Set index of B to head in feet on A, and under 64·4 on B read theoretical velocity in feet per second on D. HORSE-POWER OF WATER WHEELS. To find the effective horse-power of a Poncelet water wheel. Set 880 on C to cubic feet of flow of water per minute on D, and under height of fall in feet on C, read effective horse-power on D. For breast water wheels use 960, and for overshot wheels 775, in place of 880 as above. ELECTRICAL ENGINEERING. To find the resistance per mile, in ohms, of copper wire of high conductivity, at 60° F. the diameter being given in mils. (1 mil. = 0·001 in.). Set diameter of wire in mils. on C to 54,900 on A, and over R.H. or L.H. index of B read resistance in ohms on A. EX.—Find the resistance per mile of a copper wire 64 mils. in diameter. Set 64 on C to 54,900 on A, and over R.H. index of B read 13·4 ohms on A. To find the weight of copper wire in lb. per mile. Set 7·91 on C to diameter of wire in mils. on D, and over index of B read weight per mile on A. Given electromotive force and current, to find electrical horse-power. Set 746 on C to electromotive force in volts on D, and under current in ampères on C read electrical horse-power on D. Given the resistance of a circuit in ohms and current in ampères, to find the energy absorbed in horse-power. Set 746 on B to current on D, and over resistance on B read energy absorbed in H.P. on A. EX.—Find the H.P. expended in sending a current of 15 ampères through a circuit of 220 ohms resistance. Set 746 on B to 15 on D, and over 220 on B read 66·3 H.P. on A. COMMERCIAL. To add on percentages. Set 100 on C to 100 + given percentage on D, and under original number on C read result on D. To deduct percentages. Set R.H. index of C to 100 − the given percentage on D, and under original number on C read result on D. EX.—From £16 deduct 7½ per cent. Set 10 on C to 92·5 on D and under 16 on C, read 14·8 = £14, 16s. on D. To calculate simple interest. Set 1 on C to rate per cent. on D; bring cursor to period on C and 1 on C to cursor. Then opposite any sum on C find simple interest on D. For interest per annum. Set R.H. index on C to rate on D, and opposite principal on C read interest on D. EX.—Find the amount with simple interest of £250 at 8 per cent., and for a period of 1 year and 9 months. Set 1 on C to 8 on D; bring cursor to 1·75 on C, and 1 on C to cursor; then opposite 250 on C read £35, the interest, on D. Then 250 + 35 = £285 = the amount. To calculate compound interest.