The earth has been measured as a basis for a permanent standard of
length, and every property of metals has been investigated to guard
against any alteration of the material standards when made. To weigh
or measure any thing with modern accuracy, requires a course of
experiment and calculation in which almost every branch of physics and
mathematics is brought into requisition.

Yet, after all, the dimensions of our earth and its time of rotation,
though, relatively to our present means of comparison, very permanent,
are not so by any physical necessity. The earth might contract by
cooling, or it might be enlarged by a layer of meteorites falling on
it, or its rate of revolution might slowly slacken, and yet it would
continue to be as much a planet as before.

But a molecule, say of hydrogen, if either its mass or its time of
vibration were to be altered in the least, would no longer be a
molecule of hydrogen.

If, then, we wish to obtain standards of length, time, and mass which
shall be absolutely permanent, we must seek them not in the
dimensions, or the motion, or the mass of our planet, but in the
wave-length, the period of vibration, and the absolute mass of these
imperishable and unalterable and perfectly similar molecules.

When we find that here, and in the starry heavens, there are
innumerable multitudes of little bodies of exactly the same mass, so
many, and no more, to the grain, and vibrating in exactly the same
time, so many times, and no more, in a second, and when we reflect
that no power in nature can now alter in the least either the mass or
the period of any one of them, we seem to have advanced along the path