AN AMERICAN ASTRONOMICAL ACHIEVEMENT. 447 



The principle of the method is very readily explained. Light comes 

 to us from the heavenly bodies, as from other luminous bodies, in 

 waves, which sweep through the ether of space at the rate of about 

 185,000 miles per second. The whole of that region over which as- 

 tronomers 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 rush- 

 ing 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. 



All the celestial bodies are in motion amid the multitudinous 

 waves of space. Something can be learned respecting their motions 

 by studying the waves. If a strong swimmer were stemming a series 

 of long, rolling waves, their crests would pass him in more rapid suc- 

 cession than if he were at rest ; if, on the other hand, he reversed his 

 course, so that waves overtook instead of meeting him, their crests 

 would pass in slower succession. One can easily conceive how, if he 

 knew the exact rate at which the crests would pass him so many 

 exactly per minute were he at rest, their slower or more rapid suc- 

 cession might indicate how fast he himself was moving, either from 

 or toward them. If he were quite unconscious of his own motion, the 

 effect would be simply that the distance from crest to crest would 

 seem to be diminished in one case, lengthened in the other that is, 

 the waves narrowed or widened. Similarly with the aerial waves 

 which produce sound. They are seemingly shortened when the source 

 of sound is approaching, whether by its own motion or the hearer's, 

 and lengthened when the source of sound is receding. In the former 

 case the tone of the sound is made more acute, in the latter graver 

 than it really is. This is strikingly illustrated Avhen a swift train 

 rushes past a station, the whistle sounding all the time, for there is a 

 perceptible lowering of the whistle's note as the. engine passes a 

 nearer on the platform. While the train is appproaching him, he 

 hears a note somewhat sharper than the true note of the whistle; 

 after it has passed he hears a note somewhat flatter than the true 

 note. Still more obvious, even to non-musical ears, is the correspond- 

 ing chansre when two trains pass each other. In America, where a 

 hideously-clanging engine-bell is used, the change is very remarkable, 

 insomuch that a person unfamiliar with the arrangement actually 

 adopted would suppose a different bell was rung the moment the 

 engine passed the hearer. 



Light traveling also in waves, it is obvious that a similar effect 

 must be produced by approach or recession, if only the rate of motion 

 is sufficiently rapid. The swimmer of my first illustration must have 



