ginning of the return-stroke to the point at which the exhaust-port is closed. In a condensing-engine the steam-pressure will fall below the atmosphericline, but in a non-condensing engine the pressure cannot fall to the atmospheric line, because there is always an amount of back-pressure, due to the force required to expel the exhaust-steam through the exhaust-passages and pipe against the resistance of the atmosphere. In a condensing-engine, the deeper the line of back-pressure measures from, and the more nearly parallel it is to, the atmospheric-line, the better. In a non-condensing engine, the nearer and more parallel the line of back-pressure is to the atmospheric-line the better, as back-pressure not only means a loss of force, but it diminishes the efficiency of the engine.

Point of Compression.—F, Fig. 6, is the point of compression. This line is formed by the closing of the exhaust-port at some point before the end of the return-stroke. The advancing piston compresses the confined steam into the clearance-space and passages, and provides a cushion which absorbs the momentum of the piston, and enables its motion to be reversed without shock. The rise of pressure is shown by the rising curve at F, and the portion of the stroke between F and A is the period of compression or cushioning. Excessive compression

Fig. 13.-Diagram.

causes the confined steam to rise above its initial pressure before pre-admission commences, as shown by the loop at the admission-corner in Fig. 13, consequently, when the port is opened, part of the confined steam flows from the cylinder into the steam chest, and the pressure is reduced and the steam line is lowered, as shown in Fig. 13. In slow running engines only a small amount of cushioning is necessary, but in high-speed engines the cushioning should be so adjusted that the confined steam is compressed up to its initial pressure. The compressed steam acts as an elastic spring, and gives out by its expansion the work expended in compressing it. The effect of compression is to fill the clearance-space with compressed steam, and save steam being taken from the boiler for that purpose.

The Line of Perfect Vacuum.-This line cannot be drawn by the indicator; it must be drawn by hand, parallel with the atmospheric line, and at the proper distance below it to represent, say, 14'7 lbs. per square inch, as the average pressure of the atmosphere, according to the scale of the diagram. In measuring the diagram of a condensing-engine, the distance between the vacuum-line of the diagram and the line of perfect vacuum, will show the quantity of uncondensed steam in the cylinder or the amount of back-pressure due to imperfect vacuum, slightly varying according to the barometric pressure. The temperature of the condensed water is usually about 100° F., or about 1 lb. pressure per square inch; but the pressure of air in the condenser prevents the pressure from falling

below 2 lbs. per square inch. The usual final-pressure is at least from 4 to 5 lbs. per square inch.

The initial-pressure of steam in a cylinder is always 4 or 5 lbs. less than the boiler-pressure; but when the fall of pressure is much more than this, it is due either to bends in the steam-pipes, or to the steam-pipes being too small, or to the steam-ports being too contracted.

To find the indicated horse-power of an engine from an indicator-diagram. Divide the diagram at right angles to the atmospheric-line into 10 equal parts, take the breadth in the middle between the divisions with the scale of the indicator, add them together, and divide by 10 (the number of divisions)—the result will be the mean or average pressure per square inch on the piston during the stroke; then multiply the area of the cylinder in square inches by the mean-pressure, and by the speed of the piston in feet per minute. The product divided by 33,000 gives the indicated horse-power of the engine.

The speed of the piston in feet per minute is found thus:-Multiply the length of stroke in feet by 2, and by the number of revolutions per minute. A deduction of 2 lbs. per square inch, from the gross diagram must be made for the friction of the engine alone; but if the diagram is taken when the load is on the engine, an additional deduction must be made of 5 per cent. for friction.

A constant may be found for any particular engine, which, being multiplied by the mean-pressure, will give the horse-power. To find the constant multiplier: multiply the area of the cylinder in square inches, by the speed of the piston in feet per minute, and divide the product by 33,000. The quotient will give the number of horse-power which would be produced by 1 lb. of steam of mean-pressure.

Example. Required the power of the engine from which diagram, Fig. 14, was taken. Diameter of cylinder, 12 inches; length of stroke, 2 feet; number of revolutions, 80 per minute. The mean pressure according to the diagram is 32.2 lbs., from which deduct 2 lbs. for the

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43

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33,000

friction of the engine, leaving 30°2 lbs. pressure; the area of the cylinder is 113 inches; then 113×30 2x2x2x80 = 33, indicated horse-power.

STEAM-PRESSURE.

Pressure of Steam. The pressure of steam is equal in all directions, therefore each square inch of surface exposed to its action must be equally capable of bearing the given pressure. The pressure is measured from that of the atmosphere, or 147 lbs. per square inch.

Effective-Pressure.—In a non-condensing engine the pressure of the steam is opposed by that of the atmosphere, therefore only pressures above that of the atmosphere are effective for work, and a deduction must also be made for the resistance due to back-pressure, caused by the resistance of the exhaust-passages, which may be reckoned at 2 lbs. per square inch. In a condensing-engine the pressure of the steam is only opposed by a back pressure of from 2 to 3 lbs. per square inch, due to imperfect vacuum.

The initial-pressure of steam is its pressure when admitted to the cylinder. The final-pressure of steam is its pressure when discharged from the cylinder.

The mean-pressure is the average pressure upon the piston through the whole stroke.

The effective mean-pressure is the mean-pressure less the back-pressure. The ratio of expansion is the proportion which the final volume bears to the initial volume of steam.

The relative volume of steam is the volume of steam generated from a given volume of water divided by this volume.

The absolute-pressure of steam is the pressure of steam given by the steam-gauge plus the pressure of the atmosphere.

To find the quantity of steam used by an engine, multiply the area of the cylinder in square feet by the speed of the piston in feet per minute, and divide the result by the nominal ratio of expansion. The result will be the number of cubic feet of boiler-pressure-steam consumed per minute, to which 10 per cent. must be added for the clearance of the cylinder and capacity of the steam-passages.

To find the pressure in lbs. per square inch of the steam at any point of the period of expansion, multiply the initial-pressure by the distance moved by the piston when the steam is cut off, and divide the product by the distance of the given point from the beginning of the stroke.

To find the point to cut off the steam for a given actual ratio of expansion, add the clearance to the length of stroke and divide by the ratio of expansion; from the quotient deduct the clearance, and the remainder will be the point of the stroke at which to cut off the steam.

Steam-Table.-The temperature, weight, and relative volume of steam for various pressures are given in the following Table by Mr. D. K. Clark.

Table 1 (continued).-PROPERTIES OF SATURATED STEAM.

One atmosphere, 14'706 lbs. pressure per square inch,=29.92 inches of mercury; each lo. pressure per square inch is equal to a column of mercury 2035 inches, or 1'018 rise in a syphon-gauge.

Steam worked Expansively.-Steam, in its ordinary condition as saturated steam, although it is not a perfect gas, performs work in the cylinder of an engine practically the same as if it acted on the principle of a perfect gas. Hence the curve described by the pencil of an indicator, indicating the falling pressure of dry saturated steam expanding behind an advancing piston, is nearly hyperbolic in its nature, or such that the proaucts of the pressures at all points of the stroke, multiplied by the respecuve volumes of the steam, are equal to each other.

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