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 Tides, at all Ages of the Moon is not well computed by SeaMen and Aftronomers; moft of them reckoning, that the Moon being upon a fet Point of the Compafs, or fo many Hours paft the Meridian, makes High-Tide in such and such a Point at all Times of the Moon. As for inftance, a South-west Moon makes a full Tide at London, that is, when the Moon is three Hours paft 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 obferving more narrowly, I find that at London the Tides fall out at least two Points that is, an Hour and a half fooner in the quarters than in the New and Full Moon, and the true Time of the Tides is found to be fomewhat shorter and fhorter from the New and Full Moon to the quarters, yet not in an equal manner, neither gradually decreafing from the New and Full Moon till the quarters; but rather that there was fome little Difference of Alteration both at the New and WE 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 12 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 be tween 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 obferve the perpendicular Line that falls from that Point upon the Diameter; and the proportional Minute cut thereby, will fhew how many Hours or Minutes are to be fubftracted from the Time of High Tides at the New and Full Moon, that fo you may have the true Time of the Tides that present Day. Example, SECT. IV. WE obferved 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 fubfequent Days of the Lunation, as in the following Table: 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: fo that it is High Tide 45 Min. before fix; that is, at five Hours 15 Min. and not at fix, according to the common-Rule. The like you may do for any other Port, or Place, knowing the Time of High Water at the New and Full Moon in that Place: And you may do it the more readily, if you fet down the Time of High Water at the New and Full Moon under the Diameter, as I have done for London where it is high Tide at BUT three of the Clock: So when the Moon is fouth at three of the Clock or nine, the Perpendicular cuts the Diameter at two Hours 15 Min. which, added to the aforefaid three or nine, gives the Time of high Water as above. Thus you may easily make a Table which by the Southing of the Moon fhall readily tell you the Time of High Tide in any Place. The following is for London. BUT for Practice, it is fufficient to add to the Time of High-Water at the New Moon. For the first Day after the Change. For the fecond For the third For the fourth For the fifth For the feventh BUT this Calculation fuppofes 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 fwifter than when fhe is in her Apogee; and therefore in the former Cafe fhe prolongs the Time of the Tides, and in the later shortens them. Befides, fome 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. 44 BUT in thofe Places where it is High or LowWater when the Moon approaches fome certain Azimuth, tho' the Times may be computed by this Method, yet they are not fo accurately found. NEITHER do the Conjunctions of the Sun and Moon happen at the fame Time every Change. WE fhall fhew in Chapter xxx. how this may be done by the terreftrial Globe. WE may use a Method fomething like this, for thofe Places where the Time of the Flux is more or less than the Time of the Reflux; fuppofing the Difference be conftant. But the Confideration of the Thing itself, and Experience, will fooner teach these Particulars than Discourse, PROPOSITION XXII. The Winds very often hinder, or promote, the Courfe of the Tides in all Places; and not only the Winds that blow in thofe Places, but even those in others may have the fame Effect. THE Truth of this Propofition is fo clear, that it needs no Demonftration. PROPOSITION 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 fome are conftant and others periodical. To enumerate the most famous conftant 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 Weft to Eaft, which is contrary to the general Motion. And |