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


§ 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