The rate at which
electricity travels has been very variously estimated. Fizeau asserted
that its velocity in copper wire was 111,780 miles a second; Walker
that it only travels 18,400 miles through that medium during the same
interval; while the experiments made in the United States during the
determination of the longitudes of various stations there still further
reduced the rate of motion to some 16,000 miles a second. Whichever of
these values we adopt, however, we may take it for our present purpose,
that the transmission of a message by the electric telegraph is
practically instantaneous. But be it here noted, there is no such a
thing as a _hora mundi_ or common time for the whole world. What is
familiarly known as longitude is really the difference in time, east
or west, from a line passing through the north and south poles of
the earth; and the middle of the great transit circle is the Royal
Observatory at Greenwich. If in the latitude of London (51 deg. 30' N.),
we proceed 10 miles and 1,383 yards either in an easterly or westerly
direction, we find that the local time is respectively either one minute
faster or one minute slower than it was at our initial point.
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