In this case the division line 91 on C indicates on D that the answer
lies between 7180 and 7190. As the last figure must be 9, it is at once
inferred that the last two figures are 89.
When there are more than three figures in either or both of the factors,
the fourth and following figures to the right must be neglected. It is
well to note, however, that if the first neglected figure is 5, or
greater than 5, it will generally be advisable to increase by 1 the
third figure of the factor employed. Generally it will suffice to make
this increase in one of the two factors only, but it is obvious that in
some cases greater accuracy will be obtained by increasing both factors
in this way.
CONTINUED MULTIPLICATION.—To find the product of more than two factors,
we make use of the cursor to mark the position of successive products
(the value of which does not concern us) as the several factors are
taken into the calculation. Setting the index of C to the 1st factor on
D, we bring the line of the cursor to the 2nd factor on C, then the
index of C to the cursor, the cursor to the 3rd factor, index of C to
cursor, and so on, reading the final product on D under the last factor
on C. (Note that the 1st factor and the result are read on D; all
intermediate readings are taken on C.)
If the rule for the number of digits in a product is used, it is
necessary to note the number of times multiplication is effected with
the slide projecting to the right. This number, deducted from the sum of
the digits of the several factors, gives the number of digits in the
product. Ingenious devices have been adopted to record the number of
times the slide projects to the right, but some of these are very
inconvenient. The author’s method is to record each time the slide so
projects, by a minus mark, thus −. These can be noted down in any
convenient manner, and the sum of the marks so obtained deducted from
the sum of the digits in the several factors, gives the number of digits
in the product as before explained.