There is, indeed, no doubt that, under such circumstances, the flattening would be about four times as great as it is now, so that the difference between the equatorial and polar diameters would be about a hundred miles; and then the age of our world might be found as accurately as its distance from the sun. Now, the neighboring zone of the solar system presents an actual case very similar to the extreme one under consideration. The difference between the equatorial and polar diameters of Mars is at least three times as great as that which could be expected from his present rotation if he were in a fluid condition. On taking even the lowest values which observers give for his compression, it must be concluded that, since changing his primitive fluid state and becoming solid and inflexible, the planet must have lost about forty per cent, of its diurnal motion. It evidently follows, according to Prof. Tait's rule, that about 800,000,000 years elapsed since the solidification took place, supposing the length of the day of Mars increased at the same rate as that of our globe. But the same great number will seem scarcely adequate to express the centuries since the event, when we consider that the rotation of our planetary neighbor is checked, not by strong tidal friction, but by the more feeble impediment from the resisting medium of space.
In two publications, during 1856 and 1858, I discussed the geological consequences of the slow reduction in the earth's diurnal motion; and many reasons led me to the conclusion that the long-continued decline of centrifugal force would make our planet undergo a change of form, by the gradual retirement of water to the poles, and, after long ages, by the upheaval of the bottoms of the polar oceans. I also maintained that such upheavals of circumpolar lands would be prevented by the strength of the crust of a small planet, and that Mars would be able to preserve for an exceedingly long period the form impressed on him in the very early term of his existence. The earth's internal fluidity, which I regarded as playing a very important part in such rare paroxysmal events, has been long a favorite doctrine with geologists, and has been often invoked as a means of accounting for the oft-repeated cases of elevation and submergence in the ancient world. But, on a globe entirely solid and inflexible, there would seem to be no scope or even possibility of the vast changes recorded in geological history; and speculative astronomy, in curtailing the time and restricting the means of great physical revolutions, makes the information from organic remains difficult to be understood and deficient in value. Since the authority of Hopkins gave currency to the doctrine of the internal solidity of our globe, much scientific talent has been expended in attempting to account for the great geological changes; but the causes which have been appealed to would require millions of centuries to produce the results ascribed to them. It seems very difficult to set aside the opinion which has been formed of the high antiquity of the physical world, not only from the marks in the terrestrial crust of repeated ele-