the aspects of nature, but which, having been
drawn up according to predetermined rules, allows us to follow an
ascertained road in the eternal journey towards the truth.

Among the provisional theories which are thus willingly constructed by
scholars on their journey, like edifices hastily run up to receive an
unforeseen harvest, some still appear very bold and very singular.
Abandoning the search after mechanical models for all electrical
phenomena, certain physicists reverse, so to speak, the conditions of
the problem, and ask themselves whether, instead of giving a
mechanical interpretation to electricity, they may not, on the
contrary, give an electrical interpretation to the phenomena of matter
and motion, and thus merge mechanics itself in electricity. One thus
sees dawning afresh the eternal hope of co-ordinating all natural
phenomena in one grandiose and imposing synthesis. Whatever may be the
fate reserved for such attempts, they deserve attention in the highest
degree; and it is desirable to examine them carefully if we wish to
have an exact idea of the tendencies of modern physics.

CHAPTER II

MEASUREMENTS

Sec. 1. METROLOGY

Not so very long ago, the scholar was often content with qualitative
observations. Many phenomena were studied without much trouble being
taken to obtain actual measurements. But it is now becoming more and
more understood that to establish the relations which exist between
physical magnitudes, and to represent the variations of these
magnitudes by functions which allow us to use the power of
mathematical analysis, it is most necessary to express each magnitude
by a definite number.

Under these conditions alone can a magnitude be considered as
effectively known. "I often say," Lord Kelvin has said, "that if you
can measure that of which you are speaking and express it by a number
you know something of your subject; but if you cannot measure it nor
express it by a number, your knowledge is of a sorry kind and hardly
satisfactory. It may be the beginning of the acquaintance, but you are
hardly, in your thoughts, advanced towards science, whatever the
subject may be."

It has now become possible to measure exactly the elements which enter
into nearly all physical phenomena, and these measurements are taken
with ever increasing precision. Every time a chapter in science
progresses, science shows itself more exacting; it perfects its means
of investigation, it demands mor