TRANSFORMATION AND EQUIVALENCE OF FORCES. 215 



the reply is, It fell in the shape of rain. If we ask, How 

 came the rain to be in that position whence it fell? the reply 

 is, The vapour from which it was condensed was drifted 

 there by the winds. If we ask, How came this vapour to 

 be at that elevation ? the reply is, It was raised by evapora- 

 tion. And if we ask, What force thus raised it ? the reply 

 is, The sun's heat. Just that amount of gravitative force 

 which the sun's heat overcame in raising the atoms of water, 

 is given out again in the fall of those atoms to the same level. 

 Hence the denudations effected by rain and rivers, during 

 the descent of this condensed vapour to the level of the sea, 

 are indirectly due to the sun's heat. Similarly with the 

 winds that transport the vapours hither and thither. Con- 

 sequent as atmospheric currents are on differences of tem- 

 perature (either general, as between the equatorial and polar 

 regions, or special as between tracts of the Earth's surface of 

 unlike physical characters) all such currents are due to that 

 source from which the varying quantities of heat proceed. 

 And if the winds thus originate, so too do the waves raised 

 by them on the sea's surface. Whence it follows that what- 

 ever changes waves produce the wearing away of shores, 

 the breaking down of rocks into shingle, sand, and mud- 

 are also traceable to the solar rays as their primary cause. 

 The same may be said of ocean-currents. Generated as the 

 larger ones are by the excess of heat which the ocean in 

 tropical climates continually acquires from the Sun; and 

 generated as the smaller ones are by minor local differences 

 in the quantities of solar heat absorbed ; it follows that the 

 distribution of sediment and other geological processes 

 which these marine currents effect, are affiliable upon the 

 force which the sun radiates. The only aqueous agency 

 otherwise originating is that of the tides an agency which, 

 equally with the others, is traceable to unexpended astro- 

 nomical motion. But making allowance for the changes 

 which this works, we reach the conclusion that the slow 

 wearing down of continents and gradual filling up of seas, 



