MECHANICS. 
740 
the evaporation being always greatefl when the heat is the 
greatest. The difficulty of folving the phenomenon arifes 
from hence, that we find a heavier fluid (water) lufpended 
in a lighter fluid (air), contrary to our foregoing principles. 
Dr. Halley fuppofed, that by the adlion of the fun upon 
the furface of the water, the aqueous particles become 
formed into hollow bubbles filled with warm and rarefied 
air, fo as to make the whole bulk .fpecifically lighter than 
the air, in which cafe the particles will afcend. But there 
is a great difficulty in conceiving how this can be effected. 
And, if bubbles could be at JbJl thus formed, when they 
afcend the air within would very foon be reduced to the 
fame temperature as the air without, and they wmuld im¬ 
mediately defcend upon that effedl taking place. Another 
opinion is, that the particles of water are ieparated by a 
repulfive force, which is increafed in proportion as the 
heat is increafed, and that thus they are difperfed through 
the air ; but the fame argument may be ufed againft this 
hypothefis as againft the laft ; namely, that this effedt 
could nor continue in the cold part of the atmofphere, 
where the clouds are fufpended. The molt probable fup- 
pofition is, that evaporation is a chemicalfolution of water in 
air. We know that metals are difiolved in rnenftruums, 
and their particles difFufed and fufpended in the fluid, al¬ 
though their fpecific gravity be greater than that of the 
fluid. Heat promotes this folution ; in the day-time, 
therefore, the heat caufes a more perfect folution than 
what can, catcris paribus, take place in the night, when 
the air is colder, when the heat is frequently not fuflicient 
to keep the water in a ltate of folution, and it falls in fogs 
and dews. The vapours, thus railed by heat, afcend into 
the cold regions of the atmofphere ; and, not being there 
kept in a Hate of folution, they appear in the form of 
clouds ; and, when driven together by the agitation of 
the air, the particles run together into drops, and fall 
down in rain. If they be frozen before they form them- 
felves into drops, they defcend in fnow ; but, if the drops 
of rain themfelves be frozen, they defcend in hail. See 
Hamilton on the Afcent of Vapours. 
Springs. Marriotte fuppofed fprings to be owing to 
rain-water and melted fnow, which penetrating the fur- 
faces of hills, and running by the fide of clay or rocks 
which it cannot penetrate, at laft comes to feme place 
where it breaks out. This would account for the pheno¬ 
menon, provided the fupply from thefe caufes was fulfi- 
cient; but D. Sideleau, and others, making an eftimate 
of the quantity of rain and fnow which falls in the fpace 
of a year, to fee whether it would afford a quantity of 
water equal to that which is annually difchargtd into the 
fea by the rivers (which are fupplied by fprings), found 
that it would not. But Dr. Halley difeovered the caufe 
of a fuflicient fupply; for he has proved by experiment, 
that the vapours which are raifed alford a much greater 
fupply than is necelfary. We will give the account in 
his own words. 
“ We took a pan of water (falted to the degree as is 
common fea-vvater, by the folution of about a fortieth 
part of fait), about four inches deep, and inches dia¬ 
meter, in which we placed a thermometer; and, by means 
of a pan of coals, we brought the water to the fame de¬ 
gree of heat which is obferved to be that of the air in our 
hotteft fummers ; the thermometer nicely (bowing it. 
This done, vve affixed the pan of water, with the thermo¬ 
meter in it, to one end of the beam of the feales, and ex¬ 
actly counterpoiied it with weights in the other fcale; and, 
by the application or removal of the pan of coals, we 
found it very eafy to maintain the water in the fame de¬ 
gree of heat precifely. Doing thus, we found the weight 
of the water fenfibly to decreafe; and at the end of two 
.hours we obferved that there wanted half an ounce troy 
all but 7 grains, or 2.33 grains of water, which in that time 
had gone off in vapour ; though one could hardly per¬ 
ceive it fmoke, and the water was not fenfibly warm. 
