earth on its axis, the tide-speed of day and night. To fix our idea,
this may be taken, in our latitudes, at eighteen thousand miles per
day, or perhaps half the speed of a Mauser rifle bullet.

So fast, then, will Washington have been moving to meet the message
from Greenwich. So fast will Greenwich have been retreating from
Washington's message.

Now the ultimate effect of motion on the time-determination cannot
be calculated along any such simple lines as these. Indeed, it
cannot be exactly calculated at all, for we have not all the data.
But there is certainly _some_ effect. Suppose one rows four miles up
a river against a current of two miles per hour, at a rowing speed
of four miles per hour. This will take two hours, plainly. The
return trip with the river's gift of two miles per hour will
evidently require but forty minutes. _Two hours and forty minutes_
for the round trip, then, of eight miles.

Now then, to row eight miles in still water, according to our
supposition, would have required but _two hours_. But, some one
objects, the current must help the return trip as much as it
hindered the outgoing! Ah, here is the snare that catches
rough-and-ready common sense! How long would the double journey have
taken _if the river current had been faster than our rowing speed_?
How shall we schedule our trip if we cannot learn the correct speed,
_or if it varies from minute to minute_?

These explanations are necessarily symbolistic rather than
demonstrative, but any one who will seriously follow out these lines
of thought, or, still better, study the attitude of the hard-headed