And a third says W = (36,000 H)/(6VL), where L is here in feet. Evan Leigh’s rule is W = (66,000 × IHP)/(L × V). L = length of arc of contact upon smaller pulley in inches. V = velocity of rim in feet per minute. _A belt_ transmits its motion solely through frictional contact with the surfaces of the pulley. The lower side of the belt should be made the driving side when possible, as the arc of contact is thereby increased by the sagging of the following side. Increase of power will be obtained by increasing the size of pulleys, the same ratio being retained. Wide belts are less effective per unit of sectional area than narrow belts. Long belts are more effective than short ones. The proportion between the diameters of two pulleys working together should not exceed six to one. Convexity of pulleys to receive belt = ½ inch per foot wide. The width of pulley should equal 1·2 times width of belt. _Speed of Belts._ Belts have been employed running over 5000 feet per minute. Nothing, however, is gained by running belts much over 4000 feet per minute. About 3500 feet per minute for main belts agrees with good practice; lathe belts from 1500 to 2000 feet per minute. The life of a belt may be prolonged and its driving powers increased by keeping it in good working order. To ensure this it should be dressed on the back with castor oil every few weeks, more or less according the dryness of the _atmosphere_ in which it works. WEIGHT AND BULK OF MATERIALS. The weight of a cubic foot of any material is its specific gravity multiplied by 62·425, or the weight of a cubic foot of water in pounds. To find the specific gravity of a stone, divide its weight in air by loss of weight in water of temperature of 60° F. = specific gravity. Thus: Quartz crystal weighs in air 293·7 grains. ” ” ” water 180·1 ” ----- Loss in weight 113·6 ” Then: 293·7 / 113·6 = 2·59 = Specific gravity of quartz. One ton of quartz when solid occupies 13 cubic feet, but when broken, about 20. Rocks when solid, as compared to the same when broken, usually increase in volume in the ratio of 1 to 1·5 or 1 to 1·18, the increase depending on size and form of fragments. A dwt. of gold in a cwt. of ore = 1 oz. of gold per ton of ore. For approximate calculation a grain of gold = two pence, and a dwt., four shillings. In the following table of the chemical elements the standard of sp. gr. is hydrogen for the gaseous elements (hydrogen, oxygen, &c.) and water for the others. THE CHEMICAL ELEMENTS, THEIR SYMBOLS, EQUIVALENTS, AND SPECIFIC GRAVITIES.