disaster befalling ships on the coast, it
is not necessary to use the car, the state of the sea being such that it
is possible to go out in a boat, to furnish the necessary succor. The
boats, however, which are destined to this service must be of a peculiar
construction, for no ordinary boat can live a moment in the surf which
rolls in, in storms, upon shelving or rocky shores. A great many
different modes have been adopted for the construction of surf-boats,
each liable to its own peculiar objections. The principle on which Mr.
Francis relies in his life and surf boats, is to give them an extreme
lightness and buoyancy, so as to keep them always upon the _top_ of the
sea. Formerly it was expected that a boat in such a service, must
necessarily take in great quantities of water, and the object of all the
contrivances for securing its safety, was to expel the water after it
was admitted. In the plan now adopted the design is to exclude the water
altogether, by making the structure so light and forming it on such a
model that it shall always rise above the wave, and thus glide safely
over it. This result is obtained partly by means of the model of the
boat, and partly by the lightness of the material of which it is
composed. The reader may perhaps be surprised to hear, after this, that
the material is _iron_.

Iron--or copper, which in this respect possesses the same properties as
iron--though _absolutely_ heavier than wood, is, in fact, much lighter
as a material for the construction of receptacles of all kinds, on
account of its great strength and tenacity, which allows of its being
used in plates so thin that the quantity of the material employed is
diminished much more than the specific gravity is increased by using the
metal. There has been, however, hitherto a great practical difficulty in
the way of using iron for such a purpose, namely that of giving to these
metal plates a sufficient stiffness. A sheet of tin, for example, though
stronger than a board, that is, requiring a greater force to break or
rapture it, is still very _flexible_, while the board is stiff. In other
words, in the case of a thin plate of metal, the parts yield readily to
any _slight_ force, so far as to bend under the pressure, but it
requires a very great force to separate them entirely; whereas in the
case of wood, the slight force is at first resisted, but on a moderate
increase of it, the structure breaks down altogether. The great thing t