# Hadley's quadrant

## Definitions

• Webster's Revised Unabridged Dictionary
• Hadley's quadrant a hand instrument used chiefly at sea to measure the altitude of the sun or other celestial body in ascertaining the vessel's position. It consists of a frame in the form of an octant having a graduated scale upon its arc, and an index arm, or alidade pivoted at its apex. Mirrors, called the index glass and the horizon glass, are fixed one upon the index arm and the other upon one side of the frame, respectively. When the instrument is held upright, the index arm may be swung so that the index glass will reflect an image of the sun upon the horizon glass, and when the reflected image of the sun coincides, to the observer's eye, with the horizon as seen directly through an opening at the side of the horizon glass, the index shows the sun's altitude upon the scale; -- more properly, but less commonly, called an octant.
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## Usage

### In literature:

Hadley's quadrant came thirty years after Dampier, who must have used Davis' instrument, then about ninety years old.
"The Naval Pioneers of Australia and Walter Jeffery" by Louis Becke
Thomas Godfrey, a self-taught mathematician, great in his way, and afterward inventor of what is now called Hadley's Quadrant.
"Autobiography of Benjamin Franklin" by Benjamin Franklin
STATE-STREET, 1 best brass Sextant, latest improvement; Hadley's Quadrants, Davis' do.
"The Olden Time Series, Vol. 4: Quaint and Curious Advertisements" by Henry M. Brooks
In 1731 an account of the reflecting or Hadley's quadrant appeared in a paper given by a member of the Royal Society.
"How Britannia Came to Rule the Waves" by W.H.G. Kingston
With Hadley's quadrant, latitude could be determined within a minute, that is to say, to the third of a league.
"Celebrated Travels and Travellers" by Jules Verne
The mariner's astrolabe was superseded by John Hadley's quadrant of 1731.
"Encyclopaedia Britannica, 11th Edition, Volume 2, Slice 7" by Various
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