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t 347 3'
XVIIL Description of a Forty-feet Reflecting Telescope: By
William Herschel, LL.D. F. R. S.
Read June 11, 1795*
1 he uncommon size of my forty-feet reflecting telescope will
render a description of it not unacceptable to lovers of astro-
nomy, I shall therefore endeavour to give as complete an idea
of its construction as the limited compass of this paper will
permit, and hope that, with the assistance of the annexed
drawings, the mechanism of it will be sufficiently intelligible
to such as have been in the habit of viewing machines and
mechanical works.
It will be necessary to mention a few circumstances that
led the way to the construction of this large instrument, in
the execution of which two very material requisites were ne-
cessary : namely, the support of a very considerable expence,
and a competent experience and practice in mechanical and
optical operations.
When I resided at Bath I had long been acquainted with
the theory of optics and mechanics, and wanted only that ex-
perience which is so necessary in the practical part of these
sciences. This I acquired by degrees at that place, where in
my leisure hours^ by way of amusement, I made for myself se-
veral s.-feet, 5~feet, 7-feet, 10-feet, and 20-feet Newtonian
telescopes ; besides others of the Gregorian form, of 8 inches,
12 inches, 18 inches, a feet, 3 feet, 5 feet, and 10 feet focal
348 Dr. Herschei/s Description of a
length. My way of doing these instruments at that time,
when the direct method of giving the figure of any of the
conic sections to specula was still unknown to me, was, to
have many mirrors of each sort cast, and to finish them all as
well as I could ; then to select by trial the best of them,
which I preserved ; the rest were put by to be repolished. In
this manner I made not less than 200, 7-feet ; 150, 10-feet;
and about 80, 20-feet mirrors ; not to mention those of the
Gregorian form, or of the construction of Dr. Smith's re-
flecting microscope, of which I also made a great number.
My mechanicai amusements went hand in hand with the
optical ones. The number of stands I invented for these tele-
scopes it would not be easy to assign. I contrived and deli-
neated them of different forms, and executed the most pro-
mising of the designs. To these labours we owe my 7-feet
Newtonian telescope-stand, which was brought to its pre-
sent convenient construction about 17 years ago; a descrip-
tion and engraving of which I intend to take some future
opportunity of presenting to the Royal Society. In the year
1781 I began also to construct a 30-feet aerial reflector ; and
after having invented and executed a stand for it, I cast the
mirror, which was moulded up so as to come out 36 inches in
diameter. The composition of my metal being a little too
brittle, it cracked in the cooling. I cast it a second time, but
here the furnace, which I had built in my house for the pur-
pose, gave way, and the metal ran into the fire.
These accidents put a temporary stop to my design, and as
the discovery of the Georgian planet soon after introduced
me to the patronage of our most gracious King, the great
work I had in view was for a while postponed.
Forty-feet Reflecting Telescope. 349
In the year 1783 I finished a very good 20-feet reflector
with a large aperture, and mounted it upon the plan of my
present telescope. After two years observation with it, the
great advantage of such apertures appeared so clearly to me,
that I recurred to my former intention of increasing them
still farther ; and being now sufficiently provided with expe-
rience in the work I wished to undertake, the President of our
Royal Society, who is always ready to promote useful under-
takings, had the goodness to lay my design before the King.
His Majesty was graciously pleased to approve of it, and with
his usual liberality to support it with his royal bounty.
In consequence of this arrangement I began to construct
the 40-feet telescope, which is the subject of this paper, about
the latter end of the year 1785. The wood-work of the stand,
and machines for giving the required motions to the instru-
ment, were immediately put in hand, and forwarded with all
convenient expedition. In the whole of the apparatus none
but common workmen were employed, for I made drawings
of every part of it, by which it was easy to execute the. work,
as I constantly inspected and directed every person's labour ;
though sometimes there were not less than 40 different work-
men employed at the same time.
While the stand of the telescope was preparing I also began
the construction of the great mirror, of which I inspected the
casting, grinding, and polishing ; and the work was in this
manner carried on with no other interruption than what was
occasioned by the removal of all the apparatus and materials
from Clay-hall, where I then lived, to my present situation
at Slough.
Here soon after my arrival, I began to lay the foundation,
mdccxcv. Zz
35o Dr. Herschei/s Description of a
upon which by degrees the whole structure was raised as it
now stands ; and the speculum being highly polished and put
into the tube; I had the first view through it on Feb. 19, 1787*
I do not however date the completing of the instrument till
much later; for the first speculum, by a mismanagement of
the person who cast it, came out thinner on the centre of the
back than was intended, and on account of its weakness would
not permit a good figure to be given to it. A second mirror
was cast Jan. 26, 1788; but it cracked in cooling. Feb. 16,
we recast it with particular attention to the shape of the back,
and it proved to be of a proper degree of strength. Oct. 24, it
was. brought to a pretty good figure and polish, and I observed
the planet Saturn with it. But not being satisfied, I continued
to work upon it till Aug. 27, 1789, when it was tried upon the
fixed stars, and I found it to give a pretty sharp image. Large
stars were a little affected with scattered light, owing to many
remaining scratches in the mirror.
Aug. the 28th, 1789. Having brought the telescope to the
parallel of Saturn, I discovered a sixth satellite of that planet;
and also saw the spots upon Saturn, better than I had ever
seen them before, so that I may date the finishing of the 40-
feet telescope from that time.
Description of the Instrument. See Tab. XXIV. to XLII.
inclusively.
Fig. 1. represents a view of the telescope in a meridional
situation, as it appears when seen from a convenient distance
by a person placed towards the south-west of it.
The foundation in the ground consists of two concentric
circular brick walls, the outermost of which is 42 feet in dia-
Forty-feet Reflecting Telescope. 351
meter, and the inside one 21 feet; these measures are rec-
koned from the centre of one wall to the centre of the other.
They are 2 feet 6 inches deep under ground ; two feet 3
inches broad at the bottom, and 1 foot two inches at the top ;
and are capped with paving stones about 3 inches thick, and
iq± broad. Fig. 2. represents a section of one of them.
These walls were brought to an horizontal plane by means
of a beam turning upon a pivot fixed in the centre of the circle,
which had a roller under it at the end. Upon this beam and
over the roller was fixed a spirit level, to point out any defect
in the walls ; and by correcting every inequality that could be
perceived, they were by degrees brought to be so uniformly
horizontal that the beam would roll about every where upon
them without occasioning any alteration in the bubble of the
spirit level.
The timber of the groundwork (see fig. 3.), in the construc-
tion of which it was necessary to join strength to lightness, is
put together in the following manner : three principal beams,
A A, B B, C C, are extended from south to north, when the
telescope is in a meridional situation. They are 43 feet 2
inches long, 6 inches broad, and 6 inches thick. The distances
of the centre of the two outside ones is 17 feet. Within one
foot of the ends of them are bolted down the cross beams
DD, E E ; which serve as a foundation to the two sets of lad-
ders. These cross beams are 19 feet 2 inches long, 12 inches
broad, and 6 inches thick ; and by way of additional strength
two more, F F, G G, of the same breadth and thickness, ara
bolted against the sides of the former, resting also upon the
longitudinal beams, but these are rounded off at the ends, as
marked in the figure,
Zz 2
352 Dr, Herschei/s Description of a
The firmness of the foundation in the direction from south
to north being thus secured, it became equally necessary to
provide for strength in the support from east to west. For
this purpose there are three latitudinal cross beams, H H, I I,
K K, bolted down upon the former longitudinal ones. That
which crosses the centre is 45 feet 3 inches long, and la
inches broad ; its thickness, like that of all the ground timber,
being 6 inches. The other two are about 39 feet 9 inches
long, and 6 inches broad. They project beyond the circular
foundation wall about 8 inches, while the middle one projects
i£. The use of these cross beams is to receive six supporters
upon their respective ends, at the places which are marked
with an ellipsis ; the supportiftg beams which stand upon
them being round and inclined towards the ladders, which
they are to keep steady in the east and west direction.
Under each end of the principal beams at A A, B B, C C,
HH, II, K K, is placed a roller which rests upon the outer
foundation wall. The three latter of these beams being placed
higher than the former, have a piece of a proper thickness
under the ends to bring the bottom of them to the level with
the former. The rollers are set in iron frames, and bolted to
the beams, so as to be directed to the centre of their motion.
They are 8 inches long, and 6 in diameter. The construction
of those which are under beams that come from the centre, is
expressed in fig. 4. ; but the irons which hold the rollers un-
der beams in other directions are more or less eccentric, as
may be seen in fig. 5.
No other fastening in the whole machinery of the wood-
work has been admitted but screw bolts ; as tenons of any
kind, in an apparatus continually to be exposed to the opera
Forty-feet Reflecting Telescope. 353
air, will bring on a premature decay, by lodging wet. In
order to obtain steadiness, however, the cross timbers of the
frame, in all places where they are bolted together, are let in,
and receive each- other about \ of an inch, which makes an
entering of i|- inch into each other, and produces the required
firmness without any material weakening of the timber.
The twelve rollers whose place has been pointed out, would
not have been sufficient to support the length of the beams to
which they are fastened, the shortest of which, as we have
seen, being near 40 feet long. Eight additional rollers, there-
fore, sustain the ground timber half way towards the centre
at LMNOPQRS, They are, like the former, directed to
the pivot upon which this frame moves, and rest upon the in-
ner foundation wall.
In the centre is a large post of oak, framed together with
braces under ground, and walled fast with brick-work so as to
make it steady. The two central beams B B, II, cross each
other over this post ; and a strong iron pin, or pivot, goes
through them both into a socket within the centre of the
post, so as to permit the whole of the foundation timber to
turn freely upon this centre, when a proper force is applied for
that purpose.
Although by means of the 20 rollers, and this support in the
centre, the bottom frame of the stand to the telescope be
firmly supported, it may notwithstanding be easily seen that
there was occasion for some additional braces, in order to keep
each beam in its proper situation. For this reason 8 pieces,
aa, bb, c c 9 d d, e e, ff, gg, bb, of a proper length, 4 inches
broad and 6 deep, are applied near the end of the beams
against the sides of them ; these are held together by irons
354< ^ r - Herschei/s Description of a
that are bent, as in fig. 6. One of them, for instance, is bolted
down, at a and b, fig. 3. upon the braces that hold the beam
H in its place; upon which it is also screwed down, and makes
a firm joint of the three pieces. A small entrance of a and b
into H takes off the weight from the iron, and keeps the braces
in their places, when bolted together.
Two other braces, //, kk, are added. Their use is evident
in the horizontal motion of the telescope ; for that being ef-
fected, as will be seen hereafter by means of the strong centre
beam 1 1, the connection of the whole frame with this beam is
completed by the pieces //, A A, BB, CC, kk.
Before I proceed to explain any other part of the work in
this figure, it will be necessary to describe the construction of
the ladders and their braces.
Fig. 7. represents the front set of ladders. a,f3y$e% are six
tapering halves of three large poles, or rather small masts, cut
through the middle. Before the masts were cut they mea-
sured between 11 and 12 inches at the bottom ; but when
they had been sawed through, the pieces were flattened on the
front and back, so as to be reduced to 8 inches at the bottom,
and q\ at the top, while the other dimension, or thickness,
was left to its full extent. By trimming up and making them
pretty equal, however, they became also reduced to about
£~and 5 inches in that direction. .
The length of the ladders is 49 feet 2 inches, and their con-
struction is as follows. The top of each step is 9 inches from
that of the one below iti and, beginning 12 inches from the
bottom, there are two rounds and one flat placed alterrlately,
as far as 40 rounds, and 19 flats. In the place of the 20th
flat is the centre of the meeting of the front and back sets of
Forty-feet Refecting Telescope. 355
the ladders ; above this is another flat, with a termination
of \6 inches at the top.
