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Swimming.   87

To float a Wagon across a River.-It must be well ballasted, or it will assuredly capsize : the heavy contents should be stowed at the bottom ; the planking lashed to the axletrees, or it will float away from them ; great bundles of reeds and the empty water-vessels should be made fast high above all, and then the wagon will cross without danger. When it is fairly under weigh, the oxen will swim it across, pulling in their yokes.

Water Speetaoles.-When a man opens his eyes under water, he can see nothing distinctly ; but everything is as much out of focus, as if he looked, in air, through a pair of powerful spectacles that were utterly unsuited to him. He cannot distinguish the letters of the largest print in a newspaper advertisement ; he cannot see the spaces between the outstretched fingers, at arm's length, in clear water ; nor at a few inches' distance, in water that is somewhat opaque. I read a short paper on this subject, at the British Association in 1865, in which I showed the precise cause of this imperfection of vision and how it might be remedied. If the front of our eyeballs had been flat, we should have had the power of seeing under water as clearly as in air ; but instead of being flat, they are very convex, consequently our eye stamps a concave lens of high power into the water, and it is the seeing through this concave eyeglass which our eyeball makes for itself, that causes the indistinctness of our vision. Knowing the curvature of the eyeball, it is easy to calculate (as I did in the memoir mentioned above) the curvature of a convex lens of flint-glass that should, when plunged into water, produce effects of an exactly equal and contrary value, exactly neutralizing the effects of the concave eyeglass of water, if it were held immediately in front of the pupil of the eye. I have made several experiments with a view to obtaining serviceable spectacles, for seeing under water. The result is as follows :-experience has shown the distance from the eyeball at which spectacle-glasses can be most conveniently placed ; now at that distance, the joint effect of the concave water-lens and the convex glass spectacle-lens, is to produce an operaglass of exceedingly low magnifying power, that requires a small adjustment for accurate definition at different distances.