1270

THEORY AND PRACTICE OF ENGINEERING. Book II.

the air and condensed steam ascend through a foot-valve into the air-pump. The opening

and closing of the steam passages is performed by the eccentric, fixed on the crank shaft,the action of which communicates a reciprocal motion to the rod, which, by a bent lever,

moves the connecting rod and lever fixed at one extremity of a spindle, having a bevelled

wheel at the other, which works into another spindle of the steam-cock, admitting thesteam into the cylinder : on the fly-wheel shaft is a pair of bevelled wheels, communicatingmotion to the governor balls, which keep the valve on the steam pipes open a longer orshorter time, according to the velocity of the engine.

Application of Steam Engines for raising Water. — By means of an engine acting ex-pansively, 280,000 cube feet of water may be raised by one bushel of coals a foot inheight, and when a pump is applied for this purpose, its stroke should not exceed 8 feet,and the diameter of its cylinder 14 inches : the velocity of the piston should not exceed98 times the square root of the length of the stroke. The quantity of water in cube feetdelivered at each stroke of the pump is ’005J Sid 2 =the quantity in cube feet, / being thelength of the stroke, and d the diameter of the pump in inches. The power required to raisewater to a given height is found by taking the height in feet from the surface to the point ofdischarge, and adding a foot and a half for each lift, for the force required to give the waterthe velocity, and also add one-twentieth of the height for the friction of the piston, and callthis quantity in feet h ; then -341^d B = load in pounds, whence, if P = the mean effective

'341 A\3

force on the steam piston in pounds per circular inch, we have d= ( — — j =D, the dia-

meter of the steam piston in inches.

The quantity of work performed by one of the engines at Wheel Hope mine, Cornwall ,in 1826, was as follows : —

Diameter of the cylinderLoad per square inch on the pistonLength of stroke in cylinderNumber of strokes per minute -

60 inches.8*37 pounds.

9 feet.

5$ feet.

5 feet.

2 feet.

2 feet.

15 inches.12£ inches.11 inches.

Number of lifts, 1st, through 46 fathoms2d, 11 fathoms

3d, 11 fathoms

Diameter of pump for 1st lift -

11 fathoms11 fathoms

2d ditto3d ditto

Consumption of coals -Number of strokesLength of stroke in the pump

1242 bushels.261,890

8 feet

Number of pounds lifted one foot high by one bushel of coals - 46,838,246

Application of Steam Engines for raising Coals and Ores -A double engine of from twenty

to thirty horse-power is used, the size of the cylinder being such that the power shall beequal to the resistance when there is the greatest stress ; consequently engines require morefuel to raise such bodies to the required height, as much is lost by the stoppages and changesin motion : from 3 cwt. to 7 cwt. is usually raised at a time, and 1 pound of coal is cal-culated to lift about 70,000 pounds of ore. The engine should work expansively, andbe equalised by a fly-wheel, and regulated by a governor.

Application to Corn Mills . — The double expansive engine is usually employed, and withlow-pressure steam it will grind 14 bushels of wheat for each bushel of coals, whenworking properly; but the average is perhaps about 11^ only. To dress a bushel ofwheat, as well as grind it, per hour, is 31,000 pounds raised 1 foot per minute: thevelocity of the circumference of the millstone should be 23 feet per second, and with thatvelocity a pair of 5 feet stones should grind 4 or 5 bushels per hour.

Application to Water Works. — For this purpose, where fuel is expensive, double engineswith fly wheels are the most economical, and where it is cheap, single ones : in raisingwater care must be taken that the air-vessel should be in the direction of the motion of thefluid, for if placed on one side, the joints are liable to be torn asunder, the cranks broken,and the machinery disarranged, from the concussion which take place.

Application to Threshing Machines. — Engines of from four to six horse-power are used :the feeding rollers are 3£ inches in diameter, and make from 35 to 37£ revolutions perminute : the straw rakes are S£ inches in diameter, and make 30 revolutions per minute :the diameter of the drum is 3 feet 6 inches, and makes 300 revolutions in a minute.

The quantity of wheat threshed by a machine with feeding rollers, 4 feet broad, variesfrom 12 to 24 bushels per hour, according to its quality, and the quantity of oats from 16to 30 bushels. The power required is 100,000 pounds raised 1 foot per minute forthreshing alone, and 133,000 when winnowing is applied ; each inch of straw receives

three strokes of the beaters, and the stroke is made with a velocity of 55 feet per second, orthe beater should move 3300 feet per minute.