THE FORMER RATES OF THE EARTH'S ROTATION 

 AND THEIR BEARINGS ON ITS DEFORMATION. 



In the treatment of the earth's deformations, which is to be the subject 

 of a following paper, it is essential to know whether changes in the rate of 

 the earth's rotation must be regarded as one of the important factors or 

 not. If the rate of rotation has appreciably varied during geological history, 

 it is almost certain that the oblateness of the earth-spheroid has also 

 varied, for unless the rigidity of the earth greatly exceeds that of any 

 known substance, it must have been modified in form under changing 

 rotation so as to approach the shape it would assume if it were a perfect 

 fluid. It would be an error to assume, as is sometimes done, that the 

 earth would conform to the fluidal shape perfectly, but that it would 

 approach to this with a measurable degree of closeness seems to be beyond 

 question. If there was a change from a high rotational speed, and con- 

 sequent high degree of oblateness, to a slower speed with less oblateness, 

 the surface area of the earth must have been reduced, because the nearer 

 such a body approaches a sphere, the less the area of its surface, the greater 

 its average gravity, and hence the greater its degree of compression. This 

 is brought out numerically, with a high order of approximation, in the 

 accompanying paper of Professor Slichter. There will be occasion in the 

 course of the present paper to consider in detail the application of this 

 supposed reduction. 



Whatever therefore may be the difficulties attending a treatment of 

 past rates of rotation of the earth, it is imperative that this element of 

 the problem of deformation be recognized and evaluated so far as lies in 

 our power. 



The problem may be approached on two rather distinct lines, one of 

 which is astronomic but rests back so radically on postulates derived 

 from theories of cosmogony that it may almost be called cosmogonic, and 

 the other of which is geologic and rests on the direct or implied teachings 

 of terrestrial evidence. 



The ulterior purpose of this paper is to set forth the latter, but the 

 cosmogonic considerations can not be passed without notice, for the cogency 

 which will be thought to attach to geological evidences is certain to be 

 measured in no small degree by the presumptions which are entertained 

 on cosmogonic grounds, or on astronomic grounds with an essential cos- 

 mogonic factor. The recent literature of the subject indicates that a 

 belief in a former high rate of rotation of the earth based on cosmogonic 

 and tidal grounds has a strong hold on astronomers and, to some large 



5 



