A New Discovery for Finding the
Longitude.
For Finding the Longitude, it is to be noted, That if the true Hour, and Minute of the Day, at the Place of our first Departure, and where we are Arrived, can be both obtained, the Difference of Longitude may be found, as truly as the Difference of Latitude.
For which purpose the Spring-Movement hereunto subjoined, is most humbly proposed to effect the same, even to the Tenth of a Minute of Time. So that should the Error of a Minute happen to be found in Practice, it must proceed from the Influence or Vicissitude of the Air, and not from any Defect in the Rules by which the said Movement is composed, as will plainly appear by the Explication thereof, in the following Lines; and first of
The Description of the said Movement.
This Movement may be either of the Form and Bigness of that in a Common Clock; or else of the Smallness of a Pocket Watch, as may be thought convenient in Practice. But the larger it is, the better the Divisions in the Horologe will appear. And as for the Numbers, they may be at pleasure: Only this must be observ’d, That the swiftest Index must make just One Revolution, whilst the slow one goes but One of the Hundred Divisions, (as is common in Minute Watches, wherein 60 Minutes goes to an Hour) but the rest of the Numbers may be at pleasure, as aforesaid. And if the Slowest moves round in 3, 4, or 5 Days; and the Movement requires to be wound up once in 26 or 27 Hours, it will be sufficient: For the more Vigor the Motion is calculated for, the less the Air will affect it.
How to set it first in Motion.
When it is finished, let it go for a Month or two; after which put the two Indexes right one with the other, and both of them pointing just to 100. Then the Spring being wound up, keep it from Going (with the Key) till the Shadow of the Sun is exactly come to a Meridian Line, which must be rightly prepared for that purpose. Then let it go till the Shadow comes just to the same Line the next Day, or rather 40, 50, or 100 Days after, (the longer the Better.) At which time enter down the Centesms that the swiftest Index points to, as also the Integers and Tenths of the slowest, (but put the last first in your Numbers) by this you will be furnished with two general Numbers, viz. the said Numbers pointed to, and the Time (in Hours) spent therein, to find the Hour of the Day to the Tenth of a Minute, at any time; for that Place where it was so set going, tho’ you remove it afterwards to any Distance whatsoever. Which two Numbers ought to be entered in a Book; to be used whensoever you would find the Hour of the Day.
And it is to be noted, That if the Sun be not then in its Mean Motion, you must Add to, or Substract from what the Indexes do Give, according to the Inequality thereof: But if you try it for one whole Year, there will be no need of either. And that what is said may be the better understood, I shall give an Example.