The timber of the sides being tapering, a similar diminution
of the flats and rounds has been attended to, especially as their
size, in proportion to the sides, is far above what is generally
used in building ladders. The flats and rounds are all made
of solid English split oak.
The lowest rounds are 2 inches thick in the middle, and ii
where they enter the sides. At the 21st round the thickness
is 1^ in the middle, and if at the shoulder. About the 31st
round the thickness is \\ in the middle, and if at the shoulder,
and this size is nearly preserved up to the end. Those parts
of the rounds which enter the sides of the ladders have all been
turned in a lathe, and are about f of an inch tapering, in order
to fill the holes properly, which were also made a little tapering
so as perfectly to answer the size of the rounds.
The lowest flat, for a particular purpose in the erection of
the ladders, which will be explained hereafter, is 4f inches by
2. The next, as far as the ioth, are 3f by if ; from the 11th
to the 16th they are 2f by 1^ ; and from the 17th to the last
2i by if inches.
The two outside divisions of the ladders serve for mounting
into the gallery,- and therefore contain rounds as well as flats.
The distance of the sides, the flat parts of which, as in com-
mon ladders, are put' facing each other, is 18 inches, and re*
mains the same up to the end. But the two inside divisions,
which have no rounds, are placed with the flat face outwards,
and the distance between these faces being 2 feet 8 inches up
to the top, the parallelism of these divisions is preserved out-
side, while that of the mounting ladders is continued within.
556 £> r - Herschel's Description of a
The reason of this arrangement is, that the brackets which
support the moveable gallery rest upon the inside frames of
the ladders. These go upon 24 rollers, as will be seen here-
after, and 12 of them confining the gallery sideways, while the
other 12 support it, the parallelism was of course required
where it is placed. The mounting ladders are made parallel
within, that a moveable chair, intended to be made if required,
might be drawn up with a person seated in it, to prevent the
fatigue of mounting, or take up in safety any one who chanced
to be afraid of ascending an open ladder.
The back set is constructed like the front ; and, the ladders s
being of the same length, the only difference is that no rounds
have been put into th^m. The flats have been preserved on a
double account ;, first, that the connection of all the side tim-
ber might be firm and strong ; and secondly, that every part of
the frame might be accessible. For by means of these flats we
have steps of 27 inches, which may be ascended with tolerable
ease, when occasion requires.
The method of joining the front and back at the top, is by
passing one set of ladders through the other so as to embrace
it ; the backs, therefore, which go outside, are placed a little
farther asunder than the fronts ; and the same pins pass
through them both, at q k and /, where a section of the back
ladders is shewn, with the pins going through them. The last
flat was put into the ladders after they had been erected and
secured together.
The method of setting up and bracing the ladders was as
follows:
When the eight principal beams of the groundwork AB€
DEH I K, fig. 3. had been put together, the eastern front
Forty-feet Reflecting Telescope. 357
ladder '$e% 9 fig. 7^ with its back %o.ir (not expressed in the
figure) bolted to it, was laid down edgeways, with the ends £
and I opposite to the same letters in fig. 3. and the centre &
towards the west. In this situation the ends were properly
secured to the foundation beams, that they might not slip.
The centre 6 was then raised up to about 10 or 12 feet from
the ground, and a tackle was fastened on one side to the cross-
ing of the ladders at /, and on the other, about 2 feet from the
ground, to a tree at a convenient distance in the east. By as-
sisting a little at first, in lifting the centre, our tackle soon got
hold of the ladders, and drew them up. A rope had been pro-
vided to prevent their going farther th^n the perpendicular;
and being secured in that position, the tackle was now fast-
ened to the other ladder at y ; but instead of making use again
of our tree, the corresponding tackle was secured on the top of
the first ladder at 0; by which means we easily drew up the
second. Both sets of ladders stood now upon the ground,
within the frame, and with the front liegs, ajSy^'^. nearly
opposite to the same letters on the front beam; while the legs
of the back stood opposite the letters XpvfjoTr on the back
beam.
We now proceeded to put on the middle top cross-beam,
which is placed above the two sets of ladders in the angle
made by their crossing each other. It is expressed by points
in fig. 7. and may be seen in its place, fig. 1. The method of
keeping it there, and securing the proper distance of the lad-
ders by this beam, which is of a cylindrical form, is as follows:
twelve iron loops, shaped to the ends of the ladders, with arms
to them like lamp-irons, and a hole at the end of each arm, are
slipped down upon the ends of the ladders, till two and two of
mdccxcv. 3 A
358 Dr. Herschei/s Description of a
them, as ab, fig. 8. meet in the middle of the cross-beam c f
which is about 8 inches in diameter. Here a screw bolt, com-
ing up through the beam, passes into the holes of the two
irons, where all is screwed firmly together. By this means no
holes are made to weaken the tapering ends of the ladders,
and the centre beam takes firmly hpld of every one of them ;
so that were even the pins y and 6 pulled out, the ladders
would still remain firmly kept together.
Before, however, the ladders were screwed to the centre
cross-beam they were lifted up into their places upon the front
and back foundation beams DD, E E. This was done by a
strong lever-beam, about 25 feet long (see fig. 9.), with two
moveable iron claws, a b 9 at the end ; which took hold in two
places, equally distant from the middle division of the lowest
flat of the ladders : this flat having been made, as has been
noticed before, sufficiently strong for sustaining the whole
weight of a set of ladders. Thus they were lifted one by one
into their proper places, and supported till they could be
shaped with their lower ends to fit upon their respective bear-
ings, and were in the same manner brought to the required
parallel situation : this kind of lever affording the means of
giving some small motion to the weight it sustains, not only
upon the pivot c, but also on the support d e, which is rounded
off at the bottom.
When the ladders had been properly adjusted to their places,,
we proceeded to support them immediately by two capital side
braces. These consist of two whole masts, of nearly the same
dimensions with those which were sawed through for making
the ladders : the upper end of each was mounted, with an iron
loop a> two claws b c, and ring d, which were put on with
Forty-feet Reflecting Telescope. 359
bolts, ef 9 as expressed in fig. 1.0. The poles being drawn up,
the loops were put upon the centre pins 1 k, fig. 7. and keyed
on; while the lower ends of the poles were lifted into their
places, on the cross foundation beam 1 1, fig. 3. and fitted
upon the elliptical marks at the ends of them at 1, 2.
Before the ladders and side braces were fixed down, a line
with an hundred weight at the end, immersed in a tub of
water, was hung upon the centre at 6; and being viewed from
a considerable distance, was made to range with the flat side
$-0 of the ladder. The bottom of every end of the bracing
poles and ladders being finally adjusted by this plumb-line,
they were all screwed down by strap-bolts, as delineated in
fig. 11. The top centre cross-beam was now also screwed to
the loops on the ends of the ladders, which, as we have men-
tioned before, see fig. 8. had been already prepared.
The most essential part of the stand being now erected,
we proceeded to brace and support it finally. Four small lad-
ders, but without rounds, 22 feet 9 inches long, having been
made, were erected to support the large ones half way. They
consist of three half-poles each, placed at the same distance
from one another as the large ones, and have their faces
turned like them. These meet the former at the 10th flat
with their upper ends; while the lower parts of them rest
upon the middle foundation beam in fig. 3. at 7, 8, 9, 10, 11,
12 ; 13, 14, 15, 16, 17, 18. They are screwed both at the top
and bqttom with flat corner-irons as in fig. 13. and their situa-
tion may be seen in fig. 1.
In the next place four less poles were now added to support
the ladders sideways. They stand upon the beams H H, K K,
at 3, 4, 5, 6 ; and are fastened against the ladders at the 10th
3A 2
360 Dr. Herschei/s Description of a
flat, or half way up. The upper end is secured with an iron,
as in fig.- 10. through the loop of which passes a strap-bolt
that holds it at the same time with the triangular brace, which
will be described, to the ladders. The lower end is strap-bolted
down upon the beams, as in fig. 11.
The two long side-bracing poles are supported each by two
less poles, which meet them at one half and one quarter of
their length from the bottom ; or at a height opposite to the
5th and 10th flats of the ladders. Those poles which meet
the great ones in the middle are placed with their lower ends
upon the middle beam at ig and 20 ; and the shortest rest at
21 and 22. They add very materially to the steadiness of
the frame in the east and west, or lateral direction ; at the
bottom they are also fastened by strap-bolts as in fig. 11. and
at the top by loops, as in fig. 10. They may also be seen in
The next braces we are to describe are those of the sides of
the ladders, and these it will be seen by fig. 13. are of so simple
a nature, that a bare inspection of their representation will be
sufficient. The size of the horizontal pieces is 6 inches by %\ ;
but those which are parallel to the ladders, and are of no other
use than to keep the rest in their places, or as it were brace-
bracers, are only %\ by c i\.
Besides these there are three sets of braces, which serve to
confine the poles to their stations. The highest set meets
the side brace of fig. 13. at the 15th flat. The next meets
the middle brace at the 10th flat, and both these make with
the here mentioned side braces a triangle, in the vertex of
which is inclosed the large pole that is braced by them. At
the 5th flat a third set of braces, which incloses the two small
Forty-feet Reflecting Telescope,. 361
poles as well as the large one, is carried round with four divi-
sions. In order not to weaken the great pole by many holes,
the braces secure it by a double iron strap, abed, fig. 14; and
the small supporting poles which rest upon 19, 20,-21,- and 22,
fig. 2. are at the same time joined by a single screw-bolt, ef 9
which passes through the loop g h i at the end of them, and
through the straps which hold the braces to the great pole;
see fig. 1.
The back of the ladders is bound together by a large cross,
ab, cd, from the 10th flat to the middle braces, and by tw r o
horizontal pieces, ef, gb, as represented upon a small scale in
fig. 15. The cross is bolted in twelve places to the ladders,
and the horizontal pieces in six places each. The size of these
braces is 6 inches by 4 ; but the lowest horizontal beam, which
is used for a point pf suspension to lift the mirror in and out
of the tube, is 6 inches and a half by 5^; and the bolts that
hold it to the ladders are also very substantial
The front of the ladders, it is very evident, would admit of
no brace, and is left entirely open for the tube of the telescope
to range in. It receives, however, some confinement from the
moveable gallery, which is always hung across the front, in
the place where observations are to be made.
This gallery is next to be described. It consists of three se-
parate parts : two double side brackets with a small platform
upon them, and a middle passage. The whole of it when
joined together is properly railed in at the front by wooden
palisades ; and on the inside by light iron-capped bars. Each
of the brackets by which the gallery is supported consists of
three, frames ; a parallelogram for the bottom, with two trian-
$6% Dr. Herschei/s Description of a
gular sides erected on th6 former, and held together by a nar-
row platform on the top. Fig. 16. represents the bottom frame.
Its length, ag 9 is 8 feet 10 inches, and breadth, g b 9 2 feet 8
inches. It is made of yellow fir deal, 4 inches broad, and 2
inches thick. Six sets of brass rollers, in iron frames, con-
structed as represented in fig. 17. by means of the two small
screw-bolts, ik, are screwed under the frame, fig. 16. at c de
fgb; so that when this frame comes to be placed upon the
front of the ladders at jSy, or $s 9 fig. 3. the 6 rollers in the di-
rection m 9 fig. 17. will sustain the frame, while the other six,
in the direction /, by embracing the flat sides of the ladders,
which as has been described are turned outwards for this pur-
pose, will prevent the frame from slipping off sideways, when
it rolls up and down the ladders.
Two such triangles as delineated in fig. 18. the wood of
which is §\ by 2^ inches, are fastened to the frame, fig. 16;
one, with the side np upon eg, the other upon db. These
being joined at the top by the platform of boards, screwed
dowi} to the supporters q r s, of that which is represented here,
and of the corresponding one, which rests on db, complete the
bracket.
