To obtain a close approximation to the diameter of pulley required, multiply the diameter of the driver in inches (say) by its speed in revolutions per minute, and divide by the speed in revolutions per minute required from the driven pulley. Assuming the pulley on the engine-shaft to be 22 in. in diameter, its speed being 221) revolutions per minute, and that the speed of the driven pulley is to be 110 revolutions per minute, the diameter of the driven pulley should be (22x220)/110 = 44 in. If the required speed were 300 revolutions per minute, the pulley should be about(22x220)/300 = 16 in. (say). For greater exactness the thickness of the belt should be known; in making the calculation this should be added to the diameter of the driving pulley and subtracted from the quotient to obtain the diameter of the driven pulley.