IMPROVEMENTS IN COMPASSES FOR NAVIGATION, SURVEYING, AND SIMILAR PURPOSES
Year: 1850

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
Royal Letters Patent under the Great Seal of the United Kingdom of Great
Britain and Ireland, bearing date at Westminster, the Seventeenth day of
July, One thousand eight hundred and fifty, in the fourteenth year of Her
reign, did, for Herself, Her heirs and successors, give and grant unto me, the
said Edward John Dent, my exors, adiñors, and assigns, Her especial license,
full power, sole privilege and authority, that I, the said Edward John Dent,
my exors, adñors, and assigns, or such others as I, the said Edward John
Dent, my exors, adñors, 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 COMPASSES FOR NAVIGATION, SURVEYING, AND SIMILAR
PURPOSES;" 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, by

page_text: an instrument in writing under my hand and scal, to be inrolled in Her
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.
NOW KNOW YE, that in compliance with the said proviso, I, the said
Edward John Dent, do hereby declare that the nature of my said 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,-
Firstly, to improvements in the mode or modes of suspending the needles
of compasses and reducing or preventing their oscillation; and,
Secondly, to an improvement or improvements in the prismatic compass.
On or about the Thirtieth day of July, One thousand eight hundred and
forty-four, I obtained Letters Patent for improvements in ships compasses by
fixing the card (to which the needles were attached as usual) on a vertical
axis resting in a cup at the bottom and a pivot hole at the top, so that
the axis and card might be inverted, and then the mean of the readings on
the upper and lower surfaces of the card gave the true magnetic azimuth
of the object observed. By this mode of suspending the card, much greater
steadiness was also obtained, and all vertical oscillation of the card and
needles within the box was prevented. During the last six years I have
carried on a series of experiments with these compasses under the sanction of
the Lords of the Admiralty, and I have found them to be capable of still
further improvement, and that greater steadiness is obtained on ship board,
especially when the sea is "aft" or "on the quarter," by detaching the
needles from the card and connecting them to the axis by means of gimbals,
while the card remains fixed to the axis, which turns in a cup and pivot hole
as before. The two pairs of pivots of the gimbals should stand north and
south, and east and west respectively, when the needles are pointing north and
south, and should lie in a plane just so much above the centre of gravity of the
needles, so as to make the needles lie horizontal when at rest, before they are
magnetised.
".
Figure 1 of the Drawings hereunto annexed is a plan of a compass
constructed according to my Invention, and particularly adapted for a ship's
compass; Figure 2 is a horizontal section of the same, taken below the card;
and Figure 3 is a vertical section. Similar letters are employed to denote

page_text: similar parts in all the Figures. A is the box or bowl which contains the
compass, and is hung upon gimbals 1 and 2 in the ordinary manner. B is
the card which is fixed to the axis C, which turns freely in the pivot holes or
centres S, S'. D, D, are two needles fixed to the square frame E, which
is hung by two pins F, F, on the ring G, which again is hung on the piece H
by two pins I, I, at right angles to the first pins. The piece H is firmly
fixed to the axis C. The parts E, F, G, H, I, thus form a universal joint or
gimbals, and allow freedom of motion to the needles. The directive power of
the compass is increased by the two additional needles D', D', attached to the
first needles by the screws K, K, and nuts L, L, which serve for adjusting or
balancing the whole. The needles should be so balanced that the centre of
gravity may be in the centre line of the axis, and a little below the centre of
motion of the gimbals when the needles are horizontal. The number of needles
employed may of course be varied. The needles are thus free to follow very
nearly the direction of the dip due to the latitude of the place in which the
compass may happen to be, and no correction or adjustment is required for
the variation of the dip in passing from one latitude to another. The gimbals
allow of the inclination of the axis in any direction without altering the
position of the needles, and thus a tendency of the needles to oscillate when any
motion is communicated to the box is almost entirely obviated, and the compass
works with much greater steadiness than the ordinary compasses.
Figure 4 is a plan, Figure 5 is a horizontal section, and Figure 6 is a
vertical section of a compass, in which the needles D, D, D¹, D', are suspended by
gimbals E, F, G, I, on a tube T, which is supported by a fixed pin c. The upper
part of the tube contains an agate cap d, which rests upon the fine point of
the fixed pin c. The lower part of the tube contains a ring or hoop of agate c,
and that part of the fixed pin which it encircles is cased with a ring of agato f.
In place of constructing these parts of agate they may be made of any other
hard stone, or of porcelain or glass, and, if desired, the tube T, cap d, and ring e,
may all be made of one piece of glass tube or of one piece of porcelain or
stone. The tube may also be dispensed with, and the lower ring e supported
by a frame affixed to the card, and the needles suspended by gimbals upon this
frame. The employment of agate or other hard stone or glass in the parts e
and f has the advantage of obviating the great increase of friction which would
arise from a slight oxidation or corrosion of the tube T and pin c, if those parts
are of metal and were allowed to come into direct contact with each other.
For the purpose of obtaining what is called sluggishness of the compass,
instead of employing what is commonly termed a "heavy card," in which
there is a great pressure and friction on the point of support, I apply a

