THE ASTRONOMICAL DEDUCTIONS. 15 



those which will be reached on the assumption that the body tides are 

 merely strains in an elastico-rigid earth. So, too, if the ocean has been 

 growing in volume during the geologic ages, or has been changing in form 

 in any notable degree, the results would need to be modified accordingly. 

 Even when approached on the admirable lines of backward tracing by 

 computation, the results are therefore subject to wide variation accord- 

 ing as the postulates arising from one cosmogonic hypothesis are used or 

 those of another. 



If we were to follow carefully the first stages of the moon's evolution 

 under Darwin's hypothesis, it would be seen how critically dependent 

 that hypothesis is on an underlying theory of cosmogony. The moon is 

 assumed to have been separated from the parent earth-moon mass by 

 some form of centrifugal action. While the precise form may have been 

 either one or another of two or more alternatives, the principle of action 

 is the same up to a certain point and is best illustrated by supposing that 

 the moon-mass separated as a unit, and that just after separation it was 

 a spheroid close beside the earth-spheroid and revolving in the period of 

 the latter's rotation. An objection to this supposition will be considered 

 later. 



Now at this critical stage the earth was subject to the tidal action of 

 the sun, which, according to the fundamental theory of the hypothesis, 

 should tend to retard the earth's rotation. The earth was also subject to 

 contraction from loss of heat, which should tend to accelerate its rotation. 

 If the former was the greater influence, the lunar tide, which would have 

 begun to be generated as soon as a difference arose between the moon's 

 revolution and the earth's rotation, must have fallen behind the moon's 

 position and, according to the hypothesis, must have tended to draw it 

 backward and bring it down to the earth. To permit the evolution to 

 proceed at all it is necessary to suppose that the contraction from loss of heat 

 was a greater influence on the earth's rotation than were the solar tides. Now, 

 the earth's contraction from loss of heat at the present time is exceedingly 

 small. If, therefore, the constitution of the earth has been much the 

 same as it is now ever since its growth practically ceased, as assumed by 

 one cosmogonic hypothesis, the supposition that the tidal evolution of the 

 earth-moon system was started in the right direction for lunar recession 

 by the superiority of the influence of contraction over that of the solar 

 tides is either untenable or else the solar tidal influence was extremely 

 small. The initiation of the tidal evolution postulated by this hypothesis 

 is thus seen to be tied up with a very high rate of loss of heat in the initial 

 earth-stages, and this is only assignable under certain cosmogonic assump- 

 tions which give to the earth a very hot surface. These are, however, 

 not necessarily confined to the gaseous or meteoritic hypotheses. They 

 may possibly be made under the planetesimal hypothesis, but in any case 

 they are as speculative as the hypotheses themselves, and in the latter 

 case somewhat less well grounded, because the alternative phases of the 

 hypotheses seem to be the more probable. 



It is worth while to note further in this connection that the evolution 

 of the lunar tide, under the theory of Darwin, would be a very slow process, 



