Page:Popular Science Monthly Volume 9.djvu/695

Rh next prospectus?" "A figure of two," says Mr. Tigg, "and as many naughts after it as the printer can get into the same line."

It is hard for imagination to compass the meaning of a million, and, when that number is multiplied by hundreds, the effort is altogether beyond us. But we need not dwell on this consideration; we turn at once to the practical comments made by physical science on these and such-like opinions. The first is founded on the secular cooling of the earth.

If a red-hot ball be taken from a furnace, it begins at once to part with heat at a certain definite rate. As it becomes colder it cools more and more slowly. From the known laws of heat it is quite possible roughly to approximate to the period during which the earth has been habitable for animals and plants such as we now find upon it. Whenever a body is hotter at one part than at another, the tendency of heat is to flow from the hotter body to the colder. As the earth's crust is warmer as we go farther down, there must be a steady increase of heat from the surface to the centre, and the earth is even now losing heat at a perfectly measurable rate; therefore it is possible to calculate what was the distribution of heat a hundred thousand or a thousand thousand years ago, supposing the present natural laws to have been then in existence. According to these data, about ten millions of years ago the surface of the earth had just consolidated, or was just about to consolidate; and in the course of comparatively few thousand years after that time the surface had become so moderately warm as to be fitted for the existence of life such as we know it. If we attempt to trace the state of affairs back for a hundred millions, instead of ten millions of years, we should find that the earth (if it then existed at all) must have been liquid, and at a high white heat, so as to be utterly incompatible with the existence of life of any kind with which we are acquainted.

The next argument, namely, that founded on the earth's retardation by the tidal wave, is more recondite, and the theory that there is such a retardation at all is quite of recent date. Theoretical reasons connected with mechanics caused it to be adopted, and its establishment depends on the most refined astronomical investigation.

It is one of the peculiarities of time-measurement that, from the nature of things, no two periods of time can be compared directly one with another. The standards by which we measure time are less and less precise as we recede farther into the past. To-day we have as the standard unit of duration the interval between two successive transits of a star over the cross-wires of a fixed observatory-telescope. This measure has been considered until lately as absolutely fixed and invariable. And so it is for all practical purposes; the sidereal time of any heavenly body passing the meridian on a given day in 1880