204 PROCESS OF BLOWING OUT; BRINE PUMPS. 1-54th part. In government vessels, the engineers are instructed to blow out the boilers completely every three or four days; in other cases, it is usual to blow out a certain quantity of the contents of the boiler every two hours, and supply its place with sea water, which is introduced into the condenser, where it is mixed with the condensed steam, and carried to the boiler. By this change of water, a solution containing perhaps thirty-six per cent. of salt is discharged, and its place supplied by a solution containing only three per cent., this being the proportion in which salt exists in sea water (page 6). Messrs. Maudsley and Field have introduced a method by which the saltness of the water in the boiler is maintained at a constant degree. This consists in the application to the boiler of pumps, called brine pumps, for the purpose of discharging into the sea the supersalted water, or brine. These pumps, as well as the pumps which supply the boiler with sea water, are worked by the engine, and are consequently in perpetual action. The ingenuity of the contrivance consists in the adjustment effected by the two kinds of pump, between the supply and the discharge of salt, the brine pumps discharging as much salt dissolved in a small quantity of water, as the feed pumps are capable of supplying in a large quantity of water; the quantity of salt thus continues constant, while the difference between the quantities of water, supplied and discharged, is converted into steam, and devoted to the working of the engine. If, for instance, the discharged brine contain 5-32nds of salt, and the supplied sea water contain only 1-32nd, it will be the duty of the feed pump to supply five times as much water as the brine pump discharges; and of these five parts, four are available for the purpose of evaporation. By means of another happy contrivance of Messrs. Maudsley and Field, the brine, previously to its being discharged into the sea, is made to communicate a great portion of its heat to the supply of sea water. For this purpose, the brine is conveyed away from the boiler to the sea in a tube, contained within another tube, in which OF THE CYLINDERS OF MARINE ENGINES. 205 latter the supply is conveyed from the sea to the boiler. Thus the hot and the cold liquids pass in contrary directions, the former within the latter; by this means the temperature of the brine is reduced to about 100° previously to its being discharged into the sea. OF THE CYLINDER OF THE MARINE ENGINE. 144. Relation between the dimensions of the Cylinder, and the power of the Engine.-It has been stated that the diameter of the cylinder, in marine engines, is usually little less than its length. The power of an engine is commonly estimated by the dimensions of the cylinder; the number of inches of the diameter of the cylinder is taken as the estimate of the horse power of the engine, a cylinder of 74 inches diameter being generally considered to yield a power equal to that of 200 horses. This rule, however, must be applied with caution, for the power of an engine, as inferred from the dimensions of its cylinder, may be merely nominal, and fall far below the estimate, owing to the imperfect construction and adjustment of the other parts of the engine; whereas, on the other hand, the real power of an ably constructed engine may be far above the estimate, derived from the dimensions of its cylinder. The following table was constructed by Mr. Scott Russell from a comparison of the practice of the most eminent marine steam engine makers, with the principles of their construction. He states that, under the dimensions given, the engines of best construction will give out from one-fourth to one-third more than their nominal power; that the proper nominal power of a cylinder of 74 inches is above that of 225 horses, and that its actual effective power, as given out in the ship, is more than that of 300 horses. The variations between the nominal and the useful effect of engines are owing, not only to good or bad construction, but to certain practices which are exclusively of a mercantile character. 206 DIMENSIONS OF MARINE CYLINDERS. Table of the Dimensions of the Cylinder of a Marine Steam Engine of given Horse power. OF APPLYING STEAM IN MARINE ENGINES. 207 The table shows that the power of the steam engine increases more rapidly than the area of the cylinder or the square of the diameter. By the rule of the square of the diameter, the power of an engine of 74 inches would be about 200, instead of about 225; and 100 inches diameter would give only 333 horse power; but the same rule would give too small a diameter for the lower powers. The engines of the dimensions stated on the table will all work to more than their nominal power. In deviating from the proportions given in the table, a longer stroke will be preferable to a shorter; and, with the necessary alterations required for high velocities of the piston, a longer stroke working the steam expansively is likely to be attended with many advantages: the pressure and strain upon the working parts of the engine are lessened in proportion to a given power; all the parts of an engine may be lighter than with a shorter stroke and a greater diameter of cylinder. A short stroke has, however, this advantage-that with a given length of lever and connecting-rod, the angles of oblique pressure are smaller, and the intervals of time between maximum and minimum pressure are shorter. The velocity of the piston in the cylinder of a steam engine is generally reckoned in this country at 220 feet a minute, and all the arrangements of the engine and its work are made on that principle. Mr. Russell says, we can find no better reason for this than that a horse going at that speed, viz. two miles an hour, can draw 150 lbs. for eight hours a day, all the year round. He adds, that the rule is as universal in its acceptation as it is groundless and injurious; and that with large condensers, and large ports and valves, double the speed may be employed with great advantage.' * 145. Mode of applying the Steam in the Marine Engine.The mode by which the elastic force of the steam is communicated directly to the moving parts of the engine, may * J. Scott Russell On Steam Navigation, p. 270. 208 now be briefly described. It has been stated (p. 199), that the machinery in a steam engine, compared with that of a stationary land engine, is inverted with reference to the beam. In the land engine, the beam is placed above the cylinder, and is pushed up and pulled down by the pistonrod; in the marine engine, the beam is placed beneath the cylinder, and is pushed down and pulled up by the pistonrod. The method by which this is effected may be seen in the two following figures. 1. Fig. 77, represents a cylinder with its piston and rod; to the top of the rod is attached a cross bar ff, the extremities of which project a little beyond the sides of the cylinder; to the extremities of this cross bar are attached vertical rods fe, fe, the lower ends of which are connected with the beams, as represented at e in fig. 78; the rods ƒ e furnish the parallel motion, and serve to elevate and depress the extremities of the beams. In fig. 78, only one of the beams is seen, the other being on the opposite side of the cylinder, and connected with the opposite rod of the cylinder.-2. In the land engine, the OF APPLYING STEAM IN MARINE ENGINES. connecting-rod is pushed down and pulled up by the beam; in the marine engine, the connecting-rod is pushed up and pulled down by the beam. This is obviously the case in fig. 78, in which the beam, working on a pivot p, produces the reciprocating motion of the connecting-rod c b; this rod is connected with the crank at c, and a revolving motion of the crank is effected round the axis a in the direction of the dotted circle. The reader may compare the action here |