1803.] 
the elevation of eighty-feven feet above the 
level of the fea. A ttandard for the tempe- 
rature Mr. Kirwan himielf has difcovered, 
which feems fufficiently accurate for ail pur- 
pofes. The remaining obitacles can only be 
remedied by tie eftablifhment of Meteoro- 
logical Societies in different parts of Eu- 
rope, annually communicating: with one 
another; and by being obliged to pub- 
lifh periodical accounts of their feveral ob- 
fervations, and to compare them with 
thofe made in foreign countries. When 
the vait importance of Meteorological 
knowledge to agriculture and navigation 
is confidered, there is at prefent foine 
room to hope that eftadlifhments of this 
nature may take place at no very diftant 
period. 
After thefe, and other preliminary re- 
marks, Mr. Kirwan commences his fubjeét 
with confidering the moziffure of ihe atmo- 
Sphere, which may be confidered as rifling 
into, fubfifting in, or defcending from, 
the atmofphere. The- Firft Chapter of 
this effay treats on evaporation, which is 
defined as the converfion of a liquid (and 
even frequently of a folid) into an invifi- 
ble fluid. This converfion may be ef- 
fected either naturally or artificially: that 
produced by nature is always accompanied 
by contaét with the atmofphere; the arti- 
ficial is producible in contaét with the at- 
mofphere, or even 7” vacuo. 
In the common courfe of nature, five 
caufes concur in producing evaporation ; 
viz. heat; affinity to the atmofpheric air ; 
agitation; electricity; and light. 
. I. The evaporation produced by HEAT 
is oppofed by the attraétion of cohefion 
and external preffure. According to Sauf- 
fure, a diminution of the denfity of the 
atmo/fphere, amounting to one-third, dou- 
bles the quantity of evaporation. 
The evaporation of water expofed to 
the air, is increafed or diminifhed by va- 
rious circuimftances, fome relative to the 
wwater, and fome to the air. Thofe re- 
lative to water, are, (1) Its tempera- 
ture: (2) Its iurface; hence the increafe 
-of furface given to pans in fali-works, by 
which, from the air alone, water, contain- 
ing only one per cent. of common falt, 
is fo far evaporated that the refidue con- 
tains 20 per cent.: (3) Its purity; it is 
well known that concentrated folutions of 
moft falts, evaporate much more flowly 
than pure water in the fame circum. 
ftances. 
Evaporation of water with regard to the 
air depends, (1) On the temperature. If 
the water and air be of the fame tempera- 
ture, evaporation proceeds moft flowly : 
4 
Proceedings of Learned Societies. 
61 
if the air be warmer than the water, and 
its temperature between 60° and 70°; and 
if the water be 14° colder, or more, there 
is No evaporation: the nearer water ap- 
proaches to the temperature of the air, 
and yet remains one or two degrees below 
it, the more is évaporation accelerated. 
When the air zs colder than the water ex- 
pofed to it, the refult, with refpect to eva- 
poration, 1s exactly the reverfe of what. 
happens when the water is the colder of 
the two. Ifthe temperature of the water 
be conftant; and that of the air, in one 
cafe fome degrees hotter, and in another 
cafe colder than water, by an equal num- 
ber of degrees, the evaporation would be 
greater in the hotter air; but, on the other 
hand, hot air depofits moiiture on water, 
that is fome degrees colder than it(elf. 
TI. Of the influence of affinity. This 
attraction is limited by saturation, and is 
meafured by the hygrometer. The hygro- 
‘meter invented by Sauffure confiits of a 
fingle human hair, kept in a flate of ten- 
fion by the weight of a few grains, and 
rolled on a pulley, to which an index is 
fixed. The hair is lengthened by moif- 
ture, and contracted by crynefs. 
As faturabilicy in a given heat, and alfo 
an increale of heat, promote evaporation, 
it was neceflary to enquire which of them 
exerts the greateft influence; and it was 
found, by Sauffure, that, according toa 
fcale devifed by himfelf, the influence of 
7° of Fahrenheit is nearly equal to the in- 
fluence of a difference of one degree of fa- 
turability. Hence north and ealt winds, 
though colder than fouth and wef, pro- 
mote evaporation much more, the air they 
convey being farther from faturation. 
lil. Of the influence of wind: the ef. 
feéts of which are proportioned to its fa- 
turability, and to its velocity. Sauffure 
found that air, moving at the rate of forty 
feet per fecond, triples the quantity ef eva- 
poration that would happen in calm air. 
Hence the warmer the air, the quicker its 
motion, the greater its faturability, and 
the longer its duration, the more will it 
promote evaporation. 
iV. Of the influence of clectricity and 
light. Sauffure has fhewn that a card loft, 
when electrified, two grains and a quarter 
in an hour, while another uneleétrified loft 
only one grain and a half. Light contri- 
butes to evaporation by difengaging air 
from water. Mr. Kirwan having con- 
fidered thefe caufes feparately, gives a 
long account of their joint action, in the 
courfe of which we learn, (1) That in 
London, whofe mean annual temperature 
is about 50°, the annual evaporation from 
one 
