EVA
332
EVA
—rri 1 -.v-— ' ■ ,■■nirr rTW.
At 164° it was | of that at 212°
152 *
144 *
138 i
And in general the quantity evaporated from a given surface ofwater per minute at any temperature is to the quantity evaporatedfrom the same surface of 212°, as the force of vapour at the firsttemperature is to the force of vapour at 212°. Hence, in order todiscover the quantity which will be lost by evaporation, from wa-ter of a given temperature, we have only to ascertain the force ofvapour at that temperature. Hence we see that the presence of
atmospheric air obstructs the evaporation of water ; but this eva-poration is overcome in proportion to the force of the vapour. Mr.Dalton ascribes this obstruction to the vis inertia: of air.
3. The quantity of vapour which rises from water, even whenthe temperature is the same, varies according to circumstances.It is least of all in calm weather, greater when a breeze blows, andgreatest of all with a strong wind. The following table drawn upby Mr. Dalton, shews the quantity of vapour raised from a circu-lar surface of six inches in diameter in atmospheric temperaturesThe first column expresses the temperature ; the second the corre-sponding force of vapour; the other three columns give the num-ber of grains of water that would be evaporated from a surface ofsix inches in diameter in the respective temperatures, on the sup-position of there being previously no aqueous vapour in the at-mosphere. These columns present the extremes and the mean ofevaporation likely to be noticed, or nearly such ; for the first iscalculated upon the supposition of 35 grains loss per minute fromthe vessel of 3^ inches in diameter; the second 45, and the third55, grains per minute.
Tempe-
rature.
212°
Force ofvapour ininches.30
Evapora
120
ing force in grains154 | 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
.156
.62
.79
.97
26
.162
.65
.82
1.02
27
.168
.67
.86
1.05
28
.174
.70
.90
1.10
29
.180
.72
.93
1.13
30
.186
.74
.95
1.17
31
.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
.263
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
<40
.327
1.31
1.68
2.06
47
.339
1.36
1.75
2.13
48
.351
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
2.52
53
.415
1.66
2.13
2.61
54
.429
1.71
2.20
2.09
55
.443
1.77
2.2S
2.7S
56
.458
1.83
2.35
2.88
57
.474
1.90
2.43
2.98
58
.490
1.96
2.52
3.08
Tempe-
rature.
Force of 1vapour ininches,
Evaporating force in grains.
212°
30
120
154
189
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
2.46
3.16
3.87
66
.635
2.54
3.27
3.99
67
.655
2.62
3.37
4.12
69
.676
2.70
3.47
4.24
69
.698
2.79
3.59
4.38
70
.721
2.88
3.70
4.53
71
.745
2.98
3.83
4.68
72
.770
3.08
3 96
4.84
73
.796
3.18
4.09
5.0l)
74
.823
3.29
4.23
5.17
75
.851
3.40
4.37
5.34
76
.880
3.52
4,52
5.53
77
.910
3.65
4.63
5.72
78
.940
3.76
4.83
5.91
79
.971
3.88
4.99
6.10
80
1.00
4.00
5.14
0.29
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
5.80
7.17
85
1.17
4.68
6.07
7.46
4. Such is the quantity of vapour which would arise in differentcircumstances, oh the supposition that no vapour existed in the at-Vnosphere. But this is a supposition which can never be admitted,as the atmosphere is in no case totally free from vapour. Now'when we wish to ascertain the rate at which evaporation is goingon, we have only to lind the force of the vapour already in the at-mosphere, and subtract it from the force ol vapour at the giventemperature: the remainder gives us the actual force of evapora-tion; from which, by the table, we readily find the rate of evapora-tion. Thus, suppose we wish to know the rate of evaporation atthe temperature 59°. From the table we see tlrat the force of va-pour at 59° is 0.5 or ^ its force at 212°. Suppose we find by trialsthat the force of the vapour already existing in the atmosphere is0.25, or the half of ^j. To ascertain the rate of evaporation,we must subtract the 0.25 from 0.5; the remainder 0.25 gives nsthe force of evaporation required; which is precisely one-half ofwhat it would he if no vapour had previously existed in the atmo-sphere. By the table we sec that on that supposition a surface ofsix inches diameter would lose one grain by evaporation per mi-nute, instead of two grains, which would have been converted intovapour if no vapour had previously existed in the atmosphere, ifthe 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°, inthat case no vapour whatever would rise from the water; and ifthe force of the vapour already iu the atmosphere exceeded 0.5,instead of evaporation, moisture w ould be deposited on the surfaceof the water. These general observations, for all of which we areindebted to Mr. Dalton, account in a satisfactory manner for allthe anomalies which had puzzled preceding philosophers; and in-clude under them all the less general laws which they had disco-vered. We must consider the discoveries of Mr. Dalton as themost important additions made to the science of meteorology forthese many years.
5. An tiie force of the vapour actually in the atmosphere is sel-dom equal to the force of vapour of the temperature of the atmo-sphere, evaporation, with a few exceptions, may be considered asconstantly going on.
Various attempts have been made to ascertain the quantity eva-porated in the course of a year; but the difficulty of the problemis so great, that we can expect only an approximation towards asolution. From the experiments of Dr. Dobson of Liverpool inthe years 1772, 1773, 1774, and 1775, it appears that the mean
1 annual
v