IMPROVE- MENTS ON MERIDIAN INSTRUMENTS
Year: 1843

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, JAMES
MACKENZIE BLOXAM, of Hampstead, in the County of Middlesex, Esquire,
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 Twentieth day of June, in the seventh year of Her reign, did,
for Herself, Her heirs and successors, give and grant unto me, the said
James Mackenzie Bloxam, Her especial licence, full power, sole privilege
and authority, that I, the said James Mackenzie Bloxam, my exors, adors,
and assigns, or such others as I, the said James Mackenzie Bloxam, my
exors, admors, 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 "IMPROVE-
MENTS ON MERIDIAN INSTRUMENTS;" in which said Letters Patent is contained
a proviso that I, the said James Mackenzie Bloxam, 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

page_text: 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
James Mackenzie Bloxam, do hereby declare that the nature of my said
Invention, and the manner in which the same is to be performed, are particu-
larly 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) :-
In order that my Invention may be more readily understood, I will, in the
first place, shortly explain the principle on which the instruments or apparatus
are constructed, and then proceed to explain various forms for constructing
instruments according to my Invention. And I would here state, that the
Invention consists of so combining three reflecting planes that they are used
as one single and one double reflector, and in such manner that an observer
may see two images of a distant object, when that object is near to an imagi-
nary plane, passing through the instrument, and by the coincidence of those
images the observer may know when the distant object is in that imaginary
Figure 1 is a Diag m, by ich I shall be enabled to describe the principles
on which my instruments are constructed. Let a, c, d, b, c, represent two
rays of light proceeding from two stars, and meeting at the point c, and let e, f,
represent a transparent and reflecting plane, perpendicular to the plane a, c, b.
If the reflecting plane bisect the angle a, c, b, an observer looking in the direction
d, c, will see one star, and the reflected image of the other, as if they were
both at a. Moreover, if the reflecting plane have an angular motion, or if the
plane be supposed to be fixed, and the stars to move as from a to 6 to a¹ and
b', the reflected image of the star 6 will appear to move in the opposite
direction, or from a to a¹¹; consequently, if the transparent and reflecting
plane be fixed to a wall or other stationary object in such a position that at
some period of the day one star and the reflected image of another may appear
in the same place, it will furnish the means of making observations of the
following nature, videlicit :-If an observer look in a proper direction towards
the plane, two stars, hereafter called images, will appear to come into the field
of view from diametrically opposite points, and move in opposite directions
across the field of view, meet at or near the centre, and disappear at the points
opposite to those at which they entered, the two images appearing to be in the
same place at the moment when that point in the heavens which is midway

page_text: between the two stars corresponds with the plane of the reflecting surface. The
exact position of the observer will not be material; for if he shifts his position
a little the coincidence of the two images will take place at a different part
of the field of view, but at the same time. The recurrence of the coincidence
of the two images at intervals of exactly twenty-four sidereal hours will
furnish the means of regulating or ascertaining the rate of a clock; and if the
time of the coincidence be once ascertained it will also afford the means of
setting the clock, or ascertaining its error on any future day. It is to be
observed, however, that the reflecting plane must not be placed parallel or
nearly parallel to the earth's equator; for if it be so placed the two images
will never separate, or will do so very slowly. The nearer the plane is placed
parallel to the earth's axis the better, and suitable stars should be chosen
accordingly. It is more convenient to consider that the plane both reflects
and transmits light, though an opaque reflecting plane with a sharp edge may
be used, and one star seen by reflection at the same time that the other is
seen across the sharp edge of the plane. The above supposed instrument is
described with a view to facilitate the description of my Invention. The
instrument above described cannot be used for observing a single object unless
the instrument were furnished with adjusting screws requiring nice adjust-
ment. There would be great difficulty in fixing the instrument in a proper
position, since it is necessary that the reflecting plane e, f, should be exactly
perpendicular to the plane a, b, c, at the time of the coincidence; for if it be
not so placed, the two images will not appear to move in the same path as
indicated by the dotted line Figure 2, but in parallel directions, indicated by
the dotted lines Figure 3.
I will now proceed to describe arrangements of three reflectors so as to
obtain the objects of my Invention. Upon examination of Figure 4 it will be
readily understood that three reflecting planes A, B, B, C, and C, D, may be
placed in such a position that the rays of light falling upon the plane C, D, in
the direction b, e, would be reflected in the direction e, f, and other rays of
light from the same distant object falling upon the plane A, B, in the direction
a, c, would be reflected in the direction c, d, and would be again reflected by
the plane b, c, in the direction e, f, so that the rays of light reflected by the two
planes A, B, and B, C, and passing through the transparent plane C, D, would
coincide with or be parallel to the rays of light reflected by the plane C, D,
and an observer looking in the direction f, e, would see two images of the
same object as if they were both atg; and, moreover, that if the object move so
that the rays of light fall upon the planes in a different direction, as indicated

