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to destroy the bridge.* For this purpose threeor more piles, set in a triangular figure, shouldbe driven above each raft, boat, trestle, or rowof piles; and fastened together by strong cap-pieces of timber, well braced and planked, soas to form a powerful buttress or breakwater.When piles are to be driven for this purpose,the pile-engines must be set on boats, or rafts,if the river be so deep that a scaffold on trestlescannot be applied.

Pile-driving for bridges, will generally beundertaken for the more permanent communi-cations of an army, and be executed, at compa-rative leisure, with fit and proper means. Ishall, therefore, confine my observations at pre-sent, to those temporary constructions whichmay be suddenly required in the movementsof armies; and suggest a few expedients andsubstitutes that may be resorted to upon occa-sions which cannot be furnished with regularmeans, and when but little time can be allowedfor the execution of the work.

The first thing to be considered is a pile-en-gine, for which it is generally more difficult toprocure a block of sound, heavy wood, properfor a ram, than to get beams four to five inchessquare, for making the frame.

* Section ii. p. 02, and Section iv. p. 102.