conclusions are indispensable, since they baldly set forth our
dilemma in regard to the measurement of space and time. We can
measure neither except relatively, because they must be measured one
by the other, and no matter how they vary, these variations always
compensate one another, leaving us in the same state of ignorance
that we were in before.

Suppose that two intelligent beings, one on Mars, let us say, and
the other on the earth, should attempt to establish _the same moment
of time_, by the interchange of light signals, or by any other
method which the most rigorous science could devise. Assume that
they have for this purpose two identically similar and mechanically
perfect chronometers, and that every difficulty of manipulation were
successfully overcome. Their experiment could end only in failure,
and the measure of this failure neither one, in his own place, could
possibly know. If, after the experiment, the Martian, chronometer in
hand, could be instantly and miraculously transported to the earth,
and the two settings compared, they would be found to be different:
how different, we do not know.

The reason for the failure of any such experiment anywhere conducted
can best be made plain by a crude paraphrase of a classic
proposition from Relativity. Suppose it is required to determine the
same moment of time at two different places on the earth's surface,
as must be attempted in finding their difference in longitude. Take
the Observatory at Greenwich for one place, and the observatory at
Washington for the other. At the moment the sun is on the meridian
of Greenwich, the exact time of crossing is noted and cabled to
Washington. The chronometer at Washington is set accordingly, and
the time checked back to Greenwich. This message arrives two seconds,