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comfe to imaginary bubbles, formed in a manner ^ J^^^jjjht of 3 [- mlios v/hciuhc Ijaroineter i> ac 

 only fuppoiedj and not proved. 



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Indeed he owns, that if t!ie watery particles had no 

 repc'licnt force, they niuft precipitate in the lame man- 

 ner that dud will do after it has been railed up ; but 

 there are too many obfervations and experiments, to 

 kavc any doubt of the exigence of repellent force 

 above-mentioned. 



'' And that he cannot fhewby. any experiment. How 

 '* big the molecular of Vapour mull: be which will 

 '' exclude air from their interflices ; nor that thefe 

 nvjleculas do vary in proportion to the degree of 

 heat by an increafe of repellent force in each wa- 

 tery particle, or by a farther divifion of the parti- 

 cles Hill lefs ; but in the general we may reafona- 

 bly affirm, that the rarity of the Vapour is pro- 

 portionable to the degree of its heat, as it happens 

 in other fluids. (See Philof. Tranf. N° 270.) And 

 though the different degrees of the air's rarefac- 

 tion are alio proportionable to the heat, yet the 

 fame degree of heat rarefies vapours much more 



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Now to fliew that what has been faid will account for 

 the rife of Vapours, and formation of clouds, we 

 muH: only confider whether that degree of heat which 

 is known to rarefy water 14,000 times, being compar- 

 ed Vvith feveral of thofe degrees of heat in fummer, 

 autumn, and winter, which are capable of raifing 

 exhalations from water or ice (the rarity of Vapours 

 being confidered,) will appear to be fuch, that the 

 Vapour will rife high enough in winter, but not too 

 high in fummer, to agree with the known phasnomena. 

 That the effeds are adequate to the caufes in this 

 cafe, he thinks may be made out in the following 

 maniier, viz. 



The heat of boiling water, according to Sir Ifaac 

 Newton^s table (Philofoph. Tranfaft. N° 273,) is 34, 

 the mean heat of fummer 5, the mean heatof Ipring 

 or autumn 3, and the lealt deforce of heat, at which 

 Vapours rife in winter (alias .the mean heat _of winter) 



The rarity of Va|)0ur proportionable to tliefe four de- 

 grees of heat is 24,000, 2058, 1235, and 823. 



-Si.] 



the rarity of the air is S50. But then as the air^s hotter 

 near the ground than at the height of half a mile or a 

 mile, the Vapour will condenfc as it rifes; and a-- th^ a*. 

 when the earth is heated, is rachcr near the ground 

 than at fome height from it, the place for equilibri- 

 um will, upon thefe two accotmts, be brought much 

 lower than otherwife it would be ; as for example, to 

 the height of about a mile, which will agree widi 

 phenomena. 



In fummer the two caufes above-mentioned increafin 

 the Vapour raifed by the heat 5, (whofe place of 

 equilibrium would be 5I miles high, if the Vapour, 

 after it began to rife, was not condenfed by coolin 

 and the air was denfed clofe to the earth) will fett! 

 the height of about i-i, or two miles, which is alio 

 agreeable to phsenomena. 



Laftly, as the denfity and rarity of the Vapour is 

 chiefly owing to its degree of heat, and in a fmall 

 meafure to the increafed ordiminifhed preflure of the 

 circumambient air, when it is not confined j and the 

 denfity and rarity of the air is chiefly owing to the in- 

 creafed or diminifl:ied prefTure, by the accumulation 

 or exhaufliion of fuperior air, whilfl: heat and cold 

 alter its denfity in much lefs propordon •, the clouds 

 made of the Vapours above-mentioned, infl-ead of con- 

 forming themfelves to the altered denfity of the am- 

 bient air, will rife when it is condenfed, and fink when 

 it is rarefied ; and alfo rife or fink, when the prelTurc 

 of the air is not altered, and its denfity very little 

 changed, by their own dilatation, owing to heat and 

 cold, as may be often obfervcd by feeing them chanf^e 

 their height confiderably, whilfl: the barometer conti- 

 nues exadlly at the fame degree, and the liquor of the 

 thermometer rifes or falls very little, and Ibmetimes 

 not at all. 



