large diameter chain, this cutter will be sufficiently effective for every size of mine mooring likely to be found in practice. its efficiency may be illustrated in another way. it cuts a mine mooring with no greater pull than that afforded by the mere weight of the mine sinker. it is therefore impossible for the mine and sinker to be towed by the cutter. as an example of the performance of these cutters, the results of tests made on three cutters selected at random are appended :— cutting pull (b.) a b cc d_ | average no, i 854 | 780 | 797 | 636 | 767 no. 2 ; 945 | 833 | 833 | 847 | 864 no. 3 ' 670 749 721 693 708 these tests were made on an imitation german mine mooring rope. the loads were applied statically, starting at 300 lb. and rising by sinall increments. conditions under helm.—it is desirable to consider not only the conditions when the vessel is steaming onastraight course, but also those when she is moving under helm. as already described, an effective paravane will deflect any mine from the path of a vessel which is on a straight course, but it is conceivable that, whilst a vessel is turning, a mine may be missed by the bow paravane and hit the ship near the stern, from this it is apparent that such a ship when under full helm sweeps out a pathway about 300 ft. wide. in arranging that, as far as possible, the paravanes should sweep the sea clear of all mines which approach the outer side of a vessel swinging round under moderate and, if possible, manoeuvring helm, there are two cases to be considered. for relatively small ships, such as protected cruisers and all smaller vessels, one pair of paravanes gives practically complete immu- nity from moored mines as long as the ship is not in a heavy sea. with larger ships, however, the distance swept out by the vessel pardo bazan—paris herself when turning under full helm may be so great that one pair of protector paravanes does not give adequate protection. itorse-power a bsorbed.—the following table gives the shaft horse- power absorbed at different speeds for the two types of paravanes when using the normal (56 yd.) length of wire. of course, if the length of the wire be decreased, the resistance to motion and hence the shaft horse-power absorbed is reduced. on the other hand, if the length of wire be increased, the shaft horse-power absorbed is also increased. type op type\" c” paravane paravane speed (knots) . 16} 18] 20 22 18 22 26| 28 h.p. absorbed | 450 | 620] 870 {1,170 | 640 |1,100|1,780]| 2,230 the above figures are deduced from actual tests on the meas- ured mile, and represent the horse-power absorbed by the para- vane system. ‘the shaft horse-power absorbed on any particular ship may, however, differ slightly from the above figures on either side, seeing that the propellers will then be working under different conditions and will consequently have different effi- ciencies, but the above figures should be correct to within 5% for any particular vessel. the official record of the results obtained shows that british warship tonnage that, but for protector para- vanes, would have been either sunk or seriously damaged, totalled 583,713 tons. the otter—when the admiralty decided to equip british merchant vessels with paravane protection, a simpler form of paravane called an ‘‘ otter ” was introduced. it is in all respects the same type of device as the protector paravane, excepting that the mercury oscillator is removed. the removal of the mer- cury oscillator renders the otter unstable above 14 knots. since the average speed of a merchant vessel is 10-13 knots, the otter gave good service in practice. cilronology sept. 1914. provisional scheme and design of explosive paravane submitted to british admiralty by lieut. c.d. burncy. may 1915. high speed submarine sweep tested in 11.m].s. ‘ mas- tiff’? at harwich and approved for supply to h.m. navy. march 1915. provisional design of protector paravane submitted by lieut. c. d. burney. nov. 1915. first successful trial in h.m.s. “‘ melampus’”’ of protector paravane. feb. 1916. protector paravane successfully demonstrated to grand fleet, and bo ncoved (or supply to h.m. navy. april 1917. first merchant vessel fitted with ‘' otters.” (when the armistice was concluded, 3,000 merchant vessels had been fitted with otters.) (c. d. b.) pardo bazan, emilia (1851-1921), spanish author (see 20.800), died in madrid may 12 1921. parent, simon napoleon (1855-1920), canadian politician (see 20.801 ), died in sept. 1920.",
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