backed under it: the lanre end is then raisedin the same manner, and placed upon the bodyof the waggon, the trail of which is lashed tothe tree, and to the limber hook. The smallend of the tree is placed upon the limber, thatthe tree may move easily upon the block whenthe carriage turns out of the straight direction;which it will do the better from the weight andcontact being less, than if the large end wereput upon the limber.

Bridges of rafts being in general only tempo-rary expedients, resorted to upon sudden emer-gencies, the construction should be such as toeffect the purpose with the least means con-sistent with safety; and this we are now toconsider.

A fir tree of 4'33 feet mean girth, and 30 inlength, contains 44'9 cubic feet, and will float1262lbs. Six of these, (figs. 1 and 2, plate 7,)will float 7572 lbs. Deduct the weight of 5baulks of oak, 4 inches wide, 6 inches deep, and26 feet long; also the weight of 2-inch planks,sufficient for a floor 10 feet wide, and 12 long ;—in all about 1981 lbs. and there will remain5591 lbs. for the weight which each raft is ca-pable of bearing. If the rafts are 12 feet dis-tant from centre to centre, and the whole bridgecovered with infantry filing across, each will