Page:Popular Science Monthly Volume 7.djvu/299

Rh beginning of the present state of things on the earth, of the fitness of the earth for habitation; and then we have a probability which is an exceedingly small one, which is certainly put in this form, that we do not know any thing at all about the beginning of the universe as a whole.

The reason why I say that we do not know any thing at all of the beginning of the universe is, that we have no reason whatever for believing that what we at present know of the laws of geometry are exactly and absolutely true at present, or that they have been even approximately true for any period of time, further than we have direct evidence of. The evidence we have of them is founded on experience, and we should have exactly the same experience of them now, if those laws were not exactly and absolutely true, but were only so nearly true that we could not observe the difference, so that in making that assumption, that we may argue upon the absolute uniformity of Nature, and supposing them to have remained exactly as they are, we are assuming something we know nothing about. My conclusion then, is, that we do know, with great probability, of the beginning of the habitability of the earth about one hundred or two hundred millions of years back, but that of the beginning of the universe we know nothing at all.

Now, let us consider what we can find out about the end of things. The life which exists upon the earth is made by the sun's action, and it depends upon the sun for its continuance. We know that the sun is wearing out, that it is cooling, and although this heat which it loses day by day is made up in some measure, perhaps completely, at present, by the contraction of its mass, yet that process cannot go on forever. There is only a certain amount of energy in the present constitution of the sun, and, when that has been used up, the sun cannot go on giving out any more heat. Supposing, therefore, the earth remains in her present orbit about the sun, seeing that the sun must be cooled down at some time, we shall all be frozen out. On the other hand, we have no reason to believe that the orbit of the earth about the sun is an absolutely stable thing. It has been maintained for a long time that there is a certain resisting medium which the planets have to move through, and it may be argued from that, that in time all the planets must be gradually made to move slower in their orbits, and so to fall in toward the sun. But, on the other hand, the evidences upon which this assertion was based, the movement of Encke's comet and others, has been quite recently entirely overturned by Prof. Tait. He supposes that these comets consist of bodies of meteors. Now, it was proved, a long time ago, that a mass of small bodies traveling together in an orbit about a central body will always tend to fall in toward it, and that is the case with the rings of Saturn. So that, in fact, the movement of Encke's comet is entirely accounted for on the supposition that it is a swarm of meteors, without regarding