ch. vii.] SOURCES OF TERRESTRIAL ENERGY. 407 



inter-relations, though they may be much more important 

 than is as yet suspected, need not now detain us. Such 

 further effects as may be wrought upon the earth by polarized 

 light sent from the other planets, and by radiance from re- 

 mote stellar systems, may be left out of the account. Nor 

 need we do more than allude to the moon's gravitative force 

 as the chief cause of the oceanic tides, with their resultant 

 geologic phenomena. Passing over all these circumstances, 

 we come to the still unexpended energy represented by the 

 earth's internal heat, concerning which we need only say that 

 it is the cause of the geologic phenomena classed as igneous. 

 Volcanic eruptions, earthquake shocks, elevations and sub- 

 sidences of continents and ocean -floors, metamorphoses of 

 sedimentary rocks, boiling springs, fractures of strata, and 

 formations of metallic veins, are the various manifestations of 

 this form of terrestrial energy. 



But all these grand phenomena must be regarded as im- 

 measurably inferior in variety and importance to those which 

 are due to the transformation of solar radiance. These must 

 be described with somewhat more of detail. First, with the 

 exception of the changes wrought by the tides, all the geo- 

 logic phenomena classed as aqueous are manifestations of 

 transformed solar energy. Pulses of molecular motion pro- 

 ceeding from the sun are stored as reserved energy in masses 

 of aqueous vapour raised from the sea. This energy is again 

 partly given out as the vapour is condensed into rain and 

 falls to the ground. The portion which remains is expended 

 in the transfer of the fallen water through the soil, till it 

 collects in rivulets, brooks, and rivers, and gradually descends 

 to the ocean whence solar radiance raised it, bearing along 

 with it divers solid particles which go to form sedimentary 

 strata. The wind which blew these clouds into the colder 

 regions where they consolidated into rain-drops, was set in 

 motion by solar energy, — since all winds are caused by the 

 unequal heating of different parts of the earth's surface. 



