Practise reading values by setting 1 on C to some value on D and reading under 2, 3, 4, etc., on C, checking the readings by mental arithmetic. To the same end, find squares, square roots, etc., comparing the results with the actual values as given in tables. Practise setting both slide and cursor to values taken at random. Aim at accuracy; speed will come with practice. When in doubt as to any method of working, verify by making a simple calculation of the same form. Follow the orthodox methods of working until entirely confident in the use of the instrument, and even then do not readily make a change. If any altered procedure is adopted, first work a simple case and guard carefully against unconsciously lapsing into the usual method during the operation. Unless the calculation is of a straightforward character, time taken in considering how best to attack it (rearranging the expression if desirable) is generally time well spent. In setting two values together, set the cursor to one of them on the rule, and bring the other, on the slide, to the cursor line. In multiplying factors, as 57 × 0·1256, take the fractional value first. It is easier to set 1 on C to 1256 on D and read under 57 on C, than to reverse the procedure. When both values are eye-estimated, set the cursor to the second factor on C and read the result on D, under the cursor line. In continuous operations avoid moving the slide further than necessary, by taking the factors in that order which will keep the scale readings as close together as possible. SQUARES AND SQUARE ROOTS. We have seen that the relation which the upper scales bear to the lower set is such that over any number on D is its square on A, and, conversely, under any number on A is its square root on D, the same remarks applying to the C and B scales on the slide. Taking the values engraved on the rule, we have on D, numbers lying between 1 and 10, and on A the corresponding squares extending from 1 to 100. Hence the squares of numbers between 1 and 10, or the roots of numbers between 1 and 100, can be read off on the rule by the aid of the cursor. All other cases are brought within these ranges of values by factorising with powers of 10, as before explained. The more practical rule is the following:— _To Find the Square of a Number_, set the cursor to the number on D and read the required square on A under the cursor. The rule for _The Number of Digits in a Square_ is easily deducible from the rule for multiplication. If the square is read on the _left_ scale of A, it will contain _twice_ the number of digits in the original number _less_ 1; if it is read on the _right_ scale of A, it will contain _twice_ the number of digits in the original number. EX.—Find the square of 114.