294 CLASSICS OF MODERN SCIEx^cE 

 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 temper- 

 ature of the surface only about one-thirtieth 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 phenomena. These 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 necessity, cease. Similar currents are produced by the same cause 

 in the waters of the sea. Their power may be inferred from the in- 

 fluence 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 cold. Further, by the heat of 

 the sun, a portion of the water is converted into vapour which rises 

 in the atmosphere, is condensed to clouds, or falls in rain and snow 

 upon the earth, collects in the form of springs, brooks, and rivers, 

 and finally reaches the sea again, after having gnawed the rocks, 

 carried away the light earth, and thus performed its part in the 

 geologic changes of the earth; perhaps, besides all this it has driven 

 our water-mill upon its way. If the heat of the sun were withdrawn, 



