AKD ITS SELF-COKSEEVATION". 171 



These are three masses gravitating toward each 

 other. Assume the present distances of these bodies 

 severally from each other. Assume, also, that the 

 line joining the earth and the moon is approximately 

 at right angles with that joining the earth and the 

 sun assumptions which, on the nebular hypothesis, 

 are entirely justifiable. 



Upon these assumptions we have to ask : What 

 will be the result of their mutual attractions? 



We know, from what has already preceded, that 

 each will approach the other; that the earth and the 

 sun will move toward each other; that the earth and 

 the moon will move toward each other; and, finally, 

 that the moon and the sun will move toward each 

 other. In each couple, too, it is evident that the 

 quantity of motion of each body will be precisely 

 equal to that of the other body. But the body having 

 the less mass will, therefore, necessarily acquire the 

 greater velocity. Hence the sun, as vastly the greatest 

 mass, will acquire relatively very slight velocity as 

 toward either the earth or the moon. 



Leaving out of account, then, the velocity of the 

 sun, as a quantity relatively so small that it may be 

 neglected without vitiating the result qualitatively 

 considered, we may trace the motion of earth and 

 moon toward the sun and toward each other. 



And first, let us note that the distance between 

 the earth and the moon is so small, as compared with 

 their distance from the sun, that the intensity of 

 gravity, as between them, is necessarily correspondingly 

 greater than that between either of them and the 

 sun. Whence the velocity of approach between earth 



