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many people breathing in the fame atmosphere, thereby rendering it unfit for respiration.

It has been frequently tried, that if a gallon of air be contained in a bladder, and, by means of a blowpipe, infpired and expired into the lungs of a man, without having any communication with the external air; in the space of a minute, or little more, it becomes heated, and unfit for refpiration; and without the addition of fresh air, the perfon making the experiment would fpeedily be fuffocated. The divingbell is another instance of the fame kind, wherein a conftant fupply of fresh air must be had, to keep out the water, and refresh the people included.

Although air is abfolutely neceffary to our existence, and neceffity constrains us inevitably to breathe therein, it may be made a vehicle of moft malignant poifons, as witnefs the famous Grotto del Cani in Italy, the poisoning air by charcoal, and air impregnated with the fumes of fermenting vegetable liquors, Stagnant air, either alone or mixed with water, foon becomes very offenfive and pernicious; as in wells dug for the fupply of water, and difufed for fome time; as is the air alfo in the wells and in the holds of fhips, which is occafioned principally by what is ufually called the bulge-water, which, if the hip is tight, and not frequently pumped, becomes not only very offenfive, but fo extremely poisonous, as frequently to fuffocate thofe feamen, who, as the pumps are fubject to be clogged with filth, venture down to cleanse them; and will caufe alfo in perfons at à diftance violent head-achs, cold fweats, and frequent vomitings, which continue more or lefs, in proportion to the diftance from the well of the fhip,

when

when the injury was received, and the degree of putrefaction in the water and air.

The air, in fhips particularly, is very liable to be vitiated, not only from the bulge-water, but from too many people breathing in the fame atmosphere; especially in ships of war, hofpital-fhips, and those ufed in the Guinea trade for negroes; where a number of uncleanly people, being ftowed too close together, heat the air, make it replete with noxious effluvia, destroy the particles therein adapted to cool the lungs, particularly the acid nitrous gas. This principle is abundant in cool air, and manifefts itfelf not only from the quantity of nitrous crystallizations, which may be collected from caverns of the earth, especially those open to a northerly afpect, but also from expofing pieces of the flesh of animals fresh cut, or their blood, whereby the colours of their furfaces are foon changed from a dark deep red to a more lively and florid one. Air robbed of this valuable property, and replete with hurtful ones, not only from the people, but from the stinking water in the well and lower parts of the ship, muft produce the most putrid, if not peftilential fevers.

Although the æquilibrium within places confined is maintained by the external air, yet unless, by openings properly adapted, the air is fuffered to pafs freely enough, the external air proves as a stopple to the internal, and only mixes with that portion of it which is next in contact: this is evident from the common occurrence in privies, which are fcarcely offenfive in clear weather, but are much fo in foul or windy, from a diminution of the incumbent preffure of the atmosphere,

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atmosphere, when the vapours that have been pent up, expand themselves to a confiderable diftance.

To prevent the above-mentioned inconveniencies, and to preserve the healths and lives of the feamen, that valuable part of the nation, many fchemes have been thought of; particularly the machines of those two very worthy, ingenious, and induftrious members of this fociety, the Rev. Dr Hales, and the Rev. Dr Defaguliers; the first by an inftrument which he calls the fhip's lungs, and the latter by a machine †, which is an improvement of the Heffian bellows; but as these have been laid before the fociety by the gentlemen themselves, I fhall pafs them over, and proceed to mention the contrivance commonly made use of, I mean the wind-fails. They are made of the common fail-cloth, and are ufually between 25 and 30 feet long, according to the fize of the fhip, and are of the form of a cone ending obtufely; when they are made use of, they are hoisted by ropes tò about two thirds or more of their height, with their bafis diftended circularly by hoops, and their apex hanging downwards in the hatchways of the fhip; above each of thefe, one of the common fails is fo difpofed, that the greatest part of the air, rufhing against it, is directed into the wind-fail, and conveyed, as through a funnel, into the upper parts of the body of the fhip. These must be hung up and taken down every time they are used, and the fupply by this method is not conftant. Though custom has given a fanction to this device, it is fubject to many inconveniencies. ft, Each fhip having commonly

*See Dr Hales's treatife of ventilators. Philof. Tranf, N° 437.

+ See

three

three of thefe, (one to each maft), the feamen are a confiderable time in getting their apparatus ready, and in hoifting them up to make ufe of. 2dly, They can only be used in mild weather. 3dly, Near the equator, where fresh air is most wanted, there fometimes happen fuch ftark calms, that they are ufelefs, by not having air enough to diftend them. 4thly, The air hereby admitted paffes only into the upper and more open parts of the fhip, fo that the well, &c. receive no change therefrom; and it is obferved, that fometimes, upon ufing them after fome difcontinuance, they drive offenfive air into the cabin, and more airy parts of the fhip: like as the pouring fome fresh into ftinking water makes the whole flink, though in a lefs degree. 5thly, They are improper to be used in the night-time, when the people are fleeping between decks. And lastly, admitting they had none of the former inconveniencies, their ufe must be destructive in hospital-ships; where, though fresh air imperceptibly received, is abfolutely neceffary to preferve the crew as free as poffible from the infectious breath and exhalations of the diseased and wounded feamen; yet blafts of wind, pouring impetuously into the very places where the fick lie, must be attended with fuch confequences as are too obvious to mention.

To remedy these inconveniencies, to prevent the air proving foul even in the wells and holds of fhips, and to cause imperceptibly a large circulation of fresh air into every part of the fhip at all times, Mr Sutton has invented the following fcheme, which is ufeful not only in thefe cafes, but, by altering fome parts as particular places require, may be applied to

houfes,

houfes, the clofe parts of prifons, wells at land, privies, hofpitals, &c.

Nothing rarefies air fo confiderably as heat, which whenever it caufes a diminution in the denfity of the air, that part next in contact will rush in, and be fucceeded by a conftant fupply, till the air becomes of an equal degree of clafticity. Therefore, if a tube be laid in the well, hold, or any other part of the hip, and the upper part of this tube be fufficiently heated to rarefy the impending column of air, the xquilibrium will be maintained by the putrid air from the bottom of the tube, which being drawn out this way, a fupply of fresh air from the other parts of the fhip will fucceed in its place; which operation, being continued, will entirely change the air in all parts of the fhip. This principle, exactly conformable to the doctrines of pneumatics, is the bafis of Mr Sutton's machine, which being put in execution on board the hulk at Deptford, before the Lords of the Admiralty, Commiffioners of the Navy, our very learned and ingenious Prefident, M. Folkes, Efq; Dr Mead, &c. performed to their fatisfaction, in bringing air from the bread-room, horlop, and well of the ship at the fame time, in fuch quantity, that large lighted candles being put to the end of the tubes, the flame was immediately fucked out as fast as applied, though the end of one of the tubes was above twenty yards diftant from the fire. The method is as follows.

To boil the provifions of the fhip's company, they must have a copper, which is bigger or lefs, in proportion to the fize of the fhip, and number of the crew; this copper is fixed in fhips in the manner as

on

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