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 steam-chest 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 eccentric acts upon the bent lever to open the lower and close the upper passage. The steain 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 eccentric again acts to change the position of the sliding-valve, and thus the motion of the piston is continued indefinitely.

350. The Governor. In many engines the supply of steam to the cylinder is regulated by an apparatus called the governor. One form of this contrivance is shown in Fig. 227.

A B is a vertical axis, connected with the machine near its working point, and revolving with a velocity proportional to that of the working point; FE and GD are arms turning with the axis, and bearing heavy balls, D and E, at their extremities; the arms are attached by hinge-joints at G and F to two bars, CG and CF, and these bars are connected by hinge-joints with the axis at C. The arms, FE and GD, are also connected by hinge-joints with a ring, H, which is free to slide up and down the axis, A B.

When the axis revolves, the centrifugal force developed in the balls causes them to recede from A B, and depresses the ring, H.

Fig. 227.
This causes the lever,

BK, to turn about its fulcrum, K, and when the velocity has become sufficiently great, the lever operates to close a valve and shut off the motive power. When the velocity again diminishes, the balls approach the axis, the ring, H, rises, and the valve is opened. The governor may be adjusted so as to secure any desirable velocity at the working point.

351. Action of the Eccentric. The automatic movement of the sliding-valve by means of the eccentric needs a more detailed explanation than is given in the preceding article.

The eccentric (Fig. 228) consists of a circular piece of metal, c, so attached to the shaft of the engine that its centre does not coincide with the axis of rotation.

The eccentric is surrounded with a ring of metal which does not rotate, but follows the motion of the eccentric, thereby receiving a

T

Fig. 228.

motion back and forth in a horizontal direction. This movement is transmitted by the arm, T, to the bent lever, a b c, causing it to turn about the point, b. This rotation of the lever raises and lowers alternately the rod, d, which is connected with the sliding-valve; thus an upward and downward motion is also imparted to this valve.

352. The Locomotive. ·Fig. 229 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 filled with water and communicating with the boiler. E is the grate, and D the door for the supply of fuel.

The steam-pipe, SS, conveys the steam from the steam-dome to

the steam-chest. It may be closed by a valve, V, worked by a lever, L.

The steam-dome is an elevated portion of the boiler, the object of which is to permit the steam to enter the steam-pipe without any admixture of water, as might be the case were the steam taken from a lower level.

The cylinder, the piston, P, and the piston-rod, R, are similar to the corresponding parts of the condensing engine.

The blast-pipe, L', through which the steam is blown off after having acted upon the piston, terminates in the smoke-box, and the blast of steam from it serves to increase the draft of air through the flues, and thus promotes the combustion of fuel.

The connecting-rod, G, transmits the motion of the piston to the crank-arm, by means of which a rotary motion is imparted to the driving-wheels of the locomotive.

The manner in which steam acts to impart motion to the piston is the same as in the engine already described.

Summary.

Thermo-dynamics.

Definition.

Conservation of Energy.

Explanation.

First Law of Thermo-dynamics.

Mechanical Equivalent of Heat.
Description of Joule's Apparatus.
Mode of Operation.

Results of this Experiment.

Transformation of Energy.

Illustration by Examples.

Dissipation of Energy.

Explanation.

Illustration.

Possible Results of Dissipation.

The Steam-Engine.

Definition.

The Power of Steam.

Illustration by Experiment.

Varieties of Steam-Engines.

Condensing and Non-condensing.
Definition.

Boilers and their Appendages.

Boilers of various Shapes.

Boiler, with Appendages, of Stationary Engine, illustrated by Figure.

Open Manometer.

Closed Manometer.

Bourdon's Manometer.

Mechanism of the Condensing Engine.

Illustrated by Figure.

The Governor.

Illustrated by Figure.

The Locomotive.

353. Hygrometry. HYGROMETRY is the process of measuring the amount of moisture in the air with respect to the amount necessary to saturate it.

When a given space has taken all of the vapor that it can contain, it is said to be saturated. For example, if water be poured into a bottle filled with dry air, and the bottle be hermetically sealed, a slow evaporation will go on until the tension of the vapor given off is equal to the tendency of the remaining water to pass into vapor, when it will cease. In this case the space within the bottle is saturated.

If the temperature varies, the amount of vapor required to saturate a given space will vary also. The higher the temperature, the greater will be the quantity of vapor required to saturate the given space; and the lower the temperature, the less the quantity required for saturation.

The quantity of watery vapor in the atmosphere varies with the seasons, temperature, climate, and different local causes; but notwithstanding the continued evaporation that is taking place from lakes, rivers, and oceans, the air in the lower regions of the atmos