Water has but one natural motion; like other fluids it always descends from the higher into the lower places, unless obstructed by some intervening obstacle. When it reaches the lowest place it remains there calm and motionless, at least without some foreign causes which agitates and disturbs it. All the waters of the ocean are collected in the lowest parts of the surface of the earth, of course the motions of the sea must proceed from external causes, the principal of which is the flux and reflux, which is alternatively made in a contrary direction, and from which results a general and continual motion in the sea from east to west. These two motions have a constant and regular relation with the motions of the moon. When the moon is new, or at the full, this motion from east to west is more sensible, as well as that of the tides, which upon most shores ebb and flow every six hours and a half: that it is always high tide whenever the moon is at the meridian, whether above or below the horizon of the place; and low tide when the moon rises or sets. The motion of the sea from east to west is constant and invariable, because the ocean in its flux moves from east to west, and impels towards the west a great quantity of water, and the reflux seems to be made in a contrary direction, by reason of the small quantity of water then driven towards the west; the flux, therefore, must rather be regarded as a swelling, and the reflux as a subsiding of the water, which instead of its disturbing the motion from east to west, produces and continually restores it, although in fact it is stronger during the rise, and weaker during the fall, from the above reason. The principal circumstances of this motion are, 1. That it is more sensible when the moon is new, or at the full, than in the quadratures: in spring and in autumn it is also more violent than at any other time of the year; and it is weaker in the solstices, which, is occasioned by the combination of the attraction of the moon and sun. 2. The wind often alters the direction and quantity of this motion, particularly that which constantly blows from the same quarter. It is the same with respect to large rivers which convey their waters into the sea and produce a current there, often extending several leagues, which is strongest when the direction of the wind agrees with the general motion. Of this we have an example in the Pacific Ocean, where the motion from east to west is constant and very perceptible. 3. We must remark that when one part of a fluid moves, the whole mass receives the motion; now in the motion of the tides a great part of the ocean moves in a very sensible manner, and consequently the ocean is agitated by this motion throughout its whole extent.