EQUILIBRATION. 509 



from sphericity as is requisite to counterbalance the centrif- 

 ugal force consequent on its velocity round its axis. That is 

 to say, during the evolution of the Earth, there has been 

 reached a complete equilibrium of those forces which affect 

 its general outline. The only other process of equili- 



bration which the Earth as a whole can exhibit, is the loss of 

 its axial motion; and that any such loss is going on, we have 

 no direct evidence. It has been contended, however, by 

 Prof. Helmholtz, that inappreciable as may be its effect 

 within known periods of time, the friction of the tidal wave 

 must be slowly diminishing the Earth's rotatory motion, 

 and must eventually destroy it. Xow though it seems an 

 oversight to say that the Earth's rotation can thus be de- 

 stroyed, since the extreme effect, to be reached only in infi- 

 nite time by such a process, would be an extension of the 

 Earth's day to the length of a lunation, yet it seems clear 

 that this friction of the tidal wave is a real cause of decreasing 

 rotation. Slow as its action is, we must recognize it as ex- 

 emplifying, under another form, the universal progress to- 

 wards equilibrium. 



It is needless to point out, in detail, how those move- 

 ments which the Sun's rays generate in the air and water on 

 the Earth's surface, and through them in the Earth's 

 solid substance,* one and all teach the same general 

 truth. Evidently the winds and waves and streams, as well 

 as the denudations and depositions they effect, perpetually 

 illustrate on a grand scale, and in endless modes, that grad- 

 ual dissipation of motions described in the first section; and 

 the consequent tendency towards a balanced distribution of 

 forces. Each of these sensible motions, produced directly or 



* Until I recently consulted his " Outlines of Astronomy " on another ques- 

 tion, I was not aware that so far back as 1833, Sir John Herschel had enunci- 

 ated the doctrine that " the sun's rays are the ultimate source of almost every 

 motion which takes place on the surface of the earth." He expressly includes 

 all geologic, meteorologic, and vital actions ; as also those which we produce 

 by the combustion of coal. The late George Stephenson appears to have been 

 wrongly credited with this last* idea. 



