Evening star., 1898-12-22
COMPASS FALLIBILITI.
The Possible Cause of the Loss of the |
Steamer MoheEap.

Steamer M0heE0p.
From the London Shipping World.
The very fact that the mariners compass
is above all things a supersensitive instru-
ment renders it peculiarly liable to those
sudden and capricious changes of which
many navigators can speak from experience
and to their sorrow.
The compass of today is perhaps as per-
feet a production as the highest scientific
skill and the most delicate mechanical ma
nipuation can attain to, and yet at a criti-
cal moment and by causes beyond human
foresight. the vagaries of its readings may
lead the most experienced Seamen into utter
disaster.
If as has been said, there should exist in
the vicinity of the Manacles powerful and
uncertain magnetic field by whose influence
vessels have a times been quite ineXplica-
bly, drawn miles cut of their course, then
the terrible loss of the Mohegan might well
be explained by this hypothesis alone. And
it should be borne in mind that this sug-
gested solution of the mystery is strength
ened by reference to the case of the Pal
myra, lost on the Manacles on March II,
1885. Here the commander, Capt. Minof,
was steering, he believed, due west after
passing the Eddystone, and thought he was
some dozen miles off the Manacles and four
or five from the Lizard when his ship struck
the fatal rocks.
But there are other causes which may,
separately OF cOlJOlDlly With the SUPPOSED
magnetic field in the vicinity of the Mo-
hegan's loss, have contributed to disturb
her compasses. For instance, we are told
that the vessel had recently undergone eX-
tensive alterations If in the course of
carrying out these any considerable ham,
mering of the beams or plating were neces-
sary, then that part of the metallic struc-
ture may have had its magnetic polarity
interfered with, even to the point of re-
versal. If such were the case, il would be
interesting to know whether the compasses
were readjusted to meet the altered con-
ditiOns before the vessel proceeded on her
last and, as it proved, calamitous voyage.
Again. there is the possibility of compass
deviation due to the presence of the dyna
mo, whose large external magnetic field
would probably be lying dormant until the
lights were switched on, perhaps only some
hour or less before the Mohegan struck.
Such a DETECTION would, of course, vary
steadily-that is without oscillation of the
needle-directly as the number of lights
turned on. In the experiments on the Roy
alst one wire from the dynamo was led
under the standard compass at a distance
of thirteen feet, the return being separated
only five feet from it. With these corii-
tions it was observed that a current of
about 100 ampheres caused detection of
eight degrees in the compass. which indeed
resolved itself into an excellent dead-beat
galvanometer. the DETECTION increasing and
decreasing, without oscillation, for every
variation of the current.
on the Mohegan we do not for moment
suggest, but that these disturbing influ-
ences may have been produced on minor
scale is well within the bounds of proba-
biiity. We have seen a sensitive needle
Jump" to the influence of the magnetiZed
ribs of an otherwise inoffenSive and inno-
cent-looking umbrella. and many of US have
suffered the inconvenience of maguetized
watch-a small mass of metal. truly, but
perhaps in the pocket of an officer near
the pinnacle all sufficient to deviate the
senstively poised compass card. Hitherto,
with one exception, We have dealt only
with possible and unforeseen urbinS
causes that may have existed within the
navigator as Capt. Griift.s. OF disturbing
most troublesome of all is the ''subperma-
nent' source of error due to the constant
action of the earth endeayoring to form in
that part of the ship nearest north pole
repellent to the north point of the COM
paSs-a source of error the peculiar danger
of which arises from the fact that as II
accumulateS it produces visible effect
on the compass while the vessel is steering
a steady course, and only becomes appar-
ent on altering course. Thus, to exagger-
ate the Mohegan's course, IL Capt. Griffiths
had been steering west for some days, and
then altered his course at right angles to
south the greatest effect would have been
produced, as in every case this error causes
the vessel to in the direction of the
course previously steered An error of less
that half point in the long stretch of
sixty-two miles from the start to the LIZ
ard would have been sufficient to cause the
disaster and from any one of. the causes