forces, especially in sparsely settled countries under bad conditions as to transportation.

Because of lack of other fastening materials, spar-bridges are often assembled by lashing the trestles. In such cases it is impossible to use "rider" sills and caps, placed on the tops and bottoms of the posts; hence "ledgers," attached a short distance below the tops and above the bottoms of the posts, are employed. Stringers are also lashed, and decking is secured by means of side-rails placed over and lashed down to the outer stringers. The term "spar-bridge" is generally used to describe such a characteristically military structure. Whenever possible, however, even bridges of round timber should be roughly framed, have rider caps and sills, and be fastened with bolts, spikes, or dogs.

A special form of spar-bridge, known as the lock-spar, is a structure in which the trestle bents are tilted towards each other and locked together or to a frame placed between two bents (double lock). These are of very limited application, but may occasionally be used to advantage in spanning a deep gorge. With the double lock the bridge is practicable in spans up to 50 ft. for moderate loads.

Spar-bridges, especially when lashed, if of necessity employed in the first instance for a hasty crossing, should be replaced promptly by a better type of bridge, as they are unsuitable for continued heavy traffic.

Pile Trestles

Framed bents, as they are so easily erected without plant, will be generally employed when the bottom is firm enough to support them, and when scour can be prevented. When the bottom is of very low bearing-power, or subject to scour, as in the case of soft mud or shifting sand, piles will be required. Pile bents have the disadvantage of necessitating longer pieces, more time, and the use of a pile driver for their erection. However, they are required in many situations, especially for railroad bridges, where even a slight settlement, such as might result from the use of framed bents, would be dangerous. Accordingly, a number of efficient portable pile-drivers should form part of the equipment of an army; and, when such are available, pile trestles will often be preferred to framed ones. Pile-trestle bents are subject to the same economic considerations as framed-trestle bents, except that, being stiffer, they require less bracing.

Trusses

For military purposes the truss has all the economic advantages it possesses in civil use. It is employed to reduce the number of trestle bents, or other type of piers, when the construction of the said piers is, for any reason, particularly difficult. Trusses are also used to give greater clearance as a measure of flood protection, or to span deep chasms or streams without intermediate supports.

Military trusses must usually be erected without falsework, and with