creased of the centre of the suspension-pin from the centre of the spindle. Although wrong in principle, the arms are frequently hung away from the centre of the spindle, as in Fig. 9; and in calculating such governors, the vertical height is to be taken from the plane line, P, to the top of the cone, T, instead of the actual centre of suspension.

To find the power of a governor, multiply the weight of the balls in lbs. by the vertical height they are lifted.

To find the vertical height, H, between the point of suspension and the plane of revolution, P, divide the constant number 1875 by the number of revolutions of the governor, and square the quotient, which will give the height in inches.

Diameter of Cast-iron Balls for Ordinary Governors, B.-The weight of the balls must be sufficient to overcome the resistance of the valve and its connections. In ordinary cases the diameter of each ball may be equal to one half the height of plane line, H, in inches.

Length of Governor Arms.—First determine the vertical height from

Fig. 9.

Ad

Fig. 10.

the plane of revolutions to point of suspension of arm, H, Fig. 10; then set out the centre lines of the arms at an angle of 60°, as their position at the proper speed of the governor, and where the said centre lines of arms cut the plane line will be the centres of the balls, and the length of arm will be the distance between the centre of suspension and the centre of the ball thus found. The speed required to maintain the balls at that height is obtained by the following rule :

To find the speed of ordinary governors, divide the constant number, 1875, by the square root of the vertical height in inches between the plane of revolution and centre of suspension, and the quotient will be the number of revolutions per minute required to maintain the balls at that height.

Governors are driven from the engine crank shaft by means of pulleys or gearing, and the diameter of pulley or number of teeth in the wheel to produce the proper velocity may be found by the following rules :

To find the diameter of pulley (or number of teeth in the wheel) on the driving shaft of the governor. Multiply the number of revolutions of the engine per minute by the diameter of pulley (or number of teeth in the

C

wheel) on the engine crank shaft, and divide by the required number of revolutions per minute of the governor.

To find the diameter of pulley (or number of teeth in the wheel) on the engine crank shaft. Multiply the diameter of pulley (or number of teeth in the wheel) on the governor driving shaft by the number of revolutions per minute of the governor, and divide by the number of revolutions per minute of the engine.

Spring Governor.-In small engines, the governor is often placed horizontally, the centrifugal force being balanced by a spring placed inside the governor on the spindle. The tension of the spring is regulated by nuts to suit the required speed.

Cross-armed Governor with Centre Weight, Fig. 8.—In this class of governor, the centre of suspension must be calculated from the point where the arms cross each other in the centre-line of the spindle, and the vertical height is the distance from that point to the plane of revolution. By crossing the arms in this way the governor becomes very sensitive; when the speed is increased, the point of intersection of the crossed arms rises at the same rate as the plane of revolution, and the governor balls will remain in equilibrium in every angular position at the proper speed of the governor. This kind of governor is run at a high speed; the proportions may be calculated by the following rules for centre-weighted governors.

ENGINE GOVERNORS WITH CENTRE-WEIGHT.

Governor with Centre-weight, Fig. 11.-This form of governor requires to be driven at a high speed, so that he centrifugal force of the

balls may overcome the gravity of the centre-weight. Its advantages over the ordinary governor are: its extreme sensitiveness, whereby uniformity of speed is maintained under varying and sudden changes of the load on

the engine; and its great power, enabling a much smaller governor to be used.

To find the vertical height from the plane of revolution to the point of suspension of a governor with centre-weight. First, fix upon the number of revolutions, divide the constant number 187.5 by the number of revolutions the balls will make when the engine is at its proper speed, and square the quotient, which will give the height in inches for an ordinary governor, H; then add together the weight of the revolving balls and twice the weight of the centre-weight, which sum multiply by the height, H (found as for an ordinary governor), and divide the product by the sum of the weights of the revolving balls, the quotient will be the height of a centreweighted governor. If the centre-weight is hung by links at a point in the arm above the centre of the balls, like Fig. 12, then use the above rule, but instead of twice the weight of the centre-weight named above, use the product of twice the weight of the centre-weight, multiplied by the result of the length between the centre of suspension of the arm and the point where the link is hung on to the arm, subtracted from the length between the centre of the ball and the centre of suspension of the arm. To find the weight of the centre-weight. Find the vertical height by the above rule, both for a centre-weighted governor and for an ordinary governor, both at the same speed, then multiply the weight of the two revolving balls by the vertical height thus found for the centre-weighted governor, and divide the product by the vertical height thus found for an ordinary governor, which will give twice the weight of the centre-weight plus the two revolving balls, then subtract the weight of the two balls from that result, and divide the remainder by two, which will give the weight of centre-weight required.

The diameter of the revolving balls for governors like Fig. 11 should be equal to about 4th of the vertical height from the plane of revolution to the centre of suspension of the arm. The speed of these governors is from 200 to 300 revolutions per minute.

Example of the rules for Centre-weighted Governors.-A governor like Fig. 11 revolves at 260 revolutions per minute, the weight of the balls is 3 lbs. each, the weight of the centre weight is 84 lbs, required the vertical 1875 height. ='71, then '71 x71504, vertical height, then 5 (6+168) 260

= 87, then 14'5 inches, vertical height. Taking these particulars to

87 6

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 14'7 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 about 2 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 mean effective 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.

The temperature, weight, and relative volume of steam for various pressures are given at page 337.

LAP OF VALVE, ETC.

Lap of Valve necessary to cut the Steam off at a given part of the Stroke.-Rule: From the length of stroke in inches, deduct the distance in inches moved by the piston when the steam is cut off, divide the remainder by the stroke of the piston in inches, and extract the square root of the quotient, next multiply the result by half the stroke of the valve in inches, and deduct half the lead from the product, the remainder will be the required lap in inches.

Point of Cut-off of Steam from a given Lap.-Rule: To the lap of the valve on the steam side in inches add one half the lead, then divide by half the travel of the valve in inches, and multiply the square of the quotient by the length of stroke of the piston in inches; deduct the product from the length of stroke of the piston in inches, and the remainder will be the distance in inches the piston moves when the steam is cut off.

The steam in a compound engine, after driving the piston in one cylinder is exhausted into a second, and sometimes into a third cylinder, and acts on their pistons before being condensed in a condensing engine, or before being finally exhausted in a non-condensing engine. The saving of fuel effected by compounding is about 25 per cent. To obtain uniformity of rotative pressure upon the cranks they are placed at right angles. In a