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It is a veritable wonder that we can carry out this business

without getting into the greatest difficulties. We only need to

think of the following. If at any moment three squares meet at a

corner, then two sides of the fourth square are already laid,

and, as a consequence, the arrangement of the remaining two sides

of the square is already completely determined. But I am now no

longer able to adjust the quadrilateral so that its diagonals may

be equal. If they are equal of their own accord, then this is an

especial favour of the marble slab and of the little rods, about

which I can only be thankfully surprised. We must experience many

such surprises if the construction is to be successful.