Engines.
The strength of a steam engine is indicated in horse-powers. A horse-power is taken as the capacity of performing 33,000 foot-pounds of work in one minute; lifting 3300lb. 10 feet high, or 10lb. 3300 feet high would be 33,000 foot-pounds of work.
To obtain the Indicated Horse-power—the most usual Standard.—A diagram is taken from the engine by a small apparatus, and this diagram, when measured and averaged at different points of its length, gives the mean pressure of steam in the cylinder. Multiply this by the speed of the piston, by the area of the piston, and divide by 33,000, and the I.H.P. is to hand. 39·81 average pressure per square inch, area of piston 400 square inches, length of stroke 5-1/2 feet, strokes per minute 40 (or 11 feet both ways)—
(39·81 × 400 × 11 × 40)/33,000 = 212·32 I.H.P.
Nominal horse-power (condensing) = area of piston divided by 22; ditto high pressure = area of piston divided by 11.
2-1/2 to 3 looms, with preparation, are reckoned to 1 indicated horse-power.
Coal.—A good quality of coal should evaporate 8lb. of water for each 1lb. burnt, and for a manufacturing concern (including sizing, which takes a great amount of steam) the consumption of coal should be about 3-1/4lb. per I.H.P. per hour. Thus 600 horse-power should use about 24 tons per week; excluding sizing, 2-3/4lb. would suffice.
To find the Circumference of a Circle.—Multiply the diameter by 3·1416 or (roughly) by 3-1/7.
To find the Area of a Circular Space.—Square the diameter and multiply by ·7854.