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recourse to the difficult and scarcely practicable operation of reefing the paddles."

When paddles are deeply immersed, as in vessels heavily laden, their action is materially impeded, and a great part of the power of the engine uselessly absorbed. On the other hand, when vessels are too lightly laden, the paddle-boards do not take sufficient hold of the water, and the slip is consequently greatly increased. In the former case, when the greatest effort is required to be made to overcome the increased amount of resistance to the motion of the ship, the effect of the paddle is impeded by the greater body of water which it has to lift up in rising through the water, and hence the numerous contrivances for reefing paddles. To these evils must be added that which arises, when there is much swell, from the rolling of the vessel; one paddle being too deeply immersed, and the other not sufficiently so, and sometimes not at all.

38. The injury done to the stern of a ship by the shake of the screw is a result much to be dreaded in a general use of that implement, whatever its advantages may be in other respects, and a sufficient number of experiments have not yet been made to ascertain the effects of long continued screw-propulsion at full speed. Such experiments should therefore be made and continued d Voutrance with all classes of vessels, more particularly steam-frigates, block-ships, and the first ships of the line fitted with screws. There are indications that the results may not be satisfactory, and, if so, it should be seen by what means this defect may be obviated. The screw is no doubt preferable to the paddle-wheel as a propelling implement; the defects now under notice arise only from its being placed in the dead wood, where it is subject to the cross strains it receives in passing through a body of water in a state of perturbation.

■ In the trials of the 'Basilisk' and 'Niger' (Art. 54), the paddles of the former were reefed several times according as she became lighter; but this was found so inconvenient that she was ordered to retain them as fixed relatively with her mean immersion.

39. If the water reacted upon the screw of a steamer precisely as a nut reacts against the threads of a screw which works into it, the vessel would move through a space equal to the pitch of the screw in the time that the latter makes one revolution on its axis; but such speed is not realized in practice, first, on account of the recession of the water (the slip) from behind the screw, after being acted upon by the latter; secondly, the water in advance of the screw is not free to fill the void caused by the recession of the backwater, but arrives at the face of the screw in a disturbed state, in consequence of the displacement at the bows of the vessel, and the subsequent convergence towards the after part; thus producing cross actions and strains upon the blades of the screw, and forming an eddy round the stern of the ship, all of which affect considerably the propelling action of the screw. If the screw could be placed out of the vortices of this body of water, no doubt an important improvement would be made in screw propulsion.

40. Steam propulsion cannot be well combined with that of the wind in paddle-propelled vessels. In the first place the funnel prevents the use of the mainsail, as stated in the report of the trial between the 'Eeynard' and the 'Plumper.' Again, when the wind is a-beam in a breeze of considerable strength, the lee-paddle is too much, and the weather-paddle too little, immersed to be efficient; and in a strong breeze, a vessel will go faster before the wind with her sails alone, or with her steam alone, than with both combined. In fact the sails can scarcely be used unless the paddle-wheels are disconnected from the engine, and this process, as well as that of connecting them again, is very difficult. The expedient of removing some of the paddle-boards, and turning into the water that part of the wheel which is thus dismantled, is a tedious operation, and might be extremely detrimental in a war-steamer, which should ever be prepared to put forth all her power. The screw, on the contrary, may be disconnected with great facility at any time, and consequently screw-propelled vessels may sail more and steam less than those furnished with paddles; their steam may thus be reserved for strong head winds or calms, and in emergencies incidental to the operations of war.

41. Feathering-paddles are particularly objectionable for ships of war, as they are even more likely to be damaged by shot than paddles of the common kind," since shot has been known to pass between the spokes without disabling, or even touching, any part of the wheel, which could scarcely be the case if the boards were applied on the feathering principle.

