20 FROM NEBULA TO NEBULA 



Having now by make-believe got our moon into 

 motion, and having transported it into this region of 

 Nothing-something, we are fairly in the midst of Alice's 

 Wonderland with appetite sharpened for further mar- 

 vels. The next thing we know is that our rnoori is sud- 

 denly switched out of its straight course into a curved 

 one, but without in the least slackening speed, so that 

 in a unit of time it covers precisely the same distance 

 through space as it did before ! It is plain, is it not, that 

 on this supposition all its alleged momentum in used up 

 in prosecuting this forward motion, and that no surplus 

 energy is left to resist and counteract fresh external 

 strain? But what have we done to cause the moon 

 to turn? Why, we have suspended to it the earth as 

 a pendant, and now require it to carry this load for- 

 ever without complaining! I have already denned for 

 the reader the term "radius vector, " Suppose we rep- 

 resent the earth's attraction on the moon along this 

 line by an elastic string. Whence, I ask, is derived 

 the energy that keeps this string stretched taut? It 

 cannot be filched from the momentum of the moon, for 

 that must be religiously guarded against waste; and, 

 besides, as we have already seen, it is already wholly 

 appropriated toward linear speed. 



Asr.iin, projectile motions are in their very nature 

 fortuitous as to the three elements of location, velocity, 

 and direction. The likelihood, therefore, of all these ele- 

 ments just happening to coincide so as to produce a prac- 

 tically circular orbit is almost inconceivably remote. 

 Supposing this marvel should happen in the case of two 

 planets, which, besides, should travel in almost the same 

 plane, around the same central body, and with a definite 

 relation to each other as to distance and velocity, then 

 the supposition of mere fortuity must be discarded and 

 either divine intervention be invoked or some great fun- 



