"'The heat of rotation of the sun and planets, taken all together, would cover the solar emission for 134 years; while the heat of gravitation (that produced by falling into the sun) would cover the emission for 45,589 years. There is nothing hypothetical in these results; they follow directly and necessarily from the application of the mechanical equivalent of heat to cosmical masses.'... "But, continues Helmholtz, though the store of our planetary system is so immense as not to be sensibly diminished by the incessant emission which has gone on during the period of man's history, and though the time which must elapse before a sensible change in the condition of our planetary system can occur is totally incapable of measurement, the inexorable laws of mechanics show that this store, which can only suffer loss, and not gain, must finally be exhausted. Shall we terrify ourselves by this thought? Men are in the habit of measuring the greatness of the universe, and the wisdom displayed in it, by the duration and the profit which it promises to their own race; but the past history of the earth shows the insignificance of the interval during which man has had his dwelling here. What the museums of Europe show us of the remains of Egypt and Assyria we gaze upon in silent wonder, and despair of being able to carry back our thoughts to a period so remote. Still, the human race must have existed and multiplied for ages before the Pyramids could have been erected. We estimate the duration of human history at 6,000 years; but vast as this time may appear to us, what is it in comparison with the period during which the earth bore successive series of rank plants and mighty animals, but no man? Periods during which, in our own neighborhood (Koenigsberg) the amber tree bloomed and dropped its costly gum on the earth and in the sea; when in Europe and North America groves of tropical palms flourished, in which gigantic lizards, and after them elephants, whose mighty remains are still buried in the earth, found a home. Different geologists, proceeding from different premises, have sought to estimate the length of the above period, and they set it down from one to nine million of years. The time during which the earth has generated organic beings is again small, compared with the ages during which the world was a mass of molten rocks. The experiments of Bischoff upon basalt show that for our globe to cool down from 2,000° to 200° centigrade would require 350 millions of years. And with regard to the period during which the first nebulous masses condensed, so as to form our planetary system, conjecture must entirely cease. The history of man, therefore, is but a minute ripple in the infinite ocean of time. For a much longer period than that during which he has already occupied the world, the existence of a state of inorganic nature, favorable to man's existence, seems to be secured; so that for ourselves, and for long generations after us,