CONCLUSION. 



59 



Length of day for various lengths of earth's radius — Laplacian law of density. 



If current views of the shrinkage of the lithosphere founded on the folds 

 of the shell and on overthrust faulting are vaHd, its accelerative effects on 

 the earth's rotation are greater than the retardative effects of its water-tides 

 as hereinbefore computed, and perhaps greater than all the tides combined. 



It is possible that the earth may respond to the radiant energy of the 

 sun as a thermal engine and that its rotation may be influenced by this, 

 but the subject is obscure and elusive, if not delusive, and no attempt will 

 be made to develop it here. 



If, as seems probable, the evening sky, because of clouds, dust, etc., 

 offers more resistance than the morning sky to the passage of solar radiation 

 tangential to the earth, there is a slight preponderance of light-pressure in 

 favor of acceleration of rotation, but it must be very small. 



CONCLUSION. 



The application of the most radical and the most rigorous method of 

 estimating the frictional value of the present water-tides, a method which 

 brings to bear practically all the friction of these tides as a retardative 

 agency, irrespective of their positions or directions of motion, seems to 

 show that they have only a negligible effect on the earth's rotation. 



From the best available evidence I conclude that the tides of the litho- 

 sphere are chiefly elastic strains and have little retardative value, while 

 the tides of the atmosphere are too small to be measured. 



The accelerative influences seem to be also negligible, so far as geological 

 applications are concerned. 



In close accord with these deductions, the geological evidences indicate 

 that there has been no such change in the rate of the earth's rotation 

 during its known history as to require it to be seriously considered in the 

 study of the earth's deformations. 



I desire to acknowledge my great obligations to my colleagues, C. S. 

 Slichter, F. R. Moulton, A. C. Lunn, and W. D. MacMillan, for the indispen- 

 sable aid which their several contributions have rendered to these studies 

 and for criticisms and suggestions relative to my own paper. 



