THE ASTRONOMICAL HISTORY OF WORLDS. 577 



that, in a molten globe, the rocks which became solid should sink into 

 the fiery menstruum; that permanent solidification must have according- 

 ly commenced at the centre and ended at the surface, leaving some in- 

 ternal lakes or pockets of lava to keep up volcanic action. Now, such 

 reservoirs of molten rock should have a small size in order to receive a 

 roof in opposition to the laws of hydrostatics; and, if much of the origi- 

 nal fluidity of their contents is still preserved, notwithstanding their 

 immense losses of heat, especially during volcanic eruptions, we are 

 compelled to make a very high estimate of the time required for the 

 solidification of a fused globe 8,000 miles in diameter ; and we must 

 conclude that the time since the earth was covered by a permanent 

 crust is but a very small part of that which has elapsed since terrestrial 

 matter began its career in a gaseous or in a molten condition. 



Though deprived by recent investigations of much of the support 

 first claimed for it by Hopkins, the doctrine of the almost total solidity 

 of the internal earth is still held by many ; and, in the hands of Thom- 

 son and Tait, it is made to contribute to the evidences of the youth of 

 our planet. They consider that, with such a solid and inflexible consti- 

 tution, the terrestrial structure must have retained almost immutably, 

 to the present day, the exact shape impressed on it in the beginning. 

 As the figure which it now bears differs little from that of equilibrium, 

 and as the polar compression is nearly the same as that which the 

 present diurnal movement would occasion in a molten world, it has 

 been concluded that, during geological history, the length of our day 

 has changed little, though it would be increased by tidal friction one 

 per cent, in the course of 20,000,000 years. From such considera- 

 tions, the period since our earth assumed its terraqueous character 

 has been estimated at not more than 10,000,000 years. Yet, in the 

 vague use of this round number for marking the career of our globe, 

 there is shown a wish rather to fix a limit to geological time than to 

 adhere to strict mathematical precision. So little does the earth devi- 

 ate from a figure of equilibrium, and so imperfect are the means for as- 

 certaining the exact amount of the deviation, that it would be hazardous 

 to say whether the age obtained by this course for the terrestrial crust 

 is nearer to 1,000,000 or to 10,000,000 years. 



Far more satisfactory would be the issue in dealing with a case like 

 the hypothetical one which Prof. Tait introduces for illustration. Al- 

 luding to the ancient world he says: "Suppose, for instance, that it 

 had not consolidated at less than 1,000,000,000 years ago. Calcula- 

 tion shows that at that time, at a moderate computation, it must have 

 been rotating twice as fast as it now rotates ; that is to say, the day 

 must have been twelve hours instead of twenty-four. Now, if that had 

 been the case, and the earth still fluid throughout or even pasty, the 

 double rate of rotation would have produced four times as great centrif- 

 ugal force ; and the flattening of the earth's poles and the bulging out 

 of the equator would have been much greater than we find them to be." 

 vol. xni. 37 



