Harper's New Monthly Magazine, Vol. 3, July, 1851

by Various

In many cases of distress and 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 to 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.

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 middle of the picture, from the forcing-pump to the great cylinders on the right hand. Here the water presses upward upon the under surfaces of pistons working within the great cylinders, with a force proportioned to the ratio of the area of those pistons compared with that of one of the pistons in the pump. Now the piston in the force-pump is about one inch in diameter. Those in the great cylinders are about twelve inches in diameter, and as there are four of the great cylinders the ratio is as 1 to 576.[5] This is a great multiplication, and it is found that the force which the men can exert upon the piston within the small cylinder, by the aid of the long lever with which they work it, is so great, that when multiplied by 576, as it is by being expanded over the surface of the large pistons, an upward pressure results of about eight hundred tons. This is a force ten times as great in intensity as that exerted by steam in the most powerful sea-going engines. It would be sufficient to lift a block of granite five or six feet square at the base, and as high as the Bunker Hill Monument.

Superior, however, as this force is, in one point of view, to that of steam, it is very inferior to it in other respects. It is great, so to speak, in intensity, but it is very small in extent and amount. It is capable indeed of lifting a very great weight, but it can raise it only an exceedingly little way. Were the force of such an engine to be brought into action beneath such a block of granite as we have described, the enormous burden would rise, but it would rise by a motion almost inconceivably slow, and after going up perhaps as high as the thickness of a sheet of paper, the force would be spent, and no further effect would be produced without a new exertion of the motive power. In other words, the whole amount of the force of a hydraulic engine, vastly concentrated as it is, and irresistible, within the narrow limits within which it works, is but the force of four or five men after all; while the power of the engines of a Collins' steamer is equal to that of four or five thousand men. The steam-engine can do an abundance of great work; while, on the other hand, what the hydraulic press can do is very little in amount, and only great in view of its extremely concentrated intensity.

Hydraulic presses are consequently very often used, in such cases and for such purposes as require a great force within very narrow limits. The indentations made by the type in printing the pages of this magazine, are taken out, and the sheet rendered smooth again, by hydraulic presses exerting a force of twelve hundred tons. This would make it necessary for us to carry up our imaginary block of granite a hundred feet higher than the Bunker Hill Monument to get a load for them.[6]