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. At the end of the eighteenth
century, Delambre, in his work _Sur la Base du Systeme metrique
decimal_, clearly gives us to understand that magnitudes of the order
of the hundredth of a millimetre appear to him incapable of
observation, even in scientific researches of the highest precision.
At the present date the International Bureau of Weights and Measures
guarantees, in the determination of a standard of length compared with
the metre, an approximation of two or three ten-thousandths of a
millimetre, and even a little more under certain circumstances.

This very remarkable progress is due to the improvements in the method
of comparison on the one hand, and in the manufacture of the standard
on the other. M. Benoit rightly points out that a kind of competition
has been set up between the standard destined to represent the unit
with its subdivisions and multiples and the instrument charged with
observing it, comparable, up to a certain point, with that which in
another order of ideas goes on between the gun and the armour-plate.

The measuring instrument of to-day is an instrument of comparison
constructed with meticulous care, which enables us to do away with
causes of error formerly ignored, to eliminate the action of external
phenomena, and to withdraw the experiment from the influen