Upon the platform are fixed palisades commencing at t, ig
inches from q ; which turn the corner at the front $, and are
continued so as to meet the middle platform of the gallery.
The palisades over trs are strengthened and rendered steady,
by a seat which is fastened against them, and supported from
the floor by slight iron bars.
The other double bracket, with its platform, palisades, and
seat, which runs upon the right side of the front ladders, is in
Forty-feet Reflecting Telescope. 363
every respect the same as that on the left, except that the
entrance is here upon the right side, instead of being at the left
in the former.
The whole gallery together, the floor of which is repre-
sented in fig. 19. takes up a space of 13 feet 6 inches broad, by
6 feet \\ inches in depth ; the middle platform, however, is
cut away so as to leave sufficient room for the tube to come
forward in high altitudes. At a c and b d it is 4 feet 3 inches,
but at e/ and gb, a space of 4 feet 10 inches long, it is only 2
feet deep. The front, cfh d, contains palisades, which meet
those of the left bracket trscatc, and the similar ones of the
right at dik x These palisades are 3 feet 2 inches high. The
light iron rails on the inside pass along the edge, laegbm r
and are only 2 feet 3^ inches in "height.
The first requisite in this gallery being that it should be
drawn up to any required altitude, it became necessary to con-
nect the two double brackets and the middle platform in such a
manner as to bear some little derangement in their level, arising
from the inequality of the motion of the side brackets. With a
view to this end, the method of uniting the parts is as follows.
The dotted lines 1, 2, &c. shew the place of the joists which sup-
port the floor of the platform. At the ends, 1, 2, 5, 6, 7, 8, of
these joists are six iron hooks, shaped as in fig. 20; they are
bolted and screwed with the end n under the bottom of the
joists, and rise to the level of them with the arms p, leaving
the hooks q projecting. These enter into six proper openings
made in the side brackets ; three in each : they leave a space
of about ^ of an inch between the two brackets and the middle
platform, which permits a small irregularity in the level of the
three parts to take place without injury to either of them*
3&| Dr. Herschei/s Description of a
The hooks sink down into the floor of the sides so as to be
level with the surface ; and go over the inside of the support-
ing triangles, fig. 18. ; which, for the sake of additional
strength, and to prevent their being galled by friction, are
lined with an iron plate at the inside, in all their length.
The light ifon rail joining the bars of the inside, which are
along the margin I aegb a, fig. 19. are left moveable at the
bottom, in the places / a and mb ; where they run down into
loops ; by which means they admit of being a little displaced.
The contrivance to make the junction of the front and side
palisades moveable is by means of a front bar. This being
slipped upon pins at the end of the rails belonging to the sides,
a hole at each end of the bar, lined with an iron plate about 2
inches long, through which the pins pass, pef mits the bar to
be drawn either way. There are moreover at the ends of the
rails, which are fixed to the platform, two iron hooks ; which,
though they bind the rails to the front bar, still permit it to go
up or down a little way, as occasion may require. By this
means a deviation from the level, amounting to six or eight
inches, will occasion no injury to the wood-work. The greatest
security against such a derangement of the platform, however,
will be explained hereafter, when we come to the mechanism
by which it is moved.
There is a small staircase by which we may ascend into the
gallery, without being obliged to go up any ladder ; and as
that is strong enough to hold a company of several persons,
and can afterwards be drawn up to any altitude, observations
may be made with great conveniency : the activity of an as-
tronomer, however, will seldom require this indulgence. The
readiness with which I ascend the ladders, has even prevented
Forty-feet Reflecting Telescope. 365
my executing the projected running chair, which may easily
be added, Jto take a single person into the gallery after it has
been already drawn up to its destined situation. A view of
the staircase in fig. 1. will suffice to point out its construction.
I ought only to observe, that in the engraving the gallery is
placed higher than where it will join the staircase properly,
but that when it is lowered on purpose, it becomes then to be
just one step above the little landing-place of the staircase, and
the palisades of the former unite with the railing of the latter.
The next piece to be described, is the tube of the telescope/
This, though very simple in its form, which is cylindrical, was
attended with great difficulties in its construction. No one
will wonder at this who considers the size of the tube, and the
materials of which it is made.
Its length is gg feet 4 inches ; it measures 4 feet 10 inches
in diameter, and every part of it is of iron. Upon a moderate
computation, the weight of a wooden tube must have exceeded
an iron one at least 3000 pounds ; and its durability would
have been far inferior to this of iron.
The body of the tube is made of rolled, or sheet iron, which
has been joined together without rivets ; by a kind of seaming,
well known to those who make iron funnels for stoves. It is
represented by fig. 21. where the two sheets of iron are left a
little open at a b to shew the construction, but which being
properly compressed will become very nearly flat: the whole
outside was thus put together in all its length and breadth, so
as to make one sheet of near 40 feet long, and 15 feet 4 inches
broad. The tools, forms, and machines, we were obliged to
make for the construction of the tube were very numerous.
For instance, in the formation of this large sheet, a kind of
jmdccxcv. 3 B
Dr. Herschel's iDescription of a
table was built for its support^ which grew in size as the sheet
advanced, till when finished, it was as large as the whole of it.
In the formation of the sheet, cramping irons/seaming bars,
setting tools, and claw-screws, such as are represented in the
figures 22, 23, 24, 25, and 26, were made in great number, to
confine and stretch the parts as they were seamed together.
The small single sheets of which this large one is cortiposed,
are 3 feet 10 inches long, and about 23^ inches broad. Their
thickness is less than the 36th part of an inch ; or, what will
be a more precise measure, a square foot of it weighs about
fourteen ounces. They are joined so, that the middle of a
whole one always butts against the seam of the preceding two,
in the manner of brick- work, where joints are crossed by
bricks above and below.
When the whole sheet was formed, which was done in a
convenient barn not far from my house, the sides were cut
perfectly parallel, and afterwards bent over at the ends in con-
trary directions, as in fig. 21. to be ready to receive each
other. A number of broad hooks, such as were proper for
grasping the sides of the sheet, with loops at the other end for
cords to go through, see fig. 22. were now prepared with their
necessary tackle.
Twejve pulleys were fastened about 11 feet high, on move-
able beams, that might be drawn together ; six on each side.
The sheet was now taken up, by occasioning all the corded
hooks to be drawn at the same time, and while it was kept
suspended our large table was taken to pieces. Another kind
of support was now put under the middle of the sheet to re-
ceive it. The form of this was that of an hollow segment, or
quarter of a cylinder, cut lengthways, to the extent of a few
Forty-feet Reflecting Telescope. 367
feet more than the length of the intended tube; and the con-
cavity of which was formed by the same tadius as that of the
tube.
The sheet being let down, it rested upon the hollow gutter ;
for so we may call the machine that was placed under it. Six
moveable segments of a whole cylinder, or circular arches,
about 3 feet wide each, which had been prepared, were now
brought upon the sheet and placed at proper distances from
each other. By these the sheet was pressed down upon the
foundation, so that no injury could be done by walking upon
it. The beams which held the pulleys were now brought
close together ; which being done, we hung the pulleys of one
upon the hooks of the other beam, so as by that means to cross
the cords which held the sheet. In this operation we slack-
ened only one of the cords at a time, the rest being sufficieht
to keep the whole up.
The beams were now again separated, and the cramping
hooks by the crossing^ the cords drew the two sides of the
sheet together.
Here I must take notice, that the circular inside supports,
which resembled the machines upon which arches of brick-
work are built, were cut in two in the middle, as in fig. 27. ;
some part of the circumference being taken out, that when
they were laid down upon each other they might not fill the
tube. Four long wedges, abed, in opposite directions, were
confined two and two in the notches ef, g h ; and similar ones
at the back. By driving them in very equally, the upper half
of the arches might be forced up so as to swell to the full ex-
tent of the tube.
When all this was properly arranged, and the arches lowered,
3 B 2
$68 Dr. Herschei/s Description of a
the two sides of the sheet were gradually brought to take hold
of each other. As we proceeded, the wedges within the arches
were forced in successively, till at last, with much care and
considerable difficulty, the two sides completely embraced one
another, and were kept stretched by the swelled inside arches.
Another circular arch, closed in with boards all around, well
rounded off, and only about 2 feet 3 inches long, had a va-
cancy at the top into which we could introduce the iron seam-
ing bars, fig. 23. for indenting, and 24. for closing up the long
seam of the two sides. This arch also had its stretchers for
swelling it up, and served at the same time, as soon as the
seam was properly closed, to beat with mallets the whole
sheet all around upon its well-finished outside, in order to take
away any accidental bulge which it might have received in
the long preparations it had undergone, till it came to the
present state.
The same arch, as soon as any portion of the tube had been
done, was removed to another place, and the whole was by
this means completely seamed up.
The theory upon which the strength of so thin a cylinder of
iron is founded, is, that the sides of it must unavoidably sup-
port it, provided you can secure the cylindrical form of the
tube.
It appeared to me the most practical way to obtain this end
by the following contrivance. By a few experiments I found
that a slip of sheet iron, a little thicker than that of the tube,
and doubled to an angle of about 40 degrees, as in fig. 28.
might afterwards be made circular, as in fig. 29. The deepest
we could conveniently bend, and such as I supposed would
answer the end, was when the sides a b were about 2^ inches
Forty-feet Reflecting Telescope. 369
broad. They were shaped red hot upon a concave tool, which
had the required curvature and angle of the slips. The pieces
were long enough to form a complete quadrant of the circle,
with the ends sufficiently projecting to be seamed together.
Before they were joined the sides received another bending,
as in fig. 30. which was given them by tools of a proper con-
vexity. A back was next prepared, consisting of a slip of iron
turned up at both sides, and also bent to the circle, as in fig.
31. Last of all, the four quadrants having been put together,
and a back put round them, the whole was firmly seamed to-
gether, so as to resemble a hollow tHangular bar made into a
hoop or ring, of a proper diameter to go closely into the tube,
so as to keep it extended, and braced to the cylindrical form.
A section of the ring with the bottom seam not quite pressed
down, in order to shew it better, is represented in fig. 32.
One of these rings was put into the middle of every one of
the small sheets, which brought them to about 23 inches from
each other. They were carried in edgeways, and afterwards
turned about and forced into their respective places. In order
to get them in, as they were all obliged to go in from one side,
there was substituted, in the room of the circular arches, a
kind of temporary props, like fig. 33. that could be easily re-
moved, one at a time, and were narrow enough at a b to pass
through the hoops while they advanced ; and as soon as a ring
was in its proper place* no further support became necessary.
In this manner we secured the cylindrical form of the tube ;
and as soon as this was accomplished, we had every thing re-
moved from within and without, and began to give the tube
three or four good coats of paint, inside as well as outside ; in
sflo Dr. Herschei/s Description of a
order to secure it against the damp air, to which it was soon
to be exposed.
As the tube was now much lighter than it would be here-
after, we transported it into my garden in the following man-
ner. Many short poles, about 5 feet long each, were joined
two and two by a piece of coarse cloth, such as is used for sacks,
about 7 feet long each. This, being fastened in the middle,
left at each end part of the pole to serve as a handle for a per-
son to hold by. The cloth of one of these being put under
the tube, there was left one of the poles at each side, and four
men taking hold of the ends of the poles, might conveniently
assist in carrying the tube. When six sets of these were put
under the tube, it was with great facility lifted up by 24 men,
who carried it through an opening which we had made at one
end of the barn. The inclosure of part of my garden having
also been taken down, with some trees that were in our way,
it was safely landed upon my grass-plot ; where a proper appa-
ratus of circular blocks was put under to receive it. While it
remained in this state, we prepared every thing for its recep-
tion, and afterwards moved it into its place, and supported it
in an horizontal situation.
It will be necessary now to return to the rest of the machi-
nery, which by this time was in great forwardness.