page_text: by the dotted lines, the two images will appear to be at a" and 6", conse-
quently this instrument furnishes the means of making observations of the
same nature as those before described, except that a single object is observed
by means of its two images. The reflecting planes represented by the lines
A, B, B, C, and C, D, were supposed to be perpendicular to the plane of the
paper, and the other lines, which represent the rays of light, were supposed to
be in or parallel to the plane of the paper; but the proposition holds good if
these other lines be supposed to represent planes perpendicular to the paper,
and the rays of light to be in or parallel to those planes and rays from every
object passing through the plane a, c, or b, e, (which, owing to the distance of
the object compared to the size of the instrument, are considered as one plane,
and which I call the plane of coincidence,) will have the precise moment of
the object passing in that plane indicated by the coincidence of its two images
reflected and seen in the manner before described. Mathematically speaking,
the range of the instrument is 180°, but practically the range is less extensive,
because the extent of the reflecting planes is necessarily very limited, but an
instrument may easily be constructed having a range of 150° or 160°.. The
plane e, f, is also a plane of coincidence, that is to say, if there be a distant
object in that plane, rays of light proceeding from the object, and reflected
by the plane D, C, and rays reflected by both the planes B, C, and A, B, will,
after such reflections, coincide or be parallel; but, owing to the arrange-
ment of the reflecting planes, the parallel rays from the planes A, B,
and C, D, would not enter the eye of the observer at the same time, and
consequently the coincidence of the images could not be easily observed,
unless, indeed, the instrument be very minute, or a telescope be used whose
object glass is large enough to embrace both the planes. It is obvious that
an instrument having three reflectors, arranged and fixed in the positions
described, may be employed as a transit or meridional instrument, and some of
its advantages are the facility it affords for accurate observations, the simplicity
of its construction, the extremely small space it occupies, and especially that it
does not require to be elevated or depressed for the purpose of observing
objects of considerably different altitudes. The instrument consists of three
small reflecting planes fixed together in a suitable frame or stand, requiring no
moveable parts. The two images move with velocities equal to that of the
object, and since they move in opposite directions their relative velocity
is double the actual velocity of the object. The instrument may be
improved by the addition of a telescope, which, for the sake of convenience,
should be attached to the instrument by means of a joint, so that it can

page_text: only move in one plane. The arrangement and adjustment of the reflect-
ing planes are by no means so difficult as might be supposed. It is not
necessary that the reflecting planes should be placed at precise angles with
respect to each other. The angle B may vary between 0 and 180°. The
angle b, e, f, is equal to twice the difference between the angle B and a right
angle, so that the angle B merely determines the position of the observer with
respect to the instrument and the object. The plane D, C, produced should in
general make equal, or nearly equal, angles with the other planes. The
three planes are to be so adjusted that they are all perpendicular to some one
plane, or, in other words, so that the right lines formed by the meetings of two
contiguous planes are parallel. If they be so, the two images of the same
distant object will move in the same line as before described by reference to
Figure 2; whereas, if the planes be not so adjusted, the two images, although
they will move in parallel directions, will pass without coming into contact, as
before described by reference to Figure 3. The adjustment of the three planes
is easily examined by observing whether the two images of any distant object
pass over each other when the instrument is held in a proper position, and
moved backwards and forwards so as to cause the images to pass and repass
each other. The effect of perceptible inaccuracy in adjustment is, that
the two images of an object pass each other without ever appearing in
exact coincidence, which would prevent the time of their passing being
accurately observed; but if the adjustment be not very bad, the two images of
the sun (supposing the sun to be the object) cannot pass without coming into
contact, and small inaccuracy in the adjustment does not cause perceptible
error in the time of the contact; moreover, when the contacts of both limbs
of the two images of the sun are observed, the errors are equal and opposite,
and consequently no error is occasioned in the time of their centres passing
each other when obtained, by taking the mean of such two observations. If
the sun's semidiameter be sixteen minutes of a degree, the time of its semi-
diameter passing the meridian one minute four seconds of time, and the error
of the above adjustment of the instrument such that the two images of the sun
overlap by the space of one minute of a degree, the error in the time of the
contact of the limbs scarcely exceeds three hundredths of a second of time.
For small quantities the error of the observation varies as the square of the
error of the adjustment nearly. If the instrument be intended for the purpose
of regulating a clock, it may be fixed against a wall or other fixed point, and
adjusted to the meridian, and for that purpose azimuth and vertical or hori-
zontal adjusting screws, or other means of adjustment, may be used as in other

