The Electric Telegraph has become an instrument in the hands of the
astronomer for determining the difference of longitude between two
observatories. Thus in 1854 the difference of longitude between London
and Paris was determined within a limit of error which amounted barely
to a quarter of a second. The sudden disturbances of the magnetic
needle, when freely suspended, which seem to take place simultaneously
over whole continents, if not over the whole globe, from some
unexplained cause, are pointed out as means by which the differences of
longitude between the magnetic observatories may possibly be determined
with greater precision than by any yet known method.
So long ago as 1839 Professor Morse suggested some experiments for
the determination of Longitudes; and in June 1844 the difference of
longitude between Washington and Baltimore was determined by electric
means under his direction. Two persons were stationed at these two
towns, with clocks carefully adjusted to the respective spots; and
a telegraphic signal gave the means of comparing the two clocks
at a given instant. In 1847 the relative longitudes of New York,
Philadelphia, and Washington were determined by means of the electric
telegraph by Messrs. Keith, Walker, and Loomis.