Two solid cast-iron concave rollers, 6± inches broad, and
10 inches in diameter, are mounted upon an axle or iron bar,
2-§- inches square ; the axle in the middle being swelled out
so as to admit of a pivot 2-^ inches thick to pass through it,
without being weakened by the hole. The tube is mounted
upon this at the lower end, and as the speculum lies in this
Forty-feet Reflecting Telescope. 37>
part, great strength is requisite to support it firmly, as also an
extensive connection of this strong part with the length of the
tube. The speculum likewise is to be put in here, and when
the telescope is in use the cover of the speculum is to be taken
off, and afterwards to be put on again; for which reason a
convenient door or opening must be had. The line of collima-
tion of the mirror also requires an adjustment at this end of
the tube ; and a small side motion is required upon the pivot
of the axle, which must not only be perfectly smooth, but
equally firm and steady. All these exigencies have been pro-
vided for in the following manner.
Fig. 34, represents the back of the tube, closed up by six
iron bars, abed ef, which cross each other. The middle bar
is 4 inches broad, and i-f- thick at e, but is swelled so as to
measure 5 broad, and i§- thick at /, where it is turned at rect-
angles, and passes under the bottom of the tube. In this bar
is a square hole, through which a pivot, or pin, passes from
the inside of the tube, where it is confined by a square head,
into the hole of the axle, A B, under which at the bottom it is
keyed fast at C ; with proper washers between the joints to
allow of a very smooth motion.
The bar, eg, is of the same strength with el, and passes
over it at e. It is bent at rectangles at c and g, so as to pass
along the sides of the tube. The two bars, d m, fk, fastened
upon eg, and afterwards turned down to the back, are 2 inches*
broad, and f thick ; and are also bent at rectangles at m and
k, so as to go under the tube: the remaining two bars, bh 9
ai, cross the other three bars, with proper offsets ; and are
bent at rectangles 011 both sides, that they may turn round the
end of the tube, to go along the sides of it. At the crossing
3£2 Dr. Herschei/s Description of a
of these bars they are fastened by screws, which pass through
the upper bar, and are lodged in the lower one. The same
screws pass on through a moderate plate of sheet iron which
closes the back, and is held by nuts upon them within the
tube. The eight returning bars, at a bcgb i k, extend only to
about 6 inches along the tube ; but they are immediately re-
ceived by other eight bars of the same size, which are screwed
upon them. These bars are made tapering, so as only to mea-
sure ii inch broad, and -f thick at the ends ; and they are 9
feet 8 inches long. The middle bar is turned over about 16
inches, and made tapering ; and the bar which meets it is laid
under it, and also made tapering to answer the former. The
pivot goes through both, and they form, as it were, only one
bar ; this is soon reduced gradually, and at the end measures
i\ inches by \ ; its length being the same as the rest.
The segment cng is cut off to leave an opening, which is 2
feet broad at the sides. A cover of the same shape, with the
piece cut out of the tube, is laid upon the place, to overlap the
opening properly. But this would not have been sufficient :
for, after observations at night, this cover, though close enough
to preserve the inside of the tube from damp or wet, would
itself be covered with dew or condensed vapour. And by tak-
ing it off in order to secure the mirror, many drops of water
would unavoidably fall upon it from this wet cover. To pre-
vent this, an outward cover has been applied, which com-
pletely preserves the inner one from moisture.
The tube being much too weak, in this place, for the sup-
port of the mirror, a piece consisting of three sheets of iron,
2 feet 4 inches broad, | thick, and dove- tailed together so as
to be long enough to reach from c around b am Ik eh to g 9
Forty-feet Reflecting Telescope. 373
was added to its thickness within ; upon this again, an iron
bar, 6 inches broad, and f thick, was bent round close to the
end, from b to h only; and another bar, a'i inches broad,
and \ thick, made into a complete circle, was added to sup-
port that end of the tube which had been cut, to make the
entrance. All these pieces were well secured, by screw-bolts
passing through the nine long outside bars abcgbiklm,
next through the tube, then through the strong sheet, and at
last through the broad strap and circular bars, upon which
they were screwed down with nuts at the inside. The more
advanced parts of the long bars were secured also by screw-
bolts passing through the tube, and through circular straps of
hoop iron, about 2^ inches broad, and ± thick; one of these
being put into every sheet of the tube as far as the bars went.
As we had now secured what I call the point of support, it
was no less necessary to form a strong point of suspension.
This was obtained by grasping the tube with a system of bars
similar to that which has been employed at the bottom.
Ten bars, equally divided around the circumference, about
10 feet 4, inches long, are placed longitudinally so as to have
one;, of them at the top, and an opposite one at the bottom.
Every one of these has six sqrew-bolts, which pass through the
bar and the tube, and also through complete' circles of hoop
iron, which is of the same breadth and thickness as has been
mentioned before. The bars also, except the highest and
lowest, are of the size of those which have been used about
the point of support. They are also, like them, chamfered at
the sides, and begin to lessen in breadth and thickness about
4 feet from the front, to the same dimensions with the former*
mdccxcv. 3C
374* £*•• Herschel's description of a
The lowest bar is a little stouter in Its dimensions, but other-
wise exactly the same.
The middle bar at the top is strongest about the point of
suspension, where it is 4 inches broad, and j inch thick. In
this place it is crossed by another bar, which is a segment of
a circle, and embraces the middle one, and two other bars at
each side. This crossing bar is 5^ inches broad, and 1 inch
thick in the centre ; chamfered or sloped at the sides, and
reduced in thickness towards the ends. It passes over the
middle bar with a proper offset, and its two ends terminate
upon the two farthest bars ; but the bars next to the middle,
on each side, are made to pass over it. The middle bar re-
ceives a loop, by which the telescope is suspended, the centre
of which is 3 feet 8 inches from the mouth of the tube. The
loop is made of iron, 4 inches broad, and 1 inch thick ;
doubled together, and the ends of it opened again, so as to
cross the circular bar, and to rest upon the strong middle one 5
to which it is fastened with four large screw-bolts. These
pass through the bar into the tube, where they are well se-
cured with substantial nuts. The long middle bar is reduced
gradually after the place of the loop, the ends of which ex-
tend about 1.8 inches, till it comes at last to the breadth of 2^
inches, and thickness f.
All the ten bars are secured with six screw-bolts each,
which pass through the tube, and through iron hoops, four of
which are of the same dimensions with those which are used
about the point of support. The hoop which is under the sus-
pension is 8 inches broad, and a little thicker than the rest.
The front hoop is of a different construction : its thickness is
Forty-feet Reflecting Telescope. gy^
about two-tenths of an inch, and being bent at rectangles, that
part which is held down to the tube, and to the ten bars,
keeps it steady, while that in the other direction serves as a
ring, both to strengthen and confine the aperture. It pro-
jects about three inches all around, and leaves an opening of
4 feet 4 inches to the mouth of the telescope.
The loop of suspension stands across the tube, and receives
a round Ibar of iron, shaped as in fig. 35. which is left at li-
berty to take its own position. To the places a b are hung
two double pulleys, and at c> a single one; all turning upwards
to meet the upper set of pulleys.
On the top of the stand, and round the centre beam, passes
a ring of iron, 4 inches broad, and 1 inch thick, which con-
tains a loop resembling that on the point of suspension at the
telescope. This also receives a round iron bar, bent as in fig.
36. and supports three double pulleys at def.
Nothing can obstruct the motibn of a tackle more than the
friction of the ropes against each other ; and as the utmost
ease was required in the action of my pulleys, it was particu-
larly necessary to guard against a defect of that kind. Ano-
ther inconvenience was to be avoided, still more pernicious
than the friction of the tackle. When pulleys are set, two*
three, or four in a row, side by side, they will incline one way
when the weight is drawn up, and another when it is let down.
This may easily occasion an accident, which in the case of my
large telescope must have been exceedingly troublesome, and
probably in the end proved fatal; for by the side inclination
of the set, a rope will sometimes slip out of its place ; espe-
cially as my ropes are well soaked in melted tallow to preserve
them from moisture. This in summer will occasion dust to
gC 2
376 Dr. Herschei/s Description of a
settle upon them, and sometimes fill up a channel of the pul-
leys, so that the least deviation from the perpendicular may-
throw a rope out of its place. Should this happen in the night,
when it might not be immediately perceived, the rope would
soon be injured, or even cut through, by the continuation of
the force that acts upon it. Besides, this irregular motion of
the pulleys, when the telescope is finely suspended in the me-
ridian, will tend to produce a little deviation in right ascen-
sion, which ought to be avoided. My pulleys, therefore, are
all but one in a meridional situation, and this might also be
turned the same way if there were occasion for it. The double
pulleys are placed under each other ; by which means the
stress of the lower ones at the top, and the upper ones at the
bottom, adds to their meridional and % perpendicular steadiness.
In order to command every altitude, from the horizon to
the zenith, it was necessary that the point of support should
be moveable. Its motion is effected by a mechanism which I
shall novy explain.
Eight bars, q± inches broad, and i± thick, were cut into
teeth at the distance of 1^ inch each; and afterwards con-
nected by dips screwed against both sides of the places where
two butt together. Their length is such, that four and four
being joined make up two bars of 29 feet 8 inches long each.
Two loops which are screw- bolted to the ends of them, take
hold of the axle, fig. 34. at D and E, which in those places
is made round for that purpose.
Upon the foundation beams in fig. 3. are fixed four short cross
beams, at // mm nn 00 ; these carry the following machine,
A handle which turns a pinion of eight leaves, drives a wheel
of 20 inches diameter, with 51 teeth ; the axle of the wheel
Forty-feet Reflecting Telescope. 377
contains a pinion of 12 leaves, driving a wheel 3 feet in dia-
meter, of 88 teeth. On the axle of this latter are fixed, upon
a long bar, two lantern pinions of twelve leaves* at a distance
of 3 feet 9 inches from each other, and these are confined
down to work in the two long cut bars, which pass under
them at that distance in iron notches, to prevent their reced-
ing sideways. The long bars, are supported by narrow, slips
of timber, pp, qq, which are extended from the front to the
back ;, as otherwise the weight of these bars would bend them
down so as to render them unfit for action* The slips are co-
vered with sheet iron, that they may not be injured by friction.
The -front ends, of the bars are furnished with claws, which
keep them in their places upon the slips;
Two supporters of oak-, 29 feet 8 inches long each, 6 inches
broad, and 4, thick, are extended from r s near the pinions, to
r s at the back. These are made convex at the top so as to fit
the concavity of the iron rollers A B, fig. 34. They are also
covered with pretty thick sheet iron, to prevent their being
worn by the motion of; the weight which is to go upon them.
The distance from the centre of one to that of the other is 5
feet 4^ inches.
These things being arranged as has been described, it ap-
pears clearly that, when the handle of the first pinion is turned,
the system of wheels and pinions in the machine will-draw the
bars, a*<id consequently the point of support of the tube, for-
ward into any required, situation ; and return it back to its
former place, by turning the same handle a different way.
At ;S-', fig. 3. near the platform of boards* 1 1 vv> is placed a
barrel, 19 inches in diameter, and 17 broad, with high sides to
confine the long rope which draws up the point of suspension
378 Dr. Herschei/s Description of a
<>f the telescope by means of the pulleys that have been de^
scribed. On one side of the barrel is a wheels c i feet 3 inches
in diameter, with 91 teeth ; and a handle with a pinion of 4
leaves gives motion to it, when the telescope is to be lifted up
or let down.
" The method of stringing the pulleys is expressed in fig. 37.
The^rope A, coming from the great barrel, passes successively
over the pulleys 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ; and from B
goes to another barrel, T', fig. 3* which is also near the plat-
form t tvv, the use of which will be explained hereafter.
By the assistance of these two motions, the telescope may
be set to any altitude, up to the very zenith ; and in order to
have the direction of it at command, a foot quadrant of Mr.
