GOB OPT 
tretnely imperfedt. But bis judgment and liis ingenuity, 
and fertility in refource, are as eminent as his philofophi- 
cal ardour. He has contrived for his reflecting telefcopes 
Hands which have every property that can be defired. 
The tubes are all fuppcrted at the two ends. The mo¬ 
tions, both vertical and horizontal, are contrived with the 
utmoft fimplieity and firmnefs. We cannot more pro¬ 
perly conclude this fedtion than with a deftription of his 
40-feet telefcope, the nobled monument of fkill that the 
.world can boalt of. 
The Herfcbelian telefcope we might with fome propri¬ 
ety call the German telefcope, inafmuch as the celebrated 
contriver of its ftupendous mechanifm is a native of Ha¬ 
nover : it was however condrudted in England, and by 
Englifh workmen, except fo far as the ingenious inventor 
lent his powerful aflilfance, partly in the execution, but 
chiefly in the contrivance of the mechanjcal appendages. 
This mafterpiece of mechanifm was begun at Clay-tlall at 
the latter end of the year 1785, when, through the media¬ 
tion of the prefident of the Royal Society, the fupport of 
regal munificence had been gracioufly promifed ; and 
when the various portions of the bulky drudture, which 
employed forty workmen of different denominations, had 
been removed to Slough, near Windfor, the foundation 
was begun, which was to be the feite for the larged tele¬ 
fcope that had ever been pointed to the heavenly regions. 
We will not detain our readers by deferibing the details 
of mafonry, carpentry, and fmith’s work, which have oc