AN IMPROVEMENT IN THE MANUFACTURE OF SEXTANTS, QUADRANTS, CIRCLES, AND OTHER INSTRUMENTS USED IN TAKING OBSERVATIONS AND SURVEYS
Year: 1833

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, DAVID
ROWLAND, of Number 68, Crawford Street, in the Parish of Saint Marylebone,
in the County of Middlesex, Mechanic, send greeting.
WHEREAS His present most Excellent Majesty King William the
Fourth, by His Letters Patent under the Great Seal of Great Britain,
bearing date at Westminster, the Twentieth December, in the fourth year
of His reign, did, for Himself, His heirs and successors, give and grant
unto me, the said David Rowland, His especial license, that I, the said David
Rowland, my exors, adñiors, and assigns, or such others as I, the said David
Rowland, my exors, adiñors, and 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 "AN IMPROVEMENT IN THE MANUFACTURE OF SEXTANTS, QUADRANTS, CIRCLES,
AND OTHER INSTRUMENTS USED IN TAKING OBSERVATIONS AND SURVEYS;" in which
said Letters Patent there is contained a proviso obliging me, the said
David Rowland, by an instrument under my hand and seal, particularly to
describe and ascertain the nature of my said Invention, and in what manner
the same is to be performed, and to cause the same to be enrolled in His
Majesty's High Court of Chancery within two calendar months next and
immediately after the date of the said 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, I, the said
David Rowland, do hereby declare that the nature of my said Invention, and
the manner of carrying the same into effect, are particularly ascertained
and described as follows (that is to say) :-
My improvement in the manufacture of sextants, quadrants, circles, and
other instruments used in taking observations and surveys is applicable to the
manufacture of instruments which operate when used to measure angles in
taking observations and surveys on the principle of reflection, whether the said
reflecting instruments be such as are commonly known by the name of
sextants in which the limb or graduated arc is one sixth of a circle, or
whether the limb be only one eighth of a circle, as is really the case in the
instrument called a quadrant, or whether the limb be an entire circle, or
whether, lastly, the said reflecting instrument used in taking observations and
surveys be other than sextants, quadrants, and circles, and consist of any other
portion of a circle which is not an octant or a sextant, or an entire circle.
In order to explain the nature of my Invention, it will be necessary first
to state briefly the mode of operation of reflecting instruments. In using
such instruments one of two distant objects (the angle between which it is
desired to measure) is viewed by direct vision through an eye-piece or a
telescope, and through the upper transparent part of an horizon glass,
the lower part of which is silvered so as to be a mirror, and the other
of the two objects is seen in the silvered part of the horizon glass by
reflection from a mirror fixed to the moveable index or radial arm of the
instrument at the centre of motion thereof, the position of the index mirror
being such that its plane is perpendicular to the plane of the instrument, and
is parallel to the surface of the horizon glass when the index is at zero; and in
taking an observation the index mirror is turned about by moving the end of
the index over the graduated are on the limb of the instrument until the plane
of the index glass is brought into such a position that it will receive the image
of that one of the two objects which is on one side of the line of vision, and
then the reflected image of that object is transmitted to the horizon glass.
The index of the instrument being thus moved about its centre of motion
until both of the objects are seen by the observer in exact contact or coinci-
dence in the field of the telescope, the division on the graduated arc that the
index then points to corresponds to the magnitude of the angle that the
two objects include between them. Now in all instruments that act thus on
the principle of reflection there is a limit to the magnitude of the angle that
can be measured correctly thereby on one side of zero, or on one side of the
point of parallelism of the index and horizon glasses; because, when that

page_text: angle exceeds a certain number of degrees, the index mirror must (in order
that it may receive the image of the object that is to be seen by reflection) be
turned round from its parallelism with the horizon glass until it becomes too
nearly parallel to the line of direction in which the reflection of the image of
the object will pass from the index mirror to the horizon glass, or, in other words,
until the angle between the ray of light that brings the image of one of the
objects to the index mirror and the reflected ray that conveys the same image
to the horizon glass becomes too obtuse. And then it is found in practice that
errors of reflection arise from various causes which make it unsafe to rely on
the results obtained by any reflecting instrument of the ordinary construction
in mensuring any entire angle much above one hundred and twenty degrees at
one observation. The quadrant is graduated only to ninety degrees, and the
sextant to one hundred and twenty. The circle is graduated to observe a
larger angle, but the great obliquity of the index glass to the horizon glass
whenever the angle included between the object viewed directly and the
reflected object much exceeds one hundred and twenty degrees renders the
circle of very little more use than the other instruments named to take a
correct measure of such very obtuse angles. Now my improvement consists
in combining together two sets of the operative parts of sextants, quadrants,
circles, or any other reflecting instrument, consisting of any other portion of a
circle, so as to form a double or compound reflecting instrument, each
operating part of which may have the same range as the other, but the two
sets of operative parts will act in opposite directions; that is, the said com-
pound instrument will have, first, all the parts of a complete reflecting
instrument of its particular kind, and it will have besides an additional index
mirror (on a centre distinct from the centre of the principal or original.
