Page:Mars - Lowell.djvu/73

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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 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 contain the least. In consequence of this principle, the atmosphere of the smaller body finds itself obliged to cover relatively more surf ace, 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.

Thus, for three reasons, Mars should have a thinner air at his surface than is found on the surface of the Earth.

Calculating the effect of the above causes numerically we find that on this à priori sup-