330 
THE ECONOMIC ASPECTS OF SOIL EROSION 
may take the shape of the great tropical belt of ascending air — 
that vast, permanent, slit-like- chimney which extends almost 
completely around the earth in the tropical zone. Whether these 
ascending currents have the character of the spinning storms or 
the great permanent tropical upcast, their effect is to put the air 
in motion. Through them volumes of the atmosphere are con- 
stantly set into currents of swaying movement, the result being 
that winds (variable so far as created by the cyclone groups or 
tolerably constant Avhen due to the tropical upcast) are brought 
about. These winds, of sufficient energy to have distinct geo- 
logical value in a direct or indirect way, appear to be constantly 
at work at all times and at all parts of the earth’s surface, except 
during the long Avinter nights in the realm about either pole. 
The simplest geological Avork of the Avinds is that Avhich is 
brought about by their friction upon the AA^ater surfaces of the 
earth. For our purpose the important result of this friction is 
the formation of Avaves or undulations of Avater, in Avhich are 
stored the energy Avhich the Avinds expended in their making. 
In their greater form these Avaves may have a length of seA^eral 
miles, a Avidth of a thousand feet or more, and a height from 
trough to crest of fifty or sixty feet. Such a Avave may store 
more energy than can be applied at one time by the guns of the 
greatest Avarship. Gathering their poAver from a long-continued 
storm Avind, these Avaves can roll on for hundreds of miles after 
they have passed beyond the field of air Avhich set them in 
motion. So long as Avaves move over a deep sea they have no 
geological value. The greater part of them die out, generally 
converting the energy Avhich they represent into heat, that is 
given to the Avater or to the air. AMien, hoAvever, the surges enter 
a part of the sea Avhich is relatively shallow, they begin to do 
erosive AVork. In a depth of one thousand feet the higher Avaves 
drag a little on the bottom, brushing the sea floor lightly in a 
manner that may move the finer sediment. At a depth of tAvo 
hundred and fift}^ feet the movement is strong enough to sweep 
small, coarse sand toAvard the shore, and Avith each further step 
in the shalloAving the vigor of the scouring action increases until 
as the Avave rises in the Avail of the surf the rush has something 
like the fury of an avalanche, Avhirling before it eveiything that 
is not closely knit to the surface over Avhich it is moving. 
As the Avave comes into shalloAV Avater, and in proportion to 
the dragging action Avhich it exercises on the bottom, the surge 
becomes to a certain extent Avorn out ; it shrinks in size, so that 
