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of perfect folution, when the air will be clear, dry, and heavy, and its powers of folution fill active; in a state of beginning precipitation, when it becomes moift and foggy, its powers of folution are diminished, and it becomes lighter in proportion as its water is depofited; and 3dly, when it is completely precipitated, which may happen either by a flower procefs, when the diffolved water falls in a drizzling rain, or by a more fudden procefs, when it defcends in brifk thowers. Phil. Tranf. vol. 67, p. 257, and Phipps's Voyage towards the North Pole, p. 211.

2. A

(8.) EVAPORATION, DR HALLEY'S EXPERI MENTS AND CONCLUSIONS RESPECTING. Dr Halley, whofe theory is mentioned above, (5.) has furnished fome experiments on the cvapo ration of water, the refult of which is; 1. That water falted to about the fame degree as fea water, and expofed to a heat equal to that of a fummer's day, did, from a circular furface of about 8 inches diameter, evaporate at the rate of 6 ounces in 24 hours: whence by a calculus he finds that, in tuch circumftances, the water evaporates one roth of an inch deep in 12 hours: which quantity, he obferves, will be found abuncantly fufficient to furnish all the rains, fprings, dews, &c. By this experiment, every 10 fquare inches of furface of the water yield in vapour per day, a cubic inch of water: and each fquare foot half a wine pint; every space of 4 feet fquare, a gallon; a mile fquare, 6914 tuns; and a fquare degree, of 69 Englifh miles, will evaporate 33 millions of tuns a day; and the whole Mediterranean, computed to contain 160 fquare degrees, at leaft 5280 millions of tuns each day. Phil. Tranf. N° 189, or Abridg. vol. 2. p. 108. furface of 8 fquare inches, evaporated purely by the natural warmth of the weather, without either wind or fun, in the courfe of a whole year, 16,292 grains of water, or 64 cubic inches; confequently, the depth of water thus evaporated in one year amounts to 8 inches. But this being too little to anfwer the experiment of the French, who found that it rained 19 inches of water in one year at Paris; or thofe of Mr Townley, who found the annual quantity of rain in Lalicalhire above 40 inches: he concludes, that the fun and wind contribute more to evaporation than any internal heat or agitation of the water. In effect, Dr Halley fixes the annual Evaporation of London at 48 inches; and Dr Dobfon ftates the fame for Liverpool at 36 inches. Phil. Tranf. vol. 67. p. 252. 3. The effect of the wind is very confiderable, on a double account; for the fame obfervations thew a very odd quality in the vapours of water, viz. that of adhering and hanging to the furface that exhaled them, which they clothe as it were with a fleece of vapourous air; which once invefting the vapour, it afterwards rifes in much lefs quantity. Whence, the quantity of water loft in 24 hours, when the air is very ftill, was very fmail, in proportion to what went off when there was a Arong gale of wind abroad to diflipate the fleece, and make room for the emiflion of vapour; and this, even though the experiment was made in a place as clofe from the wind as could be contrived. Add, that the fleece of water, hanging to the furface of waters in fill weather, is the occa

fion of very ftrange appearances, by the refrac tion of the vapours differing from and exceeding that of common air: whence every thing appears raifed, as houfes like fteeples, fhips as on land above the water, the land raifed, and as it were lifted from the fea, &c. 4. The fame experiments fhew that the Evaporations in May, June, July, and Auguft, which are nearly equal, are about three times as great as thofe in November, December, January, and February. Phil. Tranf. N° 212, or obr. vol. 2. p. 110.

