This being done, see how many degrees and minutes are cut by the edge of the sight vane C, which answers to the sight hole, and to them add the degrees that are cut by the upper edge of the shade vane, and the sum is the Zenith Distance, or Complement of the Altitude.
To find the greatest altitude you must continue the observation so long as the Altitude be found to increase by the Sea appearing instead of the horizon, removing the sight vane lower; but, when the sky appears instead of the horizon, the Altitude is diminished, therefore detist, and add the degrees on the 60 Arc to the degrees and minutes upon the 30 Arc, the sum is the Zenith Distance, or Co-altitude of the Sun's upper Limb; Add 16 minutes, the Sun's semi-diameter to that produced by the observation and the sum is true Zenith Distance of the Sun's center, to which add the allowance for refraction.
If a lens be fixed in the shade vane which contracts the Rays of light, and casts them in a small bright spot on the slit of the horizon vane instead of a shade, this will be an improvement to the instrument.
Having obtained the Meridonial Altitude or Zenith distance of the Sun, seek the Sun's declination for the day, if it be North, subtract it from the Altitude and you will have the height of the Equinoctial above the Horizon, and this height taken from 90 degrees you will have the Latitude of the place. But if the Zenith distance be added to the Declination of the Sun the sum will be the latitude of the Place.
Transcribed by Lars Bruzelius
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Copyright © 1996 Lars Bruzelius.