CH. xxi. VELOCITY OF LIGHT. 173 



times too late ; and they varied, according to some regular 

 rule, as much as 16 minutes 36 seconds 6n each side of the 

 exact moment when they ought to have happened. 



At last it occurred to Roemer, and to an Italian astrono- 

 mer named Cassini, that, as Jupiter is farther away from the 

 earth at one time than another, the eclipses might be seen 

 some minutes later whenever the rays of light from the 

 moons had to cross a greater distance to reach the earth. 

 Cassini seems to have put the thought aside and not to have 

 worked it out ; but Roemer seized upon it, and by careful 

 calculations proved that it was the true answer to the diffi- 

 culty. If the earth was at E (Fig. 34) for example, when 



FIG. 34. 



Different Distances at which Jupiter's Light reaches the Earth, 

 j, Jupiter. E E', The Earth. 



Jupiter was at j, the light would not have nearly so far to 

 travel as if the earth was at E'; and in this last position the 

 1 6 minutes 36 seconds would be taken up by the light 

 crossing the earth's orbit from E to E'. This distance was 

 known to be about 190,000,000 miles, so that light travels at 

 the rate of 190,000,000 miles in 996 seconds, or about 

 190,000 miles in a second. This is nine million times as 

 fast as the quickest express train. 



Huyghens and Newton Theories of Light. The 

 time had now come when so much was known about the way 

 in which light behaves, that philosophers began to ask them- 

 selves, 'What is Light?' a question by no means so easily 

 answered as you may think ; for though it is by means of 



