278 CELESTIAL DYNAMICS. 



Pilatus into the plain below. This wooden channel which 

 was built about thirty years ago by the engineer Hupp, was 

 9 English miles in length ; the largest trees were shot down 

 it from the top to the bottom of the mountain in about two 

 minutes and a half. The momentum possessed by the trees 

 on their escaping at their journey's end from the channel was 

 sufficiently great to bury their thicker ends in the ground to 

 the depth of from 6 to 8 metres. To prevent the wood get- 

 ting 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 transpa- 

 rent bodies undergo partial absorption, which differs in de- 

 gree, however, 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 in- 

 terposition of a screen of glass. If the same experiment be 



