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.