The Davis Quadrant

In the 17th and 18th centuries the principal instrument of the navigator at sea was the Davis quadrant, also known as English quadrant, a forerunner of the sextant. It was used to measure the altitude of the sun at noon, replacing the ancient and less accurate astrolabe in the determination of latitude. Less well known is the fact that land surveyors made use of the Davis quadrant for their own solar observations with the aid of a plumb bob, where it soon gave way to the simpler surveyor’s quadrant.

Last year I gave several presentations on the surveying and mapping procedures employed during the Spanish colonial era. To make the talks more interesting I built replicas of various tools of the trade and demonstrated how they were used by navigators as well as surveyors, the two often being the same individual. As late as 1841, during the ill-fated Texas-Santa Fe expedition, a Lieutenant Hull took the latitude and longitude, from which he computed the direction and distance to Santa Fe. George W. Kendall, the chronicler of the Texans, informs us that Lieutenant Hull had been an officer in the English navy and “fully understood the use of the quadrant.” He goes on to say that the results of Hull’s observations, “from the instruments not being in order, could not be depended upon…” and the expedition became hopelessly lost. When Kendall noticed in the distance a lone wolf, he wrote in his diary “I even envied this most contemptible of the brute creation, for he knew where he was” [emphasis added].

John Davis (c.1550-1605) a native of Sandridge in Devonshire was an English navigator. Influenced by Mercator’s 1538 Map of the World he attempted to locate the Northwest Passage on three voyages in 1585, 1586 and 1587, where he discovered Davis Strait to the west of Greenland. In 1588 he commanded a ship, the Black Dog, against the Spanish Armada. Davis wrote several books, and in one of them, The Seaman’s Secret, he described his invention of a back-staff that became the forerunner of his quadrant. The back-staff was an improvement of the widely used cross-staff, the latter having the major disadvantage of forcing the observer to look directly into the sun. The “black patch” of pirate-story fame is not entirely only an embellishment of a seaman’s yarn, or colorful addition to a Halloween costume; going blind from constantly looking into the sun was an occupational hazard of navigators, and even though darkened glass was already in use, it offered no protection against the eye-damaging ultraviolet rays. The cross-staff had the advantage however of being useable for polaris observations and for measuring horizontal angles, neither of which can be done with a Davis quadrant.

Attempts to develop alternatives to looking into the blinding sun had already been made. One of the most ingenious surveying devices ever invented is the astronomical ring. In its almost embarrassing simplicity the astronomical ring can best be described as an one-inch slice off a six inch black iron pipe (at least that’s how I made it). On top of the devise is a hole supporting a swiveling ring or leather loop, allowing the instrument to hang vertically (hopefully, for that was the idea) from the observer’s thumb. The inside of the ring is graduated into degrees and fractions of a degree. At 45 degrees from the top of the ring (as measured from it’s center) is a hole of about 1/8 inch diameter through which the sun can cast a point of light onto the graduation, the 90-degree mark of which being vertically beneath the hole and the 0-degree mark horizontally opposite the hole. Hold the ring into the sun, allow it to send a beam of light through the small aperture, and the resulting point of light on the graduation on the inside of the ring will read the sun’s angle above the horizon [see photo]. The diameter of the ring is limited to about six to eight inches, as the sun will not focus through an open pinhole over a distance much greater than that, a limitation that is evident in all surveying devices that use sunlight without a focusing lens.

Davis’ back-staff had apparently never been built, as no original model is known to exist. Davis soon improved upon it by developing his quadrant. The quadrant was an immediate success and was used well until the early 19th century when it was made obsolete by the sextant. Its use is depicted in many contemporary illustrations (mostly woodcuts) reproduced in modern history texts, and more often than not shows that the illustrating artist was ignorant of how the instrument was used.

To take a reading, the navigator had to wait until the sun was on the meridian. He estimated its altitude and set the sight on the small circle on a mark five or ten degrees below his estimate. With his back turned towards the sun and the front sight of the quadrant pointed at the horizon, he tilted the instrument in a vertical plane until the sun cast a point of light onto the horizontal centerline of the front sight. He then moved the sight on the large circle until he could look through its peephole in alignment with the front sight and the horizon (difficult to do on the pitching deck of a ship). Adding the readings on both circles will give the sun’s altitude. It goes without saying that the navigator had to repeat this process several times in order to obtain an acceptable mean of his readings for altitude. It also is obvious that readings taken on land were better as those taken at sea.

Enter the land surveyor and mapper, to whom the horizon does not mean much (except in Kansas), and who uses the vertical as a line of reference. He could (and did) use the Davis quadrant by tilting it up ninety degrees and fastening a plumb bob just below the hole of the front sight, allowing it to hang across the large circle from which he removed the eyepiece. The sun would now have to shine through the front sight and onto the sight on the small circle. Subtracting the reading taken at the plumb bob string from the setting on the small circle will yield the zenith angle of the sun. A simplification of this process resulted in the construction of the surveyor’s quadrant, which is nothing but a graduated board with two protrusions, one with an aperture for the sun and one with a target mark, and used with a plumb bob.