NEWTON'S THEORY OF PLANETARY MOTIONS 95 



returns to the starting point. At all points in her orbit 

 180 degrees apart, her compass direction is exactly re- 

 versed, showing that she must arrest all her motion east- 

 ward or westward, as the case may be, before she can 

 acquire a new motion westward or eastward ; yet month 

 after month she has been doing this throughout the cen- 

 turies, without, as alleged, drawing upon her stored 

 energy to the extent of a single ounce, and without the 

 loss of a shred of speed ! 



Imagine, if you please, some marvelous bird as large 

 as the moon, whose wings could take hold upon the ether 

 as those of our terrestrial birds do on the air, to approach 

 us out of the depth of space, and to take up its course in 

 the wake of the moon at precisely the latter 's velocity; 

 and when the race was full on, imagine, further, a hunter 

 to shoot the bird dead in full flight. Would the slain bird 

 and the inanimate moon be subject alike to the same laws 

 of "celestial mechanics ", and should we then possess two 

 satellites in lieu of one ? Would it, dead or alive, possess 

 " persistent " motion by the magic spell of "entering an 

 orbit", and be able, while living, to fold its wings and, 

 without another effort, continue to fly forever in spite of 

 its ponderous weight ! Or could it, by an extra flap of its 

 wings while in the throes of death, at the same time di- 

 verging outwardly from its tangent, thereby inaugurate 

 for its carcass an unending flight into far distant space ? 



Another illustration drawn from the flight of birds 

 may here be introduced by way of sharpening the distinc- 

 tion between the Newtonian theory of circulatory motions 

 and my own. Instead of imagining one great bird, let us 

 imagine two of them, the first as large as the moon and 

 the other the size of the earth, and picture these as pur- 

 suing exactly parallel paths 238,000 miles (the mean dis- 

 tance of the moon) apart. According to Newton's first 

 law of motion, were either of the birds unaccompanied by 

 the other, once launched in motion it might placidly fold 

 its wings, yet continue on its course, at unchanging ve- 

 locity, forever. The fact of there being two of them, 

 however, alters the case completely, for the gravitational 



