over a pulley 3 feet 6 inches diameter, making 100 revolutions per minute; the weight indicated by the spring-balance being 18 lbs.

Then (168-18 lbs.) x (3'5 feet x 3'1416 x 100 revolutions).
=5,

33,000

the brake horse-power of that engine.

To prevent the belt slipping off the pulley several light clips may be

of a given horse-power at a given speed, it has

riveted on the outside of the belt, formed with ends projecting over the sides of the pulley.

Rope-Friction-Brake. -A simple and efficient form of friction-brake was used in some tests of an

66

Ajax" gas-engine of 6 nominal horse-power. It consisted of an endless rope or sling-rope, placed round the fly-wheel of the engine, having each end of the sling attached to a spring-balance as shown in Fig. 4. A weight may be used in place of the bottom spring - balance, but it is not so convenient to adjust. The rope is kept in its place by clips.

Size of FrictionBrake.-In determining the most efficient size of friction-brake to be employed for the absorption been found that the result

of the speed of the circumference of the pulley in feet per minute, multiplied by the width of its face in inches and divided by the horse-power, should not be less than 750.

THE INDICATOR.

The Indicator.-The action of steam in a cylinder can only be correctly ascertained by means of an indicator. It shows the pressure of the steam at each point of the stroke, the power and performance of the engine, the amount of back pressure or force opposed to the motion of the piston, and enables any imperfections to be detected in the construction of the valve,

steam-ports, and steam-passages. One of the best indicators is that known as Richard's Indicator.*

Indicator Diagrams. Supposing the indicator to be fixed to a cylinder, and that the drum is connected by means of a cord to some part of the engine, which has a motion co-incident with that of the piston, if the barrel be allowed to rotate before the indicator-cock is opened, a horizontal line is traced, which is called the atmospheric line, and all portions of the diagram above that line, represent steam-pressures, and all portions below that line represent vacuum.

If the indicator-cock be opened at the beginning of the stroke, when steam enters the engine-cylinder, the pencil moves upwards and traces a vertical line, and as the piston moves forward the indicator barrel rotates and a horizontal line is traced until the steam is cut off; then, as the expanding steam increases in volume, it declines in pressure, which causes the pencil to gradually fall and describe a curved line until the exhaust-port is opened, when the pencil immediately falls and describes the "toe" of the diagram. On the return-stroke the pencil traces the bottom or exhaust-line of the diagram until the closing of the exhaust-port, when cushioning commences, then the pressure rises and moves the pencil up and completes the diagram.

Theoretical Indicator-Diagram.-The rules for the expansion of steam are based upon the approximately correct law of gases, viz., that the pressure of gas varies inversely as the volume, or the product of the pressure and volume of a gas is always a constant, other conditions being unaltered; and in order to ascertain the varying pressure and volume of steam during expansion, it is necessary to construct a theoretical diagram

* The Author is indebted for some definitions to the work on this Indicator, published by Longmans & Co., London.

according to this law, the descending curve of which represents the decreasing force of the steam as it expands in volume. This curve is called a hyperbolic curve, and is the standard by which the character of all expansion curves in indicator-diagrams is determined. To draw the theoretical curve upon a diagram as shown in Fig. 5, draw the line A F, representing the line of perfect vacuum, parallel with the atmospheric line, and at the proper distance below it to represent 147 lbs.; and perpendicular to the line A F draw A O, representing the clearance space; draw the line C D, representing the period of admission of the steam; from the point D draw the vertical line D B; draw the line D E; from A to F represents the full length of stroke; divide the distance D E into a number of parts, from which points draw diagonal lines to the point A; from the points where the diagonal lines cut the vertical line D B, drawn horizontal lines; and the points where the vertical lines drawn from the points in the line D E meet these horizontal lines, will be the points of the hyperbolic curve, which may be drawn in by hand.

8

F

Atmospheric Line

E

Line of perfect vacuum

Fig. 6.-Indicator-Diagram.

Indicator-Diagrams, Fig. 6.-The lines forming the outline of a diagram during one revolution of the engine are as follows::

A to B, The admission-line.

B to C, The steam-line.

C to D, The expansion-curve.

D to E, The exhaust-line.

E to F, The line of back-pressure.
F to A, The compression-line.