instrument) to receive and transmit the image of an object from one side of
the line of collimation to an additional horizon glass. And the said additional
index mirror may be either moveable about a central pivot by means of an
additional index provided with a vernier scale to indicate a variable angle on
an additional limb or graduated arc, or the said additional mirror may. in
some cases be set by the maker of the instrument to indicate a constant angle
with the eye of the observer, all the additional parts being provided with
suitable adjustments, as I will herein-after more particularly describe. The
direction in which the additional graduated are (with its index and index
glass) operates is reversed in respect to the direction in which the correspond-
ing parts of the original instrument do operate; that is to say, if the original
index glass or mirror receives the image of objects from the right of or froin
above the line of vision or collimation, the additional index glass will be placed

page_text: so as to receive the image of bodies to the left of or below the line of vision.
And so, if the index of the original instrument be adapted to move from left to
right on the limb in going from zero upwards, the additional index will be
adapted to move from right to left; and the additional arc or limb of the
instrument will be graduated accordingly, and the divisions used or read off in
a contrary direction to that in which they are used or read off on the limb of
the original instrument. The additional horizon glass is partly silvered like
the other horizon glass, only in an inverse order; that is, the upper part of
the additional horizon glass which stands opposite the upper transparent part
of the other horizon glass is silvered, while the lower part of the additional
horizon glass which stands opposite the silvered part of the other horizon
glass is transparent. Hence the image received by one of the index mirrors
can be reflected into the mirrow or silvered part of its own horizon glass
through the transparent part of the other horizon glass, and the silvering of
the two horizon glasses is so arranged as to leave a transparent space between
the lower line of the mirror or silvered part of one horizon glass and the upper
line of the mirror or silvered part of the other horizon glass, by which arrange-
ment a distant object can be seen in the field of the telescope thro' both
horizon glasses. The object and effect of thus constructing a reflecting
instrument with such additional operative parts is to enable the observer to
divide a large angle into two smaller angles, and measure both either with or
without the aid of an intermediate object between the two objects whose
angular distance is sought, that is to say:-First, when there are three objects
in sight, and the angular distance of each from the other is sought, the
telescope is directed to the intermediate object which, as before stated, will be
visible through the transparent part of the horizon glasses.
The angle
between that intermediate object and one of the outer objects can be measured
by moving the original index until the reflected image of that object is seen
in the original horizon glass, and coincides with the image of the intermediate
object seen by direct vision through the telescope. That index being then
fixed by its clamp screw, the remaining angle between the intermediate
object and the other outer object may be measured by bringing that other
object into the field of the additional index mirror (whence it will be carried by
reflection into the additional horizon glass), and turning the additional index
(with the slow motion by means of its tangent screw) until the image of the
said object is seen in exact coincidence with the two other images that have
already been brought to coincide in the field of the telescope. The two angles
are thus measured and indicated at once on the limbs of the double instru-
ment, and the sum of the two is equal to the angle contained between the two

page_text: outer objects; and, secondly, when the angle between two objects only is
sought, and exceeds the range of any ordinary or simple reflecting instrument,
say, for instance, one hundred and eighty degrees, and there is no intermediate
object visible whereby the total angle might be found with two observations
by a common instrument, then the said angle of one hundred and eighty
degrees will be found by a compound instrument constructed according to my
improvement by using one of the index mirrors to reflect the image of one of the
bodies into the corresponding horizon glass, when the telescope is directed in
the line of some convenient imaginary point situated between the two objects.
