CHAPTER XI. Of an Obfervation, either of Sun or Star, what it is, how or with what, and when 'tis taken, and finding the Latitude thereby.

1.

A

N Obfervation, is the finding either the Sun's or Star's
Meridian Altitude (at Sea) with a Quadrant, or with a

Cross-ftaff.

2. Meridian Altitude, is the Height above the Horizon of the Sun, or Star, they being upon the Meridian of the Place of Obfervation; and that the Sun is, every Day at Noon, but the Stars at different Times, according to the Difference of their feveral Right Afcenfion from the Sun's Right Afcenfion.

3. A Quadrant is the Inftrument whereby only the Sun's Altitude (at Sea) is found; but by Hadley's Quadrant the Altitude of a Star may be determined.

4. A Cross-ftaff or Fore-ftaff, is an Inftrument whereby Mariners find either the Sun's, or Star's Altitude. The particular Defcription of Davis's Quadrant and the Crofs-ftaff I here omit, having done it largely in the Mariner's Compass Rectified, to which Book I refer the Learner, and will now fhew how to find the Latitude after you have obferved, which is called working an Obfervation, and that in this one Propofition.

Propofition. The Meridian Altitude of any Heavenly Object, and it's Declination given; to find the Latitude of the Place of Obfervation.

In this Propofition are two Cafes.

Cafe 1. Where, or when the Object obferved doth both Rife and Set in 24 Hours. The Rules are thefe.

Rule 1. The Meridian Altitude, and Declination of one Kind; that is, both North, or both South; the Difference of the Zenith -Distance, and the Declination is the Latitude required.

Note; When the Declination is greater than the Zenith Diftance, the Latitude is of the fame Name; but if lefs, then of a contrary Name to the Declination.

Rule 2. The Meridian Altitude, and Declination of contrary Names; that is, one North, and the other South, the Sum of the Zenith Distance and the Declination, is the Latitude.required, of the fame Name with the Declination.

Note 1. By Meridian Altitude South, I mean the Object obferved is to the Southward of the Obferver; and when North, then to the Northward of the Obferver.

Note 2. By Zenith Diftance underftand the Complement of the Meridian Altitude of the Object obferved.