The point where the centrifugal force -balances gravity, is so much nearer to the body, in proportion as its rotatory motion is more rapid. Supposing that the atmosphere extends itself as far as this limit, and that afterwards it contracts and condenses itself from the effect of cold at the surface of the body, the ro tatory motion would become more and more rapid, and the farthest limit of the atmosphere would ap proach continually to its centre : it will then abandon successively in the plane of its equator, fluid zones, which will continue to circulate round the body, be cause their centrifugal force is equal to their gravity. But this equality not existing relative to those par ticles of the atmosphere, distant 'from the equator, they will continue to adhere to it. It is probable that the rings of Saturn are similar zones, abandoned by its atmosphere.
If other bodies circulate round that which has been considered, or if it circulates itself round another body, the limit of its atmosphere is that point where its centrifugal force, plus the attraction of the extra neous bodies, balances exactly its gravity. Thus the limit of the moon's atmosphere, is the point where the centrifugal force due to its rotatory motion, phis the attractive force of the earth, is in equilibrium with the attraction of this satellite. The mass of the moon of that of the earth, this point is distant from the centre of the moon, about the ninth part of the distance from the moon to the earth. If, at this distance, the primitive atmosphere of the moon had not been deprived of its, elasticity, it would have been carried towards the earth, which might have retained it. This is perhaps the cause why this
atmosphere is so little perceptible." The earth's atmosphere must experience similar ' oscillations to those of the ocean, from the action of ' the sun and moon. In an atmosphere, however, like ours, which is so much agitated by other causes, the winds and variations in the barometer, which, arising from the same cause, have the same periods as the tides, must be very inconsiderable. The change in the altitude of the mercury in the barometer is only about of an inch at the equator, where it is a maximum ; though it is not improbable, that the oscillations of the atmosphere, like those of the ocean, may be in creased by local circumstances. " If we consider all the causes," says La Place, " which clistiirb the equi librium of the atmosphere ; its great mobility arising from its fluidity and elasticity ; the influence of heat and cold on its elasticity ; the great mass of vapour that it alternately absorbs and deposes; and lastly, the changes which the rotation of the earth produces in the relative velocities of its particles, which for this reason are displaced in the direction of the meridians ; we should not be surprised at the in constancy and variety of its motions, which it would be very difficult to subject to any fixed and certain laws." See ANEMOMETER, ASTRONOMY, TER, CHEMISTRY, CLIMATE, METEOROLOGY, PNEU