As for the manner how clouds are changed into rain, 

 it has been hinted at the beginning of this article ^ but 

 for farther fatisfaftion let the reader have recourfe to 

 Dr, Halley's account of it in the Philofoph. Tranfacl. 

 N" 183, which Dr. Defaguliers fays he has always 

 found agreeable to the ph.'cnonTena. •; - '• 



He adds, that fince he had for brevity's fake, only 



The rarity of air is in fummer 900, in fpringor au- j mentioned at what heights from the furface of the 



""""^ " ' * ' earth Vapours of different denfities will come to aa 



tumn 850, and in winter 860. 

 The denfity of water, compared with the above-men- 

 tioned denfities, being inverfly as one to the afore- 

 mentioned four numbers. 



The height above the earth to which the Vapours will 

 arife, and at which they v^ill be in equilibrio, in 

 an airof the_fame denfity with themfelves, will vary 



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according to the rarity of th 

 the heat of the feafon. 



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Vapour depending on 



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For the Vapour which is raifed by the winter's heat, 

 expreffed by the number 2, when the rarity of the Sir 

 is 800, will rife to (and fettle at) an height of about 

 the fixth part of a mile, when the barometer is above 

 thirty inches n!gn. 



But if the heat be greater, then the Vapours will 

 rife higher ; and pretty much higher if the fun fhines, 

 though in frofl;y weatlier, the barometer then being 

 very high. 



If the barometer falls, and thereby brings the place of 

 the equilibrium (for Vapours raifed by heat 2) nearer 

 the earth* then alfo will the heat be increafed, the 

 Vapour more rarefied, and confequently the new place 

 of equilibrium fufficiently high. ... 



It is to be obferved, that in winter when the heat is 

 only equal to 2, the air is denfed clofe to the earth, 

 w^hich has not any heat fufJicient to rarefy it near the 

 ground, as happens In warm weather •, therefore the 

 Vapour will rife gradually in an air whofe denfity de- 

 ^reafes continually from the earth upwards, neither 

 will the Vapour be hindered of its full rife by any 

 condenfation from a greater cold of the ambient air^ 

 the air being then as cold next to the ground, where 

 the vapour begins to rife, as it is at any height from 

 the earth. 



equilibrium, without giving a reafon for ftttlino- the 

 place of equilibrium, at whofe heights he thought it 

 proper here to give the method by which they may be 

 found, viz. 



As the Vapours will fettle and rife where the air is of 

 the fame denfity with themfelves, it is only required 

 to find the denfity of the air at any diftance from the 

 earth at feveral heights of the barometer, which may 

 be deduced from Dr. Halley's two tables, Philofoph. 

 Tranfaft, N° 386. (the firfl: Ihewing the altitudes to 

 given heights of the mercury, and the fecond the 

 heights of the mercury again at given altitudes,) and 

 knowing the degree of heat by the thermometer, be- 

 caufe the denfity of the Vapour depends upon the de- 

 gree of heat of the feafon, provided that proper al- 



' lowances be made for the great rarefaction of the air 

 near the earth in hot and dry weather, and the con- 

 denfation of the Vapours in their rife, by reafon of the. 

 air being colder at a little height above the earth than 



. juft at the furface of it. 

 The quantity of Vapour raifed from the fea by the 

 warmth of the fun, is far greater than one v/ould ima- 



Dr. Halley has attempted to edimate it. ■ 

 In an experiment made with that view, and defcribeci 

 in the Philofophical Tranfaftions, he found that a 

 quantity of water no warn^er than air in fummer, lofl: 

 in Vapour in the fpace of two hours, no lefs than -/.- 

 part of an inch in depth : now for -~ in two hours, 

 taking for the eafier calculation, -X. in the twelve hours 



gine. 



that the fun is up each day, it will rife -rV of ^" ^^'^h 



from the furface of the fea. 



On this fuppofition, every ten fquare inches of the - 



_ furface of the water'yield in Vapour per diem, a cubic 



I'he Vapour which is raifed by the heat of the fpring | inch of water of four feet fquare, a gallon •, a mile 

 or autumn, expreffed number 3," will rife to the j fq^uafe, 6914 tons •, a fquare degree" fuppofed of 6o- 



. • * Engliib 



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