394' Dr. J. R. Mayer on Celestial Dynamics. 



their thicker ends in the ground to the depth of from 6 to 8 

 metres. To prevent the wood getting too hot and taking fire, 

 water was conducted in many places into the channel. 



This stupendous mechanical process, when compared with 

 cosmical processes on the sun, appears infinitely small. In the 

 latter case it is the mass of the sun which attracts, and in 

 lieu of the height of Mount Pilatus we have distances of a 

 hundred thousand and more miles ; the amount of heat gene- 

 rated by cosmical falls is therefore at least 9 million times greater 

 than in our terrestrial example. 



Rays of heat on passing through glass and other transparent 

 bodies undergo partial absorption, which differs in degree, how- 

 ever, according to the temperature of the source from which the 

 heat is derived. Heat radiated from sources less warm than 

 boiling water is almost completely stopped by thin plates of 

 glass. As the temperature of a source of heat increases, its 

 rays pass more copiously through diathermic bodies. A plate of 

 glass, for example, weakens the rays of a red-hot substance, even 

 when the latter is placed very close to it, much more than it does 

 those emanating at a much greater distance from a white-hot 

 body. If the quality of the sun's rays be examined in this 

 respect, their diathermic energy is found to be far superior to 

 that of all artificial sources of heat. The temperature of the 

 focus of a concave metallic reflector in which the sun's light has 

 been collected is only diminished from one-seventh to one-eighth 

 by the interposition of a screen of glass. If the same experi- 

 ment be made with an artificial and luminous source of heat, it 

 is found that, though the focus be very hot when the screen is 

 away, the interposition of the latter cuts off nearly all the heat; 

 moreover, the focus will not recover its former temperature when 

 reflector and screen are placed sufficiently near to the source of 

 heat to make the focus appear brighter than it did in the former 

 position without the glass screen. 



The empirical law, that the diathermic energy of heat increases 

 with the temperature of the source from which the heat is radi- 

 ated, teaches us that the sun's surface must be much hotter than 

 the most powerful process of combustion could render it. 



Other methods furnish the same conclusion. If we imagine 

 the sun to be surrounded by a hollow sphere, it is clear that the 

 inner surface of this sphere must receive all the heat radiated 

 from the sun. At the distance of our globe from the sun, such 

 a sphere would have a radius 215 times as great, and an area 

 46,000 times as large as the sun himself; those luminous and 

 calorific rays, therefore, which meet this spherical surface at 



