The product given, which is that read on the rule, is obtained as
follows:—Set R.H. index of C to 42 on D, and bring the cursor to 71 on
C. Next bring the L.H. index of C to the cursor, and the latter to 1·5
on C. This multiplication is effected with the slide to the right, and a
memorandum of this fact is kept by making a mark −. Bring the R.H. index
of C to the cursor and the latter to 0·32 on C. Then set the L.H. index
of C to the cursor and read the result, 1732, on D under 121 on C, while
as a slide again projects to the right, a second − memo-mark is
recorded. There are 2 + 2 + 1 + 0 + 3 = 8 digits in the factors, and as
there were 2 − marks recorded during the operation, there will be 8 − 2
= 6 digits in the product, which will therefore read 173,200
(173,194·56).
For a very rough evaluation of the result, we note that 1·5 × 0·3 is
about 0·5; hence, as a clue to the number of figures we have
40 × 70 × 60 = 3000 × 60 = 180,000.