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tempted, and may be quickly formed of cart, orany other carriage wheels, and a few light spars.

The purpose for which the late Sir William Congreve designed this expedient was, to ac-company columns of attack, in the assaults offield works. The beam A B, fig. 20, having alight floor on the part B C, is supported, duringthe advance, by ropes: the beams C D are at-tached to A B in C, and the other ends D restupon A E. To cross a wet ditch, the machineis run up and pushed into the ditch until E, theend of the beam A E, rest on the edge of thecounterscarp; the beam A B is then let fall,when a bridge will be formed as representedin fig. 21.

This machine may also be found useful inpassing deep, dry ditches, by facilitating thedescent—forming a passage over palisades,chevaux-de-frize, &c. or serving as scaling lad-ders to ascend a rampart; or, by means ofcarriage bridges upon this principle, deep, nar-row ditches may be crossed, without descend-ing into them, if the beams A B, figs. 22, 23, bemade long enough to reach to the crest of theexterior slope, or escarp, before the wheels quitthe edge of the counterscarp. As the ditches offield-works are commonly defended by capon-niers, and reverse fire from casemated counter-