IMPROVEMENTS IN SHIPS' COMPASSES
Year: 1844

page_text: TO ALL. TO WHOM THESE PRESENTS SHALL COME, I, EDWARD
JOHN DENT, of the Strand, in the County of Middlesex, Chronometer Maker,
send greeting.
WHEREAS Her present most Excellent Majesty Queen Victoria, by Her
Letters Patent under the Great Seal of Great Britain, bearing date at West-
minster, the Thirtieth day of July, in the eighth year of Her reign, did,
for Herself, Her heirs and successors, give and grant unto me, the said
Edward John Dent, Her especial licence, full power, sole privilege and
authority, that I, the said Edward John Dent, my exors, adññiors, and assigns,
or such others as I, the said Edward John Dent, my exors, adñiors, or assigns,
should at any time agree with, and no others, from time to time and at all
times during the term of years therein expressed, should and lawfully might
make, use, exercise, and vend, within England, Wales, and the Town of
Berwick-upon-Tweed, my Invention of "IMPROVEMENTS IN SHIPS' COMPASSES"
in which said Letters Patent is contained a proviso that I, the said Edward
John Dent, shall cause a particular description of the nature of my said
Invention, and in what manner the same is to be performed, to be inrolled
in Her said Majesty's High Court of Chancery within six calendar months
next and immediately after the date of the said in part recited Letters Patent,
as in and by the same, reference being thereunto had, will more fully and at
Large appear.

page_text: NOW KNOW YE, that in compliance with the said proviso, 1, the said
Edward John Dent, do hereby declare that the nature of my Invention, and
the manner in which the same is to be performed, are fully described and
ascertained in and by the following statement thereof, reference being had to
the Drawings hereunto annexed, and to the figures and letters marked
thereon, that is to say:-
My Invention relates to that class of ships' compasses where horizontal cards
are used, whether as azimuth compasses or steering compasses; and in order
that my Invention may be most fully understood, I will proceed to describe
the same, first remarking, amongst the evils arising from the present construc-
tion of compasses, the following may be noticed:-First, the friction arising
from the imperfect mode of suspension, which is well known to be that of a
hollow cup in the centre of the needle or card, resting upon a steel pivot, in
which case it is obvious that a want of horizontality in the card will cause
considerable friction between the convex sides of the pivot and the sides of
the cup. Secondly, a considerable error is caused by the assumption that the
magnetic axis of the needle coincides with what is called the maker's axis, which
is the line determined by the marks or zero points on the extremities of the
needle, which error in flat needles, such as are usually applied to compass cards.
is frequently of such magnitude as to be quite inadmissible even in compasses
for common purposes, much less for those intended for accurate experiment.
Thirdly, another source of inconvenience and inaccuracy arises from the
unequal amount of inertia as regards the axis or horizontal line drawn through
the centre of the card, about which line it is compelled to vibrate or deviate
from its horizontal position, by means of the alternate pitching and rolling of
the vessel. To explain this, suppose a vessel steering N. or S. by compass, a
pitching motion would in this case cause the card to move about a line drawn
through the E. and W. points, and this arises from the circumstance of the
gymbal apparatus, in which the card is suspended, not completing its vibration
in the same time as the card does in its vibrations above and below the hori-
zontal plane, which difference in the time of vibration of each causes the
convex sides of the pivot upon which the card turns to come in contact with
the sides of the cup in which it moves, and thereby communicates a vibratory
motion to the card. From the position of the needle with respect to the axis
referred to it is plain that the distance of the centre of gyration, and conse-
quently the time of vibration about that axis is a maximum. Again, in a
rolling motion (that is, about the axis of the vessel), the vessel's head being in
the same direction, the card from the same cause will in this case vibrate
about an axis drawn through the N. and S. points of the card, and the time of