Bird's is fixed at the west side of the tube, near the end of it,
inclosed in an iron case ; upon the top of which is also planted
a finder, or night-glass, about 21 inches long, with cross wires
in the focus. The divisions of the quadrant are indicated by a,
spirit-level, instead of a plumb-line.
The axle, which turns the first pinion of the mechanism for
moving the point of support, carries a pallet. This gives mo-
tion to a small wheel with studs, contained in a machine fixed
to the frame of the great wheel- work, and inclosed in a little
box. The wheel with the studs carries a perpetual screw, which
moves a central wheel, upon the axis of which is fixed an index-
hand, that passes over a graduated plate of 140 divisions.
Each of these divisions answers to four turns of the handle ;
and they are large enough that a 4th part of one of them may
be distinguished. In this manner the hand will point out how
many turns of the handle have been made to move the telescope
from its most backward point of support to the most forward.
Forty-feet Refecting Telescope* 379
I call this machine the bar-index. It is of eminent use in
giving us immediately, by means of a table made for that pur-
pose, the place of the point of support for any given altitude
or zenith distance of the quadrant*
In order to come at every part of the heavens, the vertical
motion of the telescope requires the addition of the horizontal
one. This has been obtained by another very simple mecha-
nism. We have already seen that the bottom frame rests upon
20 concentric rollers, and is moveable upon a pivot.
At ww xx y 9 fig. 3. is a machine in every respect like that
Avhich has been described as giving motion to the point of sup-
port, except that instead of a bar with two lantern pinions, the
great wheel here carries an iron barrel, 2 feet 8 inches long,
and 5 inches in diameter. Near the ends of the great cross
beam 1 1, are planted two pulleys ; one at T, the other at V.
Round the outer circular wall is a gravel-walk, 12 feet broad ;
and on a grass-plot close to the margin of this walk are eight
posts of oak, in large frames, firmly buried in the ground* at
equal distances,/ so as only to shew their heads sufficiently to
admit an iron ring and pulley to be hung upon them as occa-
sion may require ; the middle beam also carries an iron loop
at each end.
A strong rope is now thrown round one of the spokes of the
wheel, next to the barrel,, which passes with one of its ends
under the bottom of it, while the other remains at the top.
As soon as the handle puts the wheel-work in motion, the bar-
rel will draw both ends of the rope, but in contrary directions.
One of the ends is then to be led to the pulley on the great
beam at T, while the other is made to pass over that at V ;
but in a contrary direction. Upon the nearest post at rectangles
380 Dr. HerschEi/s Description of a
to the great beam, towards the south, for instance, is hung a
ring with a pulley to it; while, at the same time, a similar
ring and pulley is fastened to another post, near the opposite
end of the beam, but situated towards the north. The ends of
the rope are now returned through these pulleys, and with
iron hooks, wlneh'are fastened to them, are hung in the loops
at their respective ends of the middle beam.
As soon as the ropes are sufficiently and equally stretched,
the telescope will begin its horizontal motion, which may be
continued as long as the same posts will be conveniently si-
tuated. In order to go on with the motion, the ropes are to
be slackened, and the rings being then hung upon the two next
posts, we may continue! ' at pleasure to turn the telescope to
any part of the heavens that m&y be required. The arrange-
ment is represented upon a small scale in fig. 38.
It should be noticed, that the ends of the rope must be
equally stretched, for which reason a mark ought to be made
in the place, which is to be thrown round the spoke of the
wheel. The fastenings of the pulleys also, which are joined to
the rings that are thrown over the posts, ought to have an ad-
justment by links and: hooks, to be either lengthened or short-
ened at pleasure.
With the assistance of the motions that have now been de-
scribed, I have in the year 1789, many times taken up Saturn,
2 or 3 hours before its meridian passage, and kept it in view
with the greatest facility, till 2 or 3 hours after the passage ; a
single person beibg able, very conveniently^ to continue both
the horizontal #nd vertical motions, at the command of the ob-
server. In this however ought to be included an assisting third
motion, which I am in the liext place to explain.
Forty-feet Reflecting Telescope. 381
We have seen that in fixing the ladders they were set at
8 feet 2 inches distance in front,- in order to permit the tele-
scope to have a side motion, without displacing the whole ap-
paratus, which is designed for a meridional situation,
Every celestial object, when it passes the meridian, is then
in its most favourable situation for being viewed, on account
of the greater purity of the, atmosphere in high altitudes. The
advantage also of being able to direct the instrument, by
means of the quadrant, to the spot in which we are to view
the object, is considerable, in so large an instrument as the
40-feet telescope. With unknown objects, it is likewise of the
greatest consequence to be enabled,' by a meridional situation,
to ascertain their place. But, as a single passage through the
field of view, especially with my examinations of the heavens
in zones, would not have been sufficient to satisfy the curio-
sity of an observer, when a new object presented itself, it be-
came necessary to contrive a method to lengthen this interval.
The tlibe, therefore, as we have seen, is made to rest with the
point of support in a pivot, which permits it to be turned side-
ways.
Its diameter being 4 feet 10 inches, and that part which is
generally opposite the ladders that confine it in front being
about 35 feet from the pivot, it appears that a motion of 3 feet
% inches may be had, which to the radius of 35 feet gives up-
wards of five degrees of a great circle.
Several abatements must be made on account of the dispo-
sition of the apparatus that gives this side motion, and the
shortness of the topes in high altitudes; but there remains,
notwithstanding, a sufficient quantity of this lateral motion to
mdccxcv. 3 D
382 Dr.-HERScHEi/s Description of a
answer the purpose of viewing, pretty minutely, every object
that passes the meridian.
Before I can give the particulars of this side motion, some
other things must be explained. The point of support rests in
a pivot ; but this alone could not have given steadiness to a
tube of 4 feet 10 inches in diameter, loaded with the weight
of the strengthening bars, and speculum, which rest upon it.
Two moveable supporters have therefore been provided at pq 9
fig. ,34. They consist of two solid brass rollers, 3 inches thick,
and 4^ in diameter ; set in strong frames firmly united to the
sides of the tube, and resting upon the flat face of the square
axle AB, which carries the pivot in the centre. The middle
of these rollers is applied about 2 feet 2 inches from the centre
of the pivot ; and being set so as to lose none of the motion
which they may have upon the axle, we find that there is room
for full as much angular motion of the rollers upon the axle,
as there is for the tube between the sides of. the ladders ; and
indeed more than can be wanted, as 10 minutes of time are
generally sufficient for viewing any object.
The method of observing with this telescope is by what I
have called the front view ; and the size of the instrument
being such as would permit its being loaded with a seat, there
is a very convenient one fixed to the end of it. The foot-
board or floor, is 3 feet broad, and 2 feet q± inches deep.
The seat is moveable from the height of 1 foot 7 inches to 2
feet 7 inches, not so much for the accommodation of different
observers, as for the alteration which is required at different
altitudes, and which amounts to nearly 12 inches. One half of
the seat falls down, to open an entrance at the back ; and being
Forty-feet Reflecting Telescope. 383
inclosed at the front and sides, a bar which shuts up the back
after the observer is in his place, secures the whole in such a
manner as to render it perfectly safe and convenient.
There are two strong iron quadrants with teeth, at the sides
of the seat, in which run two pinions fixed upon a bar, with a
ratchet and handle at the end of it. By turning that handle,
the seat is easily brought to an horizontal position, before the
observer enters it; or restored to it, when any considerable
alteration in the altitude of the telescope renders a change
necessary.
The focus of the object mirror, by its proper adjustment, is
brought down to about 4 inches from the lower side of the
mouth of the tube, and comes forward into the air. By this
arrangement, there is room given for that part of the head,
which is above the eye, not to interfere much with the rays
that go from the object to the mirror; the aperture of the
speculum being 4 feet, while the diameter of the tube is 4 feet
10 inches ; especially as we suppose a night observer will pre-
fer some kind of warm cap to a hat, the rim of which might
obstruct a few of the entering rays.
A long coarse screw -bar is confined in a collet, which takes
on and off; and may readily be put to the inclosing right side
of the seat, so as to present the observer with a short handle.
The other end of this bar passes into a nut, which, like the
collet, moves upon a double swivel, so as to admit of every
motion. The nut is planted upon a machine which will be
described hereafter, and may be drawn up to any altitude, so.
as to bring the nut upon a level with the swivel of the handle.
Upon turning the handle, the, observer will screw himself, the
seat, and the telescope, from the ladder ; and may thus follow
3D 2
3S4 Dr. Herschei/s Description of a
the object he wishes to pursue in its course, for as many rfii-
nutes as may be convenient If, indeed, he is inclined to give
up the meridian for some time, he may order the whole frame
to be moved by the great round motion, which ought to be in
readiness ; and may even keep his object in view, as I have
often done, by screwing the telescope backwards as fast as the
round motion advances it. Then screwing himself forward
again, he may repeat these successive motions as long as he
pleases.
In those observations, which I have called sweeps (from the
method of oscillating or sweeping over an arch, which at first
I had adopted in the way of right ascension, but which in the
year 1783 I reduced to a systematical method of sweeping over
zones of polar distance), several conveniences are required;
the principal of them are as follows.
An assistant, provided with an apparatus for writing down
observations ; with catalogues of stars, atlasses, and other re-
sources of that kind.
A small apartment, as near to the observer Its possible, in
which this apparatus, with candles and other conveniences*
may be inclosed.
A sidereal time-piece.
A right ascension BppaMtus.
A polar distance apparatus.
A polar distance clock,
A zoned catalogue of the stars.
And a ready communication between the observer and as-
sistant, both ways.
There is also wanting, a person tor give the required mo-
tions for sweeping the zones of the heavens.
Forty-feet Reflecting Telescope., 385
A micrometer-motion to perform the sweeps.
A zone-piece, to point out the required limits of the in-
tended zones.
A small apartment to inclose these motions, and the candle
which is required for the workman.
And a ready communication, for the observer to direct the
workman in the required motions*
All these conveniences were gradually brought to perfection
with my 20-feet telescope ; but here, they were at once, and
with great advantage, designed and executed in their most
improved state.
A'B'C'D'; fig. 3; is the floor of the observatory, 8 feet 5
inches by 5 feet 5. It is of a proper height, and has a double
window towards the west, with a shutter to be used at night
Fig. 1. gives a sufficient view of it
E'F'G'H'; is the floor of the working room, which is
6 feet 6 by 4 feet 5; and has two small windows, one to the
south, the other to the east. Its height is considerably less
than that of the observatory ; and a, view of this may also be
seen in fig. r*.
The distance between the observatory and the end of the
telescope, is evidently too far for aconversation in the open
air, between the observer and assistant; especially as the
latter, on account of his candles, must be inclosed ; and ought
not to leave his post at the time-piece and writing-desk. Add
to this, that when the observer is ele va ted, 30 or 40. feet above
: the assistant, a moderate breeze Will carry a.way the soun,d of
his voice very forcibly. A speaking-pipe was therefore neces-
sary, to convey the communications of the observer to their
destination.
38£ Dr. Heeschei/s Description of a
At the opening of the telescope, near the place of the eye-
glass, is the end of a tin pipe, into which at the time of obser-
vation a mouth-piece may be put, which can be adapted, by
drawing out, or turning sideways, so as conveniently to come
to the mouth of the observer, while his eye is at the glass:
This pipe is i^ inch indiameter, and runs down to the bottom
of the telescope, to which it is held by proper hooks, that go
into the tube, and are screwed fast at the inside. When it is
arriyed as near to the axle AB, fig. 34. as convenient, it goes
into a turning joint ; thence into a drawing tube, and out of
this into another turning joint ; from which it proceeds by a
set of sliding tubes towards the front of the foundation timber.
