The statements here quoted were designed to show that the physical constitution of the sun and planets is widely different from that of the earth, and that the combustion of _some_ of the elements in this indefinite variety may account for the origin of solar heat. Let us examine the facts.
According to Laplace the mass of Jupiter's first satellite is 0·000017328, that of Jupiter being 1. The diameter is 2436 miles. Hence the corresponding density is a little more than _one-fifth_ of the mean density of the earth. In other words, it is somewhat greater than the density of water, and very nearly equal to that of Jupiter himself. Professor Ennis' value is therefore erroneous.[23] In regard to the densities of the Saturnian and Uranian satellites nothing is known, and conjecture is useless. In short, Saturn has the least mean density of all the planets, primary or secondary, so far as known. This may be owing to the great extent of his atmospheric envelope. The density of the moon is but three-fifths that of the earth: it is to be borne in mind, however, that the _mass_ and _pressure_ are also much less.
With respect to meteorites the same author remarks that "like the moon, they are probably satellites of the earth; but being very small, they are liable to extraordinary perturbations, and hence strike the earth in many directions." Here, again, his _facts_ are at fault; for (1) the observed velocities of these bodies are inconsistent with the supposition of their being satellites of the earth; and (2) the amount of perturbation of such bodies does not vary with their masses: a _small_ meteorite would fall toward the earth or any other planet with no greater velocity than a _large_ one.
It has been shown in a previous chapter that immense numbers of meteoric asteroids are constantly traversing the planetary spaces--that many millions, in fact, daily enter the earth's atmosphere. Reasons are not wanting for supposing the numbers of these bodies to increase with great rapidity as we approach the center of the system. Moreover, on account of the greater force of gravity at the sun's surface the heat produced by their fall must be much greater than at the surface of the earth. It has been calculated that if one of these asteroids be arrested in perihelion by the solar atmosphere, the quantity of heat thus developed will be 9000 times greater than that produced by the combustion of an equal mass of coal. There can, therefore, be no reasonable doubt that a _portion_ of the sun's heat is produced by the impact of meteoric matter. In considering the probability that it is _chiefly_ so generated, the following questions naturally present themselves: