with as large a portion of the boiler as possible.

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.

Fig. 222 represents a side view, and Fig. 223 a crosssection of a cylindrical boiler with the heaters attached, such as are used for stationary engines.

These heaters, indicated in the figure by Bb, are filled with water, and connected with the boiler by the tubes, PPP, while the boiler is only about half full.

The flame of the furnace, c, plays directly against the heaters; the heated gases and smoke are returned under the main cylinder in the flue, O (Fig. 223), and finally discharged into the chimney

The principal appendages of the boiler are the following, as represented in Fig. 222.

Furnace, or fireplace, c.

The alarm-whistle, s, so arranged as to be opened by the float, ƒ, when the level of the water falls too low.

Another kind of indicator of the level of the water in the boiler is represented at f'. It consists of a float connected with a counterpoise by a wire passing over a pulley, and through a packing-box in the top of the boiler. The position of the counterpoise tells the height of the water.

Still another indicator, which is sometimes used, is seen at n. It

consists of a thick glass tube, bent twice at right angles, the lower end being under the water and the upper end above. The water will stand at the same level in the tube as in the boiler.

P represents the safety-valve (see Art. 319).

v, the pipe that conducts the steam to the steam-chest.

a, the pipe for the admission of feed-water to the boiler; it reaches nearly to the bottom.

h, the man-hole, an aperture by which the boiler can be repaired and cleansed.

R, the damper to regulate the draught.

C, the flue leading to the chimney. The chimney is usually of great height, so as to secure a good draught.

The MANOMETER, or pressure

gauge, for measuring the tension of steam in the boiler, is not shown in the figure.

These are not all based upon the same principle. Some are simply siphon barometers whose long branch is open, the short branch connecting directly with the boiler. The steam from the boiler forces the mercury up the long branch, and the higher the column the greater the pressure of steam.

This manometer, which is called the open manometer, answers well enough for low pressures; but for hign ones the length of tube necessary renders it very inconvenient.

The closed manometer is shown in Fig. 224, and differs from the one just described in having its vertical tube closed at the top. It is graduated on the principle enunciated in MARIOTTE'S law.

Fig. 224.

When the pressure in the boiler is one atmosphere, the mercury

in the cistern and tube are at the same level, the tension of the steam and the elastic force of the air just balancing each other. When the pressure becomes two, three, four, etc., atmospheres, the air in the closed tube will occupy one half, one third, one fourth, etc., the space it did before, allowance being made for the weight of the mercury which is forced up into the tube. The instrument having been graduated, its use is evident. When it is desired to ascertain the tension of the steam in the boiler, the cock is turned, and the height to which the mercury ascends in the tube indicates the tension in atmospheres. Any number of subdivisions may be made in either of the two manometers described.

The liability of glass tubes to break, and to lose their transparency by the mercury clinging to their sides, renders them somewhat objectionable. They are not adapted, either, to machines in motion.

The cheapness of metallic manometers has caused them to be used for a great number of boilers. We shall mention

Fig. 225.

only the one constructed by
BOURDON. The principle is
this: If we allow the steam
from the boiler to enter a
flexible and elastic tube, that
has been flattened a little
and then coiled, the pressure
will tend to uncoil it.
off the steam, and the tube,
by virtue of its elasticity, re-
sumes its original position.

Shut

Fig. 225 represents such a manometer. One end of the tube is connected with a pipe leading to the boiler; to the other end is attached a steel needle, which traverses a scale. As the pressure of steam on the interior surface increases, it gradually uncoils, and the hand points to the number of atmospheres of pressure. When the pressure is removed the needle returns to its former position.

349. Mechanism of the Condensing Engine. The essential parts of a condensing engine are shown in Fig. 226. The figure is only intended to illustrate the prin

ciples of the engine, and, for the purpose of illustration, the parts are arranged in such a manner as will best 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, 4, working through a packing-box, d, which transmits the motion of the piston to the working-beam, L.

The parallel bars, D D, and the radial bars, CE, which keep the 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.

When the crank is at its highest or lowest position the steam has no power to move it. In either of these positions, called the dead points, the machine would come to rest if it were not for the flywheel, which, by its inertia, carries the piston and crank over these points, and brings them again under the power of the steam., The steamboat and locomotive need no fly-wheel, inasmuch as the inertia of the moving mass suffices.

The eccentric, 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.