For Solving this, there are Two Ways, the First and Infallible is, when the Sun is either Rising or Setting; at which times, if the Air be clear, observe what Numbers are pointed to by the two _Indexes_, (but observe the swiftest first, because it will soon be removed) just as the Center of the Sun is in the Horizon; and enter both the Numbers down, as before directed, with the Day of the Month, and the Latitude you are then in: Which having done, by the following Cannon, say, As ℞ Is to t. c. of the Sun’s Decl. So is t. Lat. To s. c. of an Angle; which, if converted into Time (by 15) Gives the Hour and Minute of the Sun’s Rising from Midnight; and if Substracted from 12 ho. gives the Setting. Which shews the true Hour and Minute at the Place where you then are, either at Sea or Land. The Second way for Solving the said Question, is by taking the Sun’s Altitude, when about 5 or 10 degrees above the Horizon; and at the same time let another Person see what Number the Swiftest Index points to, and after that the Slowest: Then put them both down as before directed, with the Day of the Month, and the Latitude of the place where you are; then have you the Sun’s _Altitude_, the said _Latitude_, and the Sun’s _Declination_, to find the Hour and Minute of the Day at the place required. Now having by the said Observations and Cannons found the Hour and Minute of the Day for the Place where you are, when abroad; and by the Numbers pointed to, by the two Indexes, also found the Hour and Minute that it is at _London_, (if the Movement was there set going;) If you take the Lesser of these from the Greater, and bring the Remainder into Minutes, it will give the difference between the two places in ~Time~; by which you may find their difference of Longitude either _East_ or _West_ to a Minute, by the following proportion, (_viz._) Suppose the Difference of Time was 37 Minutes, then say, { Minutes of} {Minutes of} {Minutes of} {Minutes of} As { Time } : { Longitude} :: { Time } : { Longitude} { 1440. } { 21600 } { 37 } { 555.5 } {or 24 hours} { Deg. Min.} or 9 : 15.5 Required. Which 9 deg. 15.5. min. being added to, or taken from the Longitude of the Place, where the said Movement was first set in Motion, gives the Longitude you are come into, as effectually as the Latitude is obtained by Observation, as was at first proposed. So that it is evident, that if any defect should be found in practice, it must proceed from the Vicissitude of the Air; and not from any Deficiency in this Discovery, or from the Cannons relating thereunto.