Forbes', its peculiarity consisting chiefly in dividing what would be the ordinary topsail by an additional yard into two sails, making as it were double topsails with aggregate surface equal to that of the usual single topsail. This arrangement is obviously for greater ease in handling the sail. From stem to centre of foremast is 31 feet; thence to centre of mainmast 57 feet; thence to centre of mizzen mast 42 feet. The space occupied by the machinery, coal, &c., extends from the stern. post 47 feet; and the bunkers hold 80 tons of coal. Hoisting out Apparatus of the Screw.-Plate I.-The arrangement by which the screw is hoisted out of the water, consists of a shaft, on which is a capstan or hand wheel of eight arms, measuring 24 in. from the centre of the shaft to the extremity of the arm. This shaft carries an endless screw of 4 inches on the pitch circle, which works into a wheel of 324 inches diameter, 23 inches face, and 18 inch pitch of teeth. The shaft of this wheel carries a pinion of 83 inches diameter working into a wheel of 42 inches diameter, 6 inches face, and 2,3 inches pitch of teeth; the shaft of which in its turn carries a pinion of 9 inches diameter, working into a wheel of 55 inches diameter, 7 inches face, and 3 inches pitch of teeth. The diameters of all the above pinions and wheels are to their pitch circles. On the shaft of the last wheel, the upper end of the wrought iron lifting arm is keyed, the lower end encircling the projecting portion of the hub. The weight of the lifting arm is 1200 pounds. The screw is not keyed to its shaft, which latter is square in section (with the angles a little taken off to prevent turning,) and merely ships into the hub. The outboard or screw shaft is drawn by levers directly inboard into the hollow part of the next section of shaft, sufficiently far to allow it to clear the hub of the screw; the hand wheel or capstan being then turned by two men, the lifting arm revolves, carrying the screw with it, and by half a revolution lifts the screw out of water, and elevates it perpendicularly 14 feet, where it is secured by locking the gearing and by hooking braces to it from the vessel. When it is required to lower the screw, the reverse operation is performed. It required from 20 to 30 minutes to raise or lower the screw and to secure it ready for sailing or steaming. By the above arrangement it will be perceived, that in backing, the thrust of the screw is communicated by the collar on the hub to the lifting arm; this arm must therefore be made sufficiently strong to hold the thrust against its leverage of 7 feet. In going ahead, the thrust of the screw is communicated by the hub to the after end of the shaft; the after extremities of the shaft and of the bore of the hub being tapered to the form of a frustrum of a pyramid. Screw. One true screw of bronze. The hub is cast in one piece, and the arms are cast separate and riveted to the hub. The band or drum is cast in six pieces, and riveted by flanches to the arms. It will be observed that these parts of the blades pierced by the rivets are made thicker by raised ribs, in order that the strength of the blades may not be weakened by the riveting. The peculiar form of hub resulted naturally from disposing a minimum quantity of metal most effectually to resist the crossstrain of the blades. The weight of the screw is 5145 pounds: it makes one revolution to each double stroke of engine piston. Pitch of screw, Mean fraction of pitch used, 17.111 sq. feet, 47.281" 66 Length of screw, from hub to drum, in direction of axis, on a plane at right angles to axis, 8.808 " 66 34-465" 46 Radius of the centre of pressure of the screw in function Distance the centre of screw is below the surface of the water (at 15 feet draft), ENGINES. Two direct acting, inclined, condensing engines. Stroke of pistons, Space displacement of both pistons per stroke, The engines have a steam slide valve, and an independent Steam space between cut-off and slide valves, at one end of Steam space between slide valves and pistons, at one end Boilers. Two iron boilers, with circular flues behind the furnaces and horizontal iron tubes returned above the furnaces. Each boiler contains 79 iron tubes 2 inches outside diameter and 9 feet 10 inches long. The boilers are placed side by side, with one smoke chimney in common. Weight of sea water contained in the two boilers, PROPORTIONS.--Proportion of heating to grate surface, Proportion of grate surface to aggregate cross area of tubes, 46 66 Cubic feet of steam room per cubic foot of steam used per stroke of piston, 5.242 42 feet. 280 cubic feet. 25.700 pounds. 33.778 to 1.000. The natural draft of these boilers was very poor, as might be predi cated on their small calorimeter, about th the grate surface, moderate height of smoke chimney, and large proportion of heating to grate surface, with the principal part of the heating surface in small tubes of very thin metal, and therefore very efficient in transmitting the heat. To compensate this deficiency of draft, and increase the power of the boilers, a fan blast was put into the smoke chimney to urge the fires to any required degree. During the whole steaming recorded in the logs, this fan blast was constantly and strongly driven, the steam following as soon as it was stopped, and during the latter part of the steaming, when the boiler had become considerably incrusted with scale and foul, even the most violent driving of the fan would not keep up steam during the operations of blowing off and cleaning the fires, the engines frequently stopping for want of steam on those occasions. The evaporation by the boilers is calculated for a cylinder pressure 3 pounds less than the boiler pressure, and includes the loss of blowing off to maintain the sea water in the boilers at twice the natural concentration, and the loss betweeen the cut-off and steam slide valves and between the latter and pistons, but does not include the evaporation that furnished the small engine for the fan blast with steam. The total heat of steam is taken from Regnault's data. The boilers were continually giving out by the corrosion of the tubes, which were so honeycombed over the whole of their thin surface and reduced in thickness, that the scale could not be knocked off without destroying them. Remarks on the Logs.-The condition of the water is not stated in the logs, but as the wind was principally a light or moderate breeze, it is probable there was but a slight swell on or a moderate sea. The number of "consecutive hours" was taken for such lengths of time as the direction of the wind, steam pressure, double strokes of engine, pistons, &c., remained nearly constant, the extremes differing but a few per centums. The speed of the vessel was taken by the chip log, and as the top of the screw was but 16 inches below the surface of the water, its very great slip must have driven a considerable current aft, which affecting the log would cause its indications to be much too high; the true speed of the vessel must have been considerably less than what was logged. It must further be borne in mind that the force of the wind or kind of wind as recorded in the log, was judged of by the sensations of the observer on the deck of the vessel, and that what is logged as a gentle breeze ahead when under steam alone, is as really a calm, while what is recorded as a light breeze aft was in fact a moderate wind aft, and so on throughout. Inaccuracy in these particulars was unavoidable, but the log of the Massachusetts is in these respects as correct as the logs of other vessels, and the comparison is therefore not much affected. The amount of fuel consumed, though obviously inaccurately given in many cases, is what was actually put, or thought to have been put, in the furnaces during the hours recorded, and as the steaming was always commenced before and continued after those "consecutive hours," there are no corrections to be made for the heat expended in "getting up steam," or for burning down the fires before stopping. With a view to compare the performances of the vessel under different |