it being that a paddle-shaft placed at a given height from the bottom of the vessel will be enabled to receive a longer stroke of piston than by any other arrangement now in use. A more

Fig. 128.

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compact and firm connection of the cylinder with the crankshaft bearings is effected by it, and a cylinder of much greater

diameter may be applied by which the expansive action of steam may be more fully brought into play; and a more direct action of the steam-power on the crank with a less weight of materials and a greater economy of space may be obtained than by any of the arrangements of marine engines hitherto used.

(221.) Mr. Francis Humphrys has obtained a patent for a form of marine engine, by which some simplification of the machinery is attained, and the same power comprised within more limited dimensions. In this engine there is attached to the piston of the cylinder, instead of a piston-rod, a hollow casing D D (fig. 130.), which moves through a stuffing-box G, constructed in a manner similar to the stuffing-box of a pistonrod. In the figure, this casing is presented in section, but

its form is that of a long narrow slit, or opening, rounded at either end as exhibited in the plan (fig. 131) of the cylinder

Fig. 131.

cover. The crank c is driven by the other end of the connecting rod H, the crank-shaft being immediately above the centre of the piston and the connecting rod passing through the oblong opening D, and descending into the hollow piston-rod it is attached to an axis I at the bottom of the piston. A box or cover K K encloses the cross-piece or axis I with its bearings, and is

attached so as to be steam-tight to the bottom of the piston. A hollow space L L is cast in the bottom of the cylinder for the reception of the box K K, when the piston is at the bottom of the cylinder.

By this arrangement the force by which the piston is driven in its ascent and descent is communicated to the connecting rod, not, as usual, through the intervention of a piston-rod, but directly from the piston itself by the cross-pin 1, and from thence to the crank c, which it drives without the intervention of beams, cross-heads, or any similar appendage.

The slide-valves regulating the admission and eduction of steam are represented at a; the rod of the air-pump is shown at d, being worked by a crank placed on the centre of the great crank shaft.*

(222.) To obtain from the moving power its full amount of mechanical effect in propelling the vessel, it would be necessary that its force should propel, by constantly acting against the water in a horizontal direction, and with a motion contrary to the course of the vessel. No system of mechanical propellers has, however, yet been contrived capable of perfectly accomplishing this. Patents have been granted for many ingenious mechanical combinations to impart to the propelling surfaces such angles as appeared to the respective contrivers most advantageous. In most of these the mechanical complexity has formed a fatal objection. No part of the machinery of a steam-vessel is so liable to become deranged at sea as the paddle-wheels; and, therefore, that simplicity of construction which is compatible with those repairs which are possible on such emergencies is quite essential for safe practical use.

The ordinary paddle-wheel, as has been already stated, is a wheel revolving upon a shaft driven by the engine, and carrying upon its circumference a number of flat boards, called paddle-boards, which are secured by nuts and braces in a fixed position; and that position is such that the planes

* Engines on a very large scale constructed upon this principle are said to be in process of construction for an iron steam-vessel of great tonnage, which is in preparation for the New York passage. It is said that the cylinders of these engines will be one hundred and twenty inches in diameter.

of the paddle-boards diverge nearly from the centre of the shaft on which the wheel turns. The consequence of this arrangement is that each paddle-board can only act in that direction which is most advantageous for the propulsion of the vessel when it arrives near the lowest point of the wheel. In fig. 132. let o be the shaft on which the common paddle

wheel revolves; the position of the paddle-boards are represented at A, B, c, &c. ; x, y represents the water line, the course of the vessel being supposed to be from x to y; the arrows represent the direction in which the paddle-wheel revolves. The wheel is immersed to the depth of the lowest paddle-board, since a less degree of immersion would render a portion of the surface of each paddle-board mechanically useless. In the position ▲ the whole force of the paddleboard is efficient for propelling the vessel; but as the paddle enters the water in the position H, its action upon the water, not being horizontal, is only partially effective for propulsion : a part of the force which drives the paddle is expended in depressing the water, and the remainder in driving it contrary to the course of the vessel, and, therefore, by its re-action producing a certain propelling effect. The tendency, however, of the paddle entering the water at H, is to form a hollow or trough, which the water, by its ordinary property, has a continual tendency to fill up. After passing the lowest point A, as the paddle approaches the position в, where it