PIONEER AND TONTOON SERVICE. 175 



tie-beams from 5 to 6 inches thick, is laid over the pontoon, and the bridge 

 sleepers are laid only upon this scaffold. If the bridge with ordinary span 

 is to remain standing for some time, then in each pontoon a trestle, A 

 {Jig. 50), is placed, of which the cross-piece stands three inches higher 

 than the gunwale of the pontoon, so that the string-pieces rest upon the 

 trestles. 



Fig. 42 shows a movable foot bridge. Each trestle consists of two feet, 

 F {Jig. 43 shows their ground plan), with four cross sleepers, a, four 

 strutts, h, and four posts, c. To place the head, d, upon which the bridge 

 floor rests, at any required height, the posts, c, are bored with holes at 

 every foot, to receive iron bolts, which pass also through iron plating on 

 the ends of the heads. The string-pieces of the foot bridge are seventeen 

 feet long by five or six inches through ; the planks are seventeen feet long, 

 six inches broad, and two and a half inches thick. 



C5I 



