II. 



CELESTIAL DYNAMICS. 



I. INTRODUCTION. 



EVERY incandescent and luminous body diminishes in 

 temperature and luminosity in the same degree as it 

 radiates light and heat, and at last, provided its loss be not 

 repaired from some other source of these agencies, becomes 

 cold and non-luminous. 



For light, like sound, consists of vibrations which are 

 communicated by the luminous or sounding body to a sur- 

 rounding medium. It is perfectly clear that a body can only 

 excite such vibrations in another substance when its own par- 

 ticles undergo a similar movement ; for there is no cause for 

 undulatory motion when a body is in a state of rest, or in a 

 state of equilibrium with the medium by which it is sur- 

 rounded. If a bell or a string is to be sounded, an external 

 force must be applied ; and this is the cause of the sound. 



If the vibratory motion of a string could take place with- 

 out any resistance, it would vibrate for all time ; but in this 

 case no sound could be produced, because sound is essentially 

 the propagation of motion ; and in the same degree as the 



