The creators of the metrical system were persuaded
that the measurements of the meridian effected in their day could
never be surpassed in precision; and on the other hand, by borrowing
from nature a definite basis, they thought to take from the definition
of the unit some of its arbitrary character, and to ensure the means
of again finding the same unit if by any accident the standard became
altered. Their confidence in the value of the processes they had seen
employed was exaggerated, and their mistrust of the future
unjustified. This example shows how imprudent it is to endeavour to
fix limits to progress. It is an error to think the march of science
can be stayed; and in reality it is now known that the ten-millionth
part of the quarter of the terrestrial meridian is longer than the
metre by 0.187 millimetres. But contemporary physicists do not fall
into the same error as their forerunners, and they regard the present
result as merely provisional. They guess, in fact, that new
improvements will be effected in the art of measurement; they know
that geodesical processes, though much improved in our days, have
still much to do to attain the precision displayed in the construction
and determination of standards of the first order; and consequently
they do not propose to keep the ancient definition, which would lead
to having for unit a magnitude possessing the grave defect from a
practical point of view of being constantly variable.
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