Back Observation

An altitude measured on a back observation quadrant, which requires the observer to turn his back to the body observed. The instrument is an ordinary quadrant with the addition of a second horizon glass below the regular one, and the former one is looked into while measuring the altitude--the instrument being held backwards, so that the observer faces the mirror, and pulls the sliding limb towards him. This instrument is used for measuring the altitude of a heavenly body from its farthest horizon by throwing it across the observers, zenith. This process is only employed when the intervention of the land does not admit of a regular fore observation.