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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).
(m) The true Time of the Full Moon's, and also at the Tides, at all Ages of the Moon quarters, and that the greatest is not well computed by Sea. Difference fell out in the midit Men and Aftronomers; most of between them, agreeing very them reckoning, that the Moon well to a circular Proportion being upon a set Point of the after this manner. Compass, or so many Hours past 1. Divide a Circle into 12 the Meridian, makes High-Tide equal Parts, or Hours, accordin such and such a Point at all ing to the Moon's Motion, or Times of the Moon. As for in- Distance from the Sun, from Itance, a South-west Moon the New Moon to the Full. makes a full Tide at London, that 2. Let the Diameter of the is, when the Moon is three Circle be divided into go Parts, Hours past the Meridian. Now or Minutes, that is, according to this is true indeed at the New the Time of the Difference beand Full Moon, but not at any tween the New or Full Moon, other Times of the Moon, and the quarters, which is one which few take any notice of. Hour and a half.
But observing more narrow. 3. Make perpendicular Lines ly, I find that at London the cross the Diameter of the Circle Tides fall out at least two Points from Hour to 'Hour. that is, an Hour and a half soon- 4. Reckon the Time of the er in the quarters than in the Moon's coming to the South in New and Full Moon, and the the Circumference of the Cirtrue Time of the Tides is found cle, and observe the perpendicu. to be somewhat shorter and lar Line that falls from that Point shorter from the New and Full upon the Diameter; and the Moon to the quarters, yet not proportional Minute cut therein an equal manner, neither by, will fhew how many Hours gradually decreasing from the or Minutes are to be subftracted New and Full Moon ţill the from the Time of High Tides at quarters ; but rather that there the New and Full Moon, that was some little Difference of so you may have the true Time Alteration both at the New and of the Tides that present Day,
WE 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 481 Minutes, (or more accurately 48?) and every half Day 243 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:
Example. At London, on the wards, that is at six of the Clock. Day of New and Full Moon, it But now by this Rule, if you is high Tide at three of the count this Time of the Moon's Clock, that is when the Moon coming to the South in the Ciris three Hours past the Meridian, cumference, the perpendicular and so by the common Rule the Line which comes from three to Moon being about four Days nine, cuts the Diameter at 45 old it will be South about three Min. which shews that so much of the Clock, and it will be is to be abated from the Time high Tide three Hours after- of High Tide in the New and
THAT is, at the end of the first Day of the Moon's Age it is High-Water later by forty eight Minutes, &c.
Full Moon; so that it is High three of the Clock : So when Tide 45 Min. before fix; that the Moon is south at three of is, at five Hours 15 Min. and the Clock or nine, the Perpennot at fix, according to the dicularcuts the Diameter at two common Rule. ·
Hours 15 Min. which, added to The like you may do for any the aforesaid three or nine, gives other Port, or Place, knowing the Time of high Water as athe Time of High Water at the bove. New and Full Moon in that Thus you may easily make a Place :-And you may do it the Table which by the Southing more readily, if you set down of the Moon shall readily tell the Time of High Water at the you the Time of High Tide in New and Full Moon under the any Place. The following is Diameter, as I have done for for London. London where it is high Tide at
Moon Tide Moon Tide Moon Tide Moon Tide South. Lond South. Lond. South Lond. South. Lond. H.M.H. M. TH M јн. м HMH MH MH. M. XII 03 01 11 05 151 vi 017 301 ix Ou: 15
103 91 1015 21 10 7 40 III 29
vil 08 36 ofiż 37
1018 48! 2015
2ct 1 31 3019 13 30 161
409 26 40.1 291 506 28
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.
BUT this Calculation supposes the Motion of the Moon, from the Sun, to be equal, tho’ it be not ; for when she is in her Perigee she moves much swifter than when she is in her Apogee ; and therefore in the former Case the 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.
BUT in those Places where it is High or LowWater when the Moon approaches fome certain
If you find the Difference not in Philos. Tranf. N° 34. which So much between the Neap tho it be found Fault with by Tides, and the Spring Tides, MrFlamstead (in the same Trans. the Diameter must be divided N° 143) get by many it is said to into fewer Parts. This is Mr answer very well, and therefore Henry Philips's way, delivered we have transcribed it.
- - - ***-- -* · * . ...*.Azimuth,
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 same Time every Change.
W E shall shew in Chapter xxx, how this may be done by the terrestrial Globe,
WE may use a Method fomething like this, for those Places where the Time of the Flux is more or less than the Time of the Reflux; fuppofing the Difference be constant. But the Consideration of the Thing itself, and Experience, will sooner teach these Particulars than Discourse,
The Winds very often hinder, or promote, the Course
of the Tides in all Places; and not only the Winds that blow in those Places, but even those in others may have the same Effeet.
THE Truth of this Proposition is so clear, that it needs no Demonftration.
When any part of the Ocean hath a proper, or par
ticular, Motion, it is called a Current. Currents are various and dire&ted 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,