The mechanism of the first turning joint and short sliding
tube, as well as the next turning joint, is executed in brass, as
represented in fig. 39. The tube a is the continuation of the
?pipe which comes down from the observer ; at b and c it is
turned about in an angle, but the part b and £ consists of a
double brass tube, one of which may be turned within the
other, b de 9 is an arm which has two pivots, one at b 9 the
other at c ; the part d is put through, and pinned to a fasten-
ing at the tube, where it is also permitted to turn about if re-
quired. When the telescope is lifted up, the pipe a b c turns
upon the pivot b, and within the j)ipe bee ; which also turns
upon the pivot c ; so that a be may come at rectangles to
i> c e, when the telescope is turned up to the zenith. At the
same time c e sliding i n fg, will be drawn out, since be is not
in the axis of the vertical motion, which lies in AB, fig. 34.
but turns in a small arch about it. The point c will not only
be drawn back, but will also be lifted up, and therefore a se-
cond turning joint, bg 9 becomes necessary, which is of the
Forty-feet Reflecting, Telescope.. 387
same nature with the first. From hi the pipes are continued
in those joints of 9 feet 6 inches long each. These slide into
one another as far as is required, and, all together, into a
fourth pipe, when the telescope is advanced to the place where
it rests in zenith observations. The fourth pipe, which is the
largest, goes to the end of the frame H', fig. 3. where it turns
towards F; and is there again made to return to K', At this
last place it divides itself into two branches, one going into the
observatory, to X/, where it rises up through the floor; the
other going into the work-room to M'v w T here it also, ascends
through the floor, up to the level of the workman's ear, who
stands just by the place where it terminates in the usual
shape of speaking pipes. Notwithstanding the passage of the
sound through a pipe with many inflections, and not less than
115 feet in length, I find that it requires no particular exer-;
tionto be very well understood ; and that the communication
is quite sufficient for the purpose ; though undoubtedly some
advantage might have been gained if brass sliding tubes had
been used throughout the whole length. Under the long pipes
that slide into one another, is a semicircular gutter, extended
from N' to O', which keeps the pipes in their place, as they
are carried along by the motion of the telescope,, when the
point of support is advanced or drawn back ; and the large
gathering pipe is inclosed in a box, N'H', to secure it from
accidents.
The right ascension apparatus is constructed thus. Against
the sides of the tube, and 2 feet 6 inches from the mouth of
it, are fixed the centres of two rubbing plates, 3 feet 10 inches
long, 2 feet 1 inch broad, and near 2 tenths of an inch thick/
These plates are fastened to the long bars of the tube, nearest
388 Dr. Herschei/s Description of a
the top and bottom, by six arms each ; and screwed on so as
to be perpendicular to the horizon. The plate on the west is
fixed, but that on the east is adjustable, in order to be kept
perfectly vertical on every part of its surface. One of these is
visible against the tube, in fig. 1. An iron roller, i inch thick,
and 2~ in x diameter, is set in a strong frame, in such a manner
as, to allow the claw, which holds it, to be set to any direction ;
where it can be afterwards fastened by a large horned nut. This
roller is mounted upon a frame, see fig. 40. that may be drawn
up to any altitude, and lies upon the whole set of ladders on
the east; where it rolls up and down on six sets of brass rollers,
abc d ef 9 which are constructed as in fig. 17. This machine
consists of a bottom frame, and a bar, gh, at rectangles to it,
which, when the frame lies upon its rollers on the ladders,
stands also at rectangles to them, on the lowest part of the
frame : it is braced so as to make the greatest resistance from
east to west. The bar carries the iron roller, which may be
shifted to two different situations ; g almost down to the lad-
ders, and b more elevated. The latter is used in high alti-
tudes. The iron roller, standing out, is then turned so about
as to be in the direction of the length of the eastern rubbing
iron ; in which situation it is fixed by the horned nut. The
telescope is then brought forward, or backward, by the bar
machine, till the rubbing iron comes to be opposite the roller.
Upon one of the braces of this same frame at i, is also
planted the nut belonging to the lateral screw motion, which
has been described ; and its long, bar goes always with this
machine, when it is disengaged from the .observing chair, and
is laid back at k into a secure resting place.
On the opposite set of ladders is another machine, which
Forty-feet Reflecting Tele scape. 389
carries a large spring-bolt, on the end of -which is mounted an
iron roller, exactly like that which has been described. This
is also adjustable to any direction. The bolt is contrived in
such a manner as to corhe out of the frame, in which it runs^
with a pressure of 34 pounds ; and it exerts, very nearly, the
same force during the time in which it goes through every
part of the space it describes. The epnstr notion of the springs
is expressed in fig. 41 : a b a are two iron bars, 5 feet 6 inches
long ; jointed at b, like a pair of compasses, and fastened on a
pivot at a, which remains immoveable upon the frame, while
the other end is also fastened on a pivot- fixed to the bolt,
which carries the rdller/. The bolt is 7 feet 1 inch long, and
g inches square. ■ It runs in two sets of four brass rollers
each, at g and h, which embrace it completely, and prevent
friction as much as possible. The joint b is sustained by a brass
roller, whiqh runs on the iron plate ik. Two tapering steel
bars, or springs, 1m, no, are fastened against the lower ends
ac of the iron bars ; one of ^them is convex at m 9 the other
concave at ; and they exert their force against each other at
mo, where the convex one rests in the concavity of the other.
There is an adjustmejtit of the Bap which carries the bolt, see
fig, 1. by which it may be raised up, so as to become exactly
opposite to the roller on the east, when that is raised to its
highest position.
It will now be easily perceived,, that when the eastern rub-
bing plate, in its well adjusted vertical position, is pressed
against the right ascension roller, by a roller exactly opposite,
and with a force sufficient to keep it firmly poised against
that roller, a vertical motion may be given to the telescope, m
mdccxcv. 3 E
ggo Dr. HERSCHEt's Description of u
which the same meridional situation will be preserved. Ac-
cordingly, I find that the right ascension of unknown objects,
deduced from known ones, observed by the same instrument,
and in the same zone, is capable of great precision ; and this
construction will therefore answer all the ends that were pro-
posed. For it wpuld not be doing justice to the telescope to
require of it all the accuracy of a transit instrument.
The spring-bolt, as I call this latter machine, is brought to
any required situation by a rope fastened to the middle cross-
beam of the stand, which comes down, and goes through a
pulley placed upon the machine; in its return to the top, it
passes over a second pulley, and then goes down to a barrel
with a wheel and pinion, on the ground timber at Q'.
The polar distance machine, as I call the opposite one, on
account of its chief use, which remains still to be explained, is
drawn up and down in a similar manner, by the handle of
a pinion, wheel, and barrel placed at R'.
In the observatory is placed a valuable sidereal time-piece,
made by Mr. Shexton, for which I am obliged to my astrono-
mical friend Mr, Aubert, as a gift that will always be highly
esteemed. Close to it, and of the same height, is a polar
distance piece, which has a dial-plate of the same dimensions
with the time-piece; and is also divided into sixty parts on the
outside; but these are to express minutes of space. Every
tenth is marked with large figures, but every single one is also
denoted with its proper figure, in a smaller character. The
degrees are shewn in a square opening under the centre, and
change backwards and forwards as the telescope rises or falls.
This piece may be made to shew polar distance, zenith dis-
Forty-feet Reflecting Telescope. 391
tance, declination, or altitude, by setting it differently; but
in conformity with Flamsteed's British catalogue of stars, I
have generally adopted polar distance.
The construction of this piece is very simple. ; It contains
only one barrel, for the weight and line, which gives motion
to the work ; and two small index wheels. The line is con-
ducted from th§ ^polar distance machine into the observatory
at the bottom of the polar distance clock, where it rises up,
and passes over the barrel. By making this revolve, it moves
the hand upon the axle o£ it, which points out the minutes
upon the dialrplate. The hand is made adjustable in the usual
way of the minute hand of common clocks, by going upon a
pipe, kept firm by springs. The line is of considerable length ;
but the case of the clock being no larger than that of the
time-piece, a set of neat and very thin pulleys, four and four,,
are used to draw the end, after its having crossed the barrel,
It is necessary to mind, in setting these pulleys, that they
should rsm upon very thin pivots, and clear one another per-
fectly ; as otherwise their aetion might not be adequate to the
purpose ; this however is only to stretch the end of the line
freely and sufficiently, that in passing over the barrel it may
not make it turn about irregularly. There will be no-occasion
for a revolution of the line upon the barrel, as I have found a
jnere passage over it of sufficient effect in turning it ; for the
hands. must all be properly counterpoised. Each revolution
answers to one degree of change in polar distance ; the minutes
are therefore pointed out by the hand it carries. The two
small index-plates I have mentioned, are fastened upon pivots
against the back of the dial-plate, between it and the frame of
tire barrel, They are placed so, that their edges meet not far
3 E 2
39& Dr. HEKscmt's Description of a
from the centre of the square hole, I have mentioned, in the
dial-plate, for shewing the degrees; and a small square por-
tion, a little more of one than the other of the two wheels,
may therefore be seen, in front of the dial-plate, through the
opening in it.
These wheels carry contrate teeth on the inside, and a small
dial-plate on the back. The face of the dial-plate of the wheel
which presents itself at the right, carries the units of the de-
grees ; l, 2, 3, 4, 5, 6, 7, 8, g, o ; while that on the left has a
hlank which remains till the o of the first appears. Upon the
axle of the barrel, close to the frame-plate on the outside, is
fixed a long counterpoised contrate pallet ; which at every
revolution sweeps over one of the teeth of the first wheel, of
which there are ten; The shape of the pallet must be like the
barb of an arrow ; but more obtuse, that it may take as much
time in entering very obliquely into the teeth as possible, to
avoid a sudden shock. The movement will even then be found
to be quite quick enough, for shewing almost instantly the
proper degree of polar distance. But to counteract the sud-
deh stroke of the long pallet, there is over each wheel a small
lever, see a b, fig. 42. that rests with its end between the two
uppermost teeth cd; arid its shape is that of a very obtuse
angle, such as t6o degrees. The point of the angle sinking
down between the two teeth, by its slope both ways, prevents
their overshooting. The lever is held down with a very weak
spring, ef, ths point of which touches the lever at e, near
the place of its pivot. This method will even throw back the
figure upon the dial, if it should have been overshot a little.
Care must be taken to let all this work be light, that no great
force may be required in the long pallet to move it
Forty ^feet Reflecting Telescope. 393
The first wheel in turning about carries a short pallet, of a
shape similar to the long one. This must be placed low enough
to let the long pallet pass freely, and high enough toxlear the
spring-lever in going over it. The pallet, on the appearance
of the o, strikes a tooth of the second wheel, andt brings the
figure 1 into view, which with the other forms 10. The se-
cond dial-plate has a blank, and the figures 1, 2, 3, 4, 5, 6, 7,
8, 9, 10, 11, 12, engraved upon its face, and presents thirteen
teeth to the pallet on the first wheel, by which the blank and
figures are successively brought into view, along with the suc-
cession of the units on the dial-plate of the first wheel.
In this manner the degrees are shewn from o to 129/ which
includes the whole range of north polar distance in* this lati-
tude; while at the same time they are properly subdivided
into minutes. A more minute division was not thought ne-
cessary with this instrument, and indeed ought not to be
aimed at
The cord which gives motion to the polar distance clock, is,
rendered a just representative, or true index, of the angular
movement of the telescope in the following manner.
On the machine which holds the right ascension roller, is an
arm / m n> fig. 40. in an oblique direction, upon which is
fastened a brass slider, 3 feet 1 inch long* A coarse screw
passes from one end of it to the other, and is cpnfined between
its shoulders / n. At there is a handle, by which the screw
being turned, a small sliding plate m, which carries a pulley,
is drawn backwards or forwards at pleasure, along the whole
range of the slider.
