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Let us now imagine that a large number of little rods of equal

length have been made, their lengths being small compared with

the dimensions of the marble slab. When I say they are of equal

length, I mean that one can be laid on any other without the ends

overlapping. We next lay four of these little rods on the marble

slab so that they constitute a quadrilateral figure (a square),

the diagonals of which are equally long. To ensure the equality

of the diagonals, we make use of a little testing-rod. To this

square we add similar ones, each of which has one rod in common

with the first. We proceed in like manner with each of these

squares until finally the whole marble slab is laid out with

squares. The arrangement is such, that each side of a square

belongs to two squares and each corner to four squares.