40 FROM NEBULA TO NEBULA 



self. For one thing, he knows that only a minor part of 

 the energy he puts into the flinging motion goes toward 

 overcoming the air resistance ; (2) that the momentum of 

 the ball is only the energy he himself imparts, and that 

 it dies out almost instantly the moment his flinging 

 maneuver ceases; and (3) that the great bulk of the 

 energy his hand supplies is absorbed in stretching the 

 string, or keeping it taut ; that is to say, his muscle is the 

 centrifugal force. You cannot hitch in double harness a 

 bullet fired with a charge of gunpowder equivalent to 10 

 horse-power hours of energy along with the horse itself, 

 and expect them to team a load for ten hours. No more 

 can you match a projectile moon with the steady pull of 

 gravity. 



Now, it is not at all difficult to determine the approxi- 

 mate horse-power hours of energy resident in the moon's 

 "momentum". Assuming that the moon is moving at 

 the exact velocity of 3350 feet a second and that it is fall- 

 ing at the behest of the earth 's attraction at the precise 

 rate of one-nineteenth of an inch in the same space of 

 time, we need only divide the fraction into the larger 

 number and multiply the quotient by 240 trillions to get 

 the answer desired. Eemember, however, and again I 

 say remember, that, according to Newton's theory, the 

 moon has no way to recuperate lost energy ; hence, when 

 the energy of her momentum is used up in wrestling 

 against the earth's attraction, that momentum is done 

 for, for good. Now, by the conditions of our problem, 

 the moon's momental (inertial) energy is constantly 

 pitted against the earth's attraction, which is always 

 fresh, can never be used up, and is uniformly self -renew- 

 ing. Dividing, therefore, as we did above, 3350 feet, or 

 its equivalent in inches, 40200, by 1-19 we obtain the quan- 

 tity 763,800, which is the number of seconds that it should 

 take the centripetal attraction to wear out the moon's 

 momental energy completely. Raised to higher terms, 

 this period amounts to 8.8 days, w r hich is reasonably close 

 to the time generally estimated that it would take the 

 moon to fall to the earth if dropped from a state of ab- 

 solute rest. This result agrees well with the rule that 



