ON THE INTERACTION OF NATURAL FORCES. 161 



Our earth bears still the unmistakable traces of its old fiery 

 fluid condition. The granite formations of her mountains exhibit 

 a structure, which can only be produced by the crystallisation of 

 fused masses. Investigation 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 project 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 tem- 

 perature 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 instru- 

 mentality of volcanic phenomena. Those processes owe their 

 power almost wholly to the action of other heavenly bodies, 

 particularly 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 neces- 

 sity 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 warm water of the Antilles is carried to the British 

 Isles, and confers upon them a mild uniform warmth, and rich 

 moisture ; while, through similar causes, the floating ice of the 

 North Pole is carried to the coast of Newfoundland and produces 



