LUNAR CELESTIAL PHENOMENA.

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adjusts itself so that at any given height the densitycorrespbnds to the quantity of air above that height.The air above any height acts, in fact, as a weightpressing upon the air at that height, and compressingits elastic substance until it has a density proportionalto the pressure so produced. Obviously, therefore,the density of the air at any given level depends on theamount of the earth’s attraction. For every weighton the earth would be doubled if the earth’s attractionwere doubled, and halved if the earth’s attractionwere halved, and so on ; and this applies as fully tothe air as to any other matter having weight. Ac-cordingly, if the earth’s gravity were reduced to thevalue of gravity at the moon’s surface (0'16 where theearth’s gravity is represented by unity), the pressureof the air at the sea-level, and consequently the densityof the air there, would be reduced to less than one-sixth of its present value. Of course, a given quantityof air at the sea-level would then occupy more space ;and the whole atmosphere would expand correspond-ingly. Instead of having to attain a height of aboutmiles, as at present, before the pressure would bereduced to one-half that at the sea-level (or to -Jjth thatat present existing at the sea-level), it would benecessary to attain a height more than six times asgreat, or nearly 22 miles. In other words, instead ofone half of the whole atmosphere lying as at presentbelow the height of 3^ miles, the lower half of theatmosphere would then extend to a height of nearly22 miles.