o
be desired therefore in a material for the construction of boats is to
secure the stiffness of wood in conjunction with the thinness and
tenacity of iron. This object is attained in the manufacture of Mr.
Francis's boats by _plaiting_ or _corrugating_ the sheets of metal of
which the sides of the boat are to be made. A familiar illustration of
the principle on which this stiffening is effected is furnished by the
common table waiter, which is made, usually, of a thin plate of tinned
iron, stiffened by being turned up at the edges all around--the upturned
part serving also at the same time the purpose of forming a margin.

The plaitings or corrugations of the metal in these iron boats pass
along the sheets, in lines, instead of being, as in the case of the
waiter, confined to the margin. The lines which they form can be seen in
the drawing of the surf-boat, given on a subsequent page. The idea of
thus corrugating or plaiting the metal was a very simple one; the main
difficulty in the invention came, after getting the idea, in devising
the ways and means by which such a corrugation could be made. It is a
curious circumstance in the history of modern inventions that it often
requires much more ingenuity and effort to contrive a way to _make_ the
article when invented, than it did to invent the article itself. It was,
for instance, much easier, doubtless, to invent pins, than to invent the
machinery for _making_ pins.

The machine for making the corrugations in the sides of these metallic
boats consists of a hydraulic press and a set of enormous dies. These
dies are grooved to fit each other, and shut together; and the plate of
iron which is to be corrugated being placed between them, is pressed
into the requisite form, with all the force of the hydraulic piston--the
greatest force, altogether, that is ever employed in the service of man.

[Illustration: THE HYDRAULIC PRESS.]

The machinery referred to will be easily understood by the above
engraving. On the left are the pumps, worked, as represented in the
engraving, by two men, though four or more are often required. By
alternately raising and depressing the break or handle, they work two
small but very solid pistons which play within cylinders of
corresponding bore, in the manner of any common forcing pump.

By means of these pistons the water is driven, in small quantities but
with prodigious force, along through the horizontal tube seen passing
across, in the mi