INLAND AND MARINE ENGINES.

199

summer and winter, with variable loads of passengers and merchandise, one hundred and forty-six times, at a speed of eleven miles an hour, with almost perfect regularity. This vessel was afterwards transferred to the station between Great Britain and America.

140. Differences between Inland and Marine Engines.1. The inland locomotive engine is propelled by means of wheels attached to an axle, the axle being worked by two cranks; the steam ship is propelled by means of a pair of paddle wheels attached to the two extremities of an axis, or shaft, placed across the vessel, and projecting at each end beyond its timbers; the shaft is worked by two cranks. So far, the mode of propulsion in both cases is similar; in both, the cranks are placed at right angles to each other; but in the former case, the cranks are worked by a single engine with two cylinders; in the latter, by two engines, each having a single cylinder, and acting upon a common shaft. Locomotion is effected, in the former case, by the adhesion of the wheels to the rails; in the latter case, by the reaction produced by the paddles striking the water in the opposite direction to that in which the vessel is proceeding. 2. The engines used on railways, are of the highpressure kind; in marine navigation, low-pressure condensing engines are always employed in this country, and they are constructed in all respects on the general plan of Watt's double acting engine, with slight modifications in the forms and relative position of the parts. On the rivers of America, where lightness of weight is of importance, high-pressure engines are frequently employed. 3. In Watt's engine, there is only one beam, and this is placed above the cylinder and piston-rod; in the marine engine, there are two beams, and they are placed below the cylinder and piston-rod. The connecting-rods, in the latter engine, are therefore presented upwards towards the cranks which they put in motion, instead of being presented downwards, as in the former engine. In the marine engine, the machinery is, in fact, inverted with reference to the beam, the

object being to keep the bulk and weight of the engine as low as possible in the hull of the vessel (see par. 145). In the steam boats used on the American rivers, the original position of the parts is retained, and the machinery extends to a considerable height above the deck. 4. The proportion of the cylinders differs in the two kinds of engine. In the land engine, the length of the cylinder is usually double its diameter; in the marine engine, the diameter is very little less than the length. As necessary consequences of this, the stroke of the latter engine is much shorter than that of the former, in proportion to its power; the crank is shorter; and the sphere of vertical action of the moving parts is limited in a corresponding degree. 5. In both kinds of engine, there are several parts which do not essentially differ; in both there are condensers, air-pumps, and a mechanism for working the valves. In the marine engine, equalization of power is not of the same importance as in stationary land engines; the governor and its apparatus are, therefore, not required.

OF MARINE BOILERS.

141. Form of Boiler.-In the construction of the marine boiler, it is necessary to combine the greatest possible efficiency with the least weight. A very compact form of boiler is, therefore, adopted, in which a large surface is exposed to the heat of the furnace, and a proportionate increase of evaporation produced. The annexed figure represents a horizontal section of a boiler employed in some of the government vessels. It consists, in reality, of three boilers, any two of which may be used, in case of any accident happening to the third.

Fig. 75.

SALTNESS OF WATER IN BOILERS.

201

There are three pairs of furnaces F F, communicating with three flues E, which traverse the boiler in four directions, two forward and two backward, in the direction of the arrows placed in the white spaces. The intermediate dark parts represent the water contained within thin plates; by which means the heated air acts upon every portion of the water at a great advantage; it appears that the evaporation thus produced, when compared with that effected by the boiler of a land engine, is as three to two, with the same consumption of coal. The heated air having traversed these meandering flues, and having reached the points C, enters into three curved flues, repre

sented by the letters B, in the transverse section of the same boiler, fig. 76; the curved flues terminate in the chimney. Three dampers are placed in the curved flues at the point where they merge into the chimney, by which means the communication of any one of the three boilers with the chimney may be cut off at pleasure. In the plan of boiler here described, the flues are all upon the same level; in other cases, the flues are made to traverse two levels, the one placed above the other, by which means a greater heating surface is obtained.

Fig. 76.

