DRIFTING OF THE STARS. 225 



motion of recession. In the paper called " Are there any Fixed Stars ? " 

 in the Popular Science Review for October, 1868, the nature of the 

 means by which this discovery was effected was fully described and 

 explained. It may be permitted to me to mention, also, that while Dr. 

 Huggins's researches were still unannounced (or rather incomplete) I 

 was so far fortunate as to indicate the possibility of employing the 

 very method of research which Dr. Huggins was then engaged (un- 

 known to me) in applying to Sirius. I propose here briefly to describe 

 and explain the method, referring the reader, who desires fuller infor- 

 mation on these preliminary points, to the paper of October, 1868, men- 

 tioned above. I am the more desirous of doing this, because I find 

 the principle of the method not readily grasped, and that I conceive 

 the explanation I am about to offer may remove certain difficulties not 

 uncommonly experienced. 



Conceive that a person, standing on the edge of a steadily-flowing 

 stream, throws corks into it at regular intervals say one cork per 

 second. These would float down the stream, remaining always sepa- 

 rated by a constant distance. Thus, if the stream were flowing three 

 feet per second, the corks would be a yard apart (supposing, for con- 

 venience of illustration, that each cork was thrown with exactly the 

 same force and in exactly the same direction). Now, if a person a 

 mile or so down the stream saw these corks thus floating past, he 

 could infer that they had been thrown in at regular intervals ; and, 

 moreover, if he knew the rate of the stream, and that the corks were 

 thrown in by a person standing at the river's edge, he would know 

 that the interval between the throwing of successive corks was one 

 second. But, vice versa, if he knew the rate of the stream, and that 

 the corks were thrown in at intervals of one second, he could infer that 

 the person throwing 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. Suppose 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, sup- 

 pose 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 



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