_Reversed Scale Notation._—With expressions of the form 1 − _x_, or 100 − _x_, it is often convenient to regard the scales as having their notation reversed, _i.e._, to read the scale backwards. When this is done the D scale is read as shown on the lower line— Direct Notation 1 2 3 4 5 6 7 8 9 10 D Scale Reversed Notation 9 8 7 6 5 4 3 2 1 0 The new reading can be found by subtracting the ordinary reading from 1, 10, 100, etc., according to the value assigned to the R.H. index, but actually it is unnecessary to make this calculation, as with a little practice it is quite an easy matter to read both the main and subdivisions in the reversed order. Applications are found in plotting curves, trigonometrical formulæ, etc. EX.—Find the per cent. of slip of a screw propeller from 100 − S = (10133V)/(PR) taking the speed, V, as 15 knots, the pitch of the propeller, P, as 27 ft. 6 in., and the revolutions per minute, R, as 60. Set 27·5 on B to 10133 on A (N.B.—Take the setting near the _centre_ index of A); bring the cursor to 15 on B and 60 on B to cursor. Reading the L.H. A scale backwards, the slip, S, = 8 per cent. is found on A over 10 on B. _Percentage Calculations._—To increase a quantity by _x_ per cent. we multiply by 100 + _x_; to diminish a quantity by _x_ per cent. we multiply by 100 − _x_. Hence, to add _x_ per cent., set 100 + _x_ on C to 1 on D and read new values on D under original values on C. To deduct _x_ per cent. read the D scale backwards from 10 and set R.H. index of C to _x_ per cent. so read. Then read as before. GAUGE POINTS.