(say at eighty-five degrees from one, and ninety-five from the other), and
afterwards using the other index mirror to reflect the image of the other object
into its horizon glass, so that the reflected images of the two objects shall be
seen in accurate contact in the field of the telescope, and the angles indicated
on the graduated arcs respectively being read off, their sum will be the total
angle included between the two objects.
For the more complete explanation of my Invention I have hereunto annexed
two Sheets of Drawings, whereon Fig. 1, Sheet 1, represents a plan, and Fig. 2,
a side view of a double sextant constructed according to my improvement..
The parts that are the same as those of an ordinary sextant are tinged with a
shade of Indian ink, and marked with capital letters; the corresponding addi-
tional parts are colored, and marked with small letters of reference. It will
not be necessary to do more than indicate the parts of the ordinary sextant
shewn in the Drawing, as they are in nearly every respect exactly similar,.
both as to construction and adjustment, to the parts wherewith every complete
ordinary sextant is furnished, and they are therefore well known.
A, A, is the limb of the sextant, on which are circular arcs, divided into.
degrees and quarters of degrees, from 0 to 120 degrees. B is the index,
moveable about a pivot at the centre of the arc A, furnished with a vernier
scale, divided in the usual way, to read off the number of degrees, and fractions
thereof, contained in the angle observed. C is the index mirror. D, the
horizon glass, which is silvered at the lower part, and the upper part is trans-
parent. E, the dark glasses for observing the sun or other bright object. F
is the telescope, screwed into the eye piece G, the stem of which slides in a
tube H, and can be raised or lowered to adjust the height of the telescope.
by means of the screw I. K. is the tangent screw, with the usual apparatus,.
for the slow motion of the index, and a clamp screw for fixing it at any
particular division on the limb. L, L, L, is the framing of the instrument.
M, N, are legs for it to stand on when not used. O is a handle to hold it
by when in use. The corresponding small letters indicate the corresponding

page_text: additional parts, the combination of which with the ordinary sextant constitutes
my improvement applied to sextants. All these additional parts are con-
structed like the parts of an ordinary sextant, and are provided with adjust-
ments similar to those of corresponding parts of an ordinary sextant. The are
on the limb a, a, is divided into degrees and quarters of degrees, from 0 to 120
or 140 degrees from right to left (that on the limb A being graduated from
left to right). The index 6 is a flat bar of brass turning on a centre pin that
projects from the frame 1, 1, 1, of the additional part, which frame is fixed to the
ordinary sextant by standards 1, 2, 3, screwed to cocks 4, 5, and 6. The end of
the index b is furnished with a vernier scale, which is divided in the usual manner
for reading off the fractions of degrees of an angle. The said end of the index
is furnished with a tangent screw k to move the index forward or backward
with a slow motion, exactly as the index of the ordinary sextant is moved by
means of a tangent screw for the slow motion; that is, the tangent screw k
works through a cock p screwed to the under side of the index, where it
overhangs the circumference of the limb, and acts upon another cock I, the
stem of which passes up through a separate piece or slider, and through a slit
in the upper part of the index, to which it is attached by a screw s. In the
slider is bedded a piece of brass 1, screwed thereto by a screw t, with two
springs growing out of it, that bear against the under side of the limb of the
instrument. When those springs are tightened against the limb by screwing
up the tightening or clamp screw t, the end of the index is drawn tight against
the surface of the limb, so as not to be moved readily by the hand, and then
the tangent screw k, being screwed to the right or to the left, re-acts against the
slider (which has become a fixture with the limb of the instrument), and
moves the index in either direction to set the counting division of the vernier
scale exactly to its place. The index mirror c is adjusted to be parallel to its
horizon glass d when the index b is at zero. It is carried in a frame 4 screwed
to the disk 7, at the inner or central end of the index b, so that when the outer
end of the index b is moved over the divisions on the face of the limb a, a, the
index mirror e is turned about to set it more or less oblique to the horizon
glass d, that it may receive the image of an object situated more or less on
the left of, or below, the line of vision. The limb a, a, is of smaller radius
than the limb A, A, and is not concentric therewith. The horizon glass d of
the additional sextant is placed between the field of the telescope and the
horizon glass D of the ordinary sextant, being supported by a small frame
screwed to the frame of the additional sextant by a screw 8. 9 is an adjusting
