ive the total length of spars required.

For a double lock bridge, a piece of rope of a length equal to the
length of the middle bay replaces the lashing. If the banks are not
parallel, a measurement should be taken on each side of the bridge.

If desired, a section of the gap may be laid down on the ground in
full size and the lengths of spars determined by laying them in place.
This method, though given as standard by all authorities, requires
more time and more handling of material than the other and gives no
better results.

The construction of a frame is shown in Fig. 1, and the system of
marking in Fig. 2. The arrangement of frames to form a single lock
bridge is shown in Figs. 3 and 4, and a double lock bridge in Fig 6.

[Illustration: Fig. 1]

[Illustration: Fig. 2]

[Illustration: Fig. 3]

[Illustration: Fig. 4]

[Illustration: Fig. 5]

[Illustration: Fig. 6]

[Illustration: Fig. 7]

[Illustration: Fig. 8]

[Illustration: Fig. 9]

=1113. Construction of single-lock bridges=, Figs. 3, 4, and
5.--Suitable for spans of 30 ft. or less. The two frames lock together
at the center of the span; their slope must not be more than 4 on 7.
The bridge can be erected by two or three noncommissioned officers
and 20 men, one-half on each side of the gap. Heavy spars require more
men.

The footings at _A_ and _B_ must be firm, horizontal if possible, and
at right angles to the axis of the bridge. In a masonry pier they may
be cut out. In firm soil a simple trench will suffice. In yielding
soil a plank or sill must be laid in the trench. The frames are made
of such length as to give a slight camber to the bridge, which may be
increased to allow for probable settlement of the footings. The inside
dimension of one frame is made slightly greater than the outside
dimension of the other, so that one frame may fall inside of the other
when hauled into position. For a 9 ft. roadway the standards of the
narrow (inside) frame should be 9 ft. 6 ins. apart at the transom and
10 ft. 6 ins. at the ledger, in the clear, and the other (outside)
frame 1 ft. 6 ins. wider throughout.

A frame is constructed on each bank. The standards are laid on the
ground in prolongation of the bridge, butts toward the bank. The
ledgers are lashed on _above_ and the transoms _beneath_ the standards
at the positions marked. The diagonal braces are lashed to the
standards, two butts and one tip above the latter, and to each other.
Before the