page_text: meridian instruments, except where the instrument is not required to be set
up with much greater accuracy than sun dials usually are; and even when
inaccurately set up, by ascertaining the error of its position it would afford
equally accurate, though less convenient, means of obtaining the time. It has
been stated that the angle B may vary between 0 and 180°, and that the
angle e is equal to twice the difference between the angle B and a right angle.
In Figure 4 the angle B is 135°, and consequently the angle e is 90°. In
Figure 5 the angle B is a little more than 90°, and consequently the angle e is
very small. This construction affords the means of making two observations
of the same object at short and equal intervals before and after the object is
on the meridian, instead of one only when it passes the meridian; the first
observation is made when the rays of light proceed in the direction indicated
by the arrows, and the second when the object is in such a position that the
rays of light pursue the same path, but in the opposite direction. The length
of the interval between the two observations may be long or short, ascording
to the pleasure of the instrument maker. If both limbs of the sun or moon
be observed with this instrument it will furnish no less than four observations,
the means of which will give a very accurate result. In Figure 6 the angle B
is rather less than 90°, consequently the anglo e is very small, as in Figure 5,
and its properties are similar. It would appear at first sight that the head of
the observer must interrupt the light proceeding from the object towards the
instrument when made of the form represented by Figures 5 and 6, and thus
interfere with the observation, but this is not the case unless the object is
nearly or quite vertical to the reflecting plane D, C, and even in that case an
observation might be made with the assistance of another reflecting plane held in
the hand. Every form of the instrument, except when the angle B is exactly
90°, has two planes of coincidence, which, as before stated, meet each other at an
angle equal to twice the difference between the angle B and a right angle.
The plane C, D, Figure 4, may be opaque, provided it does not extend over or
conceal the whole of the plane B, C, or if the instrument be constructed with
the reflecting surfaces in the relative positions represented in Figure 7, and the
other forms admit of similar variations.
Figure 8 represents a profile view in full size of an instrument suited for
the latitude of England, and having a little more than sufficient range for
observing the meridian passage of the sun at all seasons of the year; it is
fitted up with three reflecting planes, placed in respect to each other according
to the Diagram shown at Figure 7, and explained in Figure 4; Figure 9, a
front view of the same instrument; Figure 10, a front view of the three