screw for setting the horizon glass & perpendicular to the plane of the instru-
ment; and 10, 10, are setting screws for adjusting its angular position in

page_text: respect to the line of vision. The horizon glass d is silvered at its upper part
only, and the silvering of each horizon glass is put on, as before stated, so that
the adjacent edges of the silvered parts do not quite meet, hence, on looking
through the telescope, there will be a transparent part visible through both
horizon glasses D and d in the field of the telescope, through which any distant
object may be seen. (e) are dark glasses moveable on a centre of motion at 11,
in a cock screwed to the frame 1, 1, of the additional sextant. And note, that
the same adjustment that is requisite in the parts of the ordinary sextant in
respect to each other, is requisite in the additional parts also in respect to each
other. And further, as the two sets of parts are to act in combination, each
set must be adjusted in respect to the other set, that is, the planes of
the two instruments should be correctly parallel to each other, and both the
index glasses and horizon glasses truly perpendicular to the planes of both
instruments.
The positions and construction of the several additional parts being now
pointed out, it remains only to give instructions for the use of the instrument,
which will be best done by selecting an example of an observation. Suppose
it were required to measure an horizontal angle of 140 degrees between two
towers situated on distant heights, when there is no visible object between
them. Hold the instrument by its handle, with the plane of the instrument as
nearly as may be parallel to a plane passing through the objects whose angle
is to be found. Select by the eye some convenient imaginary point in the
plane of the two objects which shall divide the total angle into two of suitable
magnitude. Direct the telescope to that imaginary point; then turn tho
index B round to the right, until the image of the tower that is on the right
of the line of vision is reflected from the index mirror C into the horizon
glass D, and back thence to the eye. Clamp the the index B fast at that part
of the arc. Now turn the index 6 from zero to the left,. until the tower that
is on the left of the line of vision is reflected from the additional index mirror c
into the additional horizon glass d; bring the said left-hand tower, by means of
the tangent screw k, to coincide exactly with the image of the other object
in the field of the telescope. Set the index b there. Then read off each angle
on its respective graduated arc, by the vernier scale of its index. The sum of
the two angles is the desired angle of 140 degrees between the two towers.
When the angle sought is vertical, instead of horizontal, the only difference in
the method of making the observation is, that the instrument must be held
with its plane vertical, and the image of the object above the line of vision will
be brought down by the index mirror of the limb A into the field of the
telescope, while the other object that is below the line of vision will be brought

page_text: up by the index mirror of the limb a into the field of the telescope. For
instance, suppose it required to measure the sun's altitude on shore with an
artificial horizon, when it is within ten degrees of the zenith, or making
an angle with the horizon of 80 degrees (an altitude which could not be
be measured at all with the ordinary sextant, because the total angle then
required to be measured would be 160 degrees, and out of the range of the
instrument). Direct the telescope to any convenient imaginary point, about
half way between the sun and its reflected image in the artificial horizon.
Bring up the reflected image of the sun from the artificial horizon by means of
the index b, to meet the eye in the field of the telescope, and bring down the
image of the sun itself to meet its reflected image by means of the other
index B; when the two images are in contact in the field of the telescope, the
sum of the angles indicated on the two limbs of the double instrument will be
the total angle included between the sun and its image in the artificial horizon,
and the half of that angle is the measure of the sun's apparent altitude. It
may be found, in some cases more convenient to use the instrument with the
index of one or other of the limbs set at a constant angle. For instance, let
the index 6 of the additional sextant be set at 90 degrees, then, to measure
an angle of 140° between two objects, hold the instrument in a plane parallel
to the plane passing through the two objects as before directed, and move it in
that plane until the reflection of the left-band or lower object is seen through
the telescope in the horizon glass d. This operation is equivalent to setting the
telescope in a right line with some object that forms an angle of 90 degrees
with the left-hand or lower object; then move the index B until the image of
the upper or right-hand object is reflected from the index mirror C into the
horizon glass D, and bring it into exact contact with the other image in
the field of the telescope by the aid of the tangent screw of the index B.
