USEFUL INFORMATION To find diameter of a circle multiply circumference by .31831. To find circumference of a circle multiply diameter by 3.1416. To find area of a circle multiply square of diameter by .7854. To find surface of a ball multiply square of diameter by 3.1416. To find side of an equal square multiply diameter by .8862. To find cubic inches in a ball multiply cube of diameter by .5236. Doubling the diameter of a pipe increases its capacity four times. A gallon of water (U. S. Standard) weighs 8½ lbs., and contains 231 cubic inches. A cubic foot of water contains 7½ gallons, 1,728 cubic inches, and weighs 62½ lbs. Double riveting is from 16 to 20 per cent, stronger than single. One cubic foot of anthracite coal weighs about 53 pounds. One cubic foot of bituminous coal weighs from 47 to 50 pounds. One ton of coal is equivalent to two cords of wood for steam purposes. There are nine square feet of heating surface to each square foot of grate surface. The average consumption of coal for steam boilers is 12 lbs. per hour for each sq. ft. of grate surface. Each nominal horse power of a boiler requires 30 to 35 lbs. of water per hour. A horse power is equivalent to raising 33,000 lbs. one ft. per minute, or 550 lbs. one ft. per second. To sharpen dull files lay them in dilute sulphuric acid until they are eaten deep.