shaped portion of air, which effects the required refraction. In the better kind of lenses the two surfaces of the glass are ground so that both shall be of exactly the same radius of curvature.
"All the lenses of our optical instruments for subaqueous use, microscopes, telescopes, &c., are made in this way. For spectacles these lenses are especially convenient, for while they are lenses of the required power under water, they have no refractive power in air, and consequently may be worn both in and out of the water; in the former case causing, in the latter not preventing, perfect vision.
"The next question that seems to have occupied the attention of the pioneers of our submarine life was that of obtaining the needful supply of air below the water; for it was extremely awkward, and indeed quite fatal to anything like residence under the water, to be obliged to come to the surface every minute or so, to get a chestful of air.
"This difficulty was overcome by the distribution of air-pipes throughout the area occupied.
"At first, as you may imagine, this was done on a very small scale. It was originally not expected that it would ever be possible to remain permanently below the water, but only for a few hours during the extreme heat of the day.
"But as time went on, the ingenuity of our engineers and mechanicians who kept themselves well posted in all the improvements in their arts that were made in Europe—for works on mechanical science formed a considerable proportion of the books rescued from the wrecks that were more frequent formerly than now,—overcame all difficulties; and as the area of inhabited space yearly extended, the net-work of air-pipes gradually spread over every portion of the vast inland sea