page_text: its vibration is then a minimum. In any other motion of the vessel the corre-
sponding motion of the card is compounded of these two effects, which pro-
duces the "waddling" or undulatory motion observed when the motion of the
vessel is considerable and irregular, and the cause of which, I believe, has
not been previously assigned. However well, therefore, the gymbal apparatus
in which the card is placed is balanced, yet as the card has a motion or time
of vibration peculiar to itself, depending upon the position of the axis of its
vertical vibrations with respect to the axis of the needle, which vibrations are
not altogether under the control of the gymbals, although its vibrations are
continually checked and its quiescence disturbed by it, in consequence of the
supporting pivot coming in contact with the sides of the cup, as before men-
tioned; yet in the present construction of the binnacle compass, the card ever
will be subject to irregular deviations from the horizontal plane arising from
this cause.
The mode by which I propose to remove these evils has been by altering
the nature of the suspension, that is, by suspending the horizontal card in
a similar way to the balance of a chronometer, and with equal delicacy,
both ends of the pivots acting on diamonds, and the holes jewelled, by which
means the card is constrained to move very nearly in the horizontal plane, since
in this respect it is entirely under the control of the gymbals. The friction
is also considerably reduced by this mode of suspension. To remove the error
arising from the non-agreement of the marked or maker's with the magnetic
axis, a simple contrivance is effected for the inversion of the card, so that
either side of it may be placed above or below. Since the marked axis of the
needle is in each of these positions at equal distances from, but on opposite
sides of, the magnetic meridian, a mean of an equal number of observations in
both positions of the card will evidently eliminate any error of this nature and
give the true magnetic azimuth of the observed object, at the time of observa-
tion, by an equal compensation of error. It is also plain that the constant
adjustment required to make a delicate needle horizontal in different magnetic
latitudes is rendered thus unnecessary.
DESCRIPTION OF THE DRAWINGS A.
These Drawings show various views of an azimuth compass constructed
according to my Invention, and as the nature of each view of the Drawings is
written thereon, it will not be necessary to repeat them here. The peculiar
character of this instrument is that the axis a of the card passes through the
needle and card, and is supported at each end in bearings let into the frameb. The
frame is screwed into the frame c. which is received into another frane d, by

page_text: means of the studs e entering slits f formed in the frame d, as is shewn, and if
desired for a steering compass, this frame d would be mounted on the ordinary
gymbals; thus it will be evident that if the front of the card at one moment was
uppermost, and its bearing observed, by reversing the face of the card, by turning
it downwards into the frame d, the other or lower face would be upwards, and
if the card pointed differently when so turned up, then the mean of the two
pointings would be the correct one, it being understood that the upper and under
faces of the horizontal card agree in respect to their placing on the needle.
In order to compensate for the weight of the needle in the direction from
north to south, the bar g is affixed at right angles to the needle, having the
ends screwed so as to allow of adjusting the weights that the errors arising
from the deflection of the card from the horizontal plane may be a minimum,
but to this means of adjustment I make no claim. It is not necessary to enter
into any of the other parts of the compass as they are of the ordinary
construction.
DESCRIPTION OF THE DRAWING B.
This Drawing shows an improvement of a steering compass, the card of
which is mounted in the ordinary manner, so far as the using a concave cup
on a point; but in order to prevent those prejudicial effects consequent on
sudden movements of the ship or vessel, or from other causes, such as the
firing of guns, I apply the cup h within a tube i, fixed to the needle in such
manner that the cup may slide freely up and down, being guided by studs or
screws ; passing through slits in the tube i, and above the cup is applied a
spring k, pressing on the upper part of the cup, as is shewn.
Having thus described the nature of my Invention, and the best means I
am acquainted with for performing the same, I would wish it to be understood
that I do not confine myself to the details shown and described, so long as the
peculiar character of either part of my Invention be retained. But what I
claim is,―
First, the so mounting the horizontal cards of ships' compasses on axes that
the faces of the cards may be reversed.
And, secondly, I claim the mode of applying the sliding cup and spring as
herein described, when suspending a compass card on a pivot.
In witness whereof, I, the said Edward John Dent, have hereunto set my
band and seal, this Twenty-ninth day of January, in the year of our
Lord One thousand eight hundred and forty-five..
E. J.
(L.S.)
DENT

page_text: AND BE IT REMEMBERED, that on the Twenty-ninth day of January,
in the year of our Lord 1845, the aforesaid Edward John Dent came before
our said Lady the Queen in Her Chancery, and acknowledged the Specifi-
cation aforesaid, and all and every thing therein contained and specified, in
form above written. And also the Specification aforesaid was stamped according
to the tenor of the Statute made for that purpose.
Enrolled the Thirtieth day of January, in the year of our Lord One
thousand eight hundred and forty-five.