CHAPTER VII 



THE SOURCES OF TERRESTRIAL 

 ENERGY 



\T the outset we may state broadly that 

 h-\ all terrestrial energy is due either to 

 -*- -*- direct gravitative force, or to the arrest 

 of the centripetal motion produced by gravitative 

 force, either in the earth or in the sun. In other 

 words, the entire series of terrestrial phenomena 

 is the complex product of the earth's internal 

 heat, combined with solar radiance, and with 

 direct gravitative force exercised - by the moon 

 and other planets. 



Beginning with the smallest and least con- 

 spicuous of these sources of energy, a mere allu- 

 sion will suffice for the effects wrought upon 

 the earth by its companion planets through the 

 medium of their tidal action upon the sun. That 

 the phenomena of the aurora borealis, as well as 

 the periodic variations in the position of the 

 magnetic needle, are dependent upon the solar 

 spots, is now a well-established doctrine ; and it 

 seems not unlikely that we shall erelong suc- 

 ceed in tracing out other dependences of this 

 sort, — as is shown, for example, in Mr. Mel- 



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