336 
THE ECONOMIC ASPECTS OF SOIL EROSION 
sinking of the shore lands, we must look to those natural forces 
which are ever, though not uniformly, at work uplifting the con- 
tinental arches above the plane of the seas. At present we seem 
to be in a period where the great lands recently in a state of ver^’’ 
general depression as regards the sea level have come to a pre- 
vailingly steadfast state. The next step may be toward a general 
gain of the continental areas on the fields of the oceans. 
\\'e turn now from the work of erosion which goes on upon the 
shores, and which, as we have seen, is due to the action of solar 
heat working through the movements that it enforces on the 
atmosphere, to another effect of the sun’s energ\% that due to the 
evaporation and precipitation of water. We have noted the fact 
that the radiation of heat is hindered by the atmosphere, one 
consequence of which is the warming of the air next the earth’s 
surface, an effect which is noticeable in a diminishing rate for a 
great height above the surfiice. To this action is also attributable 
the establishment of conditions which bring about the system of 
the rains. It fortunately happens that the adjustment of tem- 
peratures next the earth’s surface makes it possible for the process 
of evaporation to lift a large amount of water into the air. The 
quantity thus borne upward is not as yet definitely ascertained, 
but it probably amounts to not far from an average of five feet 
per annum over the surface of the seas. The greater part of this 
water after ascending to a height of perha})S a mile or more, on 
an average, is condensed and falls back to the ocean as rain or 
snow. In making this circuit work is done, but it is of no geo- 
logical value. Following the dynamic history of a pound of 
water in its up and down journey, we see that it takes five 
thousand foot-pounds of energy expressed in heat to lift it for 
the mile or so of its ascent, and that this energ}’’ is reconverted 
into heat b}' the friction which the water encounters in falling 
or by the blow which it strikes when it attains the surface. 
Owing to the conditions, the energy of position which the water 
had when at its highest point (an amount sufficient to lift one 
ton to the height of two and one-half feet) has, when it falls back 
to the sea, done no work of lasting importance. It is, as we shall 
see, quite otherwise when in the downward movement the water 
falls upon a land surface. 
The winds — those movements of the atmosphere which create 
the waves and thus bring about marine erosion — transport the 
watery vapor from its main source, the seas, so that a share of 
it, perhaps near one-half of all that is formed, is brought over 
