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CHA P. XIV.

Of the Motion of the Sea in general; and of it's Flux and Reflux in particular.

PROPOSITION I.

Water has but one natural Motion, by which it moves from a higher to a lower Place. And if the adjoining Places are of an equal Altitude, or higher than the Superficies of the Water, it naturally refts, and is not moved out of it's Place but by fome exter¬ nal Caufe.

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1OMMON Experience manifefts the Truth of this Propofition. For if you take a Vessel of Water and move it, the Water will fluctuate, and be in Motion, 'till no part of it be higher than another; that is, 'till it's Superficies be fpherical, as was faid in Chapter xiii. And although perhaps this Motion may proceed from an external Cause, viz. the Preffure of the Atmosphere, or the Motion of the Air round the Globe; yet because there are great Difputes about this Caufe, and the Motion is fo apparent in the Water itself, that it doth not feem to proceed from any external Agent, therefore it may be called natural, to distinguish it at leaft from other Motions of the Water. And this Motion is towards that Part which is more depreffed.

PROPOSITION II.

When any Part of the Ocean is moved, the whole is moved, or all the other Parts of the Ocean movė fucceffively; but the Motion is greater by how much it is nearer the Part first moved.

BECAUSE when one part of the Ocean is moved, it neceffarily changes it's Place, and leaves that it was in, to be taken up by the Water that was next it; whofe place is again fupplied by the Water next that, and so on. But the Motion becomes lefs in the more remote Parts; because that there the Water was first moved, the next to it rushes in, not from one Part, but all round about; fo that it leaves a round Space like the Periphery of a Circle, which is fupplied from a larger Periphery, and that again from a larger, and fo on. But the greater the Periphery is from whence the Water flows, the lefs is the progreffive Motion inwards, being diftributed into a larger Space. Juft as, when a Stone is thrown into the Water it forces it into a Round, and that forces the next Water to it into a larger Round and fo on; and the further thefe Peripheries are from the Immersion of the Stone, their Motion is diminished and lefs fenfible; and tho' there may at last seem to be none all, yet there will be still fome very small Undulation, except it be hindered by another Motion of the Water.

PROPOSITION III.

To find which way the Current of the Sea fets.

CHUSE a Time, if poffible, when no great Winds are stirring, and caft a Body into the Wa

ter of almoft the fame Gravity with it; mark the Place where it was thrown in, and let a Boat remain there immoveable; then, when the Body is carried by the Current a little way from the Place where it was thrown in, let another Boat be placed there; and observe how the one Boat bears from the other, and you have the Point of the Compass toward which the Current fets (a).

PROPOSITION IV.

The Motion of the Sea is either direct, vortical, or tremulous.

I call the Motion direct, when the Water runs towards a certain Point; and vortical, when it turns round in a Whirl-Pool, and is at Times abforbed and vomited up; and tremulous, when it quakes, and is troubled without the leaft Wind. We fhall defer the Confideration of the two laft to the end of this Chapter; and treat, first, of the direct Motion, which we fhall call, in general, the Motion of the Sea.

PROPOSITION V.

Of the Motions we obferve in the Sea, Some are general, fome particular, and others accidental.

(a) The Method that Sailors commonly ufe, in the Gulph of Mexico, to keep the Boat immoveable where the Sea is deep, and perhaps not to be founded is this. They fink downa Plummet of Lead about forty or fifty Pound Weight, to a certain number of Fathoms deep, as they are taught by Experience, and tho' the Lead is nothing near the Bottom, yet will the Boat turn Head against the Cur

rent, and ride as firmly as if it were faftened by the strongest Cable and Anchor to the Bottom; this Method will perhaps fucceed in feveral other Places where there are under-Currents, fuch as have been observed in the Downs, at the StreightsMouth, and in the Baltic. See Dr Stubb's Obfervations in a Voyage to the Caribbee Islands. Philof. Trans. No 27.

I call that a general Motion of the Sea, which is obferved in all it's Parts, and at all Times.

I call those proper or particular Motions, whereby only fome Parts of the Ocean are moved, which are twofold, either perpetual or anniversary; the former continue without Ceffation or Intermiffion; the latter are inconftant, and only obferved at fome certain Months or Days.

THE accidental Motions of the Sea are fuch as now and then happen, without any regular Order; and fuch as these are infinite.

PROPOSITION VI.

The Winds cause the accidental Motions of the Sea, by blowing the Waters toward fome oppofite Point; nor is the Sea ever free from fuch Motions.

THE Wind, being nothing but a violent Motion of the Air, and a Preffure of it towards the Earth, endeavours to impel the Water of the Sea out of it's Place; and because the Sea is a Fluid, and cannot refift the Force and Preffure of the Air, it is hereby moved out of it's Place, towards the oppofite Point, and drives the adjacent Water before it, and that again the Water before it, and so on.

AND fince there is always fome Wind in the Air towards one Point of the Compass or another, and very often towards different Points, in divers Countries, at the fame Time; it follows, that fome of these Motions continually affect the Sea, but more fenfibly where the Wind blows hardest; because it being a Fluid is foon put in Agitation by fa violent an Agent.

PRO.

PROPOSITION VII.

The general Motion of the Sea is twofold; the one is conftant, and from East to West: the other is compofed of two contrary Motions, and called the Flux and Reflux of the Sea, by which, at certain Hours, it flows towards the Shores, and at others back again.

THAT the Ocean is continually moving from Eaft to Weft, is chiefly proved from the Motion of the Sea which lies between the Tropics in the Torrid Zone; where it is ftrongeft, and lefs impeded by other Motions.

THIS Motion of the Sea is manifeftly perceived by thofe that fail from India to Madagascar, and Africa; alfo in the Pacific Ocean between NewSpain and China, and the Moluccas; likewife in the Ethiopic Ocean, between Africa and Brafil.

THUS the Currents fet ftrongly, and run with a rapid Motion, from Eaft to Weft, thro' the Streights of Magellan; which induced the first Difcoverer (whether Magellan, or fome other before him) to conjecture, that there were Streights thro' which they might fail out of the Atlantic into the Pacific Ocean. Ships are carried by the Currents, from Eaft to Weft, thro' the Streights of Manilha, and alfo thro' the little Chanels between the Maldivies. The Sea runs impetuoufly into the Gulph of Mexico, between Cuba and Yucatan, and flows out again, thro' a rapid Chanel, between Cuba and Florida. There is fo rapid a Flux into the Gulph of Paria, that the Streights are called the Dragon's Jaw. This Motion is alfo remarkable at the Land of Canada. The Sea feems to run out of the Tartarian Ocean thro' the Streights of Waygats, as appears by the fetting

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