Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/477

Rh PONTOON 457 was divided internally into separate compartments by partitions which were made as water-tight as possible, and also supplied with the means of pumping out water ; when transported overland with an army, a pair of demi-pontoons and the superstructure of one bay formed the load for a single carriage weighing 3110 ft when loaded. The Pasley was superseded by the Blanshard pontoon, a tin coated cylinder with hemispherical ends, for which great mobility was claimed, two pontoons and two bays super structure being carried on one waggon, giving a weight of about 5000 ft&amp;gt;, which was intended to be drawn by four horses. The Blanshard pontoon was long adopted for the British army, but it is IIOAV being discarded ; experiments made with it in peace time showed that it would probably break down under the strain of actual warfare, and efforts were constantly made to improve on it ; when immersed to a greater depth than the semi-diameter it became very unstable and lively under a passing load, a defect which Serjeant-Major Forbes proposed to remedy by giving it a triangular instead of a circular section, thus increasing the stability by presenting a continually increasing area of bearing surface up to the level of total immersion ; but the angles of these pontoons were found so liable to injury as to counterbalance any advantages over the cylinders. After many years experience of the closed pontoon the English engineers came to the conclusion that it was desirable to return to the form of the open bateau to which the engineers of all the Continental armies had meanwhile constantly adhered. Captain Fowke, R.E., invented a folding open bateau, made of water-proof canvas attached to sliding ribs, so that for transport it can be collapsed like the bellows of an accordion and for use it can be ex tended by a pair of stretchers ; it is very mobile, but it is also deficient in power of support, for whereas the buoy ancy due to the outline form out of the water is 13,600 ft the actual buoyancy in the water is only 8640 ft, because of the cavities in the canvas between the ribs which are formed by the pressure of the water outside ; moreover, the surface irregularities cause the pressure exerted by a current upon a bridge formed of these collapsible pontoons to be about three times as much as upon one of equal power formed with Blanshard s or Pasley s pontoons ; there is thus great risk of the bridge being carried away by a strong current. The following table shows the powers of various pontoons at present or recently in use by different nations. The &quot; working power of support &quot; has been calculated in most instances by deduct ing from the &quot; available buoyancy&quot; one-fourth for open and one- tenth for closed vessels : The Powers of Various Pontoon Bridges. Pontoon. to 5 H-l Displacement of Pontoon. Actual Buoyancy of Puntoon. Weight of Pontoon and One Hay of Superstructure. Available Buoyancy. Working Power of Support. Central Interval in Bridge. Power per Lineal Foot of Uoadway. Greatest Ordinary Load per Foot Lilieal. Width of Roadway. Greatest Possible Load at 100 Ih per Foot Superficial of Headway. Gribeauval : open bateau, ( a... Ft. 36-3 Cub. Ft. 593 ft 45,044 fc 8,044 ft 37.000 ft 27,750 Ft. 22-8 n&amp;gt; 1,215 ft, 840 Ft. 15-6 It) 35,568 Austrian: open, wooden, 1799 27-0 354 22,123 3,332 18,791 14,093 16-6 849 560 11-4 18,924 Aust. - Birago : open, wooden; two pieces 28-0 303 18,907 3,249 15,658 11,744 217 542 560 9 3 20,181, , , , , , three , , ,, ,, iron; two pieces 39-4 28-0 445 353 27,791 22,090 3,884 3,698 23,907 18,392 17,930 13,794 217 217 827 636 560 560 9-3 9-3 20,181 20,181 ,,, , ,, three ,, 39-4 530 33,135 4,501 28,634 21,476 217 991 560 9 3 20,181 French: open, wooden; reserve 30-9 325 20,286 3,608 16,678 12,509 197 635 560 10-5 20,685, ,, ,, advanced guard 197 156 9,734 1,506 8,228 6,171 16-4 376 560 9-3 15.252, ,, ,, general 30-9 321 20,065 3,153 16,912 12,684 197 644 560 9-8 19,306 Prussian: open, wooden; open order 237 164 10,226 2,393 7,833 5,875 15-3 384 560 9-9 15,147, , , , , close order. 