Geology. 189 



earth's rotation during its known history as to require it to be 

 seriously considered in the study of the earth's deformations " 

 (p. 59). 



The quantitative portions of the paper rest upon one on " The 

 Rotation Period of a Heterogeneous Spheroid,'''' by C. S. Slichter, 

 and another " On the Loss of Energy by Friction of the Tides,'''' 

 by William D. MacMillan. The latter uses the formulae employed 

 by engineers for the loss of head due to friction and viscosity, 

 and applies them to the ocean. His conclusion is that the day 

 would be lengthened by one second in about 500,000 } 7 ears. 

 Even if this figure be in error tenfold or a hundredfold it is still 

 in great contrast with the conclusion of Adams in the middle of 

 the last century, that the earth was losing time at the rate of 22 

 seconds per century, a figure raised to 23"4 seconds by Darwin and 

 lowered to 8*3 seconds by Newcomb. 



As Chamberlin points out, these figures have been derived from 

 a secular acceleration of the moon's mean motion, and until a per- 

 fect lunar theory is developed such a small irregularity cannot 

 safely be used for the foundation of a superstructure reaching 

 backward tens of millions of years. It is seen that MacMillan's 

 work supports on an entirely independent line of evidence the 

 previous conclusion that no appreciable change in the rate of 

 earth rotation has occurred during the intervals of time assigned 

 by geologists for the portion of earth history recorded in the 

 sedimentary formations. 



The next paper is by F. R. Mo niton, " On Certain Relations 

 among the Possible Changes in the Motions of Mutually Attract- 

 ing Spheres when Disturbed by Tidal Interactions" This deals 

 with the general problem of tidal evolution with applications to 

 the earth- moon system. The author first shows that less critical 

 minds than Darwin's have drawn more definite conclusions from 

 Darwin's work than he himself drew. He next states : 



" In questions of cosmogony, where immense intervals of time 

 are involved, the problem of tidal evolution is obviously one of 

 great importance, unless it shall some time be shown that it is 

 not a sensibly efficient factor. The two most obvious methods 

 of determining its efficiency are by direct attacks from the 

 mathematical standpoint, or by comparing its certain implica- 

 tions with as many facts given by observation as possible. The 

 first is mainly the method of Darwin, and he has written what 

 will certainly always be an extremely important chapter in the 

 question when considered in the broadest possible way." * * * 



" The second method, that of comparing the positive implica- 

 tions of the tidal theory with observed facts in as extended a 

 way as possible, is, broadly speaking, that adopted in this paper " 

 (p. 83). 



At the end of the paper Moulton gives a summary, from which 

 the following abstracts are taken : 



" The object of this investigation has been to examine the 

 theory of* tidal evolution in order to find out, if possible, not 



