174 MILITARY SCIENCES. 



The two are connected by the galleries DD. Still another enveloping 

 gallery may be thrown forward. The listening galleries, HH, run further 

 out into the country, and from them are thrown out, according to circum- 

 stances, the fourneaux, ahc. The gallery of communication, E, along the 

 capital, is called the capital-gallery. Fig. 30 shows a comj^ete system of 

 mines for a front of attack. 



3. Pontoon Service. 



The object of pontoon service is to effect the passage of armies over 

 rivers. As the building of bridges upon trestles and piles is generally imder- 

 stood, we shall occupy ourselves here only with the construction of bridges 

 of boats, or pontoon bridges. The boats or pontoons are made either of copper 

 or iron plate, or of wood sheathed with iron. A pontoon {figs. 44-49) consists 

 of the body, BC, the stem, AB, and the stern, CD, and is 30 feet long, 5 feet 

 9 inches wide above and 3 feet 8 inches below in the centre, sharpened to 

 both ends. The height is in the middle 2 feet 6 inches, at the stem 3 feet, 

 and at the stern 2 feet 10 inches. E is the floor, F, the two sides ; a are the 

 flooring-boards, b, the side-boards. Small pieces, ddefghik, serve to hold 

 the pontoon together, and it is provided also with the requisite iron platings. 

 Fig. 44 shows the side view ; fig. 45, the upper view ; fig. 46, the longitudinal 

 section ; fig. 47, the front view ; fig. 48, the cross-section, and fig. 49, the 

 rear view of a wooden pontoon whose weight is 16 cwt. The pontoneer 

 implements for the service of the pontoon are : the pontoon kedge {fig. 35), 

 a three-fluked anchor, usually four feet long ; the steering-oar {fig. 37), with 

 a sixteen feet long handle, a, the blade, b, and, when the rudder rests upon 

 the wale, the reinforcement, a {fig. 36) ; the pulling-oar {fig. 38) is only 

 ten feet long, in other respects like the steering-oar, save that the blade, b, 

 is rounded ; the paddle {fig. 39) is only five feet long, and the handle, a, 

 and blade, b, are in one piece ; at one end is the crutch, c, and at the other 

 the iron mounting, d ; the boat hooks {figs. 40 and 41) serve to hold the 

 pontoon fast to any object. 



To throw a pontoon bridge, the first step is to lay the ground sills, whose 

 upper surface must lie one foot seven inches above the level of the water. 

 Then the first pontoon on each side is placed in the proper direction, and 

 the five bridge-sleepers are laid upon these and the ground sills, when the 

 pontoon is again exactly aligned and firmly anchored. Then the chesses are 

 stretched, but not so far as to interfere with the laying of the second set of 

 sleepers. The two next pontoons are then properly placed, the sleepers laid, 

 and so on until the bridge is completed. The pontoons are attached to each 

 other by cross-ropes. Fig. 52 shows the upper view of a pontoon bridge, 

 with the ordmary span, and fig. 53, one with a greater span, for rivers 

 having little current, or where but light weights are to be passed over. In 

 the bridge with the greater space, the string-pieces rest only on three gun- 

 wales in two pontoons. To effect this, a scaffold {fig. 51) of five cross-beams, 

 a, the same thickness as the string-pieces, and 6 feet 6 inches long, and two 

 650 



