



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
places on the same night. Then the telegraph lines are usually the
property of some commercial company and while their use for a short
time might be freely granted, yet a protracted occupation of them as
necessary when the meridians are distant from each other, would prove a
serious hindrance to their regular business.

The method at this time most generally employed, is to observe at each
station a number of stars entirely independently of the other. From
these stars are deduced the clock errors and rates upon the respective
local times. Then at some prearranged period, communication is opened
between the stations, and a comparison of the clocks made which shows
their exact difference at a given instant. By applying the error to the
time as shown by the clock at this instant, the exact local time at
each station is the result, and applying the difference between the
clocks as shown by the comparison, the required difference of longitude
is readily obtained.

These methods originated, as did the electric telegraph, in the United
States, and soon after Morse's invention came into practical use, they
were extensively employed by the Coast Survey, in accurately
determining points in every part of the country that could be reached,
no pains being spared to make the determinations as accurate as
possible. Upon the completion of the first successful Atlantic cable in
1866, an expedition was organized and placed in charge of Dr. B. A.
Gould, for the purpose of measuring the meridian distance between
Greenwich and the Naval Observatory at Washington. This was
successfully carried out in spite of numerous difficulties, and the
result proved that the determinations already made upon which the most
reliance was placed were decidedly in error. The result from the
chronometric expedition in 1855 previously referred to differing over a
second of time.

In constructing charts for use at sea, the accurate determination of
latitude and longitude is of the utmost importance. The navigator
starting on a voyage must know the exact position of his destination as
well as the location of dangers to be avoided. He must know the error
and rate of his chronometer when he sets out, but as the rate is not
constant he should have some means of re-rating it at any place where
he may stop. If the longitude of this place is well determined, the
operation of obtaining the error and rate is an easy one, and may save
his vessel from loss.

Surveys, of coasts or countries must have well established starting
points, and while the latitude of a place is comparatively easy to
determine, the longitude, except when the telegraphic method is used,
is attended with more or less uncertainty.

In 1873, Commodore R. H. Wyman, U. S. N. Hydrographer to the Bureau of
Navigation, organized by permission of the Navy Department, an
expedition for the telegraphic determination of longitude in the West
Indies and Central America. The submarine cables of the West India and
Panama Telegraph Co. had just been completed, extending from Key West
through Havana and Santiago de Cuba, south to Jamaica and Aspinwall,
and east through the Virgin and Windward Islands to the northeast coast
of South America, thus affording admirable facilities for the accurate
determination of many points. It had long been known that the
longitudes of various points in the West Indies and in Central and
South America, did not harmonize, there having been no systematic
attempt to determine them with relation to each other or to a common
base. Longitudes in the western part of the Caribbean Sea depended upon
the position of the Morro lighthouse at Havana, which had been
determined by occultations. Further to the eastward, positions depended
upon that of Fort Christian at St. Thomas. This in its turn depended
upon the observatory of Major Lang in the Island of Santa Cruz about
forty miles distant. This position depended upon numerous observations
of moon culminations and occultations. Martinique and Guadeloupe in the
Windward Islands had been surveyed by French officers who based their
positions upon longitudes derived from moon culminations. The absolute
determination of these starting points would of course fix all points
derived from them.

The U. S. Steamer Fortune was designated by the Navy Department for the
conveyance of the expedition, and Lieut. Commander (now Commander), F.
M. Green, U. S. N. was placed in charge. This officer had given great