(9.) EVAPORATION, DR HAMILTON'S THEORY OF. Dr Hamilton, profeffor of philofophy in the univerfity of Dublin, tranfmitted to the Royal Society in 1765, a long Diflertation on the nature of Evaporation, in which he propofes and establishes the theory of SOLUTION; and though other writers had been prior in their conjectures, and even in their reafoning on this fubject, Dr. Hamilton affures us, that he has not represented any thing as new, which he was confcious had ever been propofed by any one before him, even as a conjecture. Dr Hamilton, having evinced the agreement between Solution and Evaporation, concludes, that Evaporation is nothing more than a gradual folution of water in air, produced and promoted by attraction, heat, and motion, just as other folutions are effected. To account for the afcent of aqueous vapours into the atmosphere, this ingenious writer obferves, that the loweft part of the air being prefied by the weight of the upper against the furface of the water, and continually rubbing upon it by its motion, attracts and diffolves thofe particles with which it is in contact, and feperates them from the reft of the water. And fince the caufe of folution in this cafe is the ftronger attraction of the particles of water towards the air, than towards each other, thofe that are already diffolved and taken up, will be ftill farther raifed by the attraction of the dry air that lies over them, and thus will diffuse them. felves, rifing gradually higher and higher, and fo leave the lowest air not fo much faturated, but that it will still be able to diffolve and take up freth particles of water; which process is greatly promoted by the motion of the wind. When the vapours are thus raifed and carried by the winds into the higher and colder parts of the atmofphere, fome of them will coalefce into small particles, which flightly attracting each other, and being intermixed with air, will form clouds; and thete clouds will float at different heights, according to the quantity of vapour born up, and the degree of heat in the upper parts of the atmofphere: and thus clouds are generally higher in fummer than in winter. When the clouds are much increased by a continual addition of vapours, and their particles are driven close together by the force of the winds, they will run into drops heavy enough to fall down in rain. If the clouds be frozen before their particles are gathered into drops, imall pieces of them, being condensed and made heavier by the cold, fail down in thin flakes of fnow. When the particles are formed into drops before they are frozen, they become bailftones. When the air is replete with vapours, and a cold breeze fprings up, which checks the folution of them, clouds are formed in the lower

parts

fel which contained the liquor was more open; but the effects did not increase in proportion to the apertures. 5. Electricity was alfo found to increafe the evaporation from folid bodies, and of confequence to augment the infenfible perfpiration of animals.

parts of the atmosphere, and compofe a mift or fog, which ufually happens in a cold morning, and is difperfed when the fun has warmed the air, and made it capable of diffolving thefe watery particles. Southerly winds commonly bring rain, because, being warm and replete with aqueous vapours, they are cooled by coming into a colder climate; and therefore they part with fome of them, and fuffer them to precipitate in rain: whereas northerly winds, being cold, and aequiring additional heat by coming into a warmer climate, are ready to diffolve and receive more va pour than they before contained; and therefore, by long continuance, they are dry and parching, and commonly attended with fair weather. Changes of the air, with respect to its density and rarity, as well as its heat and cold, produce contrary effects in the folution of water, and the confequent afcent or fall of vapours. Several experiments prove that air, when rarefied, cannot keep fo much water diffolved as it does in a condensed state; and therefore when the atmosphere is faturated with water, and changes from a denfer to a rarer ftate, the high and cold parts of it will let go fome of the water before dissolved, forming new clouds, and difpofing them to fall down in rain: but a change from a rarer to a denfer flate will ftop the precipitation of the water, and enable the air to diffolve, either in whole or in part, fome of thofe clouds that were formed before, and render their particles lefs apt to run into drops and fall down in rain. On this account, we generally find that the rarefied and condensed tates of the atmosphere are respectively attended with rain or fair weather. See Phil. Tranf. vol. 55. p. 146, or Hamilton's Philof. Effags, P. 33.

OF.

(10.) EVAPORATION, DR WISTAR'S THEORY In the Tranfactions of the American Philofojhical Society, vol. 3. p. 125, there is an ingenious paper on evaporation, by Dr Wiftar. It is there thewn, that evaporation arifes when the moist body is warmer than the medium it is inclofed in. And, on the contrary, it acquires moisture from the air, when the body is colder. This car rying off, and acquiring of moisture, it is fhewn, is by the paffage of heat out of the body, or into it.

(11.) EVAPORATION, METHODS OF PERFORMING. See CHEMISTRY, Index.