Now the angle found on the limb A, added to 90 degrees, will be the angle
sought. By this mode of using the instrument an angle may be found at one
reading. It is not necessary to multiply examples of the method of making
observations with a double sextant, constructed as above described, as the
kind of observation for which it is fitted will suggest themselves readily
to persons acquainted with the use and range of sextants of the ordinary
construction.
In respect to the application of my improvement to quadrants, to make a
compound or double quadrant, little instruction needs be given beyond the
foregoing description of the mode of applying it to a sextant, because a quad-
rant differs from a sextant in little except the length of its arc, and in being
made made with less care and accuracy. But its operation is precisely

page_text: similar in kind to that of a sextant. In constructing a
complete double
quadrant according to my improvement, the maker may therefore take the
arrangement of the double sextant exhibited in Figures 1 and 2 as his model
for the arrangement of the parts of the double quadrant. All the operative
parts of the ordinary quadrant must be retained, except the eye piece and
horizon glass used for the back observation, which may be dispensed with;
the additional parts required will be an additional limb graduated in a
direction inverse to that of the limb of the original or ordinary quadrant, an
additional index glass and index, and an additional horizon glass with suitable
dark glasses. All these additional parts must be mounted in a frame, which
may be connected with the frame of the original quadrant in a manner
similar to that which I have described in reference to the double sextant.
The additional quadrant must be furnished with adjustments similar to those
of the corresponding parts of the original quadrant, and with all the parts
requisite for it to measure an angle by itself, that is to say, in the same manner
that it is obvious that the additional sextant which I have described in refe-
rence to Figures 1 and 2, Sheet I., and which forms a part of that compound
instrument, might, if detached from the ordinary sextant and furnished with a
telescope, be used by itself to observe and measure an angle. So in con-
structing a double quadrant of the best kind, the additional quadrant will be
composed of such operative parts, that if detached from the other or original
quadrant and furnished with a telescope, it might also be used separately to
observe and measure an angle. The directions that I have given in respect
to the relative positions and adjustment of the two sets of parts of a double
sextant will apply to the like considerations in respect to the two sets of parts
of a double quadrant, that is, the two arcs must be graduated, and the two
indexes adapted to move from 0 to 90 in opposite directions, in order to bring
into the horizon glass of the original quadrant the image of an object that is
situated on one side of the line of vision, and to bring into the horizon glass
of the additional quadrant the image of an object that is situated on the oppo-
site side of the line of vision. The two index glasses must be so adjusted,
that when the indexes are at zero the planes of the index glasses will be
parallel to the planes of their horizon glasses respectively. The additional
index glass should be placed as near to the other index glass as may be con-
sistently with the condition of no part of its apparatus interfering with the
field of the tellescope, the object of placing the centre of the additional
limb thus being to get as large a radius for it as conveniently may be, without
increasing the total size of the instrument. The two horizon glasses must be
silvered, one at the upper part and the other at the lower part, as I bave

page_text: described for a double sextant, and the silvering must be put on so as to
leave a small transparent space between the adjacent edges of the aforesaid
silvered parts, through which transparent part a distant object may be viewed
through the eye-piece or telescope. The reasons for these arrangements of
the graduated arcs, index glasses, and horizon glasses of the two sets of parts
in respect to each other are precisely the same as those already fully explained
in describing the double sextant, Figures 1 and 2, and the foregoing directions
for making a double quadrant being read with reference to the description
and drawings of the double sextant will be sufficient to instruct a competent
maker of reflecting instruments. Note, in applying my improvement either
to a sextant or to a quadrant, the graduated limb a, a, and the index b of the
additional instrument might be dispensed with, and the additional index glass
and additional horizon glass only retained. The index glass c being adjusted
by the maker correctly to reflect an object from the left of the line of colli-
mation, forming a constant angle of 90 degrees for either a quadrant or a
sextant. The double instrument would then be used as I have directed when
one of the indexes is set fast at a contant angle; all the framing 1, 1, 1, would
be also done away with except so much as is necessary to support the index
glass c and horizon glass d. This construction would be suitable for a quad-
rant, as it would require the addition of but very little work to convert a
simple quadrant into a double quadrant capable of measuring an angle of
180 degrees at one observation. But I do not recommend this construction
for a double sextant, in which the saving of the graduated limb and the index
is not so important, and in which therefore the advantage of saving some
expence might be overbalanced by the inconvenience of being obliged to use
a constant angle in subdividing any angle. Note, the two index glasses for
a double sextant or for a double quadrant might be placed as shewn by the
Diagram, Figure 3, Sheet I., viz., both on one side of the telescope, provided
they be so placed that they will receive and transmit the images of objects that
are on opposite sides of the line of collimation, and provided they be moveable
in opposite directions to each other. A large radius for the additional limb may
be obtained by placing its centre in respect to the centre of the other limb, as
shewn in Figure 3, and that arrangement may in some cases be found con-
venient. But an arrangement of the parts similar to that represented in
Figures 1 and 2, is that which I have adopted in constructing a double
sextant, which has been found to answer its intended purpose.
