The Particulars themselves follow:
At Land, where by the help of good fixt Dials, and other fit Instruments, the precise Meridian of the place may be known, it is easie to find the Variation of the Needle, divers ways: As, by applying of the Needle, &c. to the Shadow of a Thread hanging perpendicular when the Sun is in the Meridian; or to the Meridian Line; or the Side of a fixt Horizontal Dial, &c.
But at Sea, in regard the Meridian is not so easie to be found to any tolerable exactness, to know the Variation of the Needle, is much more laborious and difficult. The Height of the Pole, and the Suns Declination being known; a large Ring-Dial, truly wrought, having a Box with a Compass or Needle fixt to its Meridian below, may go as near as any other Instrument, to shew the Variation required. For, when it is let to the just hour and minute of the day, the Meridian of it stands just in its due place; and so shews how far the Needle varies from it, as exactly as the largeness of the Card will permit.
But because these Dials are so rarely just, &c. though they may be used and taken notice of, yet are they not to be relied on. The thing therefore is to be performed, as followeth:
Find out the Suns Azimuthal Distance from the Meridian some hours before, or after Noon, and then its Magnetical Azimuth, or Distance from the Meridian pointed at by the Needle, and the Difference of these two Distances, is the Variation of the Needle.
To find the Suns true Azimuth, or by how many Degrees, &c. of the Horizon it is distant from the Meridian: its Declination, its Altitude, and the Elevation of the Pole, must all three be known.
