instrument used at sea for taking the altitudes of heavenly bodies. The fore-staff, called also cross-staff, takes its denomination hence, that the observer, in using it, turns his face towards the object; in opposition to the back-staff, where he turns his back to the object.

The fore or cross-staff, consists of a straight square staff, graduated like a line of tangents, and four crosses or vanes, which slide on it. The first and shortest of these vanes, is called the ten cross, or vane, and belongs to that side of the instrument on which the divisions begin at three degrees and end at ten. The next longer vane, is called the thirty cross, belonging to that side of the staff in which the divisions begin at ten degrees and end at thirty, called the thirty scale. The next vane is called the sixty cross, and belongs to the side where the divisions begin at twenty degrees and end at sixty. The last and longest, called the ninety cross, belongs to the side where the divisions begin at thirty degrees and end at ninety.

The use of this instrument is to take the height of the sun and stars, or the distance of two stars: and the ten, thirty, sixty, or ninety crosses, are to be used according as the altitude is greater or less; that is, if the altitude be less than ten degrees, the ten cross is to be used; if above ten, but less than thirty, the thirty cross is to be used, &c. Note, For altitudes greater than thirty degrees, this instrument is not so convenient as a quadrant or semicircle.

To observe an Altitude by this instrument.—Apply the flat end of the staff to your eye, and look at the upper end of the cross for the centre of the sun or star, and at the lower end for the horizon. If you see the sky instead of the horizon, slide the cross a little nearer the eye; and if you see the sea instead of the horizon, slide the cross farther from the eye; and thus continue moving till you see exactly the sun or star's centre by the top of the cross, and the horizon by the bottom thereof.

Then the degrees and minutes, cut by the inner edge of the cross upon the side of the staff peculiar to the cross you use, give the altitude of the sun or star.

If it be the meridian altitude you want, continue your observation as long as you find the altitude increase, still moving the cross nearer to the eye. By subtracting the meridian altitude thus found from 90 degrees, you will have the zenith distance. To work accurately, an allowance must be made for the height of the eye above the surface of the sea, viz. for one English foot, 1 minute; for 5 feet, 2½; for 10 feet 3½; for 20 feet, 5½; for 40 feet, 7½, &c. These minutes subtracted from the altitude observed, and added to the zenith distance observed, give the true altitude and zenith distance.

To observe the distance of two stars, or the moon's distance from a star, by the fore-staff.—Apply the instrument to the eye, and looking to both ends of the cross, move it nearer or farther from the eye till you see the two stars, the one on the one end, and the other on the other end of the cross; then the degrees and minutes cut by the cross on the side proper to the vane in use give the stars distance.