Set the L.H. index of C to the amount of £1 at the given rate of interest on D, and find the logarithm of this by reading on the reverse side of the rule, as explained on page 46. Multiply the logarithm, so found, by the period, and set the result, on the scale of equal parts, to the index on the under-side of the rule; then opposite any sum on C read the amount (including compound interest) on D. EX.—Find the amount of £500 at 5 per cent. for 6 years, with compound interest. Set L.H. index of C to £1·05 on D, and read at the index on the scale of equal parts on the under-side of rule, 0·0212. Multiply by 6, we obtain 0·1272, which, on the scale of equal parts, is placed to the index in the notch at the end of the rule. Then opposite 500 on C read £670 on D, the amount required, including compound interest. MISCELLANEOUS CALCULATIONS. To calculate percentages of compositions. Set weight (or volume) of sample on C, to weight (or volume) of substance considered, on D; then under index of C read required percentage on D. EX.—A sample of coal weighing 1·25 grms. contains 0·04425 grm. of ash. Find the percentage of ash. Set 1·25 on C to 0·04425 on D, and under index on C read 3·54, the required percentage of ash on D. Given the steam pressure P and the diameter _d_ in millimetres, of the throat of an injector, to find the weight W, of water delivered in lb. per hour from W = (_d_^2√̅P)/(0·505). Set 0·505 on C to P on A; bring cursor to _d_ on C and index of C to cursor. Then under _d_ on C read delivery of water on D. To find the pressure of wind per square foot, due to a given velocity in miles per hour. Set 1 on B to 2 on A, and over the velocity in miles per hour on D read pressure in lb. per square foot on B. To find the kinetic energy of a moving body. Set 64·4 on B to velocity in feet per second on D, and over weight of body in lb. on B read kinetic energy or accumulated work in foot-lb. on A. TRIGONOMETRICAL APPLICATIONS _Scales._—Not the least important feature of the modern slide rule is the provision of the special scales on the under-side of the slide, and by the use of which, in conjunction with the ordinary scales on the rule, a large variety of trigonometrical computations may be readily performed.