The foregoing method being an inversion of the rule for multiplication, is easily remembered and is generally advised. Another plan is, however, preferable when a series of divisions are to be effected with a constant divisor—_i.e._, when _b_ in (_a_)/(_b_) = _x_ is constant. In this case 1 on the scale is set to the index and the pointer set to _b_; then if any value of a is brought to the pointer, the quotient _x_ will be found under the index. _Combined Multiplication and Division_, as (_a_ × _b_ × _c_)/(_m_ × _n_) = _x_, can be readily performed, while cases of continued multiplication evidently come under the same category, since _a_ × _b_ × _c_ = (_a_ × _b_ × _c_)/(1 × 1) = _x_. Such cases as _a_/(_m_ × _n_ × _r_) = _x_ are regarded as (_a_ × 1 × 1 × 1)/(_m_ × _n_ × _r_) = _x_; while (_a_ × _b_ × _c_)/(_m_) = _x_ is similarly modified, taking the form (_a_ × _b_ × _c_)/(_m_ × 1) = _x_. In all cases the expression must be arranged so that there is _one more factor in the numerator_ than _in the denominator_, _1’s being introduced as often as required_. The simple operations of multiplication and division involve a similar disposition of factors, since from the rules given it is evident that _m_ × _n_ is actually regarded as (_m_ × _n_)/(1), while (_m_)/(_n_) becomes in effect (_m_ × 1)/(_n_). It is important to note the general applicability of this arrangement-rule, as it will be found of great assistance in solving more complicated expressions. As with the ordinary form of slide rule, the factors in such an expression as (_a_ × _b_ × _c_)/(_m_ × _n_) = _x_ are taken in the order:—1st factor of numerator; 1st factor of denominator; 2nd factor of numerator; 2nd factor of denominator, and so on; the 1st factor as _a_ being set to the index, and the result _x_ being finally read at the same point of reference. EX.—(39 × 14·2 × 6·3)/(1·37 × 19) = 134. Commence by setting 39 to the index, and the pointer to 1·37; bring 14·2 to the pointer; pointer to 19; 6·3 to the pointer, and read the result 134 at the index. It should be noted that after the first factor is set to the fixed index, the _pointer_ is set to each of the _dividing_ factors as they enter into the calculation, while the _dial_ is moved for each of the _multiplying_ factors. Thus the dial is first moved (setting the first factor to the index), then the pointer, then the dial, and so on. _Number of Digits in the Result._—If rules are preferred to the plan of roughly estimating the result, the general rules given on pages 21 and 25 should be employed for simple cases of multiplication and division. For combined multiplication and division, modify the expression, if necessary, by introducing 1’s, as already explained, and subtract the sum of the denominator digits from the sum of numerator digits. Then proceed by the author’s rule, as follows:— _Always turn dial to the_ LEFT; _i.e._, _against the hands of a watch_.