Figure 4, Sheet II., represents a method of applying my improvement to
the construction of a double reflecting circle, which consists (like the methods
already described in speaking of sextants and quadrants), in combining toge-

page_text: ther two sets of the operative parts of reflecting circles of the ordinary con-
struction, in such manner that while the observer looks through the telescope
at any distant real object or imaginary point, he can bring into the field of
the telescope, by means of one of the reflecting circles, the image of an object
to the right of or above the line of collimation, and afterwards, by means of the
other reflecting circle, he can bring into the field of the telescope the image of
an object that is on the left of or below the line of collimation. Therefore
each of the two circies will be capable of measuring as large an angle as an
ordinary simple reflecting circle can measure at one operation, and the two
combined will be capable of measuring double that angle at one operation.
In Figure 4, the parts that represent the original or ordinary reflecting circle
are tinged with a shade of indian ink, and marked by capital letters, and the
additional parts are colored and marked by small letters. The kind of circle
exhibited in the Drawing is that known by the name of the English reflecting
circle, or Troughton's reflecting circle. A, A, is the divided limb or circle.
B, B, B, the triple index, each end of which is provided with its vernier scale,
and the leading index with a tangent screw and apparatus for the slow
motion. C is the index glass. D, the horizon glass. E, F, are the dark
glasses of the index and horizon glasses respectively. All these glasses pro-
ject as usual from the surface of the circle opposed to that surface on which
the graduated arc is marked. L, L, L, is the framing of the circle. M, M, is
the secondary frame, which is usually applied to the back of a reflecting circle,
and attached thereto by pillars to support the glasses. The index glass C
is fixed to the end of a pivot that turns in the hollow central pillar of the
circle, and on the opposite end of that pivot the triple index is fitted in the
usual manner. The limb and framing of the additional circle a, a, 1, 1, 1, are
applied over the secondary frame M, M, and are attached to the frame of the
original circle by pillars, two of which are visible in the Drawing at 1 and 2,
the two circles being placed back to back. The index glass c of the additional
circle is carried by a frame fixed to the pivot or axis of the index b, which
pivot turns in a socket in the framing 1, 1, and the index b is fitted on its pivot
in the way represented at Figures 1 and 2, Sheet I., for fitting the index
of the sextant to its pivot. The horizon glass D is fixed to the secondary
frame M, M, of the great circle with the usual adjustments, and projects
from the back surface of the great circle A, A, towards the back sur-
face of the little circle a, a, while the horizon glass d is fixed in like
manner and with corresponding adjustments to a cock growing out of the
framing 1, 1, of the additional or little circle a, a, and projects out there-
from towards the back surface of the great circle A, A. The part of the

page_text: horizon glass D near the frame M, M, is silvered, and the part nearest
to the little circle is transparent; and the part of the horizon glass d, near
the circle a, a, is silvered; and the part nearest to the great circle is trans-
parent; that is, the transparent part of one horizon glass is opposite to the
silvered part of the other; the edges of the said silvered parts of the two
glasses being, as described for a sextant, a little distance apart, so that a
transparent space is left between them through both horizon glasses. The
reasons and objects of these arrangements are the same that I have stated
in describing a double sextant, and in the general description of the nature
of my improvement in the manufacture of reflecting instruments. The
handles are omitted in the Drawing to avoid confusion. The telescope (B)
is screwed into a ring supported in a hollow pillar which is fixed to the
frame L, L, and to the secondary frame M, M, of the great circle in the
usual manner (or the ring which carries the telescope may be fixed to both
circles). The telescope H is placed therefore between the two circles, as will be
seen by the Drawing. On the outer surface of the circle A, A, is the graduated
arc divided into degrees and portions thereof (viz, 15 or 20 seconds), and
marked in the usual way of dividing circles; viz, it is divided all round, but
only marked from zero to about 140 each way, and on the outer surface of
