260 CELESTIAL DYNAMICS. 



string communicates its vibrations to the surrounding and re- 

 sisting medium its own motion becomes weaker and weaker, 

 until at last it sinks into a state of rest. 



The sun has often and appropriately been compared to an 

 incessantly sounding bell. But by what means is the power 

 of this body kept up in undiminished force so as to enable 

 him to send forth his rays into the universe in such a grand 

 and magnificent manner ? What are the causes .which coun- 

 teract or prevent his exhaustion, and thus save the planetary 

 system from darkness and deadly cold ? 



Some endeavoured to approach " the grand secret," as 

 Sir Wm. Herschel calls this question, by the assumption that 

 the rays of the sun, being themselves perfectly cold, merely 

 cause the " substance " of heat, supposed to be contained in 

 bodies, to pass from a state of rest into a state of motion, and 

 that in order to send forth such cold rays the sun need not be 

 a hot body, so that, in spite of the infinite development of 

 light, the cooling of the sun was a matter not to be thought of. 



It is plain that nothing is gained by such an explanation ; 

 for, not to speak of the hypothetical " substance" of heat, 

 assumed to be at one time at rest and at another time in mo- 

 tion, now cold and then hot, it is a well-founded fact that the 

 sun does not radiate a cold phosphorescent light, but a light 

 capable of warming bodies intensely ; and to ascribe such 

 rays to a cold body is at once at variance with reason and 

 experience. 



Of course such and similar hypotheses could not satisfy 

 the demands of exact science, and I will therefore try to ex- 

 plain in^a more satisfactory manner than has been done up 

 to this time the connexion between the sun's radiation and its 

 effects. In doing so, I have to claim the indulgence of scien- 

 tific men, who are acquainted with the difficulties of my task. 



