Pulleys should always be of ample breadth for the power they have to transmit; and it is more economical both in power and cost, to use broad single belting than the same strength in double. If the pulley will not take a belt broad enough for the work it has to do, a second belt may be made to run on the top of the first, and will do its share of the work. Belts should be washed occasionally with soap and tepid water, and oiled with cod-oil; but if of sufficient breadth, should not require the use of rosin, or adhesive materials, to make them grip the pulley. Makers of machines often err in constructing their driving pulleys too small both in breadth and diameter. The horse-power which a belt is capable of transmitting obviously varies extremely with circumstances, but may be approximately calculated by the formula (_a_ · _v_)/66000, where _a_ is the area of contact of the belt with the smallest pulley, and _v_ its velocity in feet per minute. Another rule is, that at a velocity of 1000 ft. per min. each inch of breadth of belt should transmit 2-1/2 horse-power on metal pulleys, or 5 on wooden ones, on which the adhesion is greater. Adhesion may also be increased by covering the pulleys with leather or india-rubber. Both rules assume that the belt is of ample strength. One horse-power would be transmitted by a belt running 1000 ft. per min. with a pull of 33 lb. A good single belt should not break with a much less strain than 1000 lb. per inch of breadth, and should stand about 1/10 as much as a working strain.