OF VALUE.

107

could not serve as a measure of their relative
length : there would in that case be no com-
mon medium of comparison. lt is essential to
the discovery of the mutual relation of two ob-
jects, which cannot be directly compared, that
their respective relations to some third object
should be known: but in this case, the ratio
which the trees were found to bear would not
be to the same object, and therefore nothing
could be told as to the ratio of the trees to
each other. Itis thus indispensable, that the
instrument employed as a measure should re-
main unaltered, or be altered in a known de-
gree, during its successive applications to the
objects measured, in order to give us their re-
lations to one common object. By this means
we obtain a common term or denomination, in
which the lengths of the two trees are ex-
pressed. This is, in fact, all that is essential
to the end in view: the measurement, that is,
the actual application of the physical instru-
ment to the object, is the means, and the un-
varying length of the instrument, or its ascer-