called a horse-power, that is, a force which is capable of raising a weight of 33,000 lbs. through a height of one foot in one minute. Thus an engine that can perform a work equivalent to raising 33,000 lbs. through 10 feet in one minute, is said to be an engine of 10 horse-power.

Boilers and their Appendages.

475. The BOILER is a shell of metal, generally of wrought iron, but sometimes of copper, in which steam is generated.

Boilers are made of various shapes. One of the simplest, has the form of a cylinder, with rounded ends; it is set in a furnace, as shown in Fig. 85. Sometimes two smaller cylinders, also with rounded ends, called heaters, are placed below the main shell and connected with it by suitable pipes. The object of this arrangement is to increase the heating surface. In the Cornish boiler the cylindrical shell has a large flue passing through it, containing an internal furnace. Sometimes two such flues exist. The tubular boiler has a great number of tubes, or flues, passing through it, for transmitting the flame and heated gases from the furnace.

The principal appendages of the boiler are the furnace, or fireplace, with its flues and dampers for regulating the draft; the feed apparatus, by which water is introduced to supply the place of that converted into steam; the safety valve, to guard against danger of explosion (see Art. 229); the manometer, for measuring the tension of the steam in the boiler (see Arts. 123 and 124); the steam-guage, to indicate the height of the water in the boiler; the blow-off apparatus, usually a cock near the bottom of the boiler, which, when opened, permits the pressure of the steam in the boiler to force out the sediment and impurities that collect there; and the steam-pipe, to convey the steam from the boiler to the engine proper.

The boiler and its appendages are variously arranged in different engines, the object in all cases being to obtain the greatest amount of steam with a given quantity of fuel. In stationary engines, the furnace is usually made of brick or some other bad conductor of heat, and the flues are so arranged as to bring the flame and heated gases in contact with as large a portion of the boiler as possible. To

(475.) What is a boiler? Describe some of the forms of boilers. What are the principal appendages of boilers, and what are their uses? What arrangements are made for economising heat in stationary engines?

prevent waste of heat, the exposed surface of the boiler is covered with a jacket of coarse felt. In locomotive engines, the fire-box is made of boiler-iron, and is so constructed, that it is nearly surrounded by the water in the boiler. An additional heating surface is also obtained by means of flues, running through the boiler, for conveying the flames and heated gases.

To guard against explosion from too great a pressure of steam within the boiler, the safety valve is employed. In addition to this, a fusible safety plug is sometimes used. This consists of a plug of metal, inserted in the boiler, which is capable of being fused at that temperature beyond which there is danger of explosion. If the temperature is raised above this limit, the plug melts and falls out, permitting the water and steam to escape through the hole which it leaves.

Mechanism of the Condensing Engine.

476. The essential parts of a condensing engine are shown in Fig. 333. The figure is only intended to illustrate the principles of the engine, and, for the purpose of illustration, the parts are arranged in such a manner as best to exhibit them at a single view.

The principal parts of the condensing engine are the following: The cylinder, shown on the left, with a portion broken away. The piston, P, which receives the action of the steam, alternately on its upper and lower faces, and is thereby moved up and down in the cylinder.

The steam-chest, b, into which the steam from the boiler enters through the steam-pipe at o, and from which it passes through the -steam passages, alternately to the upper and lower ends of the cylinder.

The sliding valve, moved up and down by the rod, m, which alternately opens a communication between the steam-chest and the two steam passages leading to the top and bottom of the cylinder.

The eduction pipe, U, connecting with the cylinder at a, by which the steam, after having acted upon the piston, is conducted into the condenser, O.

The piston-rod, A, working through a packing-box, d, which transmits the motion of the piston to the working-beam, L.

The parallel bars, DD, and the radial bars, C,E, which keep the

In locomotive engines! Describe the safety plug. (476.) Describe the principal parts of a condensing engine and their uses.

piston-rod from pressing against the side of the packing-box. This arrangement is called Watt's parallel motion.

The connecting rod, I, which transmits the motion of the workingbeam to the crank arm, K, and through it imparts a motion of rotation to the shaft of the engine.

The fly-wheel, V, which obviates to a certain extent the irregularities of motion in the engine.

The excentric, e, which, acting like a crank, produces a backward and forward motion in the connecting rod, Z. This rod acting on

the bent lever, Y, causes the rod m, of the sliding valve, to move up and down.

The cold-water pump, R, worked by the rod, H, which draws cold water from a reservoir, and forces it through the pipe, T, into the condenser. This pipe, terminating within the condenser in a rose, delivers the water in the form of a shower, and condenses the steam.

The air-pump, M, worked by the rod, F, which draws the hot water and the air that is mixed with it from the condenser, and forces it into the hot well, N.

The feed-pump, Q, worked by the rod, G, which draws the water from the hot well and forces it into the boiler.

To explain the action of the engine, let the position of the parts be as represented in the figure. The steam entering the steamchest finds the upper passage open, and flowing through it, acts upon the upper face of the piston and drives it to the bottom of the cylinder. The steam below the piston meanwhile flows through the lower passage, and entering the eduction pipe at a, is conveyed to the condenser, where it is condensed. When the piston reaches the bottom of the cylinder, the excentric acts upon the bent lever to open the lower and close the upper passage. The steam from the steam-chest now flows through the lower passage, and acting upon the lower face of the piston, forces it to the top of the cylinder. Meantime the steam above the piston, flowing down the upper passage, enters the eduction pipe and is conveyed to the condenser. When the piston reaches the top of the cylinder, the excentric again acts to change the position of the sliding valve, and thus the motion of the piston is continued indefinitely.

The Locomotive.

477. The figure represents a section of a locomotive, the principal parts of which are the following:

The boiler, BB, with its flues, pp, and safety-valve, M. The dotted line represents the height of the water in the boiler.

The fire-box, A, communicating with the smoke-box, C, by means of the flues, pp. The fire-box has a double wall, the interval being

Explain the action of a condensing engine. (477.) Explain the principal parts of a locomotive engine.