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HE dissipation of energy is a continuous process, quite familiar to mankind in its main features and results, since the days of the ancient philosophers. It was recognized by them that all mechanical motions, being dissipated by friction, gradually diminish, and must finally cease unless maintained by external power. In the language of modern science, the motion which thus disappears is converted from molar into molecular motion.

It may be added that molecular energy, existing mainly in the form called heat, tends to equalization of dynamic equilibrium, after the attainment of which it is powerless to produce molar or mechanical motion, a reconversion from the condition of equilibrium being impossible.

Accordingly, the power to produce mechanical motion, exerted by the heat of the sun, which is being lavished with such prodigious prodigality, can only last while the sun continues to be hotter than the other bodies in space. At present it is well understood that all terrestrial motive power is derived from this source with the single unimportant exception of that obtained from the tides, at the expense of the earth's energy of rotation. Among the more obvious processes of conversion of the sun's molecular into terrestrial molar motion, are the expansion and contraction of the atmosphere, the evaporation and condensation of water, and the less direct method by restoration of potential chemical energy accomplished in vegetation, whence are produced food and fuel.

But it is supposed that the sun will finally grow cold, and that the resistance of the ethereal medium, the evidence of whose existence is found in the demonstration of the undulatory theory of light, will cause satellites to fall into planets, planets into suns, and suns into one common centre, after which, unless by special interposition of divine power, darkness, silence, and death, will forever prevail.

This gloomy prediction is of course inconsistent with the theory of continuous evolution, which obviously excludes from cosmical economy catastrophes or extensive destructive effects.

A careful consideration, however, of the circumstances which will be likely to accompany the falling of a satellite into its planet may lead to the conclusion that this occurrence will not necessarily be catastrophic. The process must certainly be an exceedingly slow one, no progress in it having been detected throughout all the recorded observations of the moon's motion extending over thousands of years.