276 CELESTIAL DYNAMICS. 



from 4600 to 9200 times as much heat as would be generated 

 by the combustion of an equal mass of coal. 



IV. OKIGIN OF THE SUN S HEAT. 



THE question why the planets move in curved orbits, one 

 of the grandest of problems, was solved by Newton in con 

 sequence, it is believed, of his reflecting on the fall of an ap 

 ple. This story is not improbable, for we are on the right 

 track for the discovery of truth when once we clearly recog 

 nize that between great and small no qualitative but only a 

 quantitative difference exists when we resist the suggestions 

 of an ever active imagination, and look for the same laws in 

 the greatest as well as in the smallest processes of nature. 



This universal range is the essence of a law of nature, 

 and the touchstone of the correctness of human theories. We 

 observe the fall of an apple, and investigate the law which 

 governs this phenomenon ; for the earth we substitute the 

 sun, and for the apple a planet, and thus possess ourselves of 

 the key to the mechanics of the heavens. 



As the same laws prevail in the greater as well as in the 

 smaller processes of nature, Newton s method may be used in 

 solving the problem of the origin of the sun s heat. We 

 know the connexion between the space through which a body 

 falls, the velocity, the vis viva, and the generation of heat on 

 the surface of this globe ; if we again substitute for the earth 

 the sun, with a mass 350,000 greater, and for a height of a 

 few metres celestial distances, we obtain a generation of heat 

 exceeding all terrestrial measures. And since we have suffi 

 cient reason to assume the actual existence of such mechani 

 cal processes in the heavens, we find therein the only tenable 

 explanation of the origin of the heat of the sun. 