(12.) EVAPORATION PROMOTED BY ELECTRICITY. Evaporation, according to the experiments of the Abbè Nollet, appears to be promoted by electricity. See ELECTRICITY, Index. The conclusions drawn from them are, 1. Electricity augments the natural evaporation of fluids; all that were tried, excepting mercury and oil, being found to fuffer a confiderable diminution, greater than what could be afcribed to any other caufe. 2. Electricity augments the evaporation of thofe fluids the moft which are found moft readily to evaporate spontaneously: the volatile ipirit of fal ammoniac fuffering a greater lofs than fpirit of wine or oil of turpentine, thefe two more than common water, and water more than vinegar or a folution of nitre. 3. The effects seemed always to be greatest when the veffels containing the fluids were non-electrics. 4. The increased evaporation was more confiderable when the vef

(13.) EVAPORATION, QUANTITY OF WATER NATURALLY PRODUCED BY. Evaporation is one of the great natural proceífes, by means of which the whole vegetable kingdom is fupplied with rain neceflary for its fupport. This evaporation takes place at all times, not only from the surface of the ocean, but of the earth alfo. Dr Halley, by an experiment with a pan of water kept in the heat of our fummer fun, found, that as much water might be reasonably fuppofed to evaporate from the furface of the Mediterranean fea, as would be fufficient to fupply all the rivers which run into it. (See § 8.) Dr Watfon in his Chemical Effays, has fhown, that the evaporation is not lefs confiderable from the furface of the land than from that of the fea. By inverting a glass veffel on the ground, in the time of a confiderable drought, be found that even then about 1600 gal. lons of water were raised from an acre in 24 hours; and repeating the experiment after a thunderfhower, he found that in fuch a flate an acre parted with above 1900 gallons of water in 12 hours. This evaporation is carried on not only from the ground itfelf, but from the leaves of trees, grafs, &c. with which it is covered; and great part of the water thus raised falls down in the night-time in dew, being abforbed by the vegetables which yielded it before. Thus the earth is not fo foon exhaufted of water, even for a little way below the furface, as we might imagine from the quantity raifed by evaporation : for if all that was rai. fed by the fun's heat during the time of a long drought, left the earth not to return to it for perhaps or 6 weeks, the whole vegetable kingdom, at least such as do not strike their roots very deeply into the ground, would of neceffity be deftroyed; which yet we fee is only the cafe with the moft tender grafs, and even that only on the moft elevated fituations, and when moft expofed to the fun. Dr Brownrigg, in his Art of making common falt, p. 189. fixes the evaporation of fome parts of England at 73 8 inches during May, June, July, and Auguft; and the evaporation of the whole year at more than 140 inches. But the evaporation of the 4 summer months at Liverpool, on a medium of 4 years, was found to be only 188.8 inches. Dr Hales calculates the greatest annual evaporation from the surface of the earth in England at 666 inches; and therefore the annual evaporation from a furface of water is to the annual evaporation from the furface of the earth at Liverpool, nearly as 6 to 1. See Phil. Tranf. vol. 67.

(14.) EVAPORATION, USE AND EFFECTS OF IN PRODUCING COLD. Another great use of the natural evaporation is to cool the earth, and prevent its being too much heated by the fun. This property of producing cold by evaporation has been but lately obferved by chemifts, though it has long been employed by thofe who knew not the reason of their doing fo. It has been obferved at Aleppo in Syria, that the water is always

coolest

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again confolidate itself into a much harder mass than before. See ICE.