This quantity in fo fhort a time feemed very coniiderable, 
being little lefs than 6 ounces in twenty-four hours from 
fo fmall a furface as a circle of 8 inches d/imeter. To re¬ 
duce this experiment to an exadt calyfus, and determine 
the thicknefs of the (kin of water that had lo evaporated, 
I alfume the experiment alleged by Dr. Edward Bernard, 
to have been made in the Oxford Society, viz. that the 
cube-foot Englifh of water weighs exactly 76 pounds troy ; 
this divided by 1728, the number of inches in a foot, will 
give 253^ grains, or half an ounce and 13^ grains, for the 
weight of a cube-inch of water; wherefore the weight of 
233 grains is or 35 parts in 38 of a cube-inch of water. 
Now the area ot the circle, whofe diameter is 7^5 inches, 
is 49 fquare inches ; by which, dividing the quantity of 
water evaporated, viz. of an inch, the quote, •j-f-f-2' or tV 
(hows that the thicknels of the water evaporated was ttie 
53d part of an inch; but we will fuppofe it only the 60th 
part, for the facility of calculation. If therefore water 
as warm as the air in fummer exhales the thicknefs of 
a 60th part of an inch in two hours from its whole fur¬ 
face, in twelve hours it will exhale of an inch ; which 
quantity will be found abundantly iufficient to ferve for 
all the rains, fprings, and dews, and account for the Caf- 
pian Sea’s being always at a ftand, neither wafting nor 
overflowing; as likewife for the current laid to fet always 
in, at the Straights of Gibraltar, though thofe Mediter¬ 
ranean Seas receive fo many and fo coniiderable rivers. 
“To eftiinate the quantity of water arifing in vapours 
oat of the fea, I think I ought to confider it only for the 
time the fun is up, for that the dews return in the night as 
much if not more vapours than are then emitted ; and in 
lurnmer, the days being longer than twelve hours, this 
excefs is balanced by the weaker adlion of the fun, efpe- 
cially when riling before the water is warmed ; fo that, if 
I allow of an inch of the furface of the fea to be raifed 
per diem in vapours, it may not be an improbable conjec¬ 
ture. Upon this luppofition, every 10 fquare inches of 
the furface of the water yields in vapour per diem a cube- 
inch of water; and each fquare-foot, half a wine pint j 
every fpace of 4 feet fquare, a gallon ; a mile fquare, 6914 
tons ; a fquare degree, fuppofe of 69 Engliffi miles, will 
evaporate 33 millions of tons; and, if the Mediterranean 
be eftimated at 40 degrees long and 4 broad, allowances 
being made for the places where it is broader by thofe 
where it is narrower, (and I am fure I guefs at the leal!,) 
there will be 160 fquare degrees of fea; and confequently 
the whole Mediterranean mull lofe in vapour, in a fum- 
mer’s day, atlealt 5280 millions of tons. And this quan¬ 
tity of vapour, though very great, is as little as can be con¬ 
cluded from the experiment produced ; and yet there re¬ 
mains another caufe, which cannot be reduced to the rule, 
I mean the winds, whereby the furface of the water is 
licked up, fomewliat falter than it exhales by the heat of 
the fun, as it is well known to thofe that have confidered 
thofe drying winds which blow fometimes. 
“The Mediterranean receives thefe coniiderable rivers 5 
the Iberuc, the Rhone, the Tiber, the Po, the Danube, 
the Niefter, the Borylthenes, the Tanais, and the Nile; 
all the relt being of no great note, and their quantity of 
water inconfiderable. We will fuppofe each of thefe nine 
rivers to bring down ten times as much water as the river 
Thames; not that any of them is fo great in reality, but to 
comprehend with them all the fmall rivulets that fall into 
the fea, which otherwife I know not how to allow for. 
To calculate the water of the Thames, I alfume that at 
Kingfton Bridge, where the flood never reaches, and the 
water always runs down, the breadth of the channel is 100 
yards, and its depth 3, it being reduced to an equality, 
(in both which fuppofitions I am fure I take with the 
molt.) Hence the profile of the water in this place is 300 
fquare yards: this multiplied by 48 miles, (which I allow 
the water to run in 24 hours, at a miles in an hour,) or 
84410 yards, gives 25344000 cubic yards of water to be 
evacuated every day, that is 20300000 tons per diem ; and 
I doubt not bur, in the excefs of any mealure of the chan¬ 
nel of the river, I have made more than Iufficient allow¬ 
ance for the waters of the Brent, the Wandd, the Lea, 
1 ‘ and 
