© Duane A. Cline 1999
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The Back-Staff
Although the cross-staff was popular with sailors, it had some serious defects, which were often pointed out by the English mathematicians. If the staff were not positioned correctly on the cheekbone, the eye would not be the terminal point in the axis. The result would be a misreading of the angle. Another problem was the blinding glare when a mariner was observing the meridian altitude of the sun. In the northern latitudes, the brightness of the summer nights made star sights impractical.
In about 1594 John Davis, an English captain, developed a simple back-staff which eliminated the problems of parallax and the glare of sun sights as well as the problems involved in sighting two widely separated objects simultaneously. Davis' back staff was intended to be an improvement on the mariners' quadrants, astrolabes and cross-staves. The Davis back-staff consisted of a graduated staff, a half-cross in the shape of an arc of a circle on the radius of the staff with a fixed vane, and a brass horizon vane with a slit in it at the fore-end of the staff.
The observer placed the staff on his shoulder and stood with his back to the sun. With the horizon vane lined up with the horizon, he slid the half-cross back and forth until the shadow of its vane fell across the slit in the bottom vane while the horizon was visible through the slit. By doing this the observer was able to sight both the sun and the horizon while his back was towards the sun.
Davis improved upon this design in a model with two-half-crosses, which divided an accurate scale into two parts, had the appearance of a large triangle equipped with a 30� arc at one end and a small 60� arc at the other. One scale was engraved on its upper side towards the front of the staff, the other was on its underside and at the back.
The back-staff immediately gained popularity, and during the seventeenth century it became indispensable to English as well as foreign sailors. It became known as the Davis Quadrant, by the English sailors and the English Quadrant by continental sailors. The Davis Quadrant had been in use early in the sea career of Christopher Jones. Whether he used the back-staff, or one of the other instruments in his sightings, will never be known. However, it is quite reasonable to believe there might have been a back-staff aboard Mayflower for use by any officer who might prefer its use.
Last modified October 28, 1999
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