05 8 
EVAPORATION. 
Tempe 
rature 
Force of 
vapour i 
inches. 
a Evaporating force in grai 
212° 
30 
120 
154 
189 
20° 
.129 
.52 
.67 
.82 
21 
.134 
.54 
.69 
.85 
22 
.139 
.56 
.71 
.88 
23 
.144 
.58 
.73 
-91 
24 
.150 
.60 
.77 
.94 
25 
. 1 56 
.62 
.79 
.97 
26 
.162 
.65 
,82 
1.02 
27 
.168 
.67 
.86 
1.05 
28 
.174 
.70 
.90 
1.10 
29 
.180 
1 .72 
.93 
1.13 
3.0 
.186 
.74 
.95 
1.17 
3,1 
.193 
.77 
.99 
1.21 
32 
.200 
.80 
1.03 
1.26 
33 
.207 
.83 
1.07 
1.30 
34 
.214 
.86 
1.11 
1 .35 
35 
.221 
.89 
1.14 
1.39 
36 
.229 
.92 
1.18 
1.45 
37 
.237 
.95 
1 .22 
1.49 
38 
.245 
.98 
1.26 
1 .54 
39 
.254 
1.02 
1.31 
1.60 
40 
.264 
1.05 
1 .35 
1.65 
41 
.273 
1.09 
1.40 
1.71 
42 
.283 
1.13 
1 .45 
1.78 
43 
.294 
1.18 
1,51 
1.85 
44 
.305 
1.22 
1.57 
1.92 
45 
.316 
1.26 
1.62 
1.99 
46 
.327 
1.31 
1.68 
2.06 
47 
.339 
1.36 
1.75 
2.13 
48 
.35 1 
1.40 
1.80 
2 20 
49 
' .363 
1.45 
1.86 
2,28 
50 
.375 
1.50 
1.92 
2.36 
51 
.388 
1.55 
1.99 
2.44 
52 
.401 
1.60 
2.06 
2.51 
53 
.415 
1.66 
2.13 
2.61 
54 
.429 
1.71 
2.20 
2.69 
55 
.443 
1.77 
2.28 
2.78 
56 
.458 
1.83 
2.35 
2.88 
57 
.474 
1-90 
2.43 
2,98 
58 
.490 
1.96 
2.52 
3.0.8 
59 
.507 
2.03 
2.61 
3.19 
60 
.524 
2.10 
2.70 
3.30 
61 
.542 
2.17 
2.79 
3.41 
62 
.560 
2.24 
2.88 
3.52 
63 
.578 
2.31 
2 97 
3.63 
64 
.597 
2.39 
3.07 
3.76 
65 
.616 1 
2.46 
3. 1 6 
3.87 
66 
.635 I 
2.54 
3.27 
3.99 
67 
.6.55 I 
2.62 
3.37 
4.12 
68 
.676 
2.70 
3.47 
4.24 
69 
.698. 
2.79 
3.59 
4.38 
70 
.721 
2.8S 
3.70 
4.53 
71 
.745 
2.98 
3.83 
4.68 
7-2 
.770- 
3.08 
3.96 
4.84 
73 
.796 
3.18 
4.09 
5.00 
74 
.823 
3.29 
4.23 
5.17 
75 
.851 
3.40 
4.37 
5.34 
76 
.880 
3.53 
4.52 
5.53 
7 7 
•910- 
3.65 
4.68 
5.72 
78 
.940 
3.76 
4.83 
5.91 
79 
.971 
3.88 
4.99 
6.10 
80 
1.00 
4,00 
5. 1 4 
6.29 
81 
1.04 
4. 16 
5.35 
6.54 
82 . 
1.07 
4.28 
5.50 
6.73 
83 
1.10 
4.40 
5.66 
6.91 
84 
1,14 
4.56 
5.86 
7.17 
8.5 
1.17 
4.68 
6.07 
7.46 
when we wish to ascertain the rate at which 
evaporation is going on, we have only to line! 
the force of the vapour already in the atnio- 'dies. But from the way that his expert 
sphere, and subtract it from the force of va- 
4. Such is the quantity of vapour which 
would rise in different circumstances, on the 
supposition that no vapour existed in the ah 
inosphere. But this is a supposition which 
can never be admitted, as the atmosphere is 
In no case totally free from vapour. Now 
pour at the given temperature: the remain- 
der gives us the actual force of evaporation ; 
from which, bv the table, wc readily find the 
rale of evaporation. Thus, suppose we wish 
to know the rate of evaporation at the tem- 
perature 59°. From the table we see that 
the force of vapour at 59° is 0.5, or -i_ its 
force at 212°. Suppose we lind by trials that 
the force of the vapour already existing in 
the atmosphere is 0.25, or the half of 
To ascertain the rate of evaporation, we 
must subtract the 0.25 from 0.5 ; the remain- 
der 0.25 gives us the force of evaporation re- 
quired ; which is precisely one half of what 
it would be if no vapour had previously ex- 
isted in the atmosphere. 
