Thus, a raft of 12 logs of larch, averaging 30 lbs. each, weighs 360 lbs.; this multiplied by .47, is equal to 169 lbs. very nearly, which is the weight the raft will support without sinking.  Poplar is the lightest on the list.

Specific Specific Factors to be multiplied
Gravities.  Floating Powers. into burden to find
weight of raft just
able to support it. 
Alder........ .80 .25 4.0 Ash........... .85 .18 5.7 Beech......... .85 .18 5.7 Elm....... .59 to .80 .70 to .25 1.4 to 4.0 Fir...... .47 to .60 1.13 to .66 0.9 to 1.5 Larch..... ... .53 .89 1.1 Oak........... .75 .33 3.0
" heart of.. 1.17 sinks cannot be used
Pine..... .40 to .63 1.50 to .60 0.7 to 1.7 Poplar........ .38 1.63 0.6 Willow........ .59 .70 1.4

Examples:  —­ a raft of alder, weighing 200 lbs., would just support 200 x .25 or 50 lbs.  A burden of 100 lbs. would require a raft of alder, weighing not les than 100 x 4.0, or 400 lbs. to support it.  Burning down Trees.—­Where there are no means at hand to fell trees, they should be burnt down; two men may attend to the burning of twenty trees at one and the same time.  When felled, their tops and branches, also, are to be trimmed by fire. (See “Hutting Palisades.”)

Reed Rafts.—­Mr. Andersson, in exploring the Tioughe River, in South Africa, met with two very simple forms of rafts:  the one was a vast quantity of reeds cut down, heaped into a stack of from 30 to 50 feet in diameter, pushed out into the water, and allowed to float down stream:  each day, as the reeds became water-logged, more were cut and thrown on the stack:  its great bulk made it sure of passing over shallow places; and when it struck against “snags,” the force of the water soon slewed it round and started it afresh.  On an affair of this description, Mr. Andersson, with seven attendants, and two canoes hauled up upon it, descended the river for five days.  The second reed raft was a small and neat one, and used for ferries; it was a mattress of reeds, 5 feet long, 3 broad, and some 8 inches thick, tied together with strips of the reeds themselves; to each of its four corners was fixed a post, made of an upright faggot of reeds, 18 inches high; other faggots connected the tops of the posts horizontally, in the place of rails:  this was all; it held one or two men, and nothing but reeds or rushes were used in its construction.

Rafts of distended Hides.—­“A single ox-hide may be made into a float capable of sustaining about 300 lbs.; the skin is to be cut to the largest possible circle, then gathered together round a short tube, to the inner end of which a valve, like that of a common pair of bellows, has been applied; it is inflated with bellows, and, as the air escapes by degrees, it may be refilled every ten or twelve hours.” (’Handbook for Field Service.’)