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SECT. IV. AT a quarter past eleven, in the River Thames, and other Places in England.

IT is very difficult to explain the Cause of these wonderful, and extraordinary Differences of the Tides in all Places, tho' it properly belongs to Naturalifts, and Geographers, to do it. It is likely that the various windings of the Shores, and their different Situation to the Sea-ward, the Refiftance of the Inlands, the Concurrence of several Tides, the Distance of Places from the Moon's Way, the various Winds, chiefly thofe that are general and constant, the Declivity and Shoalness of the Shores, and other things, very much contribute to this furprifing Diversity. For Example, at the Port of London the Tide rifes 'till the Moon comes to the South-Weft, when she hath South Latitude, and only then begins to ebb, not when the approaches the Meridian: for which we give this Reafon, viz. that while the Moon is moving from the Meridian of London towards Brafil (or from Brafil towards London) the Water ought not to fettle, but ftill to rife, because the Shore of America repels the Water towards England, which is drawn thitherward by the Moon, fince there is no Paffage for it to proceed any further. But it may be afked why, when the Moon hath North Latitude, it should happen to be HighWater before the approaches the Meridian of London, viz. when fhe is in the South-Eaft Point? To which I anfwer, that when the Moon hath North Latitude fhe is much nearer England than when he hath South, and therefore raifes up the Water fooner; and the Reafon why the Flux is not continued fo long as 'till the Moon approaches the Meridian is, becaufe fhe impels the Ocean more towards the American Coast, and Hudson's Bay, where the greatest Floods are then obferved.

AND for this Reafon it is High-Water along the Coasts of China, before the Appulfe of the Moon to the Meridian, because the continual East Winds drive the Sea towards the Weft.

BUT all these Allegations are not fufficient to fatisfie me in these Matters, and therefore I would have the curious Naturalifts examine them with greater Scrutiny. For to find the true Caufe, it is requifite, that we be furnished with accurate Obfervations how the Tides ebb and flow in different Places, and what Azimuth the Moon is in when it is High-Water in thofe Places; and how her Bearing varies according to her Place at the Change and Full; efpecially in thofe-Places where the Moon is vertical, and those that bear from them directly Eaft, Weft, North, and South (1). It is alfo to be diligently obferved, what height the Tides flow at thefe times; when the Moon is in the North Part of her Orbit, and moves not over fo much Sea, but over that vaft Tract of Land which lies between China, and the western Shore of Africa. For fince fhe preffes not the Water directly when she moves over these Mediterranean

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3. Obferve the Increase and Decrease of the Velocity of the Currents.

4. Measure the Height of eve ry utmost High Water and Low Water, from one Spring Tide to another.

5. Measure the exact Height of Spring Tides and Spring Ebbs.

6. Obferve the Pofition and Strength of the Wind, the State of the Weather; the Height of the Barometer, &c.

7. Calculate the Moon's Age and Place in all Respects..

See Lowthorp's Abridgment of Philof. Tranfact. p. 260. S 3

Places,

Places, I fuppofe this will caufe a fenfible Variation of the Motion of the Water. Likewise these Phænomena are to be obferved when the Moon is in the South Part of her Orbit, and moves over Brafil, or South America. For without a perfect Notion of thefe Occurrences, we fhall fcarce be able to find out the true Reafon or Caufe of the Tide.

PROPOSITION XIX,

The Sea flows to most Shores in twelve Hours twelve Minutes, and ebbs back again in as many.

IN fome few Places it takes more Time in flowing than in ebbing; and on the contrary, in others it flows in lefs Time than it ebbs: yet fo that the Time of the Flux and Reflux (or the Time between the two full Seas) make together twelve Hours, 24 Minutes, and two of thefe Times make twenty four Hours 48 Minutes, or almost twenty five Hours. So that High-Water happens every Day later by almoft an Hour than the Day before, because the Moon comes later to the Vertex, or Meridian of any Place, by almost an Hour (fifty Minutes) every Day.

WE have fufficiently explained this Propofition in our Demonftration of the eleventh ; "tho' in that we accounted it to be full Sea, when the Moon is in the Meridian of any Place; but becaufe, as we fhewed in the laft Propofition, it is found to be High-Water in feveral Places when the Moon is not in the Meridian, we do not, in this Propofition, reckon the forementioned Hours from the Time the Moon is in the Meridian of thefe Places, but from the Time fhe is found, by Experience, to be in that Vertex when it is high Water. Nevertheless this Period

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of the Flux and Reflux is not performed exactly in twelve Hours, twenty four Minutes, (or in twenty four Hours, fifty Minutes) but fome Times fooner or later, because the Moon conftantly changing her Distance from the Zenith, returns at unequal Times to the fame Vertex; but this Difference is fmall.

THEREFORE tho' the Flux and Reflux together be performed, in all Places, in about twelve Hours, twenty four Minutes (when there are no Storms); yet in fome, the Time is equally divided between the Flood and Ebb; and in others, the Time of flowing is more or less than that of ebbing.

THE Garonne, a River in France, is feven Hours in rifing, and but five in falling. And in the Port of Maccao, upon the Shore of China, the Tides flow nine Hours, and only ebb three, or lefs if the Eaft Winds blow.

ON the contrary, in the River Senegal, in Negroland, the Sea flows four Hours, and ebbs eight.

IT is hard to affign Reafons for this Difference. Some attribute it to the ftrong and swift Current of certain Rivers, or to their ordinary Flux. Thus the River Garonne refifts the Influx of the Sea with it's ftrong Current and hinders it; but helps the Reflux, and haftens it. Others will have the Flux to be prolonged another Hour, because the return of the Flood from the northern Seas, hinders it's Egrefs at the Mouth of the Garonne, and rather forces it further up the River. But it is my Opinion, that the River pours itfelf into the Sea, to a confiderable Distance, with a rapid Motion, which is obftructed in part by the Tide, and made to ftand, fome Time before the Moon forces the Sea up into the very Chanel.

THE Reason why the Sea, flows only four Hours into the River Senegal, is either because S 4

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SECT. IV. it's Chanel is extended from Weft to Eaft, or because the swiftnefs of the Stream, may hinder the Flux for two Hours. There may be perhaps other Causes which we are ignorant of, for want of Obfervations; for we are not certain whether it really leffens all the eight Hours, or only fix, and is stagnant the other two, by Reason of the Equality of the Current and the Tide,

WE are alfo to confider that low Places have apparently a longer Flux, and a fhorter Ebb.

PROPOSITION XX.

Whether it be Flood in any Place at the Inftant the Moon is in the Horizon of that Place?

THEY commonly fay it is; tho' it be not true in thofe Places where it is full Sea when the Moon is in the Meridian. For when the Moon declines from the Equator fouthward, fhe approaches the Meridian in lefs than fix Hours, and therefore the Flux must have begun when the Moon was depreffed below the Horizon: on the contrary, when the Moon has a North Declination, fhe requires more than fix Hours to move from the Horizon to the Meridian, and therefore it is Flood when she is elevated above the Horizon, or is in the Horary Circle of the fixth Hour; and fo it is obferved in moft Places, tho' it be otherwife at London, as we obferved above. It feems indeed reasonable that, tho' the Moon has a North Declination, the Flux fhould begin when she is horizontal, because fhe is then ninety Degrees diftant from the Vertex of the Place, and therefore the Preffure of the Ocean ought firft to touch here. But Obfervations are wanting to con firm this.

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