6 THE TIDAL PROBLEM. 



extent, upon geologists. The extent of this belief is due in large measure, 

 no doubt, to the masterly papers of Sir George Darwin upon the origin 

 and tidal influence of the moon. It is obvious that if the arguments in 

 favor of a former high rate of rotation are accepted as decisive in them- 

 selves, such geological data as seem to conflict with them are likely to be 

 received with skepticism, or to be given interpretations consistent with 

 the accepted conclusions. It is therefore appropriate, if not necessary, 

 to review at the outset the grounds for the conclusions that have been 

 drawn from cosmogonic postulates and from tidal and other considerations 

 based upon these, so far at least as these have been thought to be weighty. 

 There is the more reason for this in the present series of papers, because 

 of the very different basal postulates which may be grounded on the mode 

 of planetary genesis set forth in them. 



THE ASTRONOMICAL DEDUCTIONS. 



CONSIDERATIONS BASED ON THE OLDER COSMOGONIES. 



It scarcely needs to be recited that, during the past century, astrono- 

 mers and geologists almost universally accepted the hypothesis that the 

 earth was formed from the condensation of a spheroid of gas, and that 

 current doctrines as to the earth's early rates of rotation were founded 

 on premises derived from some form of this hypothesis. Under the original 

 Laplacian view it was affirmed that the rotations of the sun and the plan- 

 etary masses were progressively accelerated as they shrank from a more 

 expanded to a more dense condition. The rotation of the parent earth- 

 moon spheroid was supposed to have reached, at a certain stage, such a 

 velocity that a ring of matter was separated from its equatorial tract and 

 formed the moon by subsequent condensation. It was held that the speed 

 of rotation of the residual spheroid further increased, or tended to increase, 

 by reason of its continued contraction, and hence that the primitive rota- 

 tion of the earth was exceedingly rapid. As the present rotation of the 

 earth is relatively slow, it followed, as a necessary inference, that a very 

 marked decHne in the earth's rotatory velocity took place in the course 

 of geological history. 



In the modification of the Laplacian view introduced by Sir George 

 Darwin,^ the material of the moon is supposed to have been separated 



'G.H.Darwin: 



On the bodily tides of viscous and semi-elastic spheroids, and on the ocean tides 



upon a yielding nucleus. <Phil. Trans. Roy. Soc. Lond., part 1, 1879, pp. 1-35. 

 On the procession of a viscous spheroid, and on the remote history of the earth. 



<Phil. Trans. Roy. Soc. Lond., part 2, 1879, pp. 447-538. 

 Problems connected with the tides of a viscous spheroid. < Phil. Trans. Roy. Soc. 



Lond., part 2, 1879, pp. 539-593. 

 The determination of the secular effects of tidal friction by a graphical method. 



<Proc. Roy. Soc. Lond., No. 197, 1879, pp. 168-181. 

 On the secular changes in the elements of the orbit of a satellite revolving about a 



tidally-distorted planet. <Phil. Trans. Roy. Soc. Lond., vol. 171, part 2, pp. 



713-891, 1880; Proc. Roy. Soc, vol. 29, 1879, p. 168, and vol. 30, 1880, p. 255. 

 On the tidal friction of a planet attended by several sateUites and on the evolution of 



the solar system. <Phil. Trans. Roy. Soc. Lond., part 2, 1881, pp. 491-535. 

 Enc. Brit., article on "tides"; "The Tides," 1899. 

 Also Thomson and Tait's Natural Philosophy, 2, articles on tides. 



