e and more exactitude, and one of the
most striking features of modern physics is this constant care for
strictness and clearness in experimentation.

A veritable science of measurement has thus been constituted which
extends over all parts of the domain of physics. This science has its
rules and its methods; it points out the best processes of
calculation, and teaches the method of correctly estimating errors and
taking account of them. It has perfected the processes of experiment,
co-ordinated a large number of results, and made possible the
unification of standards. It is thanks to it that the system of
measurements unanimously adopted by physicists has been formed.

At the present day we designate more peculiarly by the name of
metrology that part of the science of measurements which devotes
itself specially to the determining of the prototypes representing the
fundamental units of dimension and mass, and of the standards of the
first order which are derived from them. If all measurable quantities,
as was long thought possible, could be reduced to the magnitudes of
mechanics, metrology would thus be occupied with the essential
elements entering into all phenomena, and might legitimately claim the
highest rank in science. But even when we suppose that some magnitudes
can never be connected with mass, length, and time, it still holds a
preponderating place, and its progress finds an echo throughout the
whole domain of the natural sciences. It is therefore well, in order
to give an account of the general progress of physics, to examine at
the outset the improvements which have been effected in these
fundamental measurements, and to see what precision these improvements
have allowed us to attain.

Sec. 2. THE MEASURE OF LENGTH

To measure a length is to compare it with another length taken as
unity. Measurement is therefore a relative operation, and can only
enable us to know ratios. Did both the length to be measured and the
unit chosen happen to vary simultaneously and in the same degree, we
should perceive no change. Moreover, the unit being, by definition,
the term of comparison, and not being itself comparable with anything,
we have theoretically no means of ascertaining whether its length
varies.

If, however, we were to note that, suddenly and in the same
proportions, the distance between two points on this earth had
increased, that all the planets had moved further from each other,
that all objects arou