the circle a, a, is the graduated are of that circle, divided also into the same
number of degrees and portions thereof as the circle A; but the divisions run
in contrary directions to those of the limb A, A (as shewn by the Drawing),
that is, where the limb A is graduated from right to left from zero upwards,
that part of the limb a which will be used in the same observation is graduated
from left to right from zero upwards, and vice versa, so that where the index
B is moved in the direction of the arrow No. 1, to bring an object from the
right of the observer into the field of the telescope, the index b will be moved
in a contrary direction, No. 1, to bring an object from the left into that field.
The centre of the circle a is placed near to the centre of the circle A, A, and
on the plan alluded to in reference to Figure 3, viz', on the same side of the
telescope or line of collineation. This is done to obtain as large a radius as
possible for the additional circle a, a, and care must be taken to place the
telescope and other parts so as not to interfere with the field of reflection;
for by placing the index glasses very near to each other, one of them may
intercept the ray of light that would fall on the other at certain angles in
taking a backward observation. They should therefore be as far apart as is
consistent with a sufficient length of radius.
In making an observation with my double-reflecting circle to subdivide a
large angle into two, and measure the two at one observation, the telescope will

page_text: be directed, in the way I have described in reference to a double sextant, to
any convenient imaginary point situated between the two objects whose angular
distance apart is sought, and the reflected image of one of the bodies brought
by moving the index of one of the circles to meet the eye in the field of the
telescope. Then the reflected image of the other of the two objects must be
brought round by moving the index of the other circle to meet the reflected
image of the first object, and form an exact contact therewith in the field
of the telescope. After an observation has thus been taken forwards, that
portion of the total angle which was measured by each circle may be mea-
sured over again by a backward observation with the same circle, as is usual in
measuring an angle with a simple reflecting circle when the observation is of a
kind that admits of a backward observation being taken with a circle. And
note, a further correction may be obtained in certain cases with a double
reflecting circle, (that is, whenever the nature of the observation admits of a
backward observation being taken with each circle) by repeating the observa-
tion of the total angle in an inverse order to that in which it was first observed,
or measuring that part of the angle by the backward observation with one of
the circles which was before taken by the forward observation with the other
circle, and vice versa. For instance, suppose an angle B, A, D, Figure 5, has
been observed by carrying the reflected image of the object D into the field of
the telescope, directed in the line A, C, to an imaginary point C, by moving the
index B of the large circle (A, A) in the direction No. 1 from zero, and then
bringing the object B to meet the object D by moving the index of the circle
(a, a) in the direction No. 1, or in a contrary direction to that of the index B,
to correct the observation of each circle by an observation made with the other,
set both indexes back to zero as before, direct the telescope to the imaginary
point C (or to any other convenient imaginary point between the objects D
and B), and carry the reflected image of the object D to meet the eye in the
field of the telescope, by moving the index 6 of the circle a, a, in the direction
of the arrow No. 2, and then bring round the reflected image of the object B
to meet the image of D, by moving the index B of the circle A, A, in the
direction of the arrow No. 2, which is contrary to the direction in which the
index b has just been moved. (Note, the direction of the arrows No. 1 of
the indexes corresponds to the observation forwards, and the direction of the
arrows No. 2 to the observation backwards). Each part of the subdivided
angle B, A, D, will thus have been measured twice over, that is, once by each
of the circles, so that in using my compound or double reflecting circle, every
observation made by one circle may be corrected by an observation made by
the other circle, - a correction which may be of advantage in some cases,

page_text: as the two circles are adjusted to act in concert, and which cannot, of course,
be made with an instrument consisting of only one circle.
