



Produced by Ron Swanson





Vol. II. No. 1.

THE NATIONAL GEOGRAPHIC MAGAZINE.




PUBLISHED BY THE

NATIONAL GEOGRAPHIC SOCIETY.

WASHINGTON, D. C.


Price 50 Cents.




CONTENTS.


On the Telegraphic Determinations of Longitude by the Bureau of
  Navigation: Lieut. J. A. Norris, U. S. N.

Reports of the Vice-Presidents:
  Geography of the Land: Herbert G. Ogden
  Geography of the Air: A. W. Greely, Chief Signal Officer, U. S. A.

Annual Report of the Treasurer

Report of Auditing Committee

Annual Report of the Secretary

National Geographic Society:
  Abstract of Minutes
  Officers for 1890
  Members of the Society

  Published April, 1890.




PRESS OF TUTTLE, MOREHOUSE & TAYLOR, NEW HAVEN, CONN.




THE NATIONAL GEOGRAPHIC MAGAZINE.

Vol. II. 1890. No. 1.




ON THE TELEGRAPHIC DETERMINATIONS OF LONGITUDE BY THE BUREAU OF
NAVIGATION.

BY LIEUT. J. A. NORRIS, U. S. N.


The following definitions are given by Chauvenet in his Spherical and
Practical Astronomy.

"The longitude of a point on the earth's surface is the angle at the
Pole included between the meridian of that point and some assumed first
meridian. The difference of longitude between any two points is the
angle included between their meridians." To describe the practical
methods of obtaining this difference or angle, by means of the electric
telegraph both overland and submarine, and especially those employed by
the expeditions sent out by the Navy department, is the object of this
paper.

       *       *       *       *       *

Before the invention of the telegraph various methods more or less
accurate in their results were employed, and are still in use where the
telegraph is not available. The one most used and giving the best
results was that in which a number of chronometers were transported
back and forth between two places the difference of whose longitudes
was required. "For," as the author quoted above says, "the
determination of an absolute longitude from the first meridian or of a
difference of longitude in general, resolves itself into the
determination of the difference of the time reckoned at the two
meridians at the same absolute instant." If a chronometer be regulated
to the time at any place _A_, and then transported to a second place
_B_, and the local time at _B_, be determined at any instant, and at
that instant the time at _A_, as shown by the chronometer is noted, the
difference of the times is at once known, and that is the difference of
longitude required. The principal objection to this plan is that the
best chronometers vary. If the variations were constant and regular,
and the chronometer always gained or lost a fixed amount for the same
interval of time, this objection would disappear. But the variation is
not constant, the rate of gain or loss, even in the best instruments,
changes from time to time from various causes. Some of these causes may
be discovered and allowed for in a measure, others are accidental and
unknown. Of the former class are variations due to changes of
temperature. At the Naval Observatory, chronometers are rated at
different temperatures, and the changes due thereto are noted, and
serve to a great extent as a guide in their use. But the transportation
of a chronometer, even when done with great care is liable to cause
sudden changes in its indications, and of course in carrying it long
distances, numerous shocks of greater or less violence are unavoidable.
Still, chronometric measurements, when well carried out with a number
of chronometers and skilled observers have been very successful. Among
notable expeditions of this sort was that undertaken in 1843, by Struve
between Pulkova and Altona, in which eighty-one chronometers were
employed and nine voyages made from Pulkova to Altona and eight the
other way. The results from thirteen of the chronometers were rejected
as being discordant, and the deduced longitude was made to depend on
the remaining 68. The result thus obtained differs from the latest
determination by 0^{s}.2.

The U. S. Coast Survey instituted chronometric expeditions between
Cambridge, Mass., and Liverpool, England, in the years 1849, '50, '51
and '55. The probable error of the results of six voyages, three in
each direction, in 1855 was 0^{s}.19, fifty chronometers being carried.

Among other methods of determining differences of time may be mentioned
the observation of certain celestial phenomena, which are visible at
the same absolute instant by observers in various parts of the globe,
such as the instant of the beginning or end of an eclipse of the moon,
the eclipses of Jupiter's satellites by the shadow of the planet, the
bursting of a meteor, and the appearance or disappearance of a shooting
star. The difficulty of identifying these last mentioned objects and
the impossibility of foretelling their occurrence prevents the extended
use of this method.

Terrestrial signals may be used and among these can be included those
sent by the electric telegraph. But when two stations are near together
a signal may be made