Page:Popular Science Monthly Volume 58.djvu/151

Rh attraction will also be doubled, because the diameter of the spherical shell is the same, while the amount of matter within it is twice as great. Hence the hydrostatic pressure per unit of surface will be four times as great, or will vary as the square of the density. The elasticity at equal temperatures being proportional to the density, it follows that were the temperature the same in the two 'masses, the elasticity would be double in the case of mass B; whereas, to balance the hydrostatic pressure it should be quadrupled. The temperature of B must, therefore, be twice as great as that of A. It follows that in the case of stars of equal volume, but of different masses, the temperature must be proportional to the mass of density.

But how will it be if we suppose the density to be always the same, and, therefore, the mass to be proportional to the volume? In this case the attraction at a given point will be proportional to the diameter of the body. If, then, we suppose one body to have twice the diameter of the other, but to be of the same density, it follows that at corresponding points of the interior, the hydrostatic pressure will be twice as great in the larger body. The density being the same, it follows that the temperature must be twice as high in order that equilibrium may be maintained. It follows that the stars of the greatest mass will be at the highest temperature, unless their volume is so great that their density is less than that of the smaller stars.

It follows from the theory set forth in the last chapter that the stars are not of fixed constitution, but are all going through a progressive change—cooling off and contracting into a smaller volume. If we accept this result, we find ourselves face to face with an unsolvable enigma—how did the evolution of the stars begin? To show the principle involved in the question, I shall make use of an illustration drawn from a former work. An inquiring person wandering around in what he supposes to be a deserted building, finds a clock running. If he knows nothing about the construction of the clock, or the force necessary to keep it in motion, he may fancy that it has been running for an indefinite time just as he sees it, and that it will continue to run until the material of which it is made shall wear out. But if he is acquainted with the laws of mechanics, he will know that this is impossible, because the continued movement of the pendulum involves a constant expenditure of energy. If he studies the construction of the clock, he will find the source of this energy in the slow falling of a weight suspended by a cord which acts upon a train of wheels. Watching the motions, he will see that the scape wheel acting on the pendulum