MEMORANDUM.
1. All Departures must be reckon’d fromLondon, if the Movement was there Set going.
2. That wheresoever they are Set going, there must be a large and exact Meridian Line prepared by a Perpendicular, set up in an Horizontal Circle, as is Mathematically taught,&c.
3. That an Equal time be spent in Winding it up, as near as possible; for which no time need be allowed, if Equated for 10 or 12 Months; because the time required will be the same for the same time, and greater for a greater,&c.
4. That the said Movement be carefully kept in one and the same Temperature of Air, as near as possible.
5. That after this Movement is fully proved, if it should not (by reason of any sudden change of Air) agree to a Minute, yet it ought not in this respect to be altered or much regarded, because the moistness of one Month, may compensate the Dryness of another: So that by such time as you are arrived to the end of your Voyage, the Movement may even Rectifie or Correct it self.
6. It is to be noted, That if the Sun comes too soon to the Meridian, it will also come too soon to the Horizon; and therefore, Watch too fast, or too slow, must be Added or Substracted, as aforesaid.
7. That it be always wound up, as near as may be, at one certain time of the Day, (viz.) if at Sea, about Twylight, or Break of Day, as may be thought expedient. And if it should happen to be omitted, so long as to stand still; you must not set it going again, but by taking it assunder, and putting the two Indexes together, and bringing the Movement to the Meridian Line, as at first directed. Therefore when at Sea be careful of such Omissions.
8. It is to be noted, That altho’ every Minute of Time, does Answer to about 15 or 16 MilesEnglish, under theEquinoctial; yet it is not above 9 or 10 Miles in our Latitude to one such Minute. And altho’ it be so many under the Equinoctial, yettherethe Sun’s Altitude will be taken very exact,because 90 degrees of Altitude, is for 6 hours of Time; whereas in our Latitude, it is but 62 degrees, for 8 hours of Time; both which Advantages will Facilitate the Practice. I shall at present conclude with acquanting,
That if a common Minute Watch (by Filing or Loading the Ballance, or otherwise) be made to go, or does already go, any unknown quantity of Time, either too Fast, or too Slow; by this Device, we may find the Hour and Minute of the Day by such a Watch, more exactly than can be done by the best timed common Watch whatever. And this even without touching it, other than by Winding up the Spring as usual.
There are other Improvements that may be made of this Discovery, but I reserve them to some other Opportunity.