On the telescope, near the bottom of the front edge of the
§94 & r * Herschei/s Description of a
eastern rubbing-plate, is a small square bar with a loop upon
it, which is adjustable, so that it may be occasionally brought
a little nearer to the mouth of the telescope, or removed
farther from it. The end of the polar distance cord is fastened
to the loop upon this bar, where it remains when the polar
distance clock is not in use. By this means the weight which
Stretches it in all its length from the telescope to where it is
suspended in the clock-case, is kept always equally exerted,
and no relaxation of the cord, which ought to be avoided, can
take place.
When the polar distance clock is to be used, the cord is
lifted into the pulley of the slider at m, and now goes from
thence to its destination as before. The right ascension roller
resting against the rubbing plate, the pulley of the slider i&
near at hand, and the cord may easily be lifted into it. The
handle o is now to be turned till the cord, which goes frotn
the loop at the telescope to the pulley upon the slider comfes
to cover a certain white line or mark upon the side of the
tube. This line when it is first made, must be placed so as
to be vertical when the radius of motion of the loop is a little
more than one degree of elevation above the horizon.
The theory of this arrangement is* that when a motion in
polar distance takes place, the tangent and the arch may be
looked upon as equal for a few degrees, in a mechanism which
aims only at minutes. And, indeed, as far as two degrees and
twenty minutes, when the motion is taken equally both ways
of the adjusting point, the deviation from truth will not even
amount to quite one second.
The coitd from the pulley of th6 polar distance machine passes
Forty-feet Refecting Telescope. g<)$
straight away to O 7 , fig. 3. where it is bent over a small pul-
ley to one just close to it, which leads it in a direct line to P',
under the polar distance clock, where it rises up to the barrel.
The barrel is of such a diameter as to answer as nearly as
possible to the length of the cord which is drawn by the mo-
tion of the telescope over one degree of polar distance ; but
as the utmost accuracy could not have been obtained in the
make of the barrel, the loop at the telescope which draws the
end of the cord, as we have described, may be slipped back-
wards or forward upon its bar, which will either lengthen or
shorten the radius of its motion, and occasion its drawing
more or less of the cord-
As there is a good quadrant upon the telescope, there re-
mains nothing else to obtain a just position of this loop than
to compare the indication of the polar distance piece with that
of the quadrant ; and when the former is regulated to a per-
fect agreement with the latter, we may safely rely upon the
truth of its report.
The time and polar distance pieces are placed so that the
assistant sits before them at a table, with the speaking-pipe
rising between them; and in this manner observations may
be written down very conveniently. The place of new objects
also may directly be noted, as their right ascension and polar
distance is before the assistant upon the table, where nothing
is required but to read them off, on the signal of the observer.
By a catalogue in zones the assistant may guide the ob-
server, who is with his back to the objects he views, and who
ought to have notice given him of such stars as have their
places well settled, in order to deduce from their appearance
the situations of other objects that may occur in the course of
396 Dr. Hersch£l's Description of a
a sweep. In the year 1783, when I began this kind of ob-
servations, no catalogue of stars in zones had ever been pub-
lished ; I therefore gave a pattern to my indefatigable assistant*
Carolina Herscmel, who brought all the British catalogue
■into zones of one degree each, from the 45th degree of north
polar distance down to the horizon, and reduced the right as-
cension of the stars in it to time, in order to facilitate obser-
vations by the clock. [This catalogue was afterwards completed
from the same degree up to the pole in zones of 5 degrees each ;
and the variation in right ascension from one degree of change
in longitude, was also reduced to time, for every star in the
catalogue. To this were added computed tables for carrying
back present observations to the time of that catalogue; which
method I preferred to bringing the stars it contains forward
to the present 'time, on account of conforming with the con-
struction of the Atlas Coelestis, which was of great service.
The evident use of such a catalogue must undoubtedly soon
have been perceived by every person who was acquainted with
the method I used for sweeping the heavens ; and as the same
is practicable, not only with my telescopes, but likewise with
transit instruments, and mural quadrants, we are now much
indebted to the Rev. Mr. Wollaston, who in the year 1789
favoured the astronomical world; with a work of nearly a si-
milar construction with that which I was in the habit of using;
but much enlarged, and enriched with stars taken from the
best authors; and moreover reduced to the time of the year
1790.
We now seem only to want an atlas on the same construc-
tion, upon a scale equally extensive, and plentifully stored with
well ascertained objects*
Forty-feet Reflecting Telescope. 337
The micrometer-motion which is required-for sweeping the
heavens, and indeed for viewing the planets or other' objects,
is obtained by means of the end of the rope B, fig. 37. which
draws up the telescope. This goes down to a barrel, S', fig. 3.
12 inches long, and 4 in diameter, joined upon the same axle
with another barrel, 12 inches long, and 12 in diameter. A
smaller rope goes from the largest barrel into the working-
room, where it is fastened to the top of a thin vertical spindle,
2 feet 6 inches long, and 3 inches in diameter, at a, fig. 43.
Another rope of equal size is fastened to the bottom of the
same spindle at 6;' and when by turning the handle c dike
rope ae is wound upon the spindle one way, the rope bf is
wound off the contrary way- This second rope bf goes out of
the work-room over a pulley, which leads it upwards to the top
of the middle cross beam of the ladders, where it descends over
another pulley, by a weight with shifters which is suspended to
the end of it. In this manner a balance is obtained between the
stress of the ropes a e and bf, which leaves the spindle at rest
in any position where it may chance to stand, and conside-
rably eases the labour of the workman, who turns this handle
a certain number of times one way, and then the same number
of times back again. By such a motion of the handle the te-
lescope is alternately depressed and elevated ; and this being
continued for as long a time as the observer chooses, enables
him to review the heavens as they pass by the telescope.
By the arrangement of the barrels, it is easy to see that the
motion is sufficiently divided; as many turns of the handle
are wanted to pass over a small space of the heavens. The
method of barrels and ropes is to be preferred to wheel-work,
on account of the smoothness as well as silence of the motion,
mdccxcv. 3 F
398 Dr. Herschei/s Description df a
both which in observations of this kind are highly necessary.
It is true that the great stress which lies on the ropes of the
micrometer-motions wears them out very fast, and they must
therefore be carefully watched, and often renewed ; but this
ought to be no objection where the end to be obtained is of
such consequence.
It would not only be troublesome to the workman, but often
bring on mistakes, were he to count the turns of the handle,
which perhaps for hours together he is moving ; a zone-clock,
therefore, has been contrived to release him from that care.
This is a machine which is placed upon a table just by the
workman. It strikes a bell when he is no longer to turn one
way ; that is, when the telescope is come to one of the limits
of the zone, which if it be after going down, is called the bot-
tom bell ; and it strikes another bell when he has made the
same number of turns in a contrary direction. The telescope
is by this motion restored to its former situation, and this se-
cond notice is called the top bell ; which marks out the other
limit of the zone, These bells not only give notice to the
workman when he is to change, but their different sound in-
dicates the position of the telescope, and prevents mistakes.
An additional precaution has been used, to make the bells
repeat their stroke, the very next turn, if by some mistake the
workman should have been inattentive to the first notice. In
a long continuation of uniform intervals of sound, we may
become so used to them as hardly to perceive them at all ;
but the coming in of an additional sound will immediately
rouze the proper attention. Another very necessary use which
I have often made of a second or third bell, is to extend the
zone, either towards the north or south, for some time, when
Forty-feet Reflecting Telescope. 399
notice has been given of a star that was a little above or below
the sweep ; for in some parts of the heavens known stars are
scarce, and it becomes necessary to take in all those that may
be come at
The construction of the zone-clock is very simple and con-
venient. The end of the axle which holds the double barrel,
fig. 3. must be left projecting at T'. Upon this a small hollow
cylindrical pipe is placed, which holds the end of the cord
that is to move the zone-clock. The pipe must be guarded
at both ends like a clock barrel, to keep on the cord, but re-
main open at the end which goes upon the axle, upon which
it must fit upon a square so as to keep firm. It should be
about if inches long, and 1 in diameter.
From this piece the cord is made co pass to the work-room,
where it rises up into the clock at a, fig. 44. It then passes
over a large narrow barrel, bed, and by means of a weight
w at its end, descends when the. handle of the micrometer-
motion turns the spindle and double barrel with which the
pipe that holds this cord is connected. At c b are two levers
that, in the usual way, occasion the hammers ef to strike the
bells g h, when the pins quit the levers which they have lifted
up. But these levers have spring joints, so as to permit the pins
to pass back again without disordering the work. The pins
which move the lever c are fastened to the barrel b c d. The
lever b must be brought out so as to be before the front of the
first frame-plate, and close to a dial-plate, which is to contain
about 40 numbers. The dial-plate must be pretty thick, and
be fixed upon a hollow arbor. The axle of the barrel, which
should be strong, must be long enough to come through the
hollow arbor, and project a little way to receive a milled nut
3 F *
400 Dr. Herscuei/s Description of a
upon its end, which must have a screw upon it. The arbor
which carries the dial-plate is then to be pinned fast upon the
axle, and an adjustable hand being put upon the projecting
arbor, a collet is slipped over it, and the milled nut screws it
down, in any position that is to be given to it.
The adjustable hand is made of a piece of springy iron, or
steel, formed as represented at ik ; but broader than clock hands
usually are. It must have a pretty large circle in the middle^
with a hole wide enough to go upon the plate-arbor. The end
k of the hand must project beyond the dial-plate a little way,
so as to permit two screws, m n 9 to pass by it into a brass plate^
with a small piece between, to allow some motion up and down
to the hand. The plate which is fixed to the hand by the
screws mn y returns under the dial-plate sufficiently to carry
three pins that are to lift the lever fr, when they come to the
proper situation, in the same manner as those on the barrel
lift the lever c. The dial-plate, close to the margin, should
have as many small holes, to receive a pin, as there are num-
bers marked upon it ; and in the hand, answering to the holes*
must be fixed a steady pin to fall into any one of them, when
the hand comes to be placed over it There must be a small
handle near the end / of the hand, by which it may be lifted
up, and moved into any situation: that shall be required ; and
care must be taken to have both ends properly counterpoised.
In order to set the zone-piece to the breadth of any parti-
cular sweep, as for instance two degrees, we make the work-
man begin at the striking of the top bell, and while he turns
the handle till the quadrant or polar distance-piece points out
a change of two degrees, we keep the hand of the zone-clock
lifted up, that the pin may be out of the holes upon the dial-
Forty -feet Reflecting Telescope. 401
plate ; for which purpose also the nut in the centre must be
unscrewed a little to permit it to pass freely; When the tele-
scope has descended two degrees the workman must stop the
handle. We then lift the hand to the place where the first
pin strikes the lever of the bottom bell. Here we let the pin
drop into its proper hole, and screw fast the central nut.
When this is done, the workman may turn backwards and
forwards from bell to bell, and the telescope will perform the
required motion of two degrees.
The work of the zone-piece is arranged in such a manner
as to make the numbers on the dial-plate answer to turns of
the working handle : this however, though convenient, is not
absolutely necessary. The number of turns to a degree varies
a little in different altitudes ; but by trial a table may be
made, which will shew with sufficient accuracy the figure on
the dial-plate to which the hand must point, that the zone-
piece may give any required breadth to a sweep, at any certain
polar distance;
By means of the speaking-pipe the workman may be di-
rected to begin, to stop, to go fast, or slow. And these, with
a very few other orders, will be all that are wanted ; which
being known to him and to the assistant, will occasion no mis-
take, notwithstanding the pipes which go into the two apart-
ments are united.
