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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 influence of even
the personality of the observer.


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