



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 at either or at an intermediate station, which can
be observed at both, the time may be noted at each of the stations and
the difference found directly. These signals may be made by flashes of
gunpowder, or the appearance and disappearance of a strong light, or a
pre-concerted movement of any object easily seen. The heliotrope
reflecting the image of the sun from one station to the other with an
arrangement for suddenly eclipsing it, is a useful and efficient
apparatus.

Various truly astronomical methods have been employed with good
results, of these may be mentioned moon-culminations, azimuths of the
moon, lunar distances, etc.

Coming now to the use of the electric telegraph for this purpose the
following is a rough outline of the methods employed. Suppose two
stations A and B connected by wire, and provided with clocks,
chronographs and transit instruments. A list of suitable fixed stars is
compiled and each observer furnished with a copy. The observer at A the
eastern station, selects a star from his list and sets his transit
instrument upon it. He is furnished with a key by which he can send
telegraphic signals over the line and also mark the time on his own
chronograph. The instant he observes the star crossing the spider line
which represents the meridian, he taps his key, thus registering the
time on his own chronograph and on that at station B and this operation
he repeats with as many stars as necessary. B has his instrument set
for the first star, and when it crosses his meridian, he taps his key
marking the time on his own chronograph and also on A's. Then,
disregarding instrumental and personal errors and the rate of the
clock, A has a record of the times at which the star passed both
meridians. The difference of these times is the difference of longitude
sought, except for an error due to the time occupied in the
transmission of the signal over the wire between the stations. B also
has a record of the same difference of time with the same error
affecting it in the opposite way. A mean of these two differences, will
be the true difference with the error of transmission eliminated. This
method has the advantage of not depending upon the computed position of
the star. The instrumental errors may be allowed for, as well as the
rate of the clocks, and the personal error may be eliminated by the
exchange of stations.

There are disadvantages inseparable from this method, however,
especially when the meridian distance is great. A star observed at the
first station, may be obscured by clouds at the time of its meridian
passage at the second. And the weather generally, at the two stations
may be cloudy, so that while stars can be observed at intervals, yet it
may be impossible to note the meridian passage of the same star at both
