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viation of the gallery from an exaX level, in cafe one of 
the brackets was elevated by its pulley fafter than the 
other; which contrivances are not eafily defcribed with¬ 
out a reference to the drawings of the feparate parts in 
the original account, or without infpeXion of the parts 
themi'e) ves. 
The tube of the telefcope, which is 39 feet 4 inches in 
length, and 4 feet 10 inches in diameter, is made entirely of 
iron ; it having been afcertained that a wooden tube would 
have exceeded an iron one in weight by at leaft 3ooolbs. 
The fheets were firft put together by a kind of (earning that 
requires no rivets; and, when the Tides of the iron platform 
were cut ftraight, it was lifted by proper tackle into a hol¬ 
low gutter, and then brought gradually, by various tools, 
into a cylindrical form. Various hoops are fixed within - 
the tube, and longitudinal bars of iron, connecting fome 
of them, were attached to the two ends of the tube, by way 
of bracing the (heets, and keeping the (hape perfeX when 
the pulleys are applied to give the neceflary elevation at 
the upper end, and that the fpeculum might be keptfecure 
in its bed at the lower. The hoop by which the upper end 
of the tube is fufpended is eight inches broad, and thicker 
than the reft ; and the fyftem of three pulleys, feen at N, 
with each a double block, has a correfponding fet at O, 
hooked to the tranfverfe pole FG ; and the bars to which 
the blocks are hooked are fo bent, that the moving ropes 
will not come in contaX ; nor will the elevated tube have 
its vertical motion dilturbed by the tackle, either in af- 
cending or defcending, which was an important precau¬ 
tion. The lower end of the tube is firmly fupported on 
rollers, that are capable of being moved forwards or back¬ 
wards by a double rack, moved by wheels and pinions at P, 
which we (hall not attempt to defcribe minutely ; but the 
life of which every mechanic will comprehend without par¬ 
ticular explanation. We (hall only juft mention, that, by 
an adjuftment at the lower extremity of the tube, the 
fpeculum is turned to afmall inclination, fo that the line 
of collimation is not coincident with the longitudinal 
axis of the tube, but erodes the tube diagonally, and 
meets the eye in the air, at about two inches from 
the edge of the tube. Hence no part of the head inter¬ 
cepts the incident rays; and the obfervation is taken with 
the face looking at the fpeculum, or by what the author 
lias called, by way of diftinXion, the front view, the 
back being always turned to the objeX to be viewed. 
The concave face of this fpeculum, or great mirror, is 
48 inches of polidied furface in diameter. The thick- 
nefs, which is equal in every part of it, remains now 
about three inches and a half; and its weight when it 
came from the call was 2118 pounds, of which it muft 
have loft a fmall quantity in polidiing. To put this fpe¬ 
culum into the tube, it is fufpended vertically by a crane 
in the laboratory, and placed on a fmall narrow carriage, 
which is drawn out, rolling upon planks, till it comes 
near the back of the tube ; here it is again fufpended and 
placed in the tube by a peculiar apparatus. There is no 
fmall fpeculum, but the magnifiers are applied imme¬ 
diately to the firft local image. 
From the opening of the telefcope, near the place of the 
eye-glafs, a fpeaking-pipe runs down to the bottom of 
the tube, where it goes into a turning joint; and, after 
feveral other inflexions, it at length divides into two 
branches, one going into the obfervatory, and the other 
into the work-room. The door of the obfervatory is 8 
feet 5 inches by 5 feet 5. It is of a proper height, and 
has a double window towards the weft, with a (hutter to 
be ufed at night. Q gives a fufficient view of it. The 
door of the working-room is 6 feet 6 by 4 feet 5 ; and has 
two fmall windows, one to the fouth, the other to the 
eaft. Its height is condderably lefs than that of the ob¬ 
fervatory; and a view of this may be feen at R. The 
diftance between'the obfervatory and the end of the te¬ 
lefcope, is evidently too far for a converlation in the open 
air, between the obferver and the adiftant; efpecially as 
the latter, on account of his candles, muft be inclofed ; 
Vol. XVII. No. 1201. 