23 7 164 10.226 2,213 8.013 6,010 11-2 535 560 9 9 11,088, , , iron ; open order 247 214 13,385 2,209 11,176 8,382 15-3 561 560 9-9 15,147, ,, ,, close order 247 214 13,385 2,029 11,356 8,517 11-2 759 560 9-9 11,C88 Italian: open, wooden; one piece 19-6 283 17,660 3,582 14,078 10,559 26-3 402 560 9-8 25,774, ,, ,, two pieces 39-2 565 35,320 4,572 30,748 23,061 26-3 878 560 9-8 25.774, modified; one piece 24-6 325 20,290 3,401 16,889 12,669 23-0 551 560 9-8 22,540 ,, ,, two pieces 49-2 649 40,580 4,489 36,091 27,068 23-0 1,178 560 9-8 22,540 open, canvas on ) open order ( wooden framework ;  close order Belgian: open, iron ; one piece 21-0 21-0 24-8 209 209 297 13,042 13,042 18,584 2,355 2,083 3,336 10,687 10,959 15,248 8,015 8,219 11,436 16-6 117 197 493 705 580 560 560 560 10-4 10-4 9-5 17,264 12,168 18,715, , , , , , two pieces 49-2 595 37,168 4,548 32,620 24,465 197 1,244 560 9-5 18,715 AmPviVin india-rubber, three open order.. -iilllCllLclll i i i ill i ( cylinders connected close order.. English Pontoons. Peninsular open, tin; reserve 20-0 20-0 18-9 130 130 209 8,125 8,125 13 092 1,980 1,824 2 374 6,145 6,301 10 718 5,530 5,761 8 039 18-0 147 16 8 307 393 477 580 560 560 11-0 11-0 lO O 19,800 18,370 16 800 equipment: ,, ,, advanced guard Pasley: closed demi-canoe; copper 15-1 25 -0 120 141 7,520 8 781 1,654 9 103 5,866 6 678 4,400 6 010 14-0 12 5 314 481 560 560 9-0 10 12,600 12 500 Blanshard: cylinder, tin; open order 22-5 109 6 785 1 600 5 185 4 667 12 - 5 373 560 lO O 12,500 ,. ,, ,, close order 22 5 109 6 785 1 408 5 377 4 839 8 3 581 560 lO O 8,300 ,, ,, ,, infantry pattern Fowke: open, collapsible, canvas; open order. Forbes : closed, spherangular, tin ; open order.. Blood : open, wooden; general 15-5 22-0 24-2 21-6 26 134 128 280 1,640 8,460 7,977 17 500 340 1,246 1,689 2 300 1,300 7,214 6,288 15 200 1,170 5,411 5,659 13 350 5-3 10-0 11-0 15 -0 220 541 514 890 280 560 560 560 7-0 10-0 10-0 10 3,710 10,000 11,000 15,000 In the English and French equipment the pontoons were originally made of two sizes, the smaller and lighter for the &quot;advanced guard,&quot; the larger and heavier for the &quot;reserve&quot;; in both equipments the same size pontoon is now adopted for general requirements, the superstructure being strengthened when necessary for very heavy weights. The Austrian and Italian pontoons are made in three pieces, two with bows and a middle piece without; not less than two pieces are ordinarily employed, and the third is introduced when great supporting power is required, but in all cases a constant interval is maintained between the pontoons. On the other hand in the Prussian, Russian, Dutch, and American and in the English Blanshard equipments greater supporting power is obtained not by increasing the number of supports but by diminish ing the central interval between the pontoons. Within certain limits it does not matter whether the buoyancy is made up of a large number of small or a small number of large vessels, so long as the water-way is not unduly contracted and the obstruction offered to a swift current dangerously increased ; but it is to be remembered that pontoon bridges have failed as frequently from being washed away as from insufficient buoyancy. XIX. --58