52 FROM NEBULA TO NEBULA 



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 ! 



Again, we are told by our learned teachers that the 

 reason the moon does not fall upon us is because of the 

 fortunate circumstance that her motion is always directed 

 tangentially. This, obviously, is a mere begging of the 

 question. The point is, what keeps it thus directed? 

 Imagine the earth and moon removed to outer space, 

 where the attractions of the rest of the universe could not 

 interfere, and started together at exactly the moon's 

 present speed, along lines potentially parallel and 240,000 

 miles apart, would, or would they not, gradually swerve 

 from the parallels and within the space of a week's time, 

 or thereabouts, collide 1 Of course they would, notwith- 

 standing their having, by supposition, a double momen- 

 tum to overcome, the earth's as well as the moon's. Mani- 

 festly, then, there is nothing in the abstract idea of tan- 

 gentially directed motion to lift our problem beyond 

 the reach of the legitimate effects of gravitational at- 

 traction. 



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 matter, and should we then have two satellites in place 

 of one? Would the bird, despite its death, not fall to the 

 earth? Or could it fold its wings and still keep in the 

 race? Would it, too, dead or alive, possess " persistent" 

 motion? 



Newton supposed that by argumentatively eliminat- 

 ing the resistance of the ether, postulating it to have ex- 

 istence but no body, he overcame the only obstacle to 

 his inertial hypothesis. In this he was culpably in error. 

 Take two pails precisely alike and provided with lids. 



