precision to which no other measurement can attain. Metrology vouches
for the hundredth of a milligramme in a kilogramme; that is to say,
that it estimates the hundred-millionth part of the magnitude studied.

We may--as in the case of the lengths--ask ourselves whether this
already admirable precision can be surpassed; and progress would seem
likely to be slow, for difficulties singularly increase when we get to
such small quantities. But it is permitted to hope that the physicists
of the future will do still better than those of to-day; and perhaps
we may catch a glimpse of the time when we shall begin to observe that
the standard, which is constructed from a heavy metal, namely,
iridium-platinum, itself obeys an apparently general law, and little
by little loses some particles of its mass by emanation.


§ 4. THE MEASURE OF TIME

The third fundamental magnitude of mechanics is time. There is, so to
speak, no physical phenomenon in which the notion of time linked to
the sequence of our states of consciousness does not play a
considerable part.

Ancestral habits and a very early tradition have led us to preserve,
as the unit of time, a unit connected with the earth's movement; and
the unit to-day adopted is, as we know, the sexagesimal second of mean
time. This magnitude, thus defined by the conditions of a natural
motion which may itself be modified, does not seem to offer all the
guarantees desirable from the point of view of invariability. It is
certain that all the friction exercised on the earth--by the tides,
for instance--must slowly lengthen the duration of the day, and must