OF VALUE.

107

could not serve as a measure of their relative
length : there would in that case be no common
 medium of comparison. lt is essential to
the discovery of the mutual relation of two objects,
 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 remain
 unaltered, or be altered in a known degree,
 during its successive applications to the
objects measured, in order to give us their relations
 to one common object. By this means
we obtain a common term or denomination, in
which the lengths of the two trees are expressed.
 This is, in fact, all that is essential
to the end in view: the measurement, that is,
the actual application of the physical instrument
 to the object, is the means, and the unvarying
 length of the instrument, or its ascer-