Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 13, MAT-MIC

716
Evapora¬
tion and
Ham.
Uefult.
M E T E O
tion. Suppofe, for inftance, it be required to know the
rate of evaporation at the temperature of 590. From the
, latl table we fee that the force of vapour at 590 is
about 0.5 or ^ its force at 21 20. Now, luppofe that by-
trials we find the force of the vapour which already
exiifs in the atmofphere to be 0.25 or of
Subtrafling the latter from the former, we have for a
remainder o.25=the force of evaporation required,
which is therefore juft the half of what it would be if
the almofphere were entirely free from vapour.
The force of vapour exifting in the atmofphere is
fcarcely ever equal to the force of vapour of the tempe-
rainre of the atmofphere. Hence evaporation may,
with a few exceptions, be confidered as going on with¬
out intermiffion. Attempts have been made to afcer-
tain the quantity of evaporation that takes place in the
courfe of a year } but the inveftigation of this problem
is fo duficult, that thefe attempts have fucceeded only
in obtaining approximations towards the truth. Mr
Dobfon of Liverpool, from a courfe of experiments made
in 1772, 1773, 1774, and 1775, concludes that the
mean annual evaporation from the furface of Avater,
amounted to 36.78 inches. The proportions for each
month are as follows.
January
February
March
April
May
June
Inches.
I.50
1-lhI
2.64
3-3°
4*34
4.41
July
Auguft
September
O&ober
November
December
Inshes.
5.01
3*i8
2*51
M1
I*49
The experiments of Mr Dalton fhew that the evapo¬
ration from the furface of water in a very dry and hot
fummer day, was rather more than two tenths of an inch.
Several experiments have been made on the quantity
of evaporation from lane}, efpecially by Mr Williams in
America, and Dr Watfon, Mr Dalton and Mr Hoyle
in Britain.
Mr Williams’s experiments appear to fhew that the
evaporation from the furface of fuch land as is covered
with trees and other vegetables is about one third
greater than the evaporation from the furface of wa¬
ter, though much reliance is not laid on thefe experi¬
ments.
R O LOGY. Chap. 111.
From an experiment made by Dr Watfon during Evapora-
fuminer, when the earth had been parched by a month’s lion and
drought, it appeared that 1600 gallons of water were , Rain-
evaporated from a fingle acre in 12 hours *. Dr Wat-
fon’s experiment, however, was of a nature that did not Evapora-
admit of great precilion. tion from
The experiments made by Mr Dalton and Mr Hoyle la, d;
in the years 1796, 1797, and 1798, are the moft exad chl^l'
that have been made on this fubjeft, and we ftiall there- Ejfay,^.
fore eonfider them more at large. They were made hi. 54’
with the following apparatus. Having procured a cy-
lindrical veflel made of tin plate, three feet deep and
ten inches in diameter, they inferted into it two pipes Dalton and
dire&ed downwards, fo that water might pafs through Hojle.
them into two bottles. One pipe was fixed near the
bottom of the veffel, and the other about an inch from
the top. I he vefiel was filled up for a few inches with
gravel and land, and all the reft with good frefti foil.
It was then put into a hole in the ground, and the
fpace around filled up with earth except on one fide, for
the convenience of putting bottles to the two pipes 5
then fome water was poured on the earth to fadden it,
and all that would drain off was fuffered to efcape.
Hence the earth may be confidered as faturated with
moifture. The foil was kept for fome weeks above the
level of the upper pipe, but after that it was conftantly
allowed to be a little below it, thus preventing any wa¬
ter from running off through that pipe. The top of
the foil for the firft year was bare ; but for the two laft
years it was covered with grafs like other turf. The
apparatus being thus prepared, a correft regifter was
kept of the quantity of rain water which ran off from
the furface of the earth by the upper pipe, as long as
that was below the earth, and alfo of the quantity of
water which paffed through the three feet of earth, and
ran off by the lower pipe ; and a rain gauge of an equal
diameter with the cylinder was kept near it, for the,
purpofe of meafuring the quantity of rain which fell in
any correfponding time. Then, by fubtra&ing the
quantity of water which paffed through the pipes from
that in the rain gauge, the remainder was confidered as
equal to the quantity evaporated from the furface of the
earth in the cylinder. The mean annual refult of thefe
experiments is Ihewn in the following table.
Water through the two pipes.
January
February
PTarch
April
May
June
July
Auguft
September
G<ftober
November
December
Rain
Evap.
1796-
Inch.
1.897—
1.778—
♦431—
.220—
2.027—
.171—
.200
6.877-
30.629-
23-723-
1797.
Inch.
.680—
.918—
.070—
.295—
2- 443 +
.726
.025
.976
.680
1.044
3- °77
1798.
Inch.
I-774+
1.122
•335
.180
.010
■5°4
I-594
1.878 +
Mean.
10.934—
38.791 —
1+57—
7-379
3I-259
23.862
Inch.
I-45° +
x-273
.279
.232
1 -493 +
.299
•o>9
.168
•325
.227
.879
1 • 718 +
8.402
Mean
Rain.
Mean
Evap.
Inch.
2- 458
1.801
902
1.717
4-x77
2.483
4-154
3- 554
3-279
2.899
2-934
3.202
3 3-5 60
Inch.
1.008
.528
.623
1.485
2.684
2.184
4-°95
3-386
2-954
2.672
2-055
1-484
25-158
It