Set 132 on C to the coefficient of friction on D, and read off the value found on D under the number of degrees in the arc of contact on C. Place this value on the scale of equal parts on the back of the slide, to the index mark in the aperture, and read the required ratio on D under the L.H. index of C. EX.—Find the tension ratio in a belt, assuming a coefficient of friction of 0·3 and an arc of contact of 120 degrees. Set 132 on C to 0·3 on D, and under 120 on C read 0·273. Place this on the scale to the index on the back of the rule, and under the L.H. index C read 1·875 on D, the required ratio. Given belt velocity and horse-power to be transmitted, to find the requisite width of belt, taking the effective tension at 50 lb. per inch of width. Set 660 on C to velocity in feet per minute on D, and opposite horse-power on D find width of belt in inches on C. Given velocity and width of belt, to find horse-power transmitted. Set 660 on C to velocity on D, and under width on C find horse-power transmitted on D. (N.B.—For any other effective tension, instead of 660 use as a gauge point:—33,000 ÷ tension.) Given speed and diameter of a cotton driving rope, to find power transmitted, disregarding centrifugal action, and assuming an effective working tension of 200 lb. per square inch of rope. Set 210 on B to 1·75 on D, and over speed in feet per minute on B read horse-power on A. EX.—Find the power transmitted by a 1¾in. rope running at 4000 ft. per minute. Set 210 on B to 1·75 on D, and over 4000 on B read 58·3 horse-power on A. Find the “centrifugal tension” in the previous example, taking the weight per foot of the rope as = 0·27_d_^2. Set 655 on C to the diameter, 1·75 in., on D, and over the speed, 4000 ft. on C, read centrifugal tension = 114 lb. on A. SPUR WHEELS. Given diameter and pitch of a spur wheel, to find number of teeth. Set pitch on C to π (3·1416) on D, and under any diameter on C read number of teeth on D. Given diameter and number of teeth in a spur wheel, to find the pitch. Set diameter on C to number of teeth on D, and read pitch on C opposite 3·1416 on D. Given the distance between the centres of a pair of spur wheels and the number of revolutions of each, to determine their diameters. To twice the distance between the centres on D, set the sum of the number of revolutions on C, and under the revolutions of each wheel on C find the respective wheel diameters on D. EX.—The distance between the centres of two spur wheels is 37·5 in., and they are required to make 21 and 24 revolutions in the same time. Find their respective diameters. Set 21 + 24 = 45 on C to 75 (or 37·5 × 2) on D, and under 21 and 24 on C find 35 and 40 in. on D as the respective diameters.