Page:Philosophical Transactions - Volume 004.djvu/63

 Sun are best taken, when it is about East and West, as hath been already intimated. But note, that in Sailing North and South you make not the Observations at the Suns rising and setting, but at its being; due East and West.

But you may, especially in such Quarters, as lye farr North or South, yea and wherever you will, put the Rule here prescribed in practise, by taking 2. equal Altitudes of some known Starr, that riseth high above the Horizon. For you shall thence, according to the mention'd Rule, know at what time by the Watches the Starr hath been in the South; and so the Right Ascension of that Starr being known, as also the Right Ascension of the Sun, you may thence easily calculate, what time it then was: Which being compar'd with the time of the Watches, as before, shall give the Longitude of the place where you were, when you had the Start in the Meridian.

If the Watches, that have gone exactly for a while, should come to differ from one another (as in length of time it may well happen, that the one or the other faile a minute, more or less;) in that case it will be belt to reckon by that, which goes fastest; unless you perceive an apparent cause, why it goes too fast; seeing it is not so easie for these Pendulum-Watches to move faster than at first, as it is to goe slower. For, the Wire, on which the Pendulum hangs, may perhaps by the violent agitation of the Ship come to stretch a little, but it cannot grow shorter; and the little Weight of the Pendulum perhaps slip clown wards, but cannot get up higher.

When you get sight of any known Country, Island, or Coast, be sure to note the Longitude thereof as exactly as you can by the help of the Rules here prescribed. First, thereby to correct the Sea-Maps, after that the Longitude of a place shall have been found at divers times to be the same, so that you doubt no more of it. For all Mapps are very defective as to the Scituation of Places in respect of East and West, chiefly where Seas are interpos'd/ secondly, to be able always to know in the prosecution of your Iourney, how farr you have sail'd from Rh