Extemporised Sight Vane for Levelling Staff.
Fig. 5. Suppose your staff 1½in. thick: take a piece of tin about 7in. wide, and of any convenient length, say nearly square; line it off as in Fig. 4, so that there shall be three divisions parallel to each other, as wide as the staff is thick, and on one side of them leave another division, about ¾in. wide, and on the other mark and cut out two projections ¾in. square, and a semicircular eyepiece, somewhat larger, with a quadrant-like aperture, as in the illustration; bend the tin at the divisional lines, so as to clasp the staff loosely, with the sight vane projecting from it; let a cord be attached to the top of the tin, and pass through a hole in the top of the staff to draw it up by, and attach another cord to the bottom to draw it down again if required. Upon the edge of the narrowest divisions draw the subdivisions of the measures already marked upon your staff, letting them commence from the level of the bottom of the aperture in the sight vane, and read downwards; then, directing the telescope of your levelling instrument towards the staff, let an assistant lower or raise the sight vane according to your signals, and then read off the number of subdivisions on the vernier until you come down to a line, marking a division on the staff; read off this, and add the fractional parts, and the excess or deficiency over or below the height of the eye will be the difference of level in feet, inches, or whatever measurement may be used.
Movable stands for instruments must be light for the sake of portability, but steadiness cannot be obtained without weight; and for this purpose a bucket of water, a bamboo filled with water or sand, a bag of stones or sand, a large stone, or lashing to a tent peg driven into the ground, may be employed, as in our illustration.