The atmosphere of Mars is relatively deeper than that of the Earth, so that we, in observing the details of its surface, are looking down through an immense thickness of an obscuring medium. And yet the details of the surface are seen with remarkable distinctness; not as clearly indeed as we can see those of the Moon, but nearly so. For instance, the "canals" appear to have a breadth of from 15 to 20 miles, corresponding to 1/16th, and 1/12th, of a second of arc, at an average opposition. The oases, as a rule, are about 120 miles in diameter, that is to say about half a second of arc. These are extraordinarily fine details to be perceived and held, even if Mars had no atmosphere at all; it would certainly be impossible to detect them unless the atmosphere were exceedingly thin and transparent. For we must remember that, though our own atmosphere is a hindrance to our observing, yet the atmosphere of the planet into which we are looking is a greater hindrance still. Like the lace curtains of the window of a house, it is a much greater obstacle to looking inward than to looking outward, and as the perfect distinctness with which we see the Moon is a proof that it is practically without an atmosphere, so the great detail visible on Mars bears unmistakable testimony to the slightness of the atmospheric veil around that planet. And when we turn again to the statistics of Mars, we see that this must inevitably be the case. Of two planets, one heavier than the other, it is not possible to suppose that the lighter should secure the greater proportional amount of atmosphere. With planets, as with persons, it is the most powerful that gets the lion's share: "to him that hath it is given, and from him that hath not is taken away even that which he seemeth to have." But if we assume that Mars has acquired an atmosphere proportional to its mass, then we see from the Table that this must be a little less than 1/9th of that of the Earth; exactly 0·107. It is distributed over a smaller surface, 0·285. Consequently the amount of air above each square inch of Martian surface is 0·107 ÷ 0·285 = 0·38. But since the force of gravity at the surface of Mars is less than on the Earth, this column of air will only weigh 0·38 × 0·38 = 0·145; or one-seventh of the column of air resting on a square inch of the Earth's surface. The pressure at the surface of Mars will therefore be 2·1 lb.; and the aneroid barometer would read 4·3 inches. (In order to express the diminished pressure of the Martian atmosphere, it is necessary to refer it to the aneroid barometer. The mercury in a mercurial barometer, or the water in a water barometer would lose in weight in consequence of the diminished force of gravity in the same proportion as the air would, and the mercurial barometer would read 11·4 inches.)