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.