CHAPTER VII. 



THE SOURCES OF TERRESTRIAL ENERGY 



At the outset we may state broadly that 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 conspicuous of these 

 sources of energy, a mere allusion 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 depen- 

 dent upon the solar spots, is now a well-established doctrine ; 

 and it seems not unlikely that we shall ere long succeed in 

 tracing out other dependences of this sort, — as is shown, for 

 example, in Mr. Meldrum's investigation of the relations 

 between sun-spots and rainfall. And whatever may be the 

 final explanation of the phenomena of sun-spots, there can be 

 little doubt that the periodicity of these phenomena is 

 conditioned by the positions of the various planets, and 

 especially of the giants Jupiter and Saturn. But these 



