By altering the angle of the blades the screw may be made to advance through the water with greater facility, and with a reduced number of revolutions on its axis: thus following up the speed obtained by the ship under canvas, and using the engines as an auxiliary power only.

The alterations in the position of the blades may be effected in a few minutes, from the deck, by mechanical means, while the vessel is under way, by one man, and in any weather.

Whatever the comparative advantages or disadvantages of the paddle and the screw, applied to the propulsion of ships of war, may be, the screw possesses so many advantages over the paddle as to give it a decided preference for general purposes.

62. The screw admits of a better, stronger, and more simple form of vessel. Relieved of the paddleboxes, the screw-propelled vessel is far less acted upon by head-winds, and less subject to the heavy rolling motion occasioned and aggravated by the oscillations consequent on the top-weights on both sides of a paddle-wheel vessel when the boxes receive the impulses and surges of the sea-such oscillations being highly unfavourable to gunnery. The screw is little affected by alterations in the trim of the ship, it is very nearly equally effective at all depths of immersion, and if entirely submerged, it may be driven by the direct action of engines placed so low in the vessel that both the moving power and the propelling machinery are safe from the damaging effects of shot: the screw allows more freely the use of sails, and consequently enables the vessel to which it is applied to retain her faculties as a sailing-ship in a much higher degree than paddle-wheels; it admits of considerable reduction in the beam or breadth of the vessel, which, besides other advantages, is an important consideration in the economy of space, in a basin or in dock, and with respect to the magnitude of the floodgates through which it has to pass. To which advantages may be added, that the decks of screw-propelled

vessels are wholly available for broadside armament, and admit of full gunnery power being retained.

63. But the screw-propeller, to be effective, requires that the shaft be driven with great rotatory speed to enable it to put forth its maximum power. To effect this, gearing" or bands, and the drum, have heretofore been much resorted to in order to multiply the revolutions of the propelling shaft, but both have been found inconvenient. The slip of the band in the experiments with the 'Rattler' amounted to no less than 2.7 per cent.; and the inconvenience of gearing consists in the impossibility of placing the machinery below the water-line."

For these and other reasons, engines acting directly on the cranks of the propelling shaft have been introduced, and will, no doubt, be generally employed.

In Penn's engine, oscillating cylinders have direct actions on the piston-rods, by which means the cross strains produced by fixed cylinders on the rods are ob

viated.

64. But direct action is not without its disadvantages. In consequence of the increased velocity given to the pistons, the diameter of the cylinder is made less than the usual standard, and the steam and eduction passages are unusually large in a condensing engine. The limit to the velocity at which the pistons may be worked is determined by the velocity of the water drawn out by the air-pump; and it appears that a velocity of 110 to 120 feet per minute-the maximum allowed according to Boulton and Watt's standard-is the greatest that can be given to the pump with safety. When the velocity of the water exceeds this, the air-pump-bucket communicates to the water a series of blows, the shock of which is very destructive to the material, the valve strikes hard, and the end of the screw-shaft is thrown upon its collars with great strain to it and to the piston rods. Various appliances have been tried to diminish the violence of the shocks to which the water is subjected, but, with fixed cylinders, nothing to remedy

The engine now drives the screw without either drum or gearing,

the strains on the piston-rods acting directly on the cranks of the propelling shafts has been discovered; and, for this reason, oscillating cylinders, such as were used many years ago for paddle-wheel vessels, are now generally applied, for the direct action, in large steamers of war.

Another disadvantage in the screw, which it has not been found possible to remove, is the heat caused by friction when the number of revolutions made by the shaft per minute amounts, to 60 or 70; the weight of the screw being from three to four tons, and in large ships six to eight tons, that friction becomes very great.