page_text: reflecting planes, shown separately from the instrument. Figure 11, a section
from A to B of Figure 10 of the three reflecting planes, and of the metallic
frame in which they are held in their proper positions against the framework
of the instrument. The same letters signify the same parts in these and the
following Figures. a, a pedestal of cast iron or other metal; b, a standard,
cast in one piece with the pedestal; c, a disc or circular plate attached to the
standard by three adjusting screws with nuts; d, the three adjusting screws
and the nuts; e, the single reflecting plane, so fixed on the disc as to receive
the rays of the sun while passing over the meridian, and to reflect them in a
plane perpendicular to the face of the disc; f, f, the two surfaces consti-
tuting the double reflector, so fixed that the rays of the sun while passing over
the meridian shall be received by the plane f, and reflected on to the plane f',
and again reflected by the plane f' in a direction parallel to the rays reflected
from e.
These planes are made of black glass or other reflecting substance.
g, a metal frame screwed to the disc c, and holding the three reflecting planes
in such position that a plane bisecting all the three shall be nearly parallel
with the plane of the equator. h, a cock fixed firmly to the standard by two
screws, having a third screw for angular adjustment (but which is not to be
altered after the instrument has been set up and adjusted for use), and an
arm to receive a secondary revolving cock; k, the fixing screws of the cock;
1, the adjusting screw of the cock; m, the secondary cock, revolving on a
tightening screw pin on the arm of the fixed cock, and carrying a telescope,
the plane of revolution being perpendicular to the plane of the disc and to the
plane of the equator; n, the pin; p, the tightening nut; q, the telescope.
This instrument, being fixed on a firm level surface, with the plane of the
standard and the face of the disc in or near to the meridian, and the plane of
coincidence adjusted to be exactly in the meridian, an observation of the sun's
transit over the meridian may be made without the telescope by turning it
aside, or with the telescope by turning it in the direction of the images. But
this form is not convenient, and is not intended to be used without a telescope.
Figure 12, a plan of an instrument fitted up with three reflecting planes,
placed so as to form nearly an equiangular; Figure 13, a front elevation of
the same; Figure 14, a profile view of the same; Figure 15, a front view
of the single reflecting transparent plane and its supporting ring, shewn
separately from the instrument. Figure 16, an edge view of the same,
together with the two reflecting planes and their supports, constituting the
double reflector; Figure 17, a back view of the ring of Figure 15, shewing
the supports of the double reflector as screwed to the ring; Figure 18, a

page_text: section from C to D of Figure 17; Figure 19, a section through the length
of one of the double reflecting planes and part of its support. r, a cast-iron or
other metal block, having one of its sides, which I call its face, inclined at an
angle of about sixty degrees with the side r', Figure 13, and elevated so that
the edge of the face s, s, Figure 13, is inclined at an angle with its base about
equal to the latitude of the place of observation, a few degrees more or less
being immaterial, the block having a recess in the face to receive the double
reflecting planes and their supports. s, the face of the block; t, the ring
screwed to the face of the block; u, the single reflecting surface, made of a picce
of transparent glass, which (by way of embellishment) is grained and rendered
opaque all around a parallelogram sufficiently large for the double reflector to
be seen through it; v, two cocks screwed to the back of the ring t, cach
having a wing inclined to the ring at the angles intended for the double
reflector; w, the screws of the cocks; x, two pieces of glass or other reflecting
substances, which, if transparent, must be parallel in their thickness, placed
against the wings of the cocks v; y, two clips and screws to hold the pieces of
glass against the wings; 2, a slot, shewn by dotted lines in Figure 17, to
allow of adjusting the two planes so as to cause the two images of the sun to
move in one right line, as explained at Figures 2 and 3; 1, holes in the ring
for the screws to pass through, which fasten it to the block. This instrument
requires no other fixing for meridional observations than being placed on a
level surface, with the face towards the meridian sun, and after being adjusted
by trial it may be fixed by any suitable cement.
Having thus described the nature of my Invention, and the best manuer I
am acquainted with for performing the same, I would remark, that I do not
confine myself to the particular details of arrangement of three reflecting planes
shewn and described, provided the peculiar character of my Invention be
retained. But what I claim is, the mode of combining three reflecting planes
whereby they are used as one single and one double reflector, and in such
manner that an observer may seo two images of a distant object when that
object is near to an imaginary plane passing through the instrument, and by
the coincidence of those images the observer may know when the distant
object is in that imaginary plane, as above described.
In witness whereof, I, the said James Mackenzie Bloxam, have hereunto
set my hand and seal, this Twenty-ninth day of June, in the year of
our Lord One thousand eight hundred and forty-three.
J. M. BLOXAM. (L.S.)

page_text: AND BE IT REMEMBERED, that on the Twenty-ninth day of June, in
the year of our Lord 1843, the aforesaid James Mackenzie Bloxam came
before our said Lady the Queen in Her Chancery, and acknowledged the
Specification 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 Twentieth day of December, in the year of our Lord One
thousand eight hundred and forty-three.