CHAPTER VIII. 



MINOR APPLICATIONS OF PHYSICAL PRINCIPLES. 



Yes, them shall mark, with magic art profound, 

 The speed of light, the circling march of sound. 



Campbell. 



O matchless Age ! that even the passing tone 

 Of epoch-making speech, or lover's sigh, 

 Kecordest for the wonder of all time ! 



F. T. Mott. 



AMONG the very numerous discoveries depending upon 

 physical principles, or on the application of physical 

 laws, a few of the more generally interesting may be 

 here noticed. 



The Radiometer, to be seen in almost every optician's 

 window, was invented by Sir AVilliam Crookes in 1873, 

 and consists of an exceedingly delicate windmill, formed 

 of four very slender arms supporting thin metal or pith 

 discs, one side of which is blackened, the whole turning 

 on a fine central point, so as to revolve with hardly any 

 friction. The little machine is enclosed in a glass bulb 

 from which nearly all the air has been extracted; and 

 when exposed to the sun, or even to diffused daylight, 

 the discs revolve with considerable speed. At first this 

 motion was supposed to be caused by the direct impact 

 of the rays of light, the almost complete vacuum only 

 serving to diminish friction; but the explanation now 

 generally adopted is that the black surfaces of the vanes, 

 absorbing heat, become slightly warmer than the white 



