Page:Popular Science Monthly Volume 9.djvu/698

670 same part of her surface. Then the liquid protuberance directed toward the moon will no longer be a cause of delay, and the retardation will cease. This cessation of effect, owing to the cause having ceased, appears to have actually happened with regard to the moon herself. At some time the moon's crust, and, indeed, her whole substance, was in a molten state. Enormous tides must have been produced by the attraction of the earth in this viscous mass of molten rock, and the time of the moon's rotation must have been quickly compelled, by the friction, to become identical with the time of its revolution round the earth, and now, as is well known, the moon always presents to the earth the same side of her sphere.

It being thus established that there is retardation of the earth's motion, and the amount of retardation being calculated, it remains only to inquire how the fact affects the question of the world's age. We know that the flattening at the poles and bulging at the equator is the result of rotation; from the amount of retardation it can be calculated how fast the earth was rotating in by-gone ages. Two thousand millions of years ago she would, according to such calculation, have been revolving twice as fast as at present, and the amount of centrifugal force at the equator would have been four times as great as now. If the earth, subjected to such strong centrifugal force, had been liquid or even pasty, when it began to rotate, the equatorial protuberance would have been much greater than it is. It therefore follows that she was rotating at about the same rapidity as now, when she became solid, and as the rate of rotation is certainly diminishing, the epoch of solidification cannot be more than ten or twelve millions of years ago.

A third argument for restricted periods is founded on an examination of the question, How long can the sun be supposed to have kept the earth, by its radiation, in a state fit to support animal and vegetable life? Here, as might be expected, a wider range of opinion exists.

It will be conceded at once that the age of organic life upon the earth must, of necessity, be more recent than the age of the sun. The several theories as to the way in which the sun may have derived his heat may be put aside in favor of that of Helmholtz, viz., that the sun has been condensed from a nebulous mass, filling at least the entire space at present occupied by the whole solar system. The gravitation theory of Helmholtz is now generally admitted to be the only conceivable source of the sun's heat. The opinion that it can be obtained from combustion is not tenable for a moment. The amount of heat radiated is so enormous that, if the sun were a mass of burning coal, it would all be consumed bodily in 5,000 years! On the other hand, a pound of coal falling on the surface of the sun