IN TIME OF WAR.

necessity for passing without delay a deep ditch less than 20feet in breadth, six or seven beams may be taken from thenearest houses or villages : when there are none within reach,some trees measuring 4 or 5 feet in girth should be instantlyfelled, and when stripped of the branches laid across theditch, at about ten inches distant from each other; andupon them a layer of boughs, or any wood that can be moreeasily procured, 12 or 14 feet in length, and 4 or 5 inchesin thickness ; and the surface made smooth with a coat ofearth or turf. If no trees large enough for this purpose canbe found, or the ditch is more than 20 feet wide, a firm andsubstantial bridge may be made by placing a trestle in themiddle of the ditch, and resting the ends of the trees uponit; or in lieu of the trestle, a thick piece of timber supportedat each side by two beams fixed in the banks; this crosspiece will be strong enough to support the ends of the treesresting on it.

36. Bridges on trestles can only be laid over canals, orrivers that are not subject to floods; as in cafe of a largeswell they would be in danger of being carried away. Thetrestles are generally made on the spot of well-seasoned tim-ber that squares from 8 to ten inches. Their height abovethe surface of the water should be at least 3 feet, and the crosspiece that forms the head 12 or 14 feet long: the distancebetween every (wo trestles is 14 or 16 feet; and when thebottom of the canal or river is not rocky or gravelly, theirfeet are nailed upon square wooden frames, to prevent theirsinking. Six pieces of timber, 7 or 8 inches square, calledbaulks , are laid on the trestles, and the ends nailed down ;the whole breadth from outside to outside of the baulks isfrom 10 to 12 feet, and over them are two layers of planksor chesses if inch in thickness, and 12 or 14 feet in length."W hen the timber is green, the thickness should be increasedby -f ; and, in lieu of planks, any wood that is 4 or 5 inchesthick may be nailed on, and covered with turf or earth as

before-mentioned (35).

37. If the river be subject to freshes or very deep, thebridge is made of boats fastened together two and two bysix well-seasoned baulks from 6 to 8 inches square, and ofsuch a length, that the ends rest on the outer gunnel ofeach boat, after leaving an interval of eight feet betweenevery two boats ; they are nailed down to tire gunnels ofthe boats, and covered with planks two inches thick and 14

feet.