42. Fixed paddle-boards, therefore, continue to be used in Her Majesty's service, but they are of improved form, for which a patent was granted to Mr. Field in 1833. The improvement consists in dividing the fixed board into several narrow slips, which are placed somewhat behind each other, with inclinations which correspond to the cycloidal curves they describe; these enter the water in immediate succession, and thus permit a great part of the water, which would otherwise be forced downwards in the descending movement, and upwards in the ascent of the boards, to escape through the spaces between them, while the slips overlapping each other in horizontal directions intercept and act upon all the horizontal filaments of the fluid, thus preventing a certain portion of the power of the engine from being wasted. The improved paddle-wheels generally used in the United States consist of a combination of two or more common paddle-wheels side by side, on each side of the vessel, and moving on the same shaft or axle, the paddle-boards being so placed that each is in a position between two boards of the collateral wheel. These wheels are of great magnitude, and being found very efficient in smooth water, are preferred to any of the expedients adopted in Europe to obviate the defects of the radiating paddles.—(Lardner, on the Steam-Engine, p. 496.)

"M'Kimion's Steam Warfare, page 221.

43. Two instruments are now generally employed to determine the power of a Marine Steam-Engine; one of them shows the expansive power of the steam in the cylinder, and the other the force of impulsion in a screw-propelled vessel hy the pressure on the screw shaft in the direction of its length. The first of these, called the Indicator, consists of a hollow cylinder of small dimensions, whose top is open, the lower end being fitted to be screwed on to the top or bottom of the enginecylinder; a stop-cock at the bottom acjs as a four-way cock, and admits either the external air or the steam; a piston works steam tight in the small cylinder, and a spiral spring is attached to the top of the piston, and to a fixed cross piece above it, so that this spring, which is contained in a tube affixed to the piston, acts against the power of steam which presses the piston up, and registers its force. The instrument being screwed on that part of the steam-cylinder from which the indication is required, and the stop-cock being opened, there is a communication between the interior of the steam-cylinder and that of the Indicator. Now if the Indicator be attached to the top of the steamcylinder, and a vacuum exist above the piston of the latter, the piston of the indicator is, by the atmospherical pressure above it, forced to the bottom of the cylinder in which it moves; and when the steam from the boiler enters the top of the steam-cylinder, to force down its piston, the steam entering the lower part of the indicator-cylinder presses its piston upwards, this motion being retarded by the spiral spring already mentioned. To the top of the piston-tube in which the spring works, and at right angles to it, is affixed an arm, the extremity of which carries a pencil, which by the reciprocating motion of the piston-tube would describe on a plane surface at rest a straight line whose extremities would indicate the greatest or least elastic forces or pressures of the steam in the cylinder of the steam engine during each stroke of its piston, but would not show the pressure at any particular portion of the stroke.

When the indicator is not in communication with the cylinder of the steam engine, the piston in its cylinder is held in equilibrio by the equal pressures of the atmosphere above and below it. In this state the spiral spring is at its greatest extension, and consequently does not press against the top of the piston. The point at which the pencil then stands on the straight line is marked zero, and is designated the atmospheric point. The line being graduated, the divisions are numbered 1, 2, 3, &c, increasing upwards and downwards from the zero point; and when the indicator, being connected with the steam-cylinder of the engine, is in action, the numbers indicate, in pounds, the elastic pressure of the steam; the upward numbers denoting pounds above, and the lower, pounds below the pressure of the atmosphere (= 14-75 lbs. on a square inch).

The pencil, however, instead of pressing against a plane surface, is made to press upon the convex surface of a cylindrical barrel which is turned by some part of the machinery on an axis parallel to the piston-rod; it consequently describes a curve of double curvature, the figure and ordinates of which indicate by inspection the varying elasticity of the steam in the cylinder of the engine.

44. The indicator should be applied both at the top and bottom of the steam cylinder, and a mean of its measures taken, as the values of the pressure above and below the piston are often different on account of differences in the lengths of the slides where the steam is introduced; also on account of the position of the crank at the time the steam is cut off. The following are the general conclusions drawn from the nature of the line described by the pencil on the cylindrical surfaces against which it presses.

1. If the pencil describes a straight line upwards or downwards, the piston is not moving; but, in the first case, the steam pressure in the engine-cylinder is increasing, in the other case it is decreasing.

2. If the line is horizontal, proceeding to the right

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