142. Indicators of Saltness of the Water in Boilers.-The amount of earthy and saline deposits which takes place in the boilers of land engines is not great, and may be easily removed. But in steam vessels, the case is very different: from the constant supply of sea water, incrustations of salt and sulphate of lime are deposited in the boiler, and, these substances being bad conductors of heat, the boiler may become red hot, leaks may be produced, or even explosion may take place. 1. The degree of saltness of the water in the boiler of an engine may be ascertained by means of the thermometer. Some experiments were made on this subject

202

SALTNESS OF WATER IN BOILERS.

by Messrs. Maudsley and Field, with the view of maintaining the water in a boiler at the same temperature, and at the same degree of concentration. They found that sea water boiling under the usual low-pressure standard of 21⁄2 pounds on the square inch, will arrive at 226° Fahr. in 24 hours; at 230° in 48 hours; and at 232° in 60 hours, &c.; in the last condition it contains 11-32nds of its bulk of salt. At 232° the solution is saturated, and salt is deposited, the concentration having constantly increased with the temperature. The density of the water increases with its concentration, and more heat is required for its evaporation. Since the temperature, therefore, increases with the concentration, under the same pressure, the thermometer becomes an indicator of the saltness of the water, and, accordingly, of the danger to which the boiler is exposed from the effects of incrustation. Mr. Dinnen states, that having carefully noted the degree of saturation of the water in the boilers (which were made of copper) at various periods of the day, during a voyage from Falmouth to the Mediterranean and back, the boilers were on each successive examination found perfectly clean and free from marine deposit, with the exception of a slight film, the thermometer being at 215°, and not having varied in excess throughout the voyage. Hence the delay of "blowing out" completely on the voyage was unnecessary. 2. Dr. Lardner has suggested the use of a self-registering instrument, which should not only indicate, but record, from hour to hour, the degree of saltness of the water in the boiler; this contrivance is founded on the difference of pressures under which the salt water of the boiler, and fresh water of the same temperature, are found to boil. "A small vessel of distilled water being immersed in the water of the boiler, would always have the temperature of that water, and the steam produced from it communicating with a steam gauge, the pressure of such steam would be indicated by that gauge, while the pressure of the steam in the boiler, under which pressure the salted water boils, might be indicated by another gauge. The difference of the pressures

PROCESS OF BLOWING OUT; BRINE PUMPS. 203

indicated by the two gauges, would thus become a test by which the saltness of the water in the boiler would be measured. The two pressures might be made to act on opposite ends of the same column of mercury contained in a siphon tube, and the difference of the levels of the two surfaces of the mercury would thus become a measure of the saltness of the water in the boiler."* 3. Another contrivance for indicating the saltness of the water in the boiler, and the proper periods for blowing out the supersalted water, has been recently adopted by the Messrs. Seaward. It consists of the steam gauge (fig. 37, page 95), by which the level of the water in the boiler is indicated. In the water of the gauge are placed two hydrometer balls of different weight, but both sufficiently heavy to sink in a solution of salt, in which the salt constitutes 1-32nd of its entire weight, which is the ordinary condition of sea water. When the solution becomes so far concentrated that the salt constitutes 5-32nds of the whole weight, the lighter ball rises to the surface, and affords an indication of the necessity of blowing out. When the quantity of salt amounts to 6-32nds, the heavier ball rises, and indicates that the concentration has proceeded to an injurious degree. When the quantity of salt amounts to 9-32nds, a deposition begins to take place.

143. Process of blowing out; Brine Pumps.-In seagoing vessels, it is customary at certain intervals to blow out the boilers. This process consists in discharging from the boiler into the sea the whole, or a portion, of the concentrated salt water, and supplying its place with sea water; this is effected by means of blow-off cocks, placed at the lower part of the boiler, where the more concentrated and denser portions of the solution collect: on opening these cocks, the weight of the water, and the pressure of the steam acting upon its surface, drive out the water from below. This process obviously involves a loss of time and a waste of fuel; the latter was estimated by Tredgold at

Lardner On the Steam Engine, page 453.