PLANETARY MOTIONS 53 



Fill one with sand, letting the other go empty, and carry 

 them together for a furlong. Being exactly alike in size, 

 they are, of course, equally resisted by the atmosphere. 

 Which arm, however, will tire first, that carrying the full 

 pail, or the other one 1 The former, by all means ! The 

 moon, too, has a load resistance to overcome, besides hav- 

 ing to buffet the medium it traverses. We have already 

 seen that this load amounts to the steady downward pull 

 of 240 millions of millions of horses, more than a million 

 times greater than the etheric resistance to the moon 

 would amount to were that medium even as dense as our 

 atmosphere at sea level. Talk about straining at gnats 

 and swallowing camels ! Returning again to the pails ; 

 would you say that by running the furlong instead of 

 walking it, you would lessen the work done proportion- 

 ally! Certainly not, and neither can the imaginary im- 

 pulse that fired the moon on her course relieve her of 

 the task of carrying her own dead weight. You cannot 

 sophistrize gravity out of existence. To support the 

 moon during the 29.5 days of her monthly journey from 

 perigee to perigee and bring her back to the same alti- 

 tude, she must from some genuine source not from 

 empty imagination or word-juggling draw just as much 

 lifting power as would be required to counteract the 

 earth's gravitation did she possess no initial tangential 

 translation whatever. 



As suggested before in the introductory chapter, it 

 has been the habit of those who make a study of the 

 heavens, from time immemorial, to anticipate finding a 

 good deal of mystery and miracle intermingled with 

 prosaic fact. The supposition that nebulae rotate of 

 themselves is a modern example of this primitive instinct. 

 So is the crude belief in the spontaneity of celestial mo- 

 tions in general, and in the inherence in them of persis- 

 tency as an abstract quality. Perhaps, however, the most 

 typical instance of this superstitious streak in modern 

 astronomers is their conception of the significance of the 

 law of the conservation of moment of momentum. By ob- 

 servation and computation, the astronomers have dis- 

 covered the fact (for such it is, subject to a modification 



