An alteration in the position, of the poles has also been suggested, but this also is clearly inadmissible. There is no evidence that the present position has ever materially varied, and there is no known law that could cause such a variation. On the contrary, all the elaborate mathematical calculations by which the motions of the sun, moon, and planets are deduced from Newton's law of gravity, tend to negative such a supposition. And what is perhaps even more convincing to a nonmathematical mind, the position of the poles implies the position of the equator, and cannot change without a corresponding change in the earth's shape. Now the earth is not a sphere, but an oblate spheroid, of almost the exact shape which a fluid mass would take revolving about the present axis. The centrifugal force arising from the greater velocity of rotation in going from the poles to the equator would pile up a protuberant belt where the velocity was greatest, and in point of fact the earth's equatorial diameter is longer than the polar diameter by about twenty-eight miles. Any displacement therefore of the poles, which carried them away from their present position, must displace the present equator to a corresponding extent. This mass of twenty-eight miles in thickness of earth and ocean must be thrown out of the old position, and driven to establish a new equilibrium in a position many degrees north or south of it in order to affect climates materially, submerging all existing lands, and leaving, until removed by denudation, miles upon miles of solid earth in unsymmetrical belts, like the moraines of retreating glaciers, as the equator shifted into new positions. And all this must have occurred, not once, but twice at least, and that with many minor vicissitudes, within the narrow limit of the quaternary period. It is unnecessary to say that nothing of the sort could by any possibility have occurred. Some evidence has recently been adduced that some very slight changes in latitude are going on at the observatories of Dorpat and Greenwich, but if confirmed these can only be of very minute amount, arising from slight changes in the position of the earth's centre of gravity owing to partial elevations and depressions, and could never have been sufficient to account for great variations of climate.[11] [11] The latest researches seem to show that these slight variations in latitude do not exceed 2" or 3", and are periodical, with a period of no longer than 300 to 310 days.