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 influence of even
the personality of the observer. This standard is no longer, as
formerly, a flat rule, weak and fragile, but a rigid bar, incapable of
deformation, in which the material is utilised in the best conditions
of resistance. For a standard with ends has been substituted a
standard with marks, which permits much more precise definition and
can be employed in optical processes of observation alone; that is, in
processes which can produce in it no deformation and no alteration.
Moreover, the marks are traced on the plane of the neutral fibres[2]
exposed, and the invariability of their distance apart is thus
assured, even when a change is made in the way the rule is supported.

[Footnote 2: The author seems to refer to the fact that in the
standard metre, the measurement is taken from the central one of three
marks at each end of the bar. The transverse section of the bar is an
X, and the reading is made by a microscope.--ED.]