210 PHYSICS. 
clouds ; consequently, bodies standing freely exposed to the heavens, exhibit 
the greatest deposit of dew. Even a piece of paper, or a fine pocket 
handkerchief, will prevent the deposit of dew. A bundle of wool lying 
underneath a piece of paper gained only two grains of dew, while another 
of equal size, but exposed freely to the sky, gained sixteen grains, or eight — 
times as much. Sig. 36, pl. 23, presents an apparent exception to the law. 
Lay a metal plate, AB, on high grass, heavy enough to press down the 
- grass and thereby to become surrounded by it, and at the same time support 
a similar plate, CD, on thin props above the average height of the grass, | 
thus exposing the plate to the open sky. The first metal plate will in many 
cases be bedewed, while the second is found to be entirely free from 
moisture. This result is unquestionably produced by the cooling influence 
of the grass on the former plate. The action of an oblique exposure to 
the sky is shown by another experiment (p/. 23, fig. 37). Under the 
elevated board, FG, Jay a small quantity of wool at C, ten grains for 
instance; on account of the board this cannot radiate heat towards the part 
AB of the sky, it will therefore be less bedewed than if it had been freely 
exposed. Around the stem and under the branches of a tree less dew falls 
than in the surrounding free exposure. In the case represented by fig. 38, 
little dew will be deposited in the space represented by AB, determined by 
verticals tangent to the outer circumference of the branches. Differently 
inclined surfaces experience different deposits of dew. Let fig. 39 be a 
vertical section of a certain region of considerable unevenness, while apart 
from this the radiating power is everywhere the same; then more dew will be 
deposited on the horizontal portion, AB, than on the slope, BC; more also 
than on CD. The least deposit will be at E. If a body be placed at H, on 
the same level with AB, it will be less covered with dew, on account of the 
intercepting action of the walls F and G, than if it lay on AB. Two 
observers would obtain very different results, one of whom should be 
stationed on a well wooded and inclosed slope, like the foreground of 
fig.40, and the other on the open height in the back-ground of the same 
picture. 
An instructive experiment is performed by suspending a glass ball at 
some height above the ground. In a perfectly calm clear night the first 
moisture will be exhibited on the very summit of the ball; by degrees the 
dew drops will extend over a great portion of the ball, constantly diminishing 
in size, as in pl. 23, fig. 41. A similar action is observed on a bent sheet 
of paper, a dead insect (fig. 42), and on the back of a sheep lying quietly 
on the earth (fig. 48); in the latter figure the greatest horizontal section, 
ABC, indicates the boundary between the more and the less bedewed 
portion. 
Since different bodies possess very different radiating properties, and 
consequently cool in a very unequal degree, it follows that under precisely 
the same circumstances the deposit of dew on these different bodies would 
be very different. Thus on plants it is more copious than on the earth ; on 
grass and leaves more than on bushes and trees; on loose gravelly land 
more than on the hard-trodden soil; on glass more than on metals, &c 
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