450 THE MULTIPLICATION OF EFFECTS. 



and in its details. Thus endless is the accumulation of 

 geological and geographical results slowly brought about 

 by this one cause the escape of the Earth's primitive heat. 

 When we pass from the agency which geologists term 

 igneous, to aqueous and atmospheric agencies, we see a like 

 ever-growing complication of effects. The denuding actions 

 of air and water have, from the beginning, been modifying 

 every exposed surface: everywhere working many different 

 changes. As already shown (69) the original source of 

 those gaseous and fluid motions which effect denudation, is 

 the solar heat. The transformation of this into various 

 modes of force, according to the nature and condition of the 

 matter on which it falls, is the first stage of complication. 

 The sun's rays, striking at all angles a sphere, that from mo- 

 ment to moment presents and withdraws different parts of 

 its surface, and each of them for a different time daily 

 throughout the year, would produce a considerable variety 

 of changes even were the sphere uniform. But falling as 

 they do on a sphere surrounded by an atmosphere in some 

 parts of which wide areas of cloud are suspended, and which 

 here unveils vast tracts of sea, there of level land, there of 

 mountains, there of snow and ice, they initiate in its several 

 parts countless different movements. Currents of air of all 

 sizes, directions, velocities, and temperatures, are set up; as 

 are also marine currents similarly contrasted in their charac- 

 ters. In this region the surface is giving off water in the 

 state of vapour; in that, dew is being precipitated; and in 

 the other rain is descending differences that arise from the 

 ever-changing ratio between the absorption and radiation of 

 heat in each place. At one hour, a rapid fall in temperature 

 leads to the formation of ice, with an accompanying ex- 

 pansion throughout the moist bodies frozen; while at an- 

 other, a thaw unlocks the dislocated fragments of these bod- 

 ies. And then, passing to a second stage of complication, 

 we see that the many kinds of motion directly or indirectly 

 caused by the sun's rays, severally produce results that vary 



