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AT a quarter past eleven, in the River Thames,. and other Places in England.
I. T is very difficult to explain the Cause of these wonderful, and extraordinary Differences of the Tides in all Places, tho' it properly belongs to Naturalists, and Geographers, to do it. It is likely that the various windings of the Shores, and their different Situation to the Sea-ward, the Resistance of the Islands, the Concurrence of several Tides, the Distance of Places from the Moon'sWay, the various Winds, chiefly those that are general and constant, the Declivity and Shoalness of the Shores, and other things, very much contribute to this surprising Diversity. For Example, at the Port of London the Tide rises 'till the Moon comes to the South-West, when she hath South Latitude, and only then begins to ebb, not when she approaches the Meridian: for which we give this Reason, viz. that while the Moon is moving from the Meridian of London towards Brafil Cor from Brasil towards London) the Water ought not to settle, but still to rise, because the Shore of America repels the Water towards England, which is drawn thitherward by the Moon, since there is no Pastage for it to proceed any further. But it may be asked why, when the Moort hath North Latitude, it should happen to be HighWater before Ihe approaches the Meridian of London, viz. when she is in the South-East Point * To which I answer, that when the Moon hath North Latitude she is much nearer England than when she hath South, and therefore raises up the Water sooner; and the Reason why the Flux is not continued so long as 'till the Moon approachesthe Meridian is, because she impels the Ocean more towards the American Coast, and Hudson's Bay, where the greatest Floods are then observed.
AND for this Reason it is High-Water along the Coasts of China, before the Appulse of the Moon to the Meridian, because the continual East Winds drive the Sea towards the West.
BUT all these Allegations are not sufficient to' satisfie me in these Matters, and therefore I would have the curious Naturalists examine them with greater Scrutiny. For to find the true Cause, it is requisite, that we be furnished with accurate Observations how the Tides ebb and flow in different Places, and what Azimuth the Moon is in when it is High-Water in those Places •, and how her Bearing varies according to her Place at the Change and Full; especially in those-Places where the Moon is vertical, and those that bear from them directly East, West, North,'and South (/). It is also to be diligently observed, what height the Tides flow at these times; when the Moon is in the North Part of her Orbit, and moves not over so much Sea, but over that vast Tract of Land which lies between China, and the western Shore of Africa: For since she presses not the Water directly when she moves over these Mediterranean
(?) The following Directions 3. Observe the Increase and
are os excellent use for observ- Decrease of the Velocity os the
ing the Tides, given by Sir Ro- Currents. >
bert Murray, in Pbilos. Trans. 4. Measure the Height of eve^
N° i7. ' . ry utmost High Water and Low
1. Observe the Situation os Water, from one Spring Tide
the Place of Observation, viz to another,
what Currents, Seas, Islands, 5. Measure the exact Height
Bays, Shores, Shelves, &c. are ofSpringTidcsandSpringEbbs.
near it. 6. Observe the Position and
2.0bserve in what proportion Strength of the Wind, the State
the Increases of the Tides from of the Weather; the Height of
the Neap to the Spring Tides, the Barometer, &c.
and their Decreases, and the Ri- 7. Calculate the Moon's Age
sings and Fallings of the Ebbs, and Place in all Respects,
happen to be in regard of one See Lowthorp's Abridgment
Mother. gfPhilos. Transact, p. 260.?
S 3 Places, Places, I suppose this will cause a sensible Variation of the Motion of the Water. Likewise these Phænomena are to be observed when the Moon is in the South Part of her Orbit, and moves over Brasil, or South America. For without a perfect Notion of these Occurrences, we shall scarce be able to find out the true Reason or Cause of the Tide.
The Sea flows to most Shores in twelve Hours twelve Minutes, and ebbs back again in as many.
IN some few Places it takes more Time in flowing than in ebbing; and on the contrary, in others it flows in less Time than it ebbs: yet so that the Time of the Flux and Reflux (or the Time between the two full Seas) make together twelve Hours, 24I Minutes, and two of these Times make twenty four Hours 48^: Minutes, or almost twenty five Hours. So that Higl>Water happens every Day later by almost 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.
W E have sufficiently explained this Proposition in our Demonstration of the eleventh; tho' in that we accounted it to be full Sea, when the Moon is in the Meridian of any Place j but because, as we shewed in the last Proposition, it is found to be High-Water in several Places when the Moon is not in the Meridian, we do not, in this Proposition, reckon the forementioned, Hours from the Time the Moon is in the Meridian of these Places, but from the Time she is found, by Experience, to be in that Vertex when it is high Water, Nevertheless this Period of the Flux and Reflux is not performed exactly in twelve Hours, twenty four Minutes, (or in twenty four Hours, fifty Minutes) but some Times sooner or later, because the Moon constantly changing her Distance from the Zenith, returns at unequal Times to the fame Vertex j but this Difference is small.
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 some, the Time is equally divided between the Flood and Ebb; and in others, the T ime of flowing is more or less than that of ebbing.
THE Garonne, a River in France, is seven Hours in rising, 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 less if the East Winds blow.
O N the contrary , in the River Senegal, in Negroland, the Sea flows four Hours, and ebbs eight.
IT is hard to assign Reasons for this Difference. Some attribute it to the strong and swift Current of certain Rivers, or to their ordinary Flux. Thus the River Garonne resists the Influx of the Sea with it's strong Current and hinders it; but helps the Reflux, and hastens it. Others will have the Flux to be prolonged another Hour, because the return of the Flood from the northern Seas, hinders it's Egress at the Mouth of the Garonne, and rather forces it further up the River. But it is my Opinion, that the River pours itself into the Sea, to a considerable Distance, with a rapid Motion, which is obstructed in part by the Tide, and made to stand, some 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 it's it's Chanel is extended from West to East, or be^ cause the swiftness of the Stream, may hinder the Flux for two Hours. There may be perhaps other Causes which we are ignorant of, for want of Observations; for we are not certain whether it really lessens all the eight Hours, or only six, and is stagnant the other two, by Reason of the Equality of the Current and the Tide,
W E are also to consider that low Places have apparently a longer Flux, and a shorter Ebb.
Whether it be Flood in any Place at the Instant the Moon is in the Horizon of that Place?
THEY commonly fay it is; tho' it be not true in those Places where it is full Sea when the Moon is in the Meridian. For when the Moon declines from the Equator southward, she approaches the Meridian in less than six Hours, and therefore the Flux must have begun when the Moon was depressed below the Horizon: on the contrary, when the Moon has a North Declination, she requires more than six 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 sixth Hour* and so it is observed in most Places, tho' it be otherwise at London, as we observed above.. It seems indeed reasonable that, tho' the Moon has a North Declination, the Flux should begin when she is horizontal, because she is then ninety Degrees distant from the Vertex of the Place, and therefore the Pressure of the Ocean ought first to touch here. But Observations are wanting to con* firm this.