Page:Popular Science Monthly Volume 9.djvu/682

654 hundreds of millions of years ago that the figures convey no practical idea to the mind, and amount in effect almost to what a distinguished geologist calls "eternity a parte ante."

The principal grounds upon which scientific opinion has recently declared itself in favor of limited periods for the duration of the solar system are based, first, on the belief that the earth is cooling—if not rapidly—at such a rate as to make it impossible that it should have existed, for very many millions of years; secondly, because there is reason to believe that the earth is not now rotating on her axis with the same rapidity as in former ages, and that, as her shape would have been different if, at the time she was in a molten state, she had been rotating more rapidly than now, she has not been rotating so long as has been supposed; thirdly, because the sun is parting with caloric at such a rate as to make it certain that he could not have continued to radiate heat at the same rate for more than a few millions of years; and lastly, because the changes in the earth's crust, stupendous and varied as they are, could have been, and probably were, accomplished in the course of much shorter periods than popular geology has hitherto considered possible.

It will, of course, be understood that any inquiry as to the date of creation must necessarily have relation only to the solar system—the sun, that is, and the planets which accompany the earth in its orbit round the central luminary.

The investigation is of necessity thus narrowed, because we have not, and cannot expect to have, any definite information as to the age of the rest of the visible universe. The stars are forever beyond our ken. If the spectroscope can bring intelligence of their component elements, it is as much as we can hope to attain; for their immeasurable distance effectually removes them from investigation. No action of gravity emanating from those distant luminaries affects the internal economy of the solar system. In the vast eternity of space the sun and his attendant satellites are altogether alone.

It is difficult to gaze upon the thousands of stars that brighten the night with their radiance, and yet realize our entire isolation. The solar system, with the radius of its orbit stretching from the sun to farthest Neptune, is but a point in a vast solitude. No star is nearer to us than 200 millions of millions of miles.

It is difficult, in dealing with such enormous numbers, to retain a definite impression on the mind. Our powers of conception are fitted rather to the wants of common life than to a complete survey of the universe.

Perhaps an intelligent may be substituted for a merely formal assent to these numbers, if they are considered on a greatly-diminished scale. Consider the figures on the scale of one mile to 100,000,000. On that scale the sun's distance from the earth will be represented by nearly one mile. Let the sun be represented by a globe