After the description I have herein-before given of my double sextant, and
of the properties that result generally from my combination of two reflecting
instruments of like kind, it is not necessary to enter more minutely into the
method of using my double reflecting circle, as the same will be apparent to
persons accustomed to the use of reflecting instruments.
In applying my improvement to the manufacture of other reflecting instru-
ments than sextants, quadrants, and circles, an efficient compound or double
instrument will be made by combining two instruments, the limb of each being
one third of a circle, as shewn in Figure 6, and graduating the arc of each
limb both ways from zero to 120, or a little more, as shewn by the Drawing,
Figure 6, that is to say, by dividing it like the two divided and numbered
sextants of a reflecting circle; and the arcs of the two limbs of the double
instrument, Figure 6, must be graduated in contrary directions in respect to
each other. A compound instrument of this kind will partake of the pro-
perties of a double sextant and of a double circle. It may be used in the
same way as a double circle, as will be apparent by examination of the
Drawing, Figure 6, and will be capable of taking a forward and backward
observation with each separate limb, and of correcting the observations made
with one limb by the observations made with the other; but as it has only
one index, it will not possess the property peculiar to the circle of correcting
the error of excentricity. All the operative parts are arranged nearly as in
the double circle, care being taken to place them so as to give the greatest
practicable range for the reflection of the images of bodies in taking observa-
tions backwards. The double reflecting instrument delineated in Figure 6
must be furnished with the adjustments requisite to the other double reflecting
instruments herein-before described; its structure will be sufficiently under-
stood by examining the Drawing, Figure 6, and the mode of using it with
effect will be also easily understood by applying to it the instructions given
for using the double sextant and the double circle, and remembering that
it will differ from the double circle in its operation in little except in not
possessing the property of making a correction for the error of excentricity.
Note, the letters on the Drawing, Figure 6, indicate corresponding parts to
those marked with the same letters on the other Drawings.
Having now described the nature of my improvement in the manufacture of
sextants, quadrants, circles, and other instruments used in taking observations
and surveys, and the manner of carrying the same into effect, I do hereby
declare that I do not claim as my Invention any particular part or parts of

page_text: the instruments I have described taken separately, but I claim the combina-
tion of the operative parts of two reflecting instruments of like kind, so as to
produce a compound or double instrument of any of the kinds I have
described, one set of the said operative parts being adapted to act to measure
or indicate angles in a direction contrary to the direction in which the other
set of operative parts does act to measure angles, in the manner and for the
purposes herein-before described.
And such my Invention being, to the best of my knowledge and belief,
entirely new, and never before used within that part of His Majesty's United
Kingdom of Great Britain and Ireland called England, His said Dominion of
Wales, or Town of Berwick-upon-Tweed, I do hereby declare this to be my
Specification of the same, and that I do verily believe this my said Specifica-
tion doth comply in all respects fully and without reserve or disguise with the
proviso in the said herein-before in part recited Letters Patent contained;
wherefore, I hereby claim to maintain exclusive right and privilege to my said
Invention.
In witness whereof, I, the said David Rowland, have hereunto set my
hand and seal, the Nineteenth day of February, in the year of our
Lord One thousand eight hundred and thirty-four.
Signed, sealed, and delivered, by the
within-named David Rowland
(being first duly stamped), in the
DAVID (L.S.) ROWLAND.
Clerk to Messrs. Baxendale, Tatham, Upton, and Johnson,
King's Arms Yard, London.
AND BE IT REMEMBERED, that on the Nineteenth day of February, in
the year of our Lord 1834, the aforesaid David Rowland came before our
said Lord the King in His Chancery, and acknowledged the Specification
aforesaid, and all and everything 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.
Inrolled the Nineteenth day of February, in the year of our Lord One
thousand eight hundred and thirty-four.