of the shipwright; for there must not only be propriety of dimensions, but suitability of materials. In other words, the dimensions may be out of proportion for the purpose for which the vessel is required; or the scantling may be stouter or slighter, the timber greener or better seasoned, or specifically heavier or lighter.

Having thus briefly considered the subject of the ship's capacity, it is desirable to treat of that of Shape, a question which involves the laws of hydraulics. We know that water in motion presses more or less upon an obstacle exposed to its action, according to the position and form of that obstacle; and that an object moves through water at rest with more or less facility, according to its shape. The hand placed in running water perpendicularly or horizontally, offers different amounts of resistance; and, moved in that position through still water, encounters different amounts of resistance. The great question then to determine was that of the form of a vessel which would oppose to the water the least possible resistance; and to that question have our various schools of ship-building applied themselves with more or less success. Bluff bows and fine runs, "the dolphin's head and mackerel's tail," were for a long period considered the main desiderata. The "bruise waters" gave way to "long bows and full afterbodies;" the "long hollow floor" to the "peg-top" pattern; narrow "cribs to excessive breadth of beam; light draught of water to deep draught; and enormous quantities of ballast to none whatever. Each and all of these systems have had their respective advocates; all bent upon ascertaining that particular form which could be most easily propelled, or in other words offer the least resistance.

Numerous experiments were made with a view of solving this question. Pieces of wood of equal bulks, but of different shapes, were drawn across the surface of a water trough by equal powers of traction, and that which performed the transit in the shortest time was considered to have furnished the desired model. But when this shape came to be tested by the ruder trial of practice, it was found to be a success only under partial circumstances. In strong breezes and smooth water it succeeded to admiration; but in opposite circumstances, and with a propelling power acting in any other direction than the parallel, it proved to be a failure. The reason of this soon became obvious. Sufficient

consideration had not been bestowed on the fact that a vessel's lines and shapes alter relatively to the state of the water through which she moves. In other words, the ship which opposed comparatively little resistance to a smooth sea, presented unexpected resistance to a broken one. The forms of fish again were supposed to supply the desired pattern; but here it was overlooked that the rapidity of the motion of the fish probably depends quite as much on muscular power, as upon peculiarity of shape. In the case of birds, for example, it is notorious that speed of flight does not always depend on equality or even similarity of bulk. All this tends to prove, that while great importance is to be attached to the question of shape, no degree of excellence in that respect will necessarily ensure rapidity in sailing. That depends on many considerations besides; such as the adaptation of the weight to be carried to capacity and form; the distribution of that weight in the vessel; harmony between shape and rig; and, above all, the handling of the ship. Craft of undoubted speed have ceased to be constant winners after a change of owners; and some of the old school ships have held their own with modern ones. That may be the result either of seamanship or of trim, or of both, - questions which will come more appropriately under notice in a different section.

One fact has been established by experiments made in reference to the question of form in our own days. It has been proved that (due attention being paid to construction) rapidity of sailing is in proportion to increase of bulk. Of this our own navy exhibits many illustrations. To select one-the Duke of Wellington is equally remarkable for speed and handiness; proving that as far as her dimensions have gone, increase of size promotes rather than hinders velocity. But the most remarkable application of the theory which modern times furnishes is presented in the instance of the "Great Eastern," now in process of construction.

In the case of this vessel, it has been carried to such a singular extreme as to warrant notice. Her distinguishing feature is length. The Atlantic waves are calculated rarely to exceed 28 feet in height, and 600 in length, whilst in moderate gales, they are but 300, and in fresh weather about 120 feet in length; and it is expected that as the ship will be water borne, even in extreme circumstances, by two waves at the same time, she will

bestride the heaviest seas, and thus avoid those descents into the troughs which compel shorter vessels to traverse additional space, and so lose time in struggling upward against a powerfully resisting medium. The annexed sketch (Fig. 4.) is on a scale of proportion.

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The ship is of iron, 692 feet long over all, 83 feet beam, 114 feet across the paddle-boxes, and nearly 60 feet deep the sides are tied together by double decks, and 10 transverse bulk-heads. As to the comparative excellence of wood and iron in shipbuilding, Mr. Fairbairn remarks: 'Iron ships of the same external dimensions as wooden are both lighter and stronger, and consequently have more space for cargo. Their original cost is less, but their comparative durability is not yet decided." But the whole of this subject of ship-building, with reference to the question of fluid resistance, is admitted by the most eminent writers and the more experienced practical men to be one, even now, of extreme difficulty. The creation and failure of the several schools of naval architecture are but so many steps towards the ultimate discovery of the right form according to which ships should be constructed. Builders of high reputation have carried out their own theories only to arrive at the discovery that results did not correspond to the expectation. Naval men have done the same, and with the same result. Both -the scientific architect and the practical seaman—have found out that scientific knowledge, and practical experience, separately, have failed to solve the problem. It was probably this which led Mr. Fincham to say: "The theory of resistances remains in about the same condition that it was left by Du Buat, and experiment has done little since the date of Colonel Beaufoy's. labours. . . And until the perfection of a theory of naval architecture shall have been sought in new inquiries on this subject, the naval architect must look chiefly to the navy itself for suggestions to improve the forms of ships."

This last acknowledgment appears to suggest the only probable plan for the construction of vessels answering under trial to the theoretic anticipations of their designers. That which has failed under separate talents, may possibly succeed under conjoint ones. Abstract science and personal experience may, when combined and assistant to each other, give us the great desideratum. Since the above quotation was written, we

have seen its idea reduced to fact in one of our own naval departments. Practical knowledge and seaman-like experience preside over one of our most important Boards; and the modern ships which grace our navy are the best illustrations of the soundness of the remark of the distinguished architect whose views I have cited.

CHAP. III.

WINDS.

THE Composition and extent of atmospheric air, its nature as far as regards subjection to the law of gravity, the absence of cohesive attraction among its particles, and their equality of pressure at every point, have already been noticed. We have now to consider its weight, compressibility, elasticity, mobility, and the different purposes to which these properties are applied in connection with naval architecture. Air is of different degrees of density throughout; a cubic foot weighing on the average 1.2 oz. The particles of the lower strata, yielding to the pressure of those above, become so much reduced in bulk that there is usually as much air within 3 miles of the earth as there is altogether beyond that altitude (Fig. 5.), and the whole body exerts a pressure of 15 pounds on every square inch of the globe's surface, equal to 15 tons on the human frame. Air expands in proportion to the diminution of pressure, and when it is entirely extracted from a close vessel, a vacuum is formed. To compress air into twice its density requires a force of 15 pounds 3 ounces, into four times its density, 45 pounds 9 ounces, and so for other diminutions less one. In a thoroughly exhausted receiver animals soon expire, vegetation and combustion cease, gunpowder will not explode, smoke descends, bodies of different densities fall with equal rapidity, water and other fluids turn to vapours, sound is indistinctly heard, and heat imperfectly transmitted. The nature of a vacuum, the elasticity of air, and the force of atmospheric pressure will readily be understood by alternately closing and opening the vent during the common operation of washing out a gun barrel.