page_text: spring M, Figures 1 and 3, which is bent at one end, and fixed to the box or
bowl A at N. A small pin O fixed to this spring projects through the side
of the box, and can be pushed in by means of a cam or excentric P, which is
moved by the small lever or index 9. By moving this lever the spring is
pushed forward until it comes in contact with the side of the axis C, and acts
as a break so as slightly to retard the motion of the axis and render the
instrument sluggish. The particular mode of advancing the spring is not
material, and a screw or wedge or other means may be employed for that
purpose. The effect of this spring is, that when the compass box is quite
still the friction against the side of the axis would hold the card some distance
away from the magnetic meridian if it were so placed, but as soon as the box
is agitated, as by a rough sea, the vibration of the axis in the pivot hole
causes the card to return to the meridian without oscillation, and this effect
is obtained with a much smaller amount of friction than when the friction is
produced by the pressure of a heavy card on the end of a blunt pivot. This
break will be found very advantageous, as, by turning the lever 9 more or less,
the resistance is at once adjusted to the weather, and the maximum of deli-
cacy consistent with steadiness is always obtained.
The second part of my Invention consists in an improvement on the pris-
matic compass, which, as usually constructed, is an ordinary compass, carrying
a graduated circle, and provided with two sights fixed on opposite sides of the
box. One of these sights is a perpendicular wire, and the other contains a
glass prism, by the aid of which the operator is enabled to see the graduated
circle and the wire at the same time. This instrument is defective in as much
as there are no means of adjusting the focus of the eye to objects at different
distances, nor of adapting the instrument to the eyes of different operators,
which renders it alınost or entirely uscless to short-sighted persons. My
improvements consist in combining a telescope with the prism so as at once
to obviate these defects.
Figure 7 is an elevation, and Figure 8 is a plan of a prismatic compass
thus constructed.-U is the telescope turning on the centre h, and capable of
being raised and lowered in the standard i by means of the pinion k and
toothed sector l. A perpendicular wire or cross wires are fixed in the
telescope at m. n is a button for adjusting the forms of the object glass.
Close to the eye-piece of the telescope is fixed a prism o, which is better seen
in Figure 9, which is a section of that end of the telescope. This end of the
telescope is perforated with two eyeholes close together, as shewn in Figure 10,
or with one large hole, by means of which the operator can see the cross
wires, the distant object, and the reflected image of the graduated circle a on

page_text: the card-B, at the same moment. The frame p which carries the telescope "
is capable of turning round the centre pin q fixed on the cover of the compass
box A. The instrument is shewn as mounted upon gimbals to facilitate its
use at sea; but when used for surveying on land or similar purposes, it will
frequently be more advantageous to mount it on parallel plates and screws,
in the way in which theodolites are generally mounted. 7 is a spirit level
attached to the telescope, and s is a cross spirit level attached to the frame p.
These levels serve for adjusting the instrument when it is mounted on
parallel plates, and for checking the correctness of the level when it is,
mounted upon gimbals. The telescope carries a vernier t, which moves over
a scale u, on the standard i, for the purpose of taking vertical angles. When
it is wished to take the bearing of a distant object, the telescope is directed
towards it, and the focus adjusted by the button n, and the direction of the
telescope adjusted until the vertical wire covers the object. The operator
then reads off the angle at once from the meeting of the image of the vertical
wire with that of the graduated circle .x. The elevation or depression of the
object may also be read off by means of the vernier t and scale u, after,
bringing the horizontal wire of the telescope on the object. The vertical
motion of the telescope is not sufficient to throw the graduated circle out of
the field of the prism. The frame p carries a vernier y, and the circumference
of the compass box is graduated for taking horizontal angles, in the same
manner as with the common theodolite. The compass employed for this
instrument may have its needles suspended by the ordinary means, or by
means of gimbals, as above described.
Having now described the nature of my Invention, and the manner in
which the same is to be performed, I wish it to be understood that I do not
claim generally the suspending the needles of compasses by means of gimbals,
as I am aware that the agate cap of an ordinary compass has been connected
by means of gimbals to the card to which the needles are fixed, but I claim as
my Invention,
Firstly, the mode of suspending the needles of compasses by means of
gimbals upon an axis moveable in pivot holes or centres, and to which axis
the card is fixed, or upon a tube or frame mounted upon a fixed pin and
carrying the card, as above described. And I claim the application of the rings
of glass, porcelain, agate, or other hard stone, marked e and f, for the lower
bearing or guide of compasses mounted on a fixed pin, as above described in
reference to Figures 4, 5, and 6. I also claim the application to compasses of
a spring or break, as above described.

page_text: EDWARD JOHN (L.s.) DENT.
AND BE IT REMEMBERED, that on the Seventeenth day of January,
in the year of our Lord 1851, 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 Seventeenth day of January, in the year of our Lord One
thousand eight hundred and fifty-one.