88 Art of Travel.
If the spectacle-lens be of flint-glass and doubly convex, each of its faces should have a curvature of not greater than 62 tenths of an inch, nor more than 82 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 spectaclemakers, 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 aia 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 ; " I 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 where the logs cross, and a few pegs and lashings. Briers, woodbines, &c.,
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