I c s. 
and Ought not to leave his pod: at the time-piece and 
writing-delk. Add to this, that, when the obferver is 
elevated thirty or forty feet above the adiftant, a mode¬ 
rate breeze will carry away the found of his voice very 
forcibly. By means of the fpeaking-pipe, therefore, the 
communications of the obferver are conveyed to the af- 
fiftant in the obfervatory, and the workman is directed 
to perform the required motions; but they could not be 
conveniently (hown in the Plate. 
In the obfervatory is placed a valuable fid-ereal time¬ 
piece, made by Mr. Shelton. Clofe to it, and of the fame 
height, is a polar diftance-piece, which has a dial-plate of 
the fame dimenfions with the time-piece : this piece may 
be made to diow polar diftance, zenith diftance, declina¬ 
tion or altitude, by fetting it differently. The time and 
polar-diftance pieces are placed fo that the adiftants fit be¬ 
fore them at table, with the fpeaking-pipe riling between 
them ; and in this manner obfervations may be written 
down very conveniently. 
This noble inftrument, with proper eye-glades, magni¬ 
fies above 6000 times. It was completed on Atiguft the 
28th, 1789 ; and on the dime day was the fixth fatellite of 
Saturn dilcovered. For all the minute particulars of the 
conftrudtion of this wonderful inftrument, fee the Phil. 
Tranf. for 1795. Part II. p. 347-409. 
Of Cajling, Grinding, mid Pulijhiug, the Specula of 
Telefcopes. 
The metal of refleXing telefcopes is generally com- 
pofed of 32 parts of copper and 15 of grain tin, with 
the addition of two parts of arfenic, to render the com¬ 
pofition more white and compaX. The Rev. Mr. Ed wards 
found, from a variety of experiments, that if one part of 
brafs, and one of filver, be added to the preceding compo- 
fition, and only one part of arfenic ufed, a molt excel¬ 
lent metal will be obtained, which is the whited, hardeft, 
and molt refleXive, that he ever met with. The fupe- 
riority of this compofition, indeed, has been completely 
evinced by the excellence of Mr. Edwards’s telefcopes, 
which excel other refleXors in brightnefs and diftinXnefs* 
and (how objeXs in their natural colours. But, as metals 
of this compofition are extremely difficult to call, as well 
as to grind and polilh, it wall be better for thofe who are 
inexperienced in the art, to employ the compofition firft 
mentioned. 
After the flafks of fand are prepared, and a mould made 
for the metal by means of a wooden or metallic pattern, 
fo that its face may be downwards, and a few fmall.holes 
left in the fand at its back, for the free egrefs of the in¬ 
cluded air, melt the copper in a crucible by itfelf, and, 
when it is reduced to a fluid (late, fufe the tin in a fepa¬ 
rate crucible, and mix it with the melted copper, by ftir- 
ring them together with a wooden fpatula. The proper 
quantity of powdered arfenic, wrapt up in a piece of pa¬ 
per, is then to be added, the operator retaining his breath 
till its noxious fumes are completely diflipated ; and 
when the fcoria is removed-from the fluid mafs, it is to be 
poured out as quickly as poflible into the fialks. As foon 
as the metal is become (olid, remove it from the fand into 
fome hot allies or coals, for the purpofe of annealing it, 
and let it remain among them till they are completely 
cold. The ingate is then to be taken from the metal by 
means of a file; and the furface of the fpeculum muft be 
ground upon a common grindftone till all the imperfec¬ 
tions and afperities are taken away. When Mr. Edwards’s 
compofition is employed, the copper and tin Ihould be 
melted according to the preceding direXions; and, when 
mixed together, Ihould be poured into cold water, which 
will leparate the mafs into a number of fmall particles. 
Thefe l'mall pieces of metal are then to be colleXed and* 
put into the crucible, along with the filver and brafs, after 
they have been melted together in a feparate crucible ; 
the proper quantity of arfenic is to be added, and a little 
powdered rofin thrown into the fluid metal before it is 
poured into the flalks. 
7 Q 
When 