65. Among the anomalies in steam navigation which practice has exhibited, may be mentioned the fact that screw-vessels, though full in the quarter, steer remarkably well, contrary to what is observed with sailing vessels and with wheel-steamers-a circumstance which is caused by the current of water from the screw acting on the rudder with considerable force. To the same cause must be ascribed the fact that screw-vessels, even with full after-bodies, have, in general, less slip than other vessels. In the experimental trials made by H. M.S.Plumper,' the slip was found to be negative; that is, the water aft of the screw, instead of receding from it, moved towards it, thus increasing the screw's power of giving motion to the vessel. In eleven trials made by that ship, in running a measured mile in Stokes Bay, the speed of the vessel always exceeded that which should be given by the power of the engine by 0.7 knots per hour on an average, the screw making from 83 to 115 revolutions per minute. In going head to wind, the slip of the screw has been thought to increase in a higher ratio than that of the paddle-wheel, but the experiments have not, as yet, been sufficiently numerous and precise to determine this point. 66. A screw-vessel clean in the run is apparently the most advantageous. The Dauntless,' steam-frigate, when tried in August, 1848, in running a measured mile, had a speed of 7.36 miles per hour. Subsequently an additional length of eight feet at the stern was

given her, in order to carry the rudder farther out of the eddy; the speed was found to be 10.266 miles per hour; the vessel being under steam only in all the trials.

67. The relative consumption of fuel in steamers of different kinds is a subject of very great importance, and, in any one steamer, it is found to vary with the vessel's draught of water and with the cube of its velocity; consequently, when a high speed is obtained, the consumption will be very great, a double velocity being produced, cæteris paribus, by an eight-fold quantity of fuel. Hence it follows that when the necessity of the service does not imperatively require great speed, a due economy would be obtained by keeping the vessel as much as possible at a low rate of motion. The minimum speed of steamers is seldom less than 3 miles per hour; a less velocity would require the power of the engine to be so far diminished that it would scarcely turn the shaft.

68. It is now proved that, in steam navigation, greater amount of locomotive power is obtained by means of the paddle-wheels than by means of the screw, with an equal consumption of fuel; of the two kinds of vessels, when moved by steam only, one with paddlewheels can, therefore, keep the sea a longer time, or perform service of longer duration, than one of the other kind; but the advantages are on the side of the screw when steam is used in both, in conjunction with, or as an auxiliary to the sail.

69. If two steamers equal in every respect, except that one is equipped with paddle-wheels and the other with a screw, are moving under sail only, with equal velocities, any required increase of speed will be obtained from steam with less expenditure of fuel by a screw, than by a paddle-wheel ship; and the fact may be accounted for by considering that the water on which a screw acts being under the stern is almost in a state of rest relatively to the ship, whereas, along the sides, where the wheels act, it is virtually receding at a rate equal to the ship's movement; and consequently a greater number of revolutions of the wheels than of the

screw, in a given time, must be made in order to obtain an equal increase of speed for the vessels.

70. Economy of fuel may be gained by working steam very expansively. But the expansive process cannot long be continued; inasmuch as the expansion is attended with diminished pressure, which causes a diminution of speed; and therefore, the economy does not exist when a given distance is to be run in a limited time.

71. In experiments made by the 'Bee,' which was fitted up to work either with screw or wheel, 42 revolutions of the screw only produced a velocity equal to 6.8 miles per hour, while 32 revolutions of the wheel-shaft produced a speed of 7.5 miles per hour; the consumption of steam, and consequently of fuel, being proportional to the number of revolutions of the shafts. Likewise, in the trials between the 'Rattler' and the Alecto,' where the advantage of speed was in favour of the former ship, the revolutions of the screw were, to those of the paddle-wheels, in the ratio of 24 to 19 very nearly, and the consumptions of fuel were, of course, in the same proportion.

72. It is evident that, when the object is to attain a given distance, steam may be worked expansively, with different degrees of expansion according to circumstances with a given increase of time an economy of fuel may be obtained by a greater space for expansion; and conversely a given diminution of time may be obtained by less space for expansion, which would require an increased consumption of fuel.

73. The shake of the screw and the consequent injury to the stern of the ship, as mentioned in Art. 38, are caused by the sudden and violent reactions of the disturbed water in that place against the blades of the screw as they enter and emerge from thence." The

a In consequence of the metal covering of the screw-shaft breaking loose on board the Royal Albert,' during her passage from the Black Sea to Malta, in 1855, by which the tubing in the stern-post was torn away, and the gland and stuffing-box were forced off, so great a quantity of water rushed into the ship that it was necessary to lay her aground in order to prevent her from sinking