THE ECONOMIC ASPECTS OF SOIL EROSION 
329 
surface eacli year is sufficient to lift a pound of matter to a 
height of many thousand feet. If all this heat could be con- 
verted into dynamic energy and applied to rending rock, such 
as granite, into sand-like material, the effect would be to break 
up the rocks in a very rapid manner. It is likel}’^ that the pro- 
cess of destruction would go on at the rate of several feet a year. 
Fortunately for the earth, this work is so organized that only a 
small part of this energy actualh’- enters into the j)rocesses which 
bring about wearing. By far the greater portion is fended off, in 
ways that we shall have to note, and sent upon other errands. 
We shall now consider the ways in which excessive erosion is 
avoided, and thus be led to see how the remnant of the forces is 
applied to such w’ork. When the tide of solar energy strikes 
our sphere, somewhere near one-half thereof is more or le.ss 
directly intercepted by the atmos})here, and does not penetrate 
to the lower realm of rock and water, but goes away again into 
space. Of tliat which comes to what is commonly called the 
surface of the earth, again the greater i>art quickly flies away by 
radiation into the realms of space. If the air ])ermitted the 
egress of heat as easily as it does the ingress of that form of 
motion the earth would never acquire the relatively high and 
tolerably stable temperatures which make it fit for organic life or 
for that work of erosion which, as we shall see hereafter, is inti- 
mately as.sociated with the existence of all proce.sses of develop- 
ment. If we trust the reckonings of certain eminent ])h}'sicists, 
this sphere would under such conditions remain at the tempera- 
ture of space, or some hundred degrees below zero on the Fahren- 
heit scale. Owing, however, to a nice adjustment of terrestrial 
conditions, the air, ])rincipally through the moisture it contains, 
hinders the outward motion of heat a little more than it does its 
incoming. It is in a small way a trap serving to retain the tem- 
])erature. Thus the surface is in general maintained in a some- 
what warmer state than that of the air. In this interesting 
condition of affairs we are now to find the origin of those j)ro- 
cesses which effect erosion. 
Owing to the warmth which the sunshine gives alike to land 
and sea, the atmosi)here next those surfaces becomes consider- 
ably heated and thereby expanded. 'I'his ])rocess leads to the 
formation of an ascending air current, which may be of a local 
nature, aj)pearing as in dust-whirls, water-spouts, cyclones, or 
hurricanes, all exhiliiting a spinning, upward movement of a 
temporary and migratory character ; or the jvseending movement 
