Page:Popular Science Monthly Volume 17.djvu/303



HE additions that are being continually made to our knowledge of the composition and physical condition of the most distant heavenly bodies may well prompt one to ask why we are still so poorly informed concerning the constitution of the planet which the Creator has assigned to us for a dwelling-place. Mines and wells have barely scratched the solid crust that conceals the mysteries of the earth's depths. Our vague and uncertain ideas regarding the condition of the interior of the earth are based on analogies and inductions from facts observed on its surface or in the heavens. Very little light do we get on this subject from direct experiment. The bowels of the earth are not, indeed, easily accessible. Whatever the poet may say, the descensus Averni is not easily made; the domain of the stars is not thus hidden from us. For about two centuries large sums have been expended in the construction of gigantic telescopes with which to sound the depths of space; but no attempt, as a purely scientific undertaking, has been made to fathom the secrets of the underground world. The object of the numerous mines in different parts of the world has been simply the discovery of mineral riches, and the depths they have reached barely exceed, even in a few rare instances, a thousand metres; i. e., hardly the six-thousandth of the earth's radius—corresponding, on a globe thirteen metres (about forty-two feet) in diameter, to a puncture one millimetre (about four one-hundredths of an inch) in depth.

Notwithstanding this paucity of positive data, it will not be