Reprint from THE "HOWTODOIT" BOOKSPRACTICAL MECHANICS FOR BOYS

To find the circumference of a circle: Multiply the diameter by 3.1416.
To find the diameter of a circle: Multiply the circle by .31831.
To find the area of a circle: Multiply the square of the diameter by .7854.
To find the area of a triangle: Multiply the base by onehalf the perpendicular height.
To find the surface of a ball: Multiply the square of the diameter by 3.1416.
To find the solidity of a sphere: Multiply the cube of the diameter by .5236.
To find the cubic contents of a cone: Multiply the area of the base by onethird the altitude.
Doubling the diameter of a pipe increases its capacity four times.
To find the pressure in pounds per square inch of a column of water: Multiply the height of the column in feet by .434.
Standard Horsepower: The evaporation of 30 pounds of water per hour from a feed water temperature of 1,000 degrees Fahrenheit into steam at 70 pounds gauge pressure.
To find the capacity of any tank in gallons: Square the diameter in inches, multiply by the length, and then by .0034.
In making patterns for aluminum castings provision must be made for shrinkage to a greater extent than with any other metal or alloy.
The toughness of aluminum can be increased by adding a small per cent. of phosphorus.
All alloys of metals having mercury are called amalgams.
A sheet of zinc suspended in the water of a boiler will produce an electrolytic action and prevent scaling to a considerable extent.
Hydrofluoric acid will not affect a pure diamond, but will dissolve all imitations.
A strong solution of alum put into glue will make it insoluble in water.
A grindstone with one side harder than the other can have its flinty side softened by immersing that part in boiled linseed oil.
One barrel contains 3 3/4 cubic feet.
One cubic yard contains 7 barrels.
To find the speed of a driven pulley of a given diameter: Multiply the diameter of the driving pulley by its speed or number of revolutions. Divide this by the diameter of the driven pulley. The result will be the number of revolutions of the driven pulley.
To find the diameter of a driven pulley that shall make any given number of revolutions in the same time: Multiply the diameter of the driving pulley by its number of revolutions, and divide the product by the number of revolutions of the driven pulley.
A piece of the wellknown tar soap held against the inside of a belt while running will prevent it from slipping, and will not injure the belt.
Boiler scale is composed of the carbonate or the sulphate of lime. To prevent the formation it is necessary to use some substance which will precipitate these elements in the water. The cheapest and most universally used for this purpose are soda ash and caustic soda.
Gold bronze is merely a mixture of equal parts of oxide of tin and sulphur. To unite them they are heated for some time in an earthen retort.
Rusted utensils may be cleaned of rust by applying either turpentine or kerosene oil, and allowing them to stand over night, when the excess may be wiped off. Clean afterwards with fine emery cloth.
Plaster of paris is valuable for many purposes in a machine shop, but the disadvantage in handling it is, that it sets so quickly, and its use is, therefore, very much limited. To prevent quick setting mix a small amount of arrow root powder with the plaster before it is mixed, and this will keep it soft for some time, and also increase its hardness when it sets.
For measuring purposes a tablespoon holds 1/2 ounce; a dessertspoon 1/4 ounce; a teaspoon 1/8 ounce; a teacupful of sugar weighs 1/2 pound; two teacupsful of butter weigh 1 pound; 1 1/3 pints of powdered sugar weigh 1 pound; one pint of distilled water weighs 1 pound.
Ordinarily, 450 drops of liquid are equal to 1 ounce; this varies with different liquids, some being thicker in consistency than others, but for those of the consistency of water the measure given is fairly accurate.
To Chapter XIV  Simplicity of Great Inventions and of Nature's Manifestation
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