(3·14)/(785 × 0·00021 × 4·3 × 64·4) = 0·0688,
may be worked by repeating as follows:—Set 7·85 on C to 3·14 on D, bring
cursor to index of C, 2·1 on C to cursor, cursor to index, 4·3 to
cursor, cursor to index, 6·44 to cursor, and under index of C read 688
on D as the significant figures of the answer.
For the number of figures in the result, we deduct the sum of the number
of digits in the several factors and add 1 for each time the slide
projects to the right, which in this case occurs once. There are 3 +
(−3) + 1 + 2 = 3 denominator digits, 1 numerator digit, and 1 is to be
added to the difference. Therefore there are 1 − 3 + 1 = −1 digits in
the answer, which is therefore 0·0688. The foregoing method of working
may confuse the beginner, who is apt to fall into the process of
continued multiplication. For this reason, until familiarity with
combined methods has been acquired, the product of the several
denominators should be first found by the continued multiplication
process, and the figures in this product determined. Then divide the
numerator by this product to obtain the result.
As the denominator product will be read on D, we may avoid resetting the
slide by bringing the numerator on C to this product and reading the
result on C _over_ the index of D. The slide and rule have here changed
places; hence if rules are followed for the number of figures in the
result, 1 must be added to the difference of digits, when the _rule
projects_ to the _right of the slide_.
The author’s method of recording the number of times division is
performed with the slide to the right is by vertical memorandum marks,
thus |. The full significance of these memo-marks will appear in the
following section.
For a rough calculation to fix the decimal point, in this example we
move the decimal points in the factors, obtaining
(3)/(0·8 × 2 × 4 × 6) = (3)/(40) = 0·075.