If the spectacle-lens be of flint-glass and doubly convex, each of its faces should have a curvature of not greater than 6 1/2 tenths of an inch, nor more than 8 1/2 tenths of an inch in radius:  within these limits, it is practicable to obtain perfectly distinct vision under water by pressing the spectacles forwards or backwards to a moderate degree.  Lenses of these high magnifying powers are sometimes sold by spectacle-makers, for persons who have undergone an operation for cataract.  I have tried, but hitherto without much success, to arrange the fittings by which the lenses are secured so that by a movement of the jaw or by an elevation of the eyebrows, I could give the necessary adjustment of the glasses, leaving my hands free for the purpose of swimming. (See also, under “Fishing;’ ‘To see Things deep under Water.’)

RAFTS AND BOATS.

Rafts.—­Rafts of Wood.—­Rafts are made of logs of wood, held together by pairs of cross-bars, one of each pair lying above the raft and the other below; then, the whole may be made quite firm by a little judicious notching wherethe logs cross, and a few pegs and lashings.  Briers, woodbines, etc., will do for these.  If the logs are large, they should be separately launched into the river, and towed into their proper places.  Outriggers vastly increase the stability of a raft.  The raft-fastening in common use is shown in f. 1:  it is a stout, little wand, bent over the cross piece, and wedged into holes in the framework.

[Sketch of raft].

[Fig 1 and Fig 2 show fastening arrangements].

The rafts of European rivers are usually built on shore, and launched into the water:  three slides are laid for the purpose, on the sloping bank of the river; upon these are laid the four poles, secured together by their ends, which are to form the framework of the raft (fig. 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.