2).
Other poles are put in between, until the whole is complete.
Bamboo rafts.--Where bamboo is plentiful, it is preferable to any other
material for rafts. A few bamboos lashed into the shape of an ordinary
field gate, but with two diagonals, and with handfuls of grass thrown on
to make a platform, is very buoyant and serviceable.
Floating power of various Woods.--The floating power of a raft depends on
the buoyancy of the wood of which it is made. I give, in a Table (p. 90),
a list of the specific gravities of a few well-known woods; and have
annexed to them a column of what may be called their "specific floating
powers."*
[Footnote] *Specific floating power = (1/Spec. Gr.)--1. (Mem., the Table
of these, in previous editions is incorrect.) Burden = weight of raft x
specific floating power. Weight of wood required to support a given burden =
Burden x (Spec. Gr./1-Spec. Gr.); the last column gives the latter
factor.
Hence, to find the actual floating power of a raft, it is simply
necessary to multiply its weight into the specific floating power of the
wood of which it is made.
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.
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