In Fig 6, A is the point of pre-admission, the steam having been admitted a little before the beginning of the steam-stroke, due to the lead of the valve, to ensure having the full pressure of steam in the cylinder at the beginning of the stroke.

Admission-Line, Fig. 6.-A to B is the admission-line. This line is formed by the rise of pressure in the cylinder as the port is opened for the

admission of steam; the full pressure of the steam should come on to the piston at the beginning of the stroke, and the admission corner should be sharp. When it is rounded as at A in Fig. 7, or when it slants, as at B, it shows that the steam is admitted too late and the momentum of the piston at the commencement of the stroke is imparted by the engine. To remedy this, the valve requires more lead. When the valve has excessive

A

C

Fig. 7.-Diagram.

Fig. 8.-Diagram.

lead, and steam enters too soon, it will produce a slanting line like C, Fig. 8; to remedy this tue valve requires less lead.

Steam-Line.-B to C, Fig. 6, is the steam line or period of admission of the steam. This line is formed by the advance of the piston while the port remains open for the admission of steam; the full pressure of steam should be maintained in the cylinder during the whole period of admission, and the steam line should be straight and horizontal, or parallel with the atmospheric line up to the point of cut off; when this line falls, like D in Fig. 9,

D

Fig. 9.-Diagram.

Fig. 10.-Diagram.

the fall is due either to condensation in the cylinder, or to the ports and steam pipes being too small, which wiredraws and reduces the pressure of the steam.

The Point of Cut-Off, Fig. 6.-C is the point of cut off or suppression. As expansion does not properly commence until the port is closed, the action of the valve in cutting off the steam should be sharp and sudden, and the pressure should fall as little as possible during the closing of the port. The point of cut off should be sharp and clear. When this corner is rounded, like E in Fig. 10, it shows that the valve does not close quickly enough, and that the expansion arrangements are defective. When the steam is cut off slowly it causes a fall of pressure in the cylinder before the port is completely closed. When this corner shows a gradually descending line like F in Fig. 10, it shows that some steam has entered the cylinder after it was supposed to have been cut off.

The Expansion-Curve.-C to D, Fig. 6, is the expansion curve or period of expansion. In a condensing-engine this curve is partly above and partly below the atmospheric-line, but in a non-condensing engine the whole of the curve is above the atmospheric-line. This curve should

approach as nearly as possible in form to that of the theoretical diagram, unless it be filled up by leaky valves, or diminished by steam leaking past the piston. When the cylinder is not properly protected, there will be great loss of heat from radiation, and fall of pressure during expansion, which will cause the expansion curve to fall below the theoretical curve. When the curve rises above the theoretical curve, it is generally due to a leaky valve, owing either to the valve being defective in rigidity, which causes it to bend into the ports in passing over them, or to the valve being deficient in wearing surface. When the expansion curve rises above the theoretical curve towards the end of the stroke, it shows that the steam has been condensed at the beginning of the stroke, and evaporated by the walls of the cylinder towards the end of the stroke.

Point of Pre-release.-D, Fig. 6, is the point of exhaust or prerelease, the exhaust-port being opened before the end of the stroke. The pre-release should allow all the steam in the cylinder to escape before the piston arrives at the end of the stroke, so that during the return-stroke the back-pressure may be as low as possible.

Exhaust-Line.-D to E, Fig. 6, is the exhaust-line. The full expansive force of the steam during the steam-stroke, should be employed as nearly as possible to the end of the stroke, and then the steam should be discharged as rapidly as possible, so as not to hinder the return of the piston. When the exhaust-pipe and exhaust-passages are cramped, or

H

Fig. 11.-Diagram.

Fig. 12.-Diagram.

when the exhaust is too late, all the steam cannot escape properly before the end of the return-stroke, which will cause a bad exhaust-line, and the expansion-curve will be continued to the end of the diagram. The exhaustline will be shown slanting gradually downwards, as at G, Fig. 11, as the piston advances on its return stroke, instead of being horizontal. When the exhaust is too soon, the exhaust-line will slope down, as shown at H, Fig. 12.

Line of Back-Pressure.-E to F in Fig. 6 is the line of back-pressure, or period of exhaust, during the return-stroke. This line extends from the be