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The state of things thus disclosed by observation, the cloudlessness and the rim of limb- light, turns out to agree in a most happy manner with what probability would lead us to expect; for the most natural supposition to make a priori about the Martian atmosphere is the following. When each planet was produced by fission from the parent nebula, we may suppose that it took with it as its birthright its proportion of chemical constituents; that is, that its amount of oxygen, nitrogen, and so forth was proportional to its mass. Doubtless its place in the primal nebula would to a certain extent modify the ratio, just as the size of the planet would to a certain extent modify the relative amount of these elements that would thereupon enter into combination. Supposing, however, that the ratio of free oxygen and so forth to the other elements remained substantially the same, we should have in the case of any two planets the same relative quantity of atmosphere. But the size of the planet would entirely alter the distribution of this air. Three causes would all combine to rob the smaller planet of efficient covering, on the general principle that he that hath little shall have less. In the first place, the smaller the planet, the greater would be its volume in proportion to its mass, because the materials of which it was composed, being subjected to less pressure owing to a lesser pull, would not be crowded so closely together. This is one reason why Mars should have a thinner atmosphere than is the case with our earth. Secondly, of two similar bodies, spheres or others, the smaller has the greater surface for its volume, since the one quantity is of two dimensions only, the other of three. An onion will give us a good instance of this. By stripping off layer after layer we reach eventually a last layer which is all surface, inclosing nothing. We may, if we please, observe something analogous in men, among whom the most superficial have the least in them. In consequence of this principle, the atmosphere of the smaller body finds itself obliged to cover relatively more surface, which still further thins it out. Lastly, gravity being less on the surface of the smaller body, the atmosphere is less compressed, and, being a gas, seizes that opportunity to spread out to a greater height, which renders it still less dense at the planet's surface. Calculating the effect of the above causes numerically, we find that on this a priori supposition Mars would have at his surface an atmosphere of about fourteen hundredths, wow 1 u c 4 at Observation supports this general supposition; for the cloudless character of the Martian skies is precisely what we should look for in a rare air. Clouds are congeries of globules of water or particles of ice buoyed up by the air about them. The smaller these are, the more easily are they buoyed up, because gravity, which tends to pull them down, acts upon their mass, while the resistance they oppose to it varies as their surface, and this, as we saw just now, is relatively greater in the smaller particles. The result is that the Thus for three reasons Mars should have a thinner air at his surface than is found on the surface of the earth. or one seventh the density of our terrestrial one.