266 CELESTIAL DYNAMICS. 



Is this restoring agency a chemical process? 



If such were the case, the most favourable assumption 

 would be to suppose the whole mass of the sun to be one 

 lump of coal, the combustion of every kilogramme of which 

 produces 6000 units of heat. Then the sun would only be 

 able to sustain for forty-six centuries its present expenditure 

 of light and heat, not to mention the oxygen necessary to 

 keep up such an immense combustion, and other unfavourable 

 circumstances. 



The revolution of the sun on his axis has been suggested 

 as the cause of his radiating energy. A closer examination 

 proves this hypothesis also to be untenable. 



Rapid rotation, without friction or resistance, cannot in 

 itself alone be regarded as a cause of light and heat, espe- 

 cially as the sun is in no way to be distinguished from the 

 other bodies of our system by velocity of axial rotation. The 

 sun turns on his axis in about twenty-five days, and his diam- 

 eter is nearly 112 times as great as that of the earth, from 

 which it follows that a point on the solar equator travels but 

 a little more than four times as quickly as a point on the 

 earth's equator. The largest planet of the solar system, 

 whose diameter is about ^th that of the sun, turns on its axis 

 in less than ten hours ; a point on its equator revolves about 

 six times quicker than one on the solar equator. The outer 

 ring of Saturn exceeds the sun's equator more than ten times 

 in velocity of rotation. Nevertheless no generation of light 

 or heat is observed on our globe, on Jupiter, or on the ring 

 of Saturn. 



It might be thought that friction, though undeveloped in 

 the case of the other celestial bodies, might be engendered by 

 the sun's rotation, and that such friction might generate enor- 

 mous quantities of heat. But for the production of friction 

 two bodies, at least, are always necessary which are in imme- 

 diate contact with one another, and which move with differ- 

 ent velocities or in different directions. Friction, moreover, 



