58 THE TIDAL PROBLEM. 



MINOR EVIDENCES. 



If the earth's rotation were much more rapid than now in early times, 

 the gyratory component affecting the courses of the winds would have been 

 strengthened and probably trees would have required a corresponding 

 strengthening of the trunks, branches, and roots to meet this successfully. 

 Such provisions are not certainly detectable. In the coal-accumulating 

 eras trees grew to great heights without tap roots, and in some cases they 

 appear to have grown on accumulations of vegetal debris which could not 

 have furnished a very secure hold, and yet there is no evidence that they 

 were especially subject to overthrow. In no way is it clear that the life of 

 the early ages, either vegetal or animal, was adapted to atmospheric move- 

 ments essentially different from those of to-day. 



A more rapid rotation should have caused a stronger deflection of the 

 streams to the right hand in the northern hemisphere and to the left in the 

 southern. This should have resulted in tilted aggradation planes. How- 

 ever, these might not now be capable of detection, even if present. 



It is probable that some changes would arise from the shortness of the 

 day and night, but it is not clear just what these would be nor what would 

 be the criteria for their detection. 



It seems safe to say, in summation, that no geological evidence of any 

 unquestionable kind, or even probable kind, is found that supports the 

 theoretical postulate of a former high rate of rotation of the earth. 



The geological criteria are not delicate enough, however, to forbid the 

 belief that the rotation of the earth has changed in some minor degree 

 during the time over which the record extends. If the deformative effects 

 of such changes were small compared with those of the other diastrophic 

 agencies, they might be so far masked as to escape ready detection. 



ACCELERATIVE AGENCIES. 



There are some agencies, apparently not very potent ones, which tend 

 to accelerate the earth's rotation and to offset the influences of the tides. 

 Of these the most familiar is the shrinking of the earth. It was noted in 

 the review of the hypothesis of Darwin that in the initial stage the shrink- 

 age of the earth was made more effective rotationally than the tides of the 

 sun. It was of course assumed as a basis for this that the loss of heat at 

 that stage was quite exceptionally great. The computations of Wood- 

 ward ^ and others have shown that the present rotational effects of loss of 

 heat, assuming the correctness of current estimates, is exceedingly small. 

 Even if the estimates of loss of heat need to be increased, as seems probable, 

 such loss can not be a very efficient agency. Shrinkage from other sources, 

 as molecular rearrangement, atomic reconstruction, or other agencies, may 

 have a more considerable effect. The rotational results of the contraction 

 of the body of the earth from a radius of 4,160 miles to 3,960 miles, with 

 intervening stages, as computed by Dr. MacMillan on the assumption that 

 the Laplacian law of density is maintained, are as follows: 



* " The effects of secular cooling and meteoric diist on the length of the terrestrial 

 day." < Astro. Jour., No. 502, 1901: " From this it appears safe to conclude that the length 

 of the day will not change, or has not changed, as the case may be, by so much as a half 

 eecond in the first ten million years after the initial epoch. " p. 174. 



