it is hardly credible until the figures which express it and the grounds on which they are based have received due attention. Let us think of a meteor which is moving, as such bodies do when near the earth, with a speed perhaps a hundred times as great as that of a bullet from a rifle, or even from one of the most finished pieces of artillery. The energy of the meteor, depending as it does upon the square of the velocity, will be, therefore, about ten thousand times that of the bullet of the same size. It seems that the energy thus possessed by a meteor one pound in weight is as much as could be developed by the explosion of a ton weight of gunpowder. Doubtless, in the vicinity of the sun, the meteors are more numerous, and they move with a vastly higher velocity than the meteors near the earth. It is therefore plain that the quantity of energy contributed to the sun from this source must be very large in amount. It can, however, be shown that there are not enough meteors in existence to supply a sufficient quantity of heat to the sun to compensate the loss by radiation. The indraught of meteoric matter may indeed certainly tend in some small degree to retard the ultimate cooling of the great luminary, but its effect is so small that we can quite afford to overlook it from the point of view that we are taking in these pages.
It is to Helmholtz we are indebted for the true solution of the long-vexed problem. He has demonstrated, in the clearest manner, where the source of the sun's heat lies. It depends upon a cause which, at the first glance, would seem an insignificant one, but which the arithmetical test, that is so essential, at once raises to a position of the greatest importance. It is sufficiently obvious that the sun is in no sense to be regarded as a
