Page:Philosophical Transactions - Volume 003.djvu/32

 out to be somewhat shorter and shorter, from the New and Full Moone unto the Quarters; yet not in an equall manner, neither gradually decreasing from the New and Full Moone untill the Quarters; but rather, that there was some little difference of alteration both at the New and Full Moones, 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: (See Fig. 5.)

First, Divide a Circle into 12 Equal parts, or hours, according to the Moones motion or distance from the Sun, from the New Moone to the Full.

Secondly, Let the Diameter of the Circle be divided into 90 parts or min. that is, according to the time of the difference of Tides between the New or Full Moone, and the Quarters, which is one hour and an halfe.

Thirdly, Make perpendicular lines cross the Diameter of the Circle, from hour to hour.

Fourthly, Reckon the time of the Moones 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 proportionall Minutes, cut thereby, will shew, how many Houres, or Minutes are to be subtracted from the time of High-tides at the New and Full Moone, that so you may have the true time of the Tides that present day.

For Example; At London, on the day of New and Full Moone, it is High-Tide at London at 3 of the Clock, that is, when the Moone is three hours past the Meridian: and so by the Common Rule, the, Moone being about four dayes old, it will be South about three of the Clock, and it will be High-tide three houres afterwards, that is, at 6 of the Clock. But now by this Rule, if you count this time of the Moones coming to the South in the Circumference, the perpendicular-line, which comes from 3 to 9, cuts the Diameter in the halfe, at 45 min, which shews, that so much is to be abated from the time of High-tide in the New and Full Moones; So that it is High-tide 45 min. before 6 of the Clocke, that is, at 5. hours 15 min. and not at 6 of the clock, according to the common-Rule.

The like you may do for any other Port or place, knowing the time