Military Bridges, a/nd the Passage of Rivers. 293 



rule (page 14) for that at the bottom, or determining the mean velocity, with a 

 rod. From this we may form some estimate what a decrease of velocity may 

 have in rendering the bed more foul ; or an increase in the velocity of the 

 current in removing soft banks formed of substances actually deposited. For 

 rivers may, after floods, become passable, where they were before too muddy 

 to be forded ; or a decrease of velocity may occasion a deposit of mud and 

 slime, where it was before clean. Rivers which are not any where fordable 

 perpendicularly across, may sometimes be found passable in a slanting direc- 

 tion between two sinuosities. In searching for fords, therefore, when, from 

 the size of the river, their existence is doubtful, the trial should be made 

 somewhere between any two sinuosities, not far distant from each other." 



To perform this, it may in general be remarked, that the 

 bank should be first entered at about one-third the length of 

 the reach, supposed moderate, not exceeding half a mile or a 

 mile, below the first sinuosity or convex turn of the bank, and 

 ought to be turned slanting downwards towards the opposite 

 convex bank, as much above it as the entrance was below the 

 first. 



** Thus crossing obliquely the transit of the current from side to side, both 

 extremes wiU be avoided, and I have frequently found rivers fordable in this 

 manner, which could not be crossed at any point perpendicularly. The Spa- 

 nish army with which I served forded the Esla, in the campaign of 1812, 

 without loss or difficulty, by taking advantage of this circumstance ; and in 

 the same manner I forded the Douro near Zamora, and several other formi- 

 dable rivers." 



The author terminates the first section with many similar ju- 

 dicious remarks on the passage of rivers. 



The second section of the work is devoted to the properties 

 of the pontoon, and to the manner of laying a pontoon bridge. 

 A pontoon, it is well known, is a portable boat, conveyed on an 

 appropriate carriage from one place to another, along with an 

 army. When a sufficient number of these boats, with their ap- 

 purtenances, can be carried with a given number of soldiers, 

 they serve the purpose of constructing a bridge over any unford- 

 able river in the course of their march. In the British service, 

 the pontoons are generally made of tin, supported by a proper 

 frame. There are two kinds usually made for our service. The 

 larger-sized pontoon weighs about 9 cwt. The appurtenances, 

 constituting the flooring, &c. for the bridge, consisting of beams, 

 strong deals, boards, oars, an anchor, a grapnel, bolts, b'nes, 

 pickets, &c. weigh about 13 cwt. ; and the four-wheeled carriage 

 upon which each pontoon is transported weighs also about 13 



