168 ABSORBING AND EMITTING SURFACE. 
apparent quantity of absorbing, radiating, and reflecting sur- 
face; but the real extent of that surface is very variable, 
depending, as it does, upon its configuration, and the bulk and 
form of the adventitious objects it bears upen it; and, besides, 
the true superficies remaining the same, its power of absorp- 
tion, radiation, reflection, and conduction of heat will be much 
affected by its consistence, its greater or less humidity, and its 
color, as well as by its inclination of plane and exposure. An 
acre of clay, rolled hard and smooth, would have great reflect- 
ing power, but its radiation would be much increased by break- 
ing it up into clods, because the actually exposed surface would 
be greater, though the outline of the field remained the same. 
The inequalities, natural or artificial, 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 reac- 
tion is not susceptible of measurement. 
Analogous effects are produced by other objects, of whatever 
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 vegeta- 
bles, as well as to other prominent bodies. If we suppose 
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 leaves 
lying scattered on the ground, would somewhat augment the 
suin-total.* On the other hand, the growing leaves of trees 
generally form a succession of stages, or, loosely speaking, 
* “(he Washington elm at Cambridge—a tree of no extraordinary size—was 
some years ago estimated to produce a crop of seven millions of leaves, ex- 
posing a surface of two hundred thousand square feet, or about five acres of 
foliage.” —Gray, First Lessons in Botany and Vegetable Physvology. 
