M.Benoit, the learned Director of the International Bureau of Weights and Measures, has furnished in various reports very complete details on the subject, from which I here borrow the most interesting.
We know that in France the fundamental standard for measures of length was for a long time the _Toise du Chatelet_, a kind of callipers formed of a bar of iron which in 1668 was embedded in the outside wall of the Chatelet, at the foot of the staircase.

This bar had at its extremities two projections with square faces, and all the _toises_ of commerce had to fit exactly between them.

Such a standard, roughly constructed, and exposed to all the injuries of weather and time, offered very slight guarantees either as to the permanence or the correctness of its copies.

Nothing, perhaps, can better convey an idea of the importance of the modifications made in the methods of experimental physics than the easy comparison between so rudimentary a process and the actual measurements effected at the present time.
The _Toise du Chatelet_, notwithstanding its evident faults, was employed for nearly a hundred years; in 1766 it was replaced by the _Toise du Perou_, so called because it had served for the measurements of the terrestrial arc effected in Peru from 1735 to 1739 by Bouguer, La Condamine, and Godin.

At that time, according to the comparisons made between this new _toise_ and the _Toise du Nord_, which had also been used for the measurement of an arc of the meridian, an error of the tenth part of a millimetre in measuring lengths of the order of a metre was considered quite unimportant.