ATOM. 475 



We have seen that the sun stops but a very small fraction of the cor- 

 puscules which enter it. The earth, being a smaller body, stops a still smaller 

 proportion of them. The proportion of those which are stopped by a small 

 body, say a 1 Ib. shot, must be smaller still in an enormous degree, because its 

 thickness is exceedingly small compared with that of the earth. 



Now, the weight of the ball, or its tendency towards the earth, is produced, 

 according to this theory, by the excess of the impacts of the corpuscules which 

 come from above over the impacts of those which come from below, and have 

 passed through the earth. Either of these quantities is an exceedingly small 

 fraction of the momentum of the whole number of corpuscules which pass through 

 the ball in a second, and their difference is a small fraction of either, and yet 

 it is equivalent to the weight of a pound. The velocity of the corpuscules 

 must be enormously greater than that of any of the heavenly bodies, other- 

 wise, as may easily be shewn, they would act as a resisting medium opposing 

 the motion of the planets. Now, the energy of a moving system is half the 

 product of its momentum into its velocity. Hence the energy of the corpuscules, 

 which by their impacts on the ball during one second urge it towards the earth, 

 must be a number of foot-pounds equal to the number of feet over which a 

 corpuscule travels in a second, that is to say, not less than thousands of millions. 

 But this is only a small fraction of the energy of all the impacts which the 

 atoms of the ball receive from the innumerable streams of corpuscules which fall 

 upon it in all directions. 



Hence the rate at which the energy of the corpuscules is spent in order 

 to maintain the gravitating property of a single pound, is at least millions of 

 millions of foot-pounds per second. 



What becomes of this enormous quantity of energy ? If the corpuscules, 

 after striking the atoms, fly off with a velocity equal to that which they had 

 before, they will carry their energy away with them into the ultramundane 

 regions. But if this be the case, then the corpuscules rebounding from the body 

 in any given direction will be both in number and in velocity exactly equiva- 

 lent to those which are prevented from proceeding in that direction by being 

 deflected by the body, and it may be shewn that this will be the case what- 

 ever be the shape of the body, and however many bodies may be present in 

 the field. Thus, the rebounding corpuscules exactly make up for those which 

 are deflected by the body, and there will be no excess of the impacts on any 

 other body in one direction or another. 



602 



