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

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 ℔ 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 The Pasley was superseded by the Blanshard pontoon, a tin coated cylinder with hemispherical ends, for which great mobility was claimed, two pontoons and two bays superstructure being carried on one waggon, giving a weight of about 5000 ℔, which was intended to be drawn by four horses. The Blanshard pontoon was long adopted for the British army, but it is now 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 extended 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 ℔ the actual buoyancy in the water is only 8640 ℔, 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 “working power of support” has been calculated in most instances by deducting from the “available buoyancy” one-fourth for open and one-tenth for closed vessels:—

The Powers of Various Pontoon Bridges.

Pontoon.	Length.	Displace-  ment of Pontoon.	Actual Buoyancy of Pontoon.	Weight of pontoon and one bay of Super- structure.	Available Buoyancy.	Working Power of  Support.	Central Interval in Bridge.	Power per lineal foot  of Roadway.	Greatest ordinary Load per foot lineal.	Width of Roadway.	Greatest possible load at 100 ℔ per  foot superficial  of roadway.
	Ft.	Cub. Ft.	℔	℔	℔	℔	Ft.	℔	℔	Ft.	℔
Gribeauval: open bateau, oak	36·3	593	45,044	8,044	37,000	27,750	22·8	1,215	840	15·6	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	21·7	542	560	9·3	20,181
„   „   „three „	39·4	445	27,791	3,884	23,907	17,930	21·7	827	560	9·3	20,181
„   „   iron; two pieces	28·0	353	22,090	3,698	18,392	13,794	21·7	636	560	9·3	20,181
„   „  „   three „	39·4	530	33,135	4,501	28,634	21,476	21·7	991	560	9·3	20,181
French: open, wooden; reserve	30·9	325	20,286	3,608	16,678	12,509	19·7	635	560	10·5	20,685
„   „  „ advanced guard	19·7	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	19·7	644	560	9·8	19,306
Prussian: open, wooden; open order	23·7	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	24·7	214	13,385	2,209	11,176	8,382	15·3	561	560	9·9	15,147
„   „  „  close order	24·7	214	13,385	2,029	11,356	8,517	11·2	759	560	9·9	11,088
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	3401	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
Russian ${\displaystyle {\Big \{}}$ open, canvas on wooden framework; ${\displaystyle {\Big \}}}$ open order close order	21·0	209	13,042	2,355	10,687	8,015	16·6	493	560	10·4	17,264
	21·0	209	13,042	2,083	10,959	8,219	11·7	705	560	10·4	12,168
Belgian: open, iron; one piece	24·8	297	18,584	3,336	15,248	11,436	19·7	580	560	9·5	18,715
„   „  „    two pieces	49·2	595	37,168	4,548	32,620	24,465	19·7	1,244	560	9·5	18,715
American ${\displaystyle {\Big \{}}$ india-rubber,  three; cylinders connected; ${\displaystyle {\Big \}}}$ open order close order	20·0	130	8,125	1,980	6,145	5,530	18·0	307	580	11·0	19,800
	20·0	130	8,125	1,824	6,301	5,761	14·7	393	560	11·0	18,370
English Pontoons.
Peninsular  equipment ${\displaystyle {\Big \{}}$ open, tin; reserve   „  „   advanced guard	18·9	209	13,092	2,374	10,718	8,039	16·8	477	560	10·0	16,800
	15·1	120	7,520	1,654	5,866	4,400	14·0	314	560	9·0	12,600
Pasley: closed demi-canoe; copper	25·0	141	8,781	2,103	6,678	6,010	12·5	481	560	10·0	12,500
Blanshard: cylinder, tin; open order	22·5	109	6,785	1,600	5,185	4,667	12·5	373	560	10·0	12,500
„   „  „   close order	22·5	109	6,785	1,408	5,377	4,839	8·3	581	560	10·0	8,300
„   „  „   light pattern	15·5	26	1,640	340	1,300	1,170	5·3	220	280	7·0	3,710
Fowke: open, collapsible, canvas; open order	22·0	134	8,460	1,246	7,214	5,411	10·0	541	560	10·0	10,000
Forbes: closed, spherangular, tin; open order	24·2	128	7,977	1,689	6,288	5,659	11·0	514	560	10·0	11,000
Blood: open, wooden; general	21·6	280	17,500	2,300	15,200	13,350	15·0	890	560	10·0	15,000

In the English and French equipment the pontoons were originally made of two sizes, the smaller and lighter for the “advanced guard,” the larger and heavier for the “reserve”; 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 diminishing 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.