Larger image
Fig. 408.—Section.
Larger image
853.—Sighted Levels.—A mechanic's level is commonly made with a hole longitudinally through it of about ½ inch diameter, closed at one end, except a small hole of 1/30 inch or so, and a cross upon a piece of glass at the other end. This plan permits a sight to be taken through it which gives an approximate level. Occasionally the same form of sight as that described is hinged on the top surface at each end of the level. The author has found a better plan of sighting to be given by a pair of sights placed on a centre upon the ends of the level to turn up when required for use, as shown Fig. 408, P S one of the pair of points. This, when turned up, shoulders on the stop-piece A B. The stop-piece is made of sufficient thickness to admit the point in the hole near B for protection when it is folded away out of use. The section of the level, as shown by the end view D, is the same as that of the level, Fig. 404. Very fair accuracy may be obtained by making these sights appear coincident upon a distant staff or rod.
Fig. 409.—Boning-rod.
Fig. 410.—Boning-rod with standard.
Larger image
854.—Boning-Rods, Fig. 409. These are very commonly employed with mechanics' levels. They are made somewhat like a stout T-square of 3 feet to 4 feet in length, about 3 inches in width, and ¾ inch in thickness both of the stem and head. They are at first placed at a distance apart, 9 or 10 feet, and a straight-edge of this length is laid from one to the other, upon which the mounted level is afterwards placed, the boning-rod being tapped down in the ground till the bubble is in the centre of its run. A third boning-rod is then placed at the same distance as the first pair, and the straight-edge with the level upon it is reversed end for end. This, if the work be fairly down, leaves the two outer boning-rods level, however imperfect the straight-edge and level may be, if the run of the bubble be taken correctly. By removing the central boning-rod from the outer pair of rods, levels may be continued by sighting over them, or boning forward as it is termed. On the Continent boning-rods are commonly fixed by driving a separate standard into the ground, which has a pair of brass slings by its side to hold the rod, Fig. 410. This is a much neater plan than that in common use of blocking the rod up with stones. Boning-rods are also sometimes used conveniently with a proper surveying level, from the tops of water-pipes, etc.