DRIFTING LIGHT WAVES. 99 



swiftly, and the same way round, only a small portion of 

 Venus's motion ever appears as a motion of approach to- 

 wards or recession from the earth. Still, Venus is some 

 times approaching and sometimes receding from the earth, 

 at a rate of more than 8 miles per second. Her light is 

 much brighter than that of Jupiter or Saturn, and accord- 

 ingly this smaller rate of motion would be probably more 

 easily recognized than the greater rate at which the giant 

 planets are sometimes approaching and at other times 

 receding from the earth. At least, the Greenwich ob- 

 servers seem to have confined their attention to Venus, 

 so far as motions of planets in the line of sight are con- 

 cerned. The moon, as a body which keeps always at 

 nearly the same distance from us, would of course be the 

 last in the world to be selected to give positive evidence 

 in favour of the new method; but she serves to afford a 

 useful test of the qualities of the instruments employed. 

 If when these were applied to her they gave evidence of 

 motions of recession or approach at the rate of several miles 

 per second, when we know as a matter of fact that the 

 moon's distance never * varies by more than 30,000 miles 

 during the lunar month, her rate of approach or recession 

 thus averaging about one-fiftieth part of a mile per second, 

 discredit would be thrown on the new method not, indeed, 

 as regards its principle, which no competent reasoner can 

 for a moment question, but as regards the possibility of 

 practically applying it with our present instrumental means. 



Observations have been made at Greenwich, both on 

 Venus and on the moon, by the new method, with results 

 entirely satisfactory. The method shows that Venus is 

 receding when she is known to be receding, and that she 

 is approaching when she is known to be approaching. Again, 

 the method shows no signs of approach or recession in the 

 moon's case. It is thus in satisfactory agreement with the 



* It varies more in some months than in others, as the moon's orbit 

 changes in shape under the various perturbing influences to which she is 

 subject. 



