NEWS FROM THE STARS. 
383 
them was standing still. For let us consider what would 
happen, if the cork-thrower sauntered up-stream or down- 
stream while throwing corks at intervals of one second. Sup- 
pose he moved up-stream at the rate of a foot per second; 
then, when he has thrown one cork, he moves a foot up-stream 
before he throws the next ; and the first cork has floated three 
feet down stream ; hence the second cork falls four feet behind 
the first. Thus the common distance between the corks is 
now four feet instead of three feet. Next suppose he saunters 
down-stream at the rate of a foot per second ; then, when he 
has thrown one cork, he moves a foot down-stream before he 
throws the next; and the first cork has floated three feet 
down-stream ; hence the second cork falls only two feet behind 
the first. Thus the common distance between the corks is now 
two feet instead of three feet. It is clear, then, that the person 
standing a mile or so down-stream, if he knows that the 
stream is flowing three feet per second, and that his friend up- 
stream is throwing one cork in per second, can be quite sure 
that his friend is standing still if the corks come past with a 
common interval of three feet between them. Moreover, he 
can be equally sure that his friend is sauntering up-stream, if 
the corks come past with a common interval exceeding three 
feet ; and that he is sauntering down-stream, if the common 
interval is less than three feet. And if, by some process of 
measuring, he can find out exactly how much greater or how 
much less than three feet the interval is, he can tell exactly 
how fast his friend is sauntering up-stream or down-stream. It 
would not matter how far down-stream the observer might be, 
so long as the stream’s rate of flow remained unchanged ; nor, 
indeed, would it matter, even though the stream flowed at a 
different rate past the observer than past the cork-thrower, so 
long as neither of these two rates were liable to alteration. 
Now, we may compare the emission of light-waves by a 
luminous object to the throwing of corks in our illustrative 
case. The rate of flow for light-waves is indeed infinitely 
faster that that of any river, being no less than 185,000 miles 
per second. The successive light-waves are set in motion at 
infinitely shorter time-intervals, since for extreme red light there 
are no less than 458,000,000,000,000 undulations per second, 
and for extreme violet no less than 727,000,000,000,000 ; but 
these specific differences do not affect the exactness of the 
illustration. It is obvious that all that is necessary to make 
the parallel complete is that the flow of light-waves shall 
reach the observer at a constant rate (which is the actual 
case), and that he shall know, in the case of any particular and 
distinguishable kind of light, what is the rate at which the 
wave-action is successively excited, and be able to compare 
