ogy
was vague, but they certainly had in view the supply of a standard for
commercial transactions, and it is quite evident that in barter what
is important to the buyer as well as to the seller is not the
attraction the earth may exercise on the goods, but the quantity that
may be supplied for a given price. Besides, the fact that the founders
abstained from indicating any specified spot in the definition of the
kilogramme, when they were perfectly acquainted with the considerable
variations in the intensity of gravity, leaves no doubt as to their
real desire.

The same objections have been made to the definition of the
kilogramme, at first considered as the mass of a cubic decimetre of
water at 4 deg. C., as to the first definition of the metre. We must
admire the incredible precision attained at the outset by the
physicists who made the initial determinations, but we know at the
present day that the kilogramme they constructed is slightly too heavy
(by about 1/25,000). Very remarkable researches have been carried out
with regard to this determination by the International Bureau, and by
MM. Mace de Lepinay and Buisson. The law of the 11th July 1903 has
definitely regularized the custom which physicists had adopted some
years before; and the standard of mass, the legal prototype of the
metrical system, is now the international kilogramme sanctioned by the
Conference of Weights and Measures.

The comparison of a mass with the standard is effected with a
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.

Sec. 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 li