We start with the first row of the grid, where we find the number 12. We
look at the numbers in the left-hand column of the ruler: 1, 4, 9, 16, .... We
can stop before we go over 12 (we want to approximate the square root of
12 from below). In our case, we can stop at the third line, where we find 9
(the next, 16, is larger than 12). The 9 is then written below the 12, and
the number 3, which corresponds to the number of the row, is written next
to the 9, but outside the grid: it is the first digit of the answer we are
looking for. On the same row of the ruler, in the middle column, we find
the number 6. So we take Napier’s bone numbered 6 and place it to the
left of the ruler of squares: we will need it for the next part of the