By the table we see that on that supposi- 
tion a surface of six niches diameter would 
lose one grain by evaporation per minute, in- 
stead of two grains, which would .have been 
converted into vapour if no vapour had pre- 
viously existed in the atmosphere. If the 
force of the vapour in the atmosphere had 
amounted to 0.5, which is equal to the force 
of vapour at the temperature of 59°, in that 
case no vapour whatever would rise from 
the water; and if the force of the vapour al- 
ready in the atmosphere exceeded 0.5, in- 
stead of evaporation, moisture would be de- 
posited on the surface of the water. 
These general observations, for all bf which 
we are indebted to Mr. Dalton, account in a 
satisfactory manner for all the anomalies 
which had puzzled preceding philosophers; 
and include under them all the less general 
laws which they had discovered. We must 
consider the discoveries of Mr. Dalton as the 
most important additions made to the science 
of meteorology for these many years. 
5, As the force of the vapour actually in 
the atmosphere is seldom equal to the force 
of vapour of the temperature of the atmo- 
sphere, evaporation, with a few exceptions, 
may be considered as constantly going on. 
Various attempts, have been made to as- 
certain the quantity evaporated in the course 
of a year ; but the difficulty of the problem 
is so great, that we can expect only an ap- 
proximation towards a solution. From the 
experiments of Dr. Dobson of Liverpool in 
the years 1772, 1773, 1774, and 1775, it ap- 
pears that the mean annual evaporation from 
the surface of water amounted to 36.78 in- 
dies. The proportion for every month was 
the following : 
January 
February 
March 
April 
May - 
June 
v Mr. Dalton found the evaporation from the 
surface of water in one of the driest aud hot- 
test days of summer rather more than 0.2 of 
an inch. 
If we believe Mr. Williams, the evapora- 
tion from the surface of land covered with 
trees and other vegetables is one-third great- 
er than from the surface of water; but this 
has not been confirmed by other philoso- 
phers. From his experiments it appears. 
that in Bradford in New England the evapo- 
ration, during 1772, amounted to 42.65 fo- 
Inches. 
Inches. 
- 1.50 
July 
- 5.11 
- 1.77 
August - 
- 5.01 
- 2.64 
September 
- 3.18 
- 3.30 
October 
- 2.51 
- 4,34 
November 
- 1.51 
- 4.41 
December 
- 1.49 
ments were conducted, the amount was pro- 
bably too great. 
From an experiment of Dr. Watson, made 
on the 2d of June 1779, after a month’s 
drought, it appears, that the evaporation 
from a square inch of a grass plot amounted 
to 1.2 grains in an hour, or 28:8 grains in 
24 hours, which is 0.061 of an inch. In ano- 
ther experiment, after there had been no 
rain for a week, the heat of the earth being 
1 10°, the evaporation was found almost twice 
as great, or 0.108 of an inch in the day. 
T he mean of these two experiments is 0.084 
inches, amounting for the whole of June to 
2.62 inches. If we suppose this to bear (he 
same proportion to the whole \ ear that the 
evaporation in Dr. Dobson’s experiments for 
June do to the annual evaporation, we shall 
obtain an annual evaporation, amounting to 
about 22 inches. This is much smaller than 
that obtained by Mr. Williams. But Dr. 
Watson’s method was not susceptible of pre- 
cision. Fie collected the vapour raised on 
the inside of a drinking-glass; but it was im- 
possible that the glass could condense much 
more than one half of what did rise, or would 
have been raised in other circumstances. 
But then the experiments were made in the 
hottest part of the day, when much more va- 
pour is raised than during any part of it. 
The most exact set of experiments on the 
evaporation from the earth was made by Mr. 
Dalton and Mr, Hoyle, during 1796, and 
the two succeeding years. T he method 
which they adopted was this : Having got a. 
cylindrical vessel of tinned iron, ten inches in 
diameter, and three feet deep, there were in- 
serted into it two pipes turned downwards 
for the water to run off into bottles: the one 
pipe was near the bottom of the vessel, the 
other was an inch from the top. The vessel 
was tilled up for a few inches with gravel and 
sand, and all the rest with good fresh soil. It 
was then put into a hole in the ground, and 
the space around filled up with earth, except 
on one side, for the convenience of putting 
bottles to the two pipes; then some water- 
was poured on to sadden the earth, and as 
much of it as would was suffered to run 
through without notice, by which the earth 
might be considered as saturated with water,. 
For some weeks the soil was kept above the 
level of the upper pipe, but latterly it was 
constantly a little below it, which precluded 
any water running off through it. For the 
first year the soil at top was bare ; but for the 
two last years it was covered with grass the 
same as any green field. Things being thus 
circumstanced, a regular register was kept 
of the quantity of rain water that ran off from 
the surface of the earth through the upper- 
pipe (whilst that took place), and also of the 
quantity of that which sunk down through the 
three feet of earth, and ran out through the 
lower pipe. A rain-gauge of the same dia- 
meter was kept close by to find the quantity 
of rain for any corresponding time. The 
weight of the water which ran through the 
pipes being subtracted from the water in the 
rain-gauge, the remainder was considered as 
the weight of the water evaporated from the 
earth in the vessel. The following table ex- 
hibits the mean annual result of these experi- * 
meats. . 
