=Double-lock Bridge.=--This is used for spans of 30 to 45 feet. In laying out the section and making the spars for the double-lock bridge it must be remembered that the road-bearing transoms, _TT_, in this case are not the transoms lashed to the frames, but those which are subsequently lashed on the top of the distance-pieces. [Illustration: FIG. 133.] The frames are of equal widths. In making the bridge the frames after being launched are held by the back guys a little higher than their ultimate position. The distance-pieces are then hauled across, the proper positions for the road-bearing transoms having been previously marked upon them; these transoms are then sent out and lashed in position, after which the back guys can be eased off and the bridge allowed to lock; the roadway is then laid. ESTIMATE OF TIMBER. -----------+------+------+--------------+--------------------------- Kind of |No. of|Length| Diam. in In. | Bridge. |Spars.|in Ft.| | | | +-------+------+ Purpose. | | |At Tip.| Mean.| -----------+------+------+-------+------+--------------------------- Double-lock| 4 | 20 | 7 | |Standards. | 2 | 25 | | 10 |Distance-pieces. (40 to 45 | 2 | 15 | | 10 |Road-transoms. Ft. Span.) | 2 | 15 | | 6 |Main transoms. | 4 | 15 | |4 to 6|Ledgers and shore-transoms. | 4 | 20 | 3 | |Braces. | 15 | 20 | | 6 |Balks. | 6 | 20 | 3 | |Ribbands. -----------+------+------+-------+------+--------------------------- [Illustration: FIG. 134.] =Single-sling Bridges, used to 50 feet.=--The frames for a single-sling bridge have an upper (locking) and lower (road-bearing) transom. In laying out the section the frames must be made to lock at such a height as to have at least 9 feet of headway between the roadway and the upper transoms. As soon as the spars of the narrow frame are in position for lashing the distance from out to out of its standards at the upper transom must be measured, and the broad frame made of sufficient width to receive them. A snatch-block with a fall rove through it is lashed to the tip of each standard of the narrow frame. The frames having been locked, the fork-transom is hauled into position by means of the falls and men working in the crutch. The falls are then used to sling the centre transom temporarily into position. The slings are then arranged by men working in the crutch and astride the slung transom, one of the guys being generally used for the purpose; three or four turns are sufficient, care being taken that they do not ride. The slings, if too long, can be subsequently twisted up with handspikes to give the requisite camber to the roadway. [Illustration: FIG. 135.]