1 66 THE BORDERLAND OF SCIENCE. 



pared with which all the motions familiar to us are 

 as absolute rest. Now, it is a well-known law of 

 motion that each kind of movement possessed by 

 a body takes place independently of all the others. 

 The moon circles round the earth as if the earth were 

 not circling round the sun. A body would circle round 

 the moon while she circles round the earth (and with 

 the earth around the sun), precisely as though the 

 moon were at perfect rest. So that the motions of 

 the bodies dependent on any star take place quite 

 independently of the motion by which the star is 

 sweeping onwards amid the depths of the star-system. 

 Our earth, for instance, pursues her course round the 

 sun as steadily as though the sun were at rest, instead 

 of being in rapid motion with all his cortege of planets. 

 And the power which a star has of communicating 

 velocity to an approaching dependent body, and of 

 withdrawing velocity from a receding body, has no 

 reference to the motion which the body shares with 

 the star. Take the case of Sirius, for instance. In 

 what I said of him above I regarded him as at rest ; 

 and I stated, justly, that he could communicate 

 to a body approaching him from a state of rest an 

 enormous velocity, the whole of which he would with- 

 draw during the recession of the body. But Sirius is, 

 in reality, travelling with great velocity amid the star- 

 depths ; and if we conceive the case of a meteoric 

 body circling close around Sirius with the enormous 

 velocity already referred to, we must remember further 

 that that body shares also with Sirius the great 



