30 THE VELOCITY OF LIGHT. 



CHAPTER III. 



THE VELOCITY OF LIGHT. 



IT may seem Btrange, as, indeed, it really is, that, since 

 light moves at such a velocity as to carry it seven times 

 around the earth in a second, there can be any possible 

 way by which its velocity can be measured. But many 

 ways of doing this have been discovered or devised. 



Some of these methods are astronomical that is, the 

 velocity of light is determined by observations of certain 

 movements and appearances among the heavenly bodies, 

 and by computations made from them. Fully to under- 

 stand these computations, and the astronomical principles 

 on which they are based, requires a degree of mathematical 

 and astronomical knowledge which few persons have time 

 to acquire. But astronomers have given such abundant 

 proof of the soundness and trustworthiness of their meth- 

 ods, in the exactness to a second with which their pre- 

 dictions in respect to eclipses, transits, occultations, and 

 other celestial phenomena are always fulfilled, that when 

 they agree in assuring us that they have determined any 

 point connected with celestial phenomena, we have every 

 possible reason for placing confidence in the result. 



A general idea, moreover, of one of the methods adopt- 

 ed can be obtained by the aid of the following engraving. 

 The method consists in making first an exact computation 

 of the time when some astronomical phenomenon will actu- 

 ally occur, and then observing the difference in the time 

 in which it is seen to occur by an observer on the earth 

 when the earth is on opposite sides of its orbit. The phe- 



