ABSORBING AND EMITTING SURFACE. 
145 
An acre of chalk, rolled hard and smooth, would have great 
reflecting power, bnt its radiation would be much increased by 
breaking it np into clods, because the actually exposed surface 
would be greater, though the outline of the field remained the 
same. The area of a triangle being equal to its base multi¬ 
plied by half the length of a perpendicular let fall from its 
apex, it follows that the entire superficies of the triangular 
faces of a quadrangular pyramid, the perpendicular of whose 
sides should be twice the length of the base, would be four 
times the area of the ground it covered, and would add to the 
field on which it stood so much surface capable of receiving 
and emitting heat, though, in consequence of obliquity and 
direction of plane, its actual absorption and emission of heat 
might not be so great as that of an additional quantity of level 
ground containing four times the area of its base. The lesser 
inequalities which always occur in the surface of ordinary 
earth affect in the same way its quantity of superficies acting 
upon the temperature of the atmosphere, and acted on by it, 
though the amount of this action and reaction is not suscep¬ 
tible of measurement. 
Analogous effects are produced by other objects, of what¬ 
ever form or character, standing or lying upon the earth, and 
no solid can be placed upon a flat piece of ground, without 
itself exposing a greater surface than it covers. This applies, 
of course, to forest trees and their leaves, and indeed to all 
vegetables, as well as to other prominent bodies. If we sujd- 
pose forty trees to be planted on an acre, one being situated in 
the centre of every square of two rods the side, and to grow 
until their branches and leaves everywhere meet, it is evident 
that, when in full foliage, the trunks, branches, and leaves 
would present an amount of thermoscopic surface much 
greater than that of an acre of bare earth ; and besides this, 
the fallen trees lying scattered on the ground, would some- 
Hence we see that in examining the calorific effects of clearing forests, it 
is important to take into account the properties of the soil laid bare. 
Beoquerel, Dee Climate et dee Sole boisee , p. 137. 
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