190 Scientific In telligen ce. 



what might take place under certain assumed conditions, but how 

 important this process has been in the actual development of our 

 system. The aim has been to avoid, as far as possible, assump- 

 tions regarding the uncertain factors depending upon the physi- 

 cal conditions of the bodies involved. In order to compare the 

 theory with the actual facts the various methods of testing it 

 have been carried to quantitative results." * * * 



"One of the conclusions reached by Darwin was that it is 

 probable that the earth and moon have developed from an 

 original mass by fission. One critical test of this hypothesis is 

 the determination of the smallest distance at which the bodies 

 could have revolved around each other consistently with the 

 present moment of momentum and energy. This test has been 

 worked out quantitatively, first with the problem simplified so 

 that the conclusions are absolutely certain under the hypotheses ; 

 then the effects of various modifying conditions, which seem 

 more or less probable, have been examined, one after another, 

 and their influence upon the final result determined" (p. 127). 



It is found that, under the simplified conditions: 



"The month has always been increasing and that it cannot 

 pass beyond 47*7 of our present days, at which period the month 

 and day will be equal and the system move as a rigid body. 

 Tnere is no way of telling by this investigation how long a time 

 will be required for the system to reach that state. But it is a 

 more interesting fact that the month can never have been less 

 than 4*93 of our present hours, this being the period of revolu- 

 tion when the distance from the center of the earth to the center 

 of the moon was 9,194 miles. Consequently we must suppose 

 that when the moon broke off from the earth it was at this dis- 

 tance from it, or 5,236 miles from its present surface. Or, includ- 

 ing the radius of the moon and supposing that both the earth 

 and moon were of the same density and shape as at present, the 

 distance from the surface of one body to the surface of the other 

 was immediately after fission 4,155 miles. Since this result is 

 altogether incompatible with the obvious implications of the 

 fission theory, we must either abandon the theory or show that 

 this number would be very largely reduced by including the 

 effects of the neglected factors. Consequently we examine the 

 effects of various neglected conditions and influences" (pp. 128, 

 129). 



As a result 



" It is seen that the one factor which makes the moon's initial 

 distance less than that found in the first computation is not only 

 of no particular consequence, but also that it is less than some of 

 the factors which increase it. Using all those factors whose 

 effects have been computed when they have been supposed to act 

 separately, and supposing that they would be essentially the same 

 when acting jointly, we find that the smallest possible distance of 

 the moon compatible with present conditions is 9,241 miles." 



