396 Dr. J. R. Mayer on Celestial Dynamics, 



The quality of the sun's rays, as dependent on his tempera- 

 ture, is of the greatest importance to mankind. If the solar heat 

 were originated by a chemical process, and amounted near its 

 source to a temperature of a few thousand degrees, it would be 

 possible for the light to reach us, whilst the greater part of the 

 more important calorific rays would be absorbed by the higher 

 strata of our atmosphere and then returned to the universe. 



In consequence of the high temperature of the sun, however, 

 our atmosphere is highly diathermic to his rays, so that the latter 

 reach the surface of our earth and warm it. The comparatively 

 low temperature of the terrestrial surface is the cause why the 

 heat cannot easily radiate back through the atmosphere into the 

 universe. The atmosphere acts, therefore, like an envelope, 

 which is easily pierced by the solar rays, but which offers consi- 

 derable resistance to the radiant heat escaping from our earth ; 

 its action resembles that of a valve which allows liquid to pass 

 freely in one, but stops the flow in the opposite direction. 



The action of the atmosphere is of the greatest importance as 

 regards climate and meteorological processes. It must raise the 

 mean temperature of the earth's surface. After the setting of 

 the sun — in fact, in all places where his rays do not reach the 

 surface, the temperature of the earth would soon be as low as 

 that of the universe, if the atmosphere were removed, or if it did 

 not exist. Even the powerful solar rays in the tropics would be 

 unable to preserve water in its liquid state. 



Between the great cold which would reign at all times and in 

 all places, and the moderate warmth which in reality exists on 

 our globe, intermediate temperatures may be imagined; and it 

 is easily seen that the mean temperature would decrease if the 

 atmosphere were to become more and more rare. Such a rare- 

 faction of a valve-like acting atmosphere actually takes place as 

 we ascend higher and higher above the level of the sea, and it is 

 accordingly and necessarily accompanied by a corresponding di- 

 minution 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 therefore 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 in- 



