The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1866-02-08
Errors of the Mariners Compass.

| compass or magnetic needle, tue practical expe.
| IienCe OF voyaRers demonstrated that it Is not
always the inlallible and true guide at first
believed.
The variations Of the needle from the true
| north, arising from general as well as local
causes, have attracted the attention OF the
scientific ana curious, and weakened the conh-
| dence OF hardy mariners who have perilled
their lives in an abiding faith of its accuracy.
The dstrust created in the mind of the seaman
has not been removed by the dissemination Of
trustworthy information in popular form cal
culated to reach and benefit those whose iBte
rests are moIe deeply involved. Efforts have
been made by theorists (some OF them specua-
tive ana very unsCund) to advance their own
ideas by patented inventions and otherwise:
still 1t is doubtful ii much good has resulted, ii
we except the labors which have been carried
on since the introduction of IRON as material
IN shipbullding.
The north pole of the earth, geographically, is
the south pole of magnet supposed to pass from
pole TO pole. 1t attracts the north pole of the
needle in the mariners compass, and owing to
the coDvexity OF the earth, causes what is known
as the dip OF the needle, by which the north END
IS DETECTED in northern latitudes, and the south
end in southern latitudes. This influence extend
ing to all the iron of which the ship is COM
posed, tends to give south polarity to the upper
ends OF iron. and consequently to attract the
north pole OF the needle while the ship is in
north latitude. In south latitude all this is
reversed, the upper ends of the iron of which
the vessel is constructed acquiring north polar
ity, and attracting the south pole OF the ships
needle.
The deviations resulting fro n these causes are
governed somewha. by the location Of the COM
pass. With the binnacle aft, the attraction Will
be towards the bows of the ship; with the bin.
nacle placed forward the attraction VIII be to-
wards the stern. Thus a vessel in north latitude
(eHcludlng other causes), with the pinnacle att
and steering east, would have the north pole of
the needle detected TO the eastward, and ii steer
ing east b. compass would be making a course
south of the true east. II steering west the
needle would no dpflppfpd IN thn ua nd
course made would be south of the true west. In
south latitude, the magnetic influence is re.
versed, and vessel steering east by west by
compass would make course north of the true
east or west, as the case may be- The attraction,
when steering north or south, being in q line
with the vessels keel would not cause a mate.
rial deviation, but as II would vary at all the
intermediate points of the compass, it should be
the subject Of careful observation by the intelll-
gent commander.
To most persons II probably seems to be neces-
sary to direct attention to the ships Compasses
only when the vessel has reached her compile.
tion, or is about to encounter the perils OF navi
CATION. Recent experiments, however, show the
Importance of careful observation from tie lay-
ing OF the keel; for not only the position IN which
the vessel is built, but the density of the iron,
and even the amount of hammering, are points
which may control the direction oil the needle,
and have effect upon the compass
Soft iron acquires magnetic properties with
great facility, and parts with them as readily;
while hard iron by hammering may acquire a
polarity which it does not readily part with The
influence of the position in which a vessel is
built Is not confined to iron vessels, for wooden
ones thus acquire a magnetism of their own,
which frequently is not wholly lost until after
numerous voyages. The loss of some vessels on
their first voyage may have resulted from the un-
known quantity of this disturbance.
1t will be seen that the influences operating
upon the needle within the vessel are varied by
latitude and the direction or course made. The
relative position OF the mass of matter compos-
ing the vessel has an important bearing Thus
ship heeled to port would in north latitude
have the needle detected to the starboard
owing to the increased mass of matter brought
into the line of attractlon-the reverse occurring
II she was heeled to starboard.
Two means have been chiefly adopted to cor
rect the errors which may result from devla-
tions OF the needle. First antagonizing the
ascertalned errors by compensating influences
in the shape of local magnets or masses of soft
IRON, SO placed as to cause the needle to point
correctly, or nearly so. Second swinging the
vessel in position remote from extraneous in
huences, and carefully noting the deviations on
each point of the compass, which, noted and
compiled in table OF corrections, enable the
master TO calculate his true course in much the
same manner as he calculates time by his chro-
nometer, knowing the rate.
VI the merits of the two systems we make no
comparison, for while either may afford correct
indications at the place where the vessel is
swung, both are subject to be controlled by
MAGNETIC influences of so varying o nature to
deDar ue from placing implicit confidence in
either. The tabular method may be the means
OF accUmuating important data for future COM
parison, and were the corrections by compen-
satlon always arranged at the same place and a
careful record of each vessel kept, It would be
the means OF accumuating store of facts for
future use and reference.
Provision for a correct knowledge of the errors
OF the compass should begin with the ship
builoer, who may do much to neutraize the
elects OF the inherent magnetism of vessels. 1t
seems now to be demonstrated that vessels
should be built, as far as practicable with their
sterns or bows to the south, and metalled in a
reverse position. The pinnacle should be remote
from any upright masses of iron, masts, or fun
nels, and not over horizontal beams of iron,
The shipmaster should make himself familiar
with the local causes of deviation of the needle;
and, while using either the plan of com pensation
or of tabular corrections, should be constantly |
on the watch for the various counteracting in. |
huences to which his vessel may be exposed, by |
change OF latitude, by proximity to coasts, by |
heelng to port or starboard, or by the influence |
OF cargo having magnetic power.
Constant care and watchfulness are REQUIRED Of :
the intelligent commander, in this as well as hi'
other duties, and If in doubt as to his position ne
should always head of shore until assured of his
reckoning. The observance of this precaution'
would nave saved number OF valuable vessels
recently lost on our coasts, as we'll as many lives
-Aeoo York EUeaing Posi-