282 CELESTIAL DYNAMICS. 



the level of the sea, and it is accordingly and necessarily ac- 

 companied by a corresponding diminution of temperature. 



This well-known fact of the lower mean temperature of 

 places of greater altitude has led to the strangest hypotheses. 

 The sun's rays were not supposed to contain all the conditions 

 for warming a body, but to set in motion the "substance" 

 of heat contained in the earth. This "substance" of heat, 

 cold when at rest, was attracted by the earth, and was there- 

 fore found in greater abundance near the centre of the globe. 

 This view, it was thought, explained why the warming power 

 of the sun was so much weaker at the top of a mountain than 

 at the bottom, and why, in spite of his immense radiation, he 

 retained his full powers. 



This belief, which especially prevails amongst imperfectly 

 informed people, and which will scarcely succumb to correct 

 views, is directly contradicted by the excellent experiments 

 made by Pouillet at different altitudes with the pyrheliometer. 

 These experiments show that, everything else being equal, 

 the generation of heat by the solar rays is more powerful in 

 higher altitudes than near the surface of our globe, and that 

 consequently a portion of these rays is absorbed on their pas- 

 sage through the atmosphere. Why, in spite of this partial 

 absorption, the mean temperature of low altitudes is never- 

 theless higher than it is in more elevated positions, is ex- 

 plained by the fact that the atmosphere stops to a far greater 

 degree the calorific rays emanating from the earth than it 

 does those from the sun. 



V. CONSTANCY OF THE SUN'S MASS. 



NEWTON, as is well known, considered light to be the 

 emission of luminous particles from the sun. In the contin- 

 ued emission of light this great philosopher saw a cause tend- 



