the
legal and the scientific definition of the metre.

Perhaps it may not be useless to briefly indicate here the reasons of
the disagreement which had taken place. Two definitions of the metre
can be, and in fact were given. One had for its basis the dimensions
of the earth, the other the length of the material standard. In the
minds of the founders of the metrical system, the first of these was
the true definition of the unit of length, the second merely a simple
representation. It was admitted, however, that this representation had
been constructed in a manner perfect enough for it to be nearly
impossible to perceive any difference between the unit and its
representation, and for the practical identity of the two definitions
to be thus assured. 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.

We may even consider that, looked at theoretically, its permanence
would not be assured. Nothing, in fact, proves that sensible
variations may not in time be produced in the value of an arc of the
meridian, and serious difficulties may arise re