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PROPOSITION XXI.

Having the Time of High or Low-Water given, on the Day of the Change or Full, in any Place where the Sea ebbs and flows regularly, (viz. in twelve Hours, forty eight Minutes) to find, at any Age of the Moon, the Time of High and Low-Water {m).

WE

{m) The true Time of the Tides, at all Ages of the Moon is not well computed by SeaMen and Astronomers; molt of them reckoning, that the Moon being upon a set Point of the Compass, or so many Hours past the Meridian, makes High-Tide in such and such a Point at all Times of the Moon. As for instance, a South-west Moon makes a full Tide at London, that is, when the Moon is three Hours past the Meridian. Now this is true indeed at the New and Full Moon, but not at any other Times of the Moon, which few take any notice of.

But observing more narrowly, I find that at London the Tides fall out at least two Points that is, an Hour and a half sooner in the quarters than in the New and Full Moon, and the true Time of the Tides is founds to be somewhat shorter and shorter from the New and Full Moon to the quarters, yet not in an equal manner, neither gradually decreasing from the New and Full Moon till the quarters; but rather that there was some little Difference of Alteration both at the New and

Full Moons, and also at the quarters, and that the greatest Difference fell out in the midst between them, agreeing very well to a circular Proportion after this manner.

1. Divide a Circle into iz equal Parts, or Hours, according to the Moon's Motion, or Distance from the Sun, from the New Moon to the Full.

2. Let the Diameter of the Circle be divided into 90 Parts, or Minutes, that is, according to the Time of the Difference between the New or Full Moon, and the quarters, which is one Hour and a half.

3. Make perpendicular Lines cross the Diameter of the Circle from Hour to Hour.

4. Reckon the Time of the Moon's coming to the South in the Circumference of the Circle, and observe the perpendicular Line that falls from that Point upon the Diameter; and the proportional Minute cut thereby, will shew how many Hours or Minutes are to be Abstracted from the Time of High Tides at the New and Full Moon, that so you may have the true Time of the Tides that present Day,

Example,

W E observed before, that the Time of High and Low-Water (if we reckon by the mean Motion of the Moon from the Sun) is every Day 48% Minutes, (or more accurately 48+) and every half Day 24; Minutes later than the preceding.

IF therefore it be High-Water in any Place, on the Day of the New or Full Moon, at twelve o’ Clock, it will be full Tide on the subsequent

Days of the Lunation, as in the following Table:

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THAT is, at the end of the first Day of the Moon's Age it is High-Water later by forty eight Minutes, &c.

BUT

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BUT for Practice, it is sufficient to add to the Time of High-Water at the New Moon.

For the first Day after the Change.

For the second

For the third

For the fourth ■

„ For the fifth

For the sixth

For the seventh ———

For the eighth

For the ninth • —

For the tenth

For the eleventh -;

For the twelfth

For the thirteenth

For the fourteenth

For the fifteenth

BUT this Calculation supposes the Motion of the Moon, from the Sun, to be equal, tho' it be not; for when (he is in her Perigee she moves much swifter than when she is in her Apogee j and therefore in the former Cafe stie prolongs the Time of the Tides, and in the later shortens them. Besides, some of the Lunar Months exceed thirty Days, and others are less than twenty nine, but the mean is twenty nine Days, twelve Hours, forty four Minutes.

B U T in those Places where it is High or LowWater when the Moon approaches some certain

If you find the Difference not in Pbilos. Trans. N° 34. which j

so much between the Neap tho" it be sound fault with byj

Tides, and the Spring Tides, MrFIamstead (in thtsame Trans.

the Diameter must be divided N°' yet by many itiisaidto'

into fewer Parts. This is Mr answer very well, and tkerefort'

Henry Philips'/ way, delivered we have transcribed it.

'-~r ~r" :" Azimuth,

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Azimuth, tho' the Times may be computed by this Method, yet they are not so accurately found.

NEITHER do the Conjunctions of the Sun and Moon happen at the fame Time every Change.

W E shall shew in Chapter xxx. how this may be done by the terrestrial Globe.

W E may use a Method something like this, for those Places where the Time of the Flux is more or less than the Time of the Reflux •, supposing the Difference be constant. But the Consideration of the Thing itself, and Experience, will sooner teach these Particulars than Discourse.

PROPOSITION XXII.

The Winds very often hinder, or promote, the Course os the Tides in all Places; and not only the Winds that blow in those Places, but even those in others may have the fame Effect.

THE Truth of this Proposition is so clear, that it needs no Demonstration.

P ROPOS ITION XXIII.

When any Part of the Ocean hath a proper, or particular, Motion, it is called a Current. Currents are various and directed towards different Parts of the Ocean, of which some are constant and others ■periodical. To enumerate the most famous constant ones.

1. THE most extraordinary Current of the Sea is that by which Part of the Atlantic or African Ocean moves about Guinea from Cape Verd towards the Curvature or Bay of Africa, which they call Fernando Poo, viz. from West to East, which is contrary to the general Motion.

And

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