The ropes that come from the gallery, each bracket of
which is separately drawn up, go through a double pulley,
hung to the top cross beam, and a double pulley fastened to
the upper end of the gallery bracket ; after this they pass over
a single pulley at the top, down to two barrels placed under
the back of the ladders, one on each side. Each barrel is moved
405 Dr. Herschei/s Description of a
by a handle, on the axle of which is fixed a pinion of four
leaves : this works in a wheel, on one side of the barrel, of 61
teeth, and 18 inches in diameter.
The barrels are s^j inches long, and 12 inches in diameter,
that the rope may not be doubled often, which might hurt the
uniformity of drawing up the gallery. They are made exactly
alike, and draw an equal length of rope at every stroke of the
handle-; but as one of the persons who draw the gallery might
go on quicker than the other, each of the handles strikes a
bell at every turn, going up as 'well, as going down ; the dif-
ferent tone of the bells easily shews, by sounding in regular
alternate succession, when the gallery is properly moved;
which therefore may be safely done in the dark. The mecha-
nism of the bell- work at each handle is in a little box, to keep
it dry, but sufficiently open at the side to throw out the sound.
A single bell being suspended, a& in fig. 45. upon a plate of
iron, at a, there is a cock, be, planted upon it; between which,
at d and e, are inserted two axles continued outwards. On the
outside of the cock, and upon these axles, are two inverted
hammers suspended, with lever arms, fg. These are made
with spring joints, like those that have been described in the
zone-clock. The axle which moves the barrel has a pallet
aipon it, and the plate with the bell apparatus being presented
at rectangles to the axle, so as to make the pallet play in the
notch of the plate between/ and g, where the lever arms meet,
it will make the bell give a stroke, either with one hammer
going up, or with the other coming down.
It is necessary to preserve the pliability of the ropes, for
which reason no tar has been used with any of them that are
about the telescope. To preserve them, however, they are
Forty-feet Reflecting Telescope, 403
passed through very hot melted tallow, and kept a sufficient
time immersed in it, that they may be thoroughly penetrated.
In this state they will last a considerable time, especially when
care is taken not to relax them often. The gallery being sus-
pended by ropes in this state, it would be unpleasant to trust
entirely to them. Each bracket therefore is furnished with
four strong broad iron hooks, two of which take hold of one
of the flats of the division j3y, fig. 7. while on the opposite
side two more take hold of the corresponding flat of J e. When
the gallery has been drawn up to the required altitude, the
hooks are let down, and the ropes slackened a little, so as to
permit it to hang in the hooks. The other two hooks on each
side serve for an elevation between the flats half way from
one to the other. They are upon the same centre with the
former, and fall back as the others do when the gallery is to
go down.
For the safety of the tube also, there is a strong chain,
which will sustain it, in case the ropes by which it is suspended
should give way. This is fastened into a loop near the point
of suspension. The other end of it is hooked upon a flat, and
passes round one of the side beams of the ladder at a certain
elevation above the telescope, and is sufficiently long to per-
mit the tube to move a few inches more than is necessary.
By this means a fall can never be considerable : if the ropes
were to break in the worst part of a sweep of 2% degrees
broad, the telescope would hardly descend two feet.
The construction of the great mirror is as in fig. 46. The
metal itself is 49I- inches in diameter, but on the rim at a b is
an offset of f inch broad, and 1 inch deep, which reduces the
concave face of it to a diameter of 48 inches of polished sur*
404 Dr. Herschei/s Description of a
face. The thickness, which is equal in every part of it, re-
mains now about 3^ inches ; and its weight, when it came
from the cast, was 2118 pounds, of which it must have lost a
small quantity in polishing.
An iron ring, 49^ inches in diameter within, 4 inches
broad, and if thick, has at the face of it on the inside a strong
head or rim added to ks thickness, which fits the offset in the
speculum, but is not quite so deep a§ that. A cross of the
same substance of iron as the ring, goes over its back, and
when the speculum is placed into the ring, so as to rest upon
the offset, the cross over the back confines it in the ring. By
the addition of a thin cover of sheet iron on the back, and
another of -tin on the face, the rim makes a complete case for
the mirror.
Three strong handles are fixed against the sides of the ring,
by which the speculum may be lifted horizontally, or using
only one of them, vertically, as occasion may require.
To put the speculum into the tube, there is provided a
small narrow carriage, going upon two rollers. It has upright
sides, between which the speculum, when suspended vertically
by a crane in the laboratory, is made to pass in at one end,
and being let down, is bolted in. The carriage is then drawn
out, rolling upon planks, till it comes near the back of the te-r
lescope. The tube must be put back as far as, the bar-machine
will permk it to go. Two beams connected together so as to
form a parallelogram of 8 feet 6 inches long, and 2 feet broad,
are sloped away on one end, while the other contains two
hooks, by which it may be hooked into two holes at the end
of the foundation timber, fig. 3. in the middle between the
rolling beams r s. This affords a passage of an easy ascent to
Forty-feet Reflecting Telescope. 405
the speculum carriage, which must now be brought into a
proper position for rolling up. When this is done, the carriage
is to be tied to the axle of the point of support, A B, fig. 34.
and by turning the bar-machine handle, the speculum with
its carriage will be drawn up to the foundation beams E E,
A A, fig. 3. which are 16 inches above the foundation wall.
By the time that the carriage comes near to the top, there
will be room for six 3-inch planks that are provided, to be laid
one after another upon the rolling beams rs> which will form
a platform of 5 feet 10 inches by 5 feet 5, for the reception of
the carriage. But these planks must not be put down till the
telescope has been first brought back, and fixed again close to
the carriage, which must be sustained in its place while this is
doing. Then, advancing again, the platform is laid down,
board by board, till completed, while at the same time the
carriage will be drawn upon it.
As soon as that is safely landed, a strong rope is to be
hooked into a loop, fixed upon the beam at a, fig. 15. This
going down to a pulley with a swivel hook at the bottom,
which is put through one of the three handles of the speculum,
returns to a pulley hung upon the hook 6. From that pulley
it goes forward to a leading pulley at V', upon the foundation
timber, fig. 3. This directs the end of the rope to the barrel,
which serves for the great round motion of the whole tele-
scope. When the handle of that machine is moved, the spe-
culum will be lifted up in its carriage, which being eased,
must now be turned about while the mirror is yet partly rest-
ing upon it, so as to become parallel with the back of the tube,
and close to it. As soon as the mirror is fairly suspended, the
carriage must be unbolted, and drawn sideways from under it.
mdcgxcv. 3 G
4p6 Dr. Herschei/s Description of a
At the same time the platform must be gradually removed,
that the tube may be brought back by the bar-motion, when-
ever the mirror is high enough to pass over the back of it.
Then letting down gently the round motion handle, and guid-
ing the mirror properly, it is to be placed upon a small hollow
square, with a sloping back, which is planted under its support.
The height of the square frame is such as will bring the centre
of the mirror into the centre of the tube ; and the sloping
back receives it in going down, and throws it from the back of
the tube> just as much as is required to make the adjustment
at the top act properly.
When the mirror is in its place, two loops which are pre-
pared are to be screwed fast to it. They contain the collets
that receive the adjusting screws from the back, through the
strong upper bar eg, fig. 34,. and as soon as these are fastened
the pulleys may be unhooked, and all the apparatus that has
been used removed. The six planks are then to be laid upon
the same rolling bars at n a, where a passage across the work
is wanted, and where they may remain till zenith sweeps re-
quire them to be moved.
The method of preserving the speculum from damp is by
having a flat cover of tin soldered upon a rim of iron, about
if inches broad, and •§• thick, the diameter of which is equal to
the iron ring which holds the speculum. Upon the flat part
of the rim is cemented, all around, some close-grained cloth of
an equal breadth with the rim. The cover has two handles
near the upper end, and under them two flaps that project
about an inch and are six inches broad. When the cover is
hung or laid upon the speculum, so that the two flaps are close
to the ring which incloses it, the rim of the cover, as far as it
Forty -feet Reflecting Telescope. 407
is lined with cloth, will rest against the edge of the' iron ring,
and fit it all around very closely.
In six places are painted white marks which divide the cir-
cumference equally ; and six claw-screws are provided, of the
shape that is represented ki fig. 47. These are applied to the
six mariced places. The end a being put over the iron ring
hXo take hold of the back, the screw c is then fastened so as
to press upon the outside of the cover and rim, till the lining
of it is brought into close contact with the iron ring.
To take off and put on again the cover, a small ladder is
provided, which being set at the outside against the back of
the tube, the person who is to uncover it goes up, and de-
scends into the tube by means of a board with steps. This
board goes across the mirror in a parallel direction with it,
and being narrow, does not interfere with the work of
loosening the screws to take them off. When~ they are re-
moved, the person comes out of the tube the same way, still
leaving the speculum covered, but when at the top of the
ladder brings out the inside board-steps. The two handles of
the cover now present themselves at the back, so that two
persons can easily lift it off, without suffering it to touch the
mirror in any place. It must then immediately be carried
into the observatory, and remain there till the mirror is to be
covered again ; but first of all the inner and outer cover of the
tube ought to be carefully closed up.
When the speculum is to be covered again, great care is re-
quired to see that no drops of dew may fall from the outer
cover of the tube upon the inner one ; or at least that these
may not find their way to the mirror ; and to let the first
3G2
4q8 Dr< Herschei/s Description of a
object be to put its own cover upon it before any thing be
done about fixing it there.
In very high observations the tube will not fall down again
readily, and in the zenith, by its great weight added to that
of the mirror, will even tilt backwards. A counterpoise there-
fore: is applied by a meridional post, 7 feet high, well fastened
by a frame in the ground ; and placed about 20 yards from
the front of the telescope. To this is fastened at the top on
the back an arm, which carries a pulley, and at the bottom on
the front, a barrel moved by a wheel and pinion. A rope with
a weight fastened to the end of it, goes over the pulley on the
post, and towards the mouth of the telescope. At the end of
the tube on each side is a loop, into which a chain is hung
with both ends. It is long enough to go round the seat to a
considerable distance, and holds a pulley in the middle, over
which the rope from the weight is made to pass back again to
the barrel at the post. Here it may be drawn up till the
weight is lifted sufficiently td keep the telescope steady, and
to make it fall again, when its own motions lower it. In ze-
nith sweeps 300 weight are required for that purpose, but one
hundred of that quantity is in Shifters that may be taken off
in lower altitudes, when less is sufficient.
A similar post and apparatus is fixed about the same dis-
tance from the telescope towards the north, to be used when
the instrument is turned about for high observations in the
northern meridian.
Another inconvenience to be removed in very high alti-
tudes, is that the long bars, which bring the v point of support
forwards, begin to project beyond their supporters. When this
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Forty-feet Reflecting Telescope, 40a
comes to take place, a light iron frame with two small wheels,
or rather rollers, is pinned to the ends of them. This not only
keeps them together, but also supports them * sufficiently as far
as they are to come out*
A slider, upon an adjustable foundationals planted at the
mouth of the telescope so as to be directed towards the centre
of the mirror. It carries a brass tube, into which all the single
eye-glasses, or micrometers, are made to slide. When they
are nearly brought to the focus, a milled head under the end
of the tube turns a bar, the motion of which adjusts them
completely.
The focus of the great mirror is directed to its proper place,
by putting two plates with springs upon the rim that limits
the aperture of the tube, into two places which are marked.
Then a cap with a small hole being put into the sliding tube,
an assistant'with a proper handle must screw in or out one or
other of the adjusting screws at the back of the mirror, till
the plates upon the aperture in front of the telescope become
both visible ; for they are contrived so that when the mirror
is not properly adjusted, either one or both will vanish. At
the same time these plates, by their situation, serve to inform
us which of the screws, whether that to the right or that to
the left, is in fault, by which means the adjustment becomes a
very easy operation.
Slough, near Windsor, WM, HERSCHEL.
May 18, 1795.
• "/'/a. 20.
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Utiles. Trans. M DC CXCV, Tak XXXH/?, 40*
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