YAP 
cGiirfe to imaginary bubbles, formed in a manner 
only fuppofed, and not proved.- ' 
Indeed he owns, that if the watery particles had no 
repellent force, they muft precipitate in the fame man- ' 
ner that dull will do after it has been railed up •, but 1 
there are too many obfervations and experiments, to 
leave any doubt of the exiftence of repellent force 
above-mentioned. 
44 And that he cannot fhew by any experiment, how 
46 big the molecuke of Vapour muft be which will 
44 exclude air from their interftices ; nor that thefe 
44 moleculae do vary in proportion to the degree of 
44 heat by an increafe of repellent force in each wa- 
44 tery particle, or by a farther divifion of the parti- 
44 cles ftill lefs ; but in the general we may reafona- 
44 bly affirm, that the rarity of the Vapour is pro- 
44 portionable to the degree of its heat, as it happens 
44 in other fluids. (See Philof. Tranf. N° 270.) And 
44 though the different degrees of the air’s rarefac- 
44 tion are alfo proportionable to the heat, yet the 
44 fame degree of heat rarefies vapours much more 
44 than air.” 
Now to fhew that what has been faid will account for 
the rife of Vapours, and formation of clouds, we 
muft only confider whether that degree of heat which 
is known to rarefy water 14,000 times, being compar- 
ed with 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 phenomena. 
That the effefts are adequate to the caufes in this 
cafe, he thinks may be made out in the following 
manner, viz. 
The heat of boiling water, according to Sir Ifaac 
Newton’s table (Philofoph. Tranfad. N° 273,) is 34, 
the mean heat of fummer 5, the mean heat of fpring 
or autumn 3, and the leaft degree of heat, at which 
Vapours rife in winter (alias the mean heat of winter) 
is 2. 
The rarity of Vapour proportionable to thefe four de- 
grees of heat is 24,000, 2058, 1235, and 823. 
The rarity of air is in fummer 900, in fpring or au- 
tumn 850, and in winter 800. 
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 will be in equilibrio, in 
an air of the fame denfity with themfelves, will vary 
according to the rarity of the Vapour depending on 
the heat of the feafon. 
For the Vapour which is raifed by the winter’s heat, 
expreffed by the number 2, when the rarity of the air 
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 high. 
But if the heat be greater, then the Vapours will 
rife higher •, and pretty much higher if the fun ftiines, 
though in frofty weather, 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 increased, 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, 
which has not any heat fufficient to rarefy it near the 
ground, as happens in warm weather ; therefore the 
Vapour will rife gradually in an air whofe denfity de- 
creafes continually from the earth upwards, neither 
will the Vapour be hindered of its full rife by any 
eondenfation from a greater cold of the ambient air ; 
the air being then as cold next to the ground, where 
the vapour begins to rift, as it is at any height from 
the earth. 
The Vapour which is raifed by the heat of the fpring 
or autumn, expreffed number 3, will rife to the 
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height of 3-f miles when the barometer is at 30, and 
the rarity of the air is 850. But then as the air is hotter 
near the ground than at the height of half a mile ora 
mile, the Vapour will condenfe as it rifts; and as the air 
when the earth is heated, is rather near the ground 
than at forne height from it, the place for equilibri- 
um will, upon thefe two accounts, be brought much 
lower than otherwife it would be ; as for example, to 
the height of about a mile, which will agree with 
phsenomena. 
In fummer the two caufes above-mentioned increafing, 
the Vapour railed by the heat 5, (whofe place of 
equilibrium would be 54. miles high, if the Vapour, 
after it began to rife, was not condenfed by cooling, 
and the air was denfed eloft to the earth) will fettle at 
the height of about 1 4, or two miles, which is alio 
agreeable to phenomena. 
Laftly, as the denfity and rarity of the Vapour is 
chiefly owing to its degree of heat, and in a final! 
meafure to the increafed or diminifhed preffure of the 
circumambient air, when it is not confined ; and the 
denfity and rarity of the air is chiefly owing to the in- 
creafed or diminifhed preffure, by the accumulation 
or exhauftion of fuperior air, whilft heat and cold 
alter its denfity in much lefs proportion ; the clouds 
made of the Vapours above-mentioned, inftead 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 rift or fink, when the preffure 
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 obferved by feeing them change 
their height confiderablv, whilft the barometer conti- . 
Hues exactly at the fame degree, and the liquor of the 
thermometer rifts or falls very litde, and fometimeS' 
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 fatisfadion let the reader have recourfe to 
Dr. Halley’s account of it in the Philofoph. Tranfad. 
N° 183, which Dr. Defaguliers fays he has always 
found agreeable to the phenomena. 
He adds, that fince he had for brevity’s fake, only 
mentioned at what heights from the furface of the 
earth Vapours of different denfities will come to an 
equilibrium, without giving a reafon for fettling the 
place of equilibrium, at whoft 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. 
Tranfad. N° 386. (the firft fhewing 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 tile de- 
gree of heat of the feafon, provided that proper al- 
lowances be made for the great rarefadion 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 would ima- 
gine. Dr. Halley has attempted to eftimate it. 
In an experiment made with that view, and defcribed’ 
in the Philofophieal Tranfadions, he found that a 
quantity of water no warmer than air in fummer, loft 
in Vapour in the fpace of two hours, no lefs than -V 
part of an inch in depth : now for T V in two hours 
taking for the eafier calculation, 44 in the twelve hours 
that the fun is up each day, it will rife of an inch 
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 
inch of water of four feet fquare, a gallon ; a mile 
fquare, 6qia tons ; a fquare degree fuppofed of 60 
‘ English? 
I 
