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they approach the centre. We have besides seen, that this is proved by experiments on pendulums, by the measurement of degrees, and by the attractions of mountains. It appears then, that the equilibrium of the ocean is stable, and if, (as seems certain) the waters have formerly covered continents, which at present are elevated much above its level, we must not search for the cause in the want of stability in their equilibrium. I have likewise discovered, by the means of analysis, that this stability would cease to exist, if the mean density of the sea exceeded that of the earth, so that the stability of the equilibrium of the ocean, and the excess of the density of the terrestrial globe above that of the waters which cover it, are reciprocally connected one with the other.

CHAP. XII.

Of the Oscillations of the Atmosphere.

To arrive at the ocean, the action of the Sun and Moon must traverse the atmosphere, which must necessarily be subject to their influence, and experience similar oscillations to those of the ocean. From hence arise winds and variations in the barometer, the periods of which are the same as those of the flux and reflux of the ocean. But these winds are very inconsiderable, and almost insensible, in an atmosphere so much agitated by other causes. The extent in the oscillations of the barometer, is only one millimetre at the equator, where it is the greatest. Nevertheless, as local circumstances may considerably augment the oscillations of the ocean, they may equally increase the os

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cillations of the barometer, the observation of which merits the notice of philosophers.

We may here remark, that the action of the Sun and Moon, produce neither in the ocean nor in the atmosphere, any motion from east to west. That which is observed in the atmosphere, between the tropics, under the appellation of the trade-winds, proceeds therefore from some other cause —this seems to be the most probable :

The Sun, which we will suppose, for the sake of simplicity, in the plane of the equator, there rarifies by its heat the columns of air, and elevates them above their natural level, they should then re-descend by their weight, and be carried towards the poles in the superior part of the atmosphere, but at the same time, a current of cool air should arrive from the climates near the poles, to replace that which has been rarefied at the equator. Thus, two opposite currents of air are established, one in the inferior, the other in the superior part of the atmosphere. But

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the real velocity of the air, due to the rotation of the Earth, is so much the less as it is nearer the pole; it ought there-fore, in advancing towards the equator, to turn slower than the corresponding parts of the Earth, and bodies placed at the terrestrial surface, should strike against it with the excess of their velocity, and experience by its re-action a resistance contrary to their motion of rotation: thus, to an observer who thinks himself immoyable, the wind seems to blow in a direction opposite to the rotation of the Earth, that is, from west to east, which in fact is the direction of the trade winds.

If we consider all the causes which disturb the equilibrium 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 di

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rection of the meridians; we should not be surprized at the inconstancy and variety of its motions, which it would be very difficult to subject to any fixed and certain laws.

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