coolest when the weather is moft warm and the power of the fun exceffive. The heats in that part of the world are fometimes almoft EVAPORATOR, n. s. an apparatus calculated intolerable; and at that time the evaporation for expediting the procefs of evaporation. A mofrom the outfide of the jars, which are made of del of it was prefented by the inventor, Mr porous clay, is very copious; and in proportion Browne of Derby, to the Society for the encouto the quantity of water evaporated from with- ragement of Arts, &c. who conferred on him out, is the degree of cold in the liquor within. their gold medal. From Mr Browne's communiThe reafon of this is, that vapour is compofed of cation, inferted in the Society's volume for 1794, fire and water united. The confequence is, that we extract the following account: "I herewith wherever there is any quantity of latent heat a- (fays he) fend you a plan and model of a furnace bove 32° of Fahrenheit contained in any body, I ufe for evaporation, and have found more ferthe water in contact with the surface, or contain- viceable for that purpose than any copper or boiled in the pores of the body, will gradually ab- er I ever faw; and am of opinion it might be forb it, and converting it into latent heat, will advantageously applied to the drying malt, as the thus be rendered specifically lighter than the com- heat is more equally difperfed, and the vapour mon atmosphere and fly off into it. Thus part carried off much quicker, than by the mode now of the fenfible heat of the body will be carried practifed. I have not obferved the exact quantity off; and as fubfequent quantities of water always of moisture which may be exhaled in a given time fly off with more and more of the fenfible heat, it by a given quantity of fuel; but I can with fafety is plain, that by continued evaporation of water fay that at leaft one half of the fuel, and a great almost all the fenfible heat above 32° of Fahrenheit deal of trouble, is faved by this contrivance, as it will be carried off. If instead of water, fpirit of does not require near the attendance that boilers wine be used, a much greater degree of cold may in general do, in fupplying it with liquor or fuel, be produced than by the evaporation of mere wa- which need only be done twice in 24 hours; for ter; and if instead of spirit of wine, we use ether, the fire, being confined in the first instance to the which is still more volatile than spirit of wine, an bottom, and the evaporation being regular, a cerexceffive degree of cold, fcarcely inferior to that tain quantity either of fuel or liquor may be put which congeals mercury, may be produced. This in at certain times: but the greatest advantage method of producing cold by the expentive li- this furnace poffetfes, and the only part I flatter quids of ether and spirit of wine, cannot be em- myfelf may be called new, is, the atmosphere beployed excepting merely for experiment: but that ing rendered of an equal heat with the liquor; by by the evaporation of water may be applied to which means more moisture is carried away by very useful purposes in warm countries; and it the current of hot air, than by any other means has been customary with failors to cool their casks I am acquainted with. The utility of this Evaof liquors by fprinkling them with fea water. porator, therefore, is in my opinion twofold: 1k, From the theory of evaporation laid down, we the evaporation is much quicker, with a lefs quanmay easily fee the reafon why, in a very warm tity of fuel, than in the generality of the boilers temperature, animal bodies have the power of now in ufe; 2dly, the operator, as well as the producing cold. A vapour, called infenfible PER- whole neighbourhood, cannot in the leaft be affectSPIRATION, Continually iffues from the bodies of ed or annoyed, let the vapour or steam be ever fo animals, from human bodies especially, which, pernicious. That the evaporation is much greater carrying off great quantities of their fenfible heat, by this mode, will appear very plain, when the enables them, according to its quantity, to pre- courfe of the heat is pointed out: it is firft carried ferve the fame temperature in many different de- under the veffel, then reverted back on the fides, grees of atmospherical heat. For the fame rea- and finally it is carried over the furface; by which fan alfo we fee why the continual fprinkling with means the air, that is in contact with the liquor, cold water is fo very powerful in depriving the is fo heated and highly rarefted, that the fluid is human body of the heat necellary for the fupport raifed into vapour or team, much quicker, and of life, even though the temperature of the water with lefs fuel, than if the atmosphere was cold; fhould not be below what can be easily born. It and, as the air neceffary to keep the fuel in comhas already been shown, that by the evaporation buftion paffes over the furface of the liquor, every of water, a degree of cold not much inferior to pernicious vapour is carried with it into the fire, that of freezing water may be produced; and where it is decompofed, or at leaft fo changed as confequently, by continual fprinkling of the body to be no longer pernicious. As the diminution with water, the whole might in time be reduced of labour in all operations is fo much to be wishto nearly the degree of cold in which water ed, I think it necessary to add, that by this confreezes. But this is what no human body can trivance one man can do more work than 3 can in bear and hence we may understand why forms the ufual method, the fire-place being fo contriof rain and fnow are often fatal; and likewife ved, that as much fuel may be put on at one time why, in cafes of shipwreck, people have died by as will ferve 12, or even 24 hours; and the fame being expofed for a few hours to the fpray of the may be faid of the fupplying the vessel with fresh fea. The theory of the evaporation of water, fur- liquor."—" A view of the Evaporator is exhibi- nithes us alfo with a folution of a very curious ted in Plate CXLII. where Fig. 1. gives a complete phenomenon, inexplicable on any other principle, view of the whole. Fig. 2. A fection lengthwife viz. why melting ice will freeze other pieces to- of the boiler, fire-place, &c. Fig. 3. A transverse gether more ftrongly; and, if a confiderable de- fection of the boiler and flues, looking towards gree of heat is not continued for fome time, will the fire-place. Fig. 4. A plan from A to B of fig.

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