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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

      In Mr. Francis's presses, the dies between which the sheets of iron or copper are pressed; are directly above the four cylinders which we have described, as will be seen by referring once more to the drawing. The upper die is fixed—being firmly attached to the top of the frame, and held securely down by the rows of iron pillars on the two sides, and by the massive iron caps, called platens, which may be seen passing across at the top, from pillar to pillar. These caps are held by large iron nuts which are screwed down over the ends of the pillars above. The lower die is movable. It is attached by massive iron work to the ends of the piston-rods, and of course it rises when the pistons are driven upward by the pressure of the water. The plate of metal, when the dies approach each other, is bent and drawn into the intended shape by the force of the pressure, receiving not only the corrugations which are designed to stiffen it, but also the general shaping necessary, in respect to swell and curvature, to give it the proper form for the side, or the portion of a side, of a boat.

      It is obviously necessary that these dies should fit each other in a very accurate manner, so as to compress the iron equally in every part. To make them fit thus exactly, massive as they are in magnitude, and irregular in form, is a work of immense labor. They are first cast as nearly as possible to the form intended, but as such castings always warp more or less in cooling, there is a great deal of fitting afterward required, to make them come rightly together. This could easily be done by machinery if the surfaces were square, or cylindrical, or of any other mathematical form to which the working of machinery could be adapted. But the curved and winding surfaces which form the hull of a boat or vessel, smooth and flowing as they are, and controlled, too, by established and well-known laws, bid defiance to all the attempts of mere mechanical motion to follow them. The superfluous iron, therefore, of these dies, must all be cut away by chisels driven by a hammer held in the hand; and so great is the labor required to fit and smooth and polish them, that a pair of them costs several thousand dollars before they are completed and ready to fulfill their function.

      The superiority of metallic boats, whether of copper or iron, made in the manner above described, over those of any other construction, is growing every year more and more apparent. They are more light and more easily managed, they require far less repair from year to year, and are very much longer lived. When iron is used for this purpose, a preparation is employed that is called galvanized iron. This manufacture consists of plates of iron of the requisite thickness, coated on each side, first with tin, and then with zinc; the tin being used simply as a solder, to unite the other metals. The plate presents, therefore, to the water, only a surface of zinc, which resists all action, so that the boats thus made are subject to no species of decay. They can be injured or destroyed only by violence, and even violence acts at a very great disadvantage in attacking them. The stroke of a shot, or a concussion of any kind that would split or shiver a wooden boat so as to damage it past repair, would only indent, or at most perforate, an iron one. And a perforation even, when made, is very easily repaired, even by the navigators themselves, under circumstances however unfavorable. With a smooth and heavy stone placed upon the outside for an anvil, and another used on the inside as a hammer, the protrusion is easily beaten down, the opening is closed, the continuity of surface is restored, and the damaged boat becomes, excepting, perhaps, in the imagination of the navigator, as good once more as ever.

      Metallic boats of this character were employed by the party under Lieut. Lynch, in making, some years ago, their celebrated voyage down the river Jordan to the Dead Sea. The navigation of this stream was difficult and perilous in the highest degree. The boats were subject to the severest possible tests and trials. They were impelled against rocks, they were dragged over shoals, they were swept down cataracts and cascades. There was one wooden boat in the little squadron; but this was soon so strained and battered that it could no longer be kept afloat, and it was abandoned. The metallic boats, however, lived through the whole, and finally floated in peace on the heavy waters of the Dead Sea, in nearly as good a condition as when they first came from Mr. Francis's dies.

      The seams of a metallic boat will never open by exposure to the sun and rain, when lying long upon the deck of a ship, or hauled up upon a shore. Nor will such boats burn. If a ship takes fire at sea, the boats, if of iron, can never be injured by the conflagration. Nor can they be sunk. For they are provided with air chambers in various parts, each separate from the others, so that if the boat were bruised and jammed by violent concussions, up to her utmost capacity of receiving injury, the shapeless mass would still float upon the sea, and hold up with unconquerable buoyancy as many as could cling to her.7

      A curious instance occurred during the late war with Mexico which illustrates the almost indestructible character of these metallic boats.

      The reader is probably aware that the city of Vera Cruz is situated upon a low and sandy coast, and that the only port which exists there is formed by a small island which lies at a little distance from the shore, and a mole or pier built out from it into the water. The island is almost wholly covered by the celebrated fortress of St. Juan de Ulloa. Ships obtain something like shelter under the lee of this island and mole, riding sometimes at anchor behind the mole, and sometimes moored to iron rings set in the castle walls. At one time while the American forces were in possession of the city, an officer of the army had occasion to use a boat for some purpose of transportation from the island to the shore. He applied to the naval authorities in order to procure one. He was informed that there was no boat on the station that could be spared for such a purpose. In this dilemma the officer accidentally learned that there was an old copper life-boat, lying in the water near the castle landing, dismantled, sunk, and useless. The officer resolved, as a last resort, to examine this wreck, in hopes to find that it might possibly be raised and repaired.

      He found that the boat was lying in the water and half filled with rocks, sand, and masses of old iron, which had been thrown into her to sink and destroy her. Among the masses of iron there was a heavy bar which had been used apparently in the attempt to punch holes in the boat by those who had undertaken to sink her. These attempts had been generally fruitless, the blows having only made indentations in the copper, on account of the yielding nature of the metal. In one place, however, in the

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