78 PLEASANT WA YS IN SCIENCE. 



have had the principle of the method imperfectly explained 

 to them. 



Light travels from every self-luminous body in waves 

 which sweep through the ether of space at the rate of 

 185,000 miles per second. The whole of that region of 

 space over which astronomers have extended their survey, 

 and doubtless a region many millions of millions of times 

 more extended, may be compared to a wave-tossed sea, only 

 that instead of a wave-tossed surface, there is wave-tossed 

 space. At every point, through every point, along every 

 line, athwart every line, myriads of light-waves are at all 

 times rushing with the inconceivable velocity just mentioned. 



It is from such waves that we have learned all we know 

 about the universe outside our own earth. They bring to 

 our shores news from other worlds, though the news is not 

 always easy to decipher. 



Now, seeing that we are thus immersed in an ocean, 

 athwart which infinite series of waves are continually 

 rushing, and moreover that we ourselves, and every one of 

 the bodies whence the waves proceed either directly or after 

 reflection, are travelling with enormous velocity through this 

 ocean, the idea naturally presents itself that we may learn 

 something about these motions (as well as about the bodies 

 themselves whence they proceed), by studying the aspect of 

 the waves which flow in upon us in all directions. 



Suppose a strong swimmer who knew that, were he at 

 rest, a certain series of waves would cross him at a particular 

 rate ten, for instance, in a minute were to notice that 

 when he was swimming directly facing them, eleven passed 

 him in a minute : he would be able at once to compare 

 his rate of swimming with the rate of the waves' motion. 

 He would know that while ten waves had passed him on 

 account of the waves' motion, he had by his own motion 

 caused yet another wave to pass him, or in other words, had 

 traversed the distance from one wave-crest to the next. 

 Thus he would know that his rate was one-tenth that of 

 the waves. Similarly if, travelling the same way as the waves, 



