《Natural History（2 自然史第2卷）》：

　　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 cause 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 severalleagues， 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.

　　Perfectly to comprehend this we must attend to the nature of the power which produces the tides. We have observed that the moon acts upon the earth by a power called attraction by some， and by others gravity： this force penetrates through the globe， is exactly proportioned to the quantity of matter， and decreases as the square of the distance increases. Let us next examine what must happen to the waters when the moon is at the meridian of any one place.-The surface of the waters being immediately under the moon is then nearer that planet than any other part of the globe； hence this part of the sea must be elevated towards the moon， by forming an eminence， the summit of which must be opposite to the moon's centre， for the formation of this eminence the waters at the bottom， as well as at the surface， contribute their share， in proportion to the proximity they are in of the moon， which acts upon them in the inverse ratio of the squares of their distances： thus the surface of that part of the sea is first raised； the surface of the neighbouring parts will be likewise elevated， but to a less height， and the water at the bottom of all these parts will be raised by the same cause； so that all this part of the sea growing higher and forming an eminence， it is necessary that the water of the remote parts， and on which this force of attraction does not act， proceeds with precipitation to replace the waters which are thus elevated and drawn towards the moon.

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