ONE of the chief difficulties which has been experienced by the builders of life-boats, has been that of making the air-cases and compartments, which form their extra buoyancy, perfectly water-tight. Different expedients have been tried, but without success :—wood —metals—patent artificial materials of different kinds—all have been adopted, but each having some drawback. Wood expands with moisture and shrinks from excessive dryness, which opens the joints and seams; metals corrode, and to have sufficient strength must be made too heavy.

Gutta percha would doubtless answer for detached cases, as being quite impervious to water, but, to be of sufficient thickness to have the requisite solidity and strength, is very expensive, and also heavy. The most prevailing mode of constructing detached air-cases is, however, of wood, carefully made, and covered with sail-canvas sewn on and well-painted. This is the manner practised in the life-boats on the Norfolk and Suffolk coasts; but, after a while, they are liable to become leaky, as if the canvas gets cut or chafed through, the water will force its way between it and the wood, and will penetrate the joints of the case. A better and safer mode of covering detached aircases, and which makes them perfectly water-tight, has, however, now been introduced, which we will briefly describe for the information of constructors of life-boats who may not be acquainted with it.

The cases should be made of a light, dry wood, sufficiently strong to bear a severe blow without injury, but yet not heavier than is necessary to give the required strength, and should be carefully put together.

The next process is to spread over them a thick coating of the now well-known material, "marine glue." Strips of light sail-canvas are then placed over the joints, and, being pressed on their outer side by a heated laundress's iron, the glue is drawn through the substance of the canvas, which becomes immoveably attached to the case; another coating of the glue is now spread over these strips, and the whole case is then covered with canvas in the same manner, There is no need of sewing, as the canvas will adhere with the utmost tenacity to the wood; forming, even before painting, a perfectly waterproof covering in itself, and having two thicknesses of the canvas over all the seams and corners. Two or more coats of paint are then laid over all.

A case of this kind will be perfectly and durably water-tight, and there will be no need of valves, or other contrivance, to let out water which might have leaked into it.

Some practice is required to perform the operation well, and it is necessary to be careful that, whilst the iron is sufficiently heated to thoroughly melt and draw the glue through the canvas, it must not be hot enough to burn either the one or the other.

This mode of covering is especially applicable to detached air-cases or boxes; but we are of opinion, that, to as great an extent as practicable, the extra buoyancy of a life-boat should be formed of such, as being less liable to damage, from concussion or other cause, than when built into the boat.