ON THE INTERACTION OF NATURAL FORCES. 181 



duced by the crystallisation of fused masses. Investiga- 

 tion still shows that the temperature in mines and 

 borings increases as we descend ; and if this increase is 

 uniform, at the depth of fifty miles a heat exists sufficient 

 to fuse all our minerals. Even now our volcanoes pro- 

 ject from time to time mighty masses of fused rocks from 

 their interior, as a testimony of the heat which exists 

 there. But the cooled crust of the earth has already 

 become so thick, that, as may be shown by calculations of 

 its conductive power, the heat coming to the surface 

 from within, in comparison with that reaching the earth 

 from the sun, is exceedingly small, and increases the 

 temperature of the surface only about ^th of a degree 

 Centigrade ; so that the remnant of the old store of force 

 which is enclosed as heat within the bowels of the earth 

 has a sensible influence upon the processes at the earth's 

 surface only through the instrumentality of volcanic 

 phsenomena. Those processes owe their power almost 

 wholly to the action of other heavenly bodies, particu- 

 larly to the light and heat of the sun, and partly also, in 

 the case of the tides, to the attraction of the sun and moon. 

 Most varied and numerous are the changes which we 

 owe to the light and heat of the sun. The sun heats our 

 atmosphere irregularly, the warm rarefied air ascends, 

 while fresh cool air flows from the sides to supply its 

 place : in this way winds are generated. This action is 

 most powerful at the equator, the warm air of which 

 incessantly flows in the upper regions of the atmosphere 

 towards the poles ; while just as persistently at the 

 earth's surface, the trade-wind carries new and cool air 

 to the equator. Without the heat of the sun, all winds 

 must of necessity cease. Similar currents are produced 

 by the same cause in the waters of the sea. Their 

 power may be inferred from the influence which in some 

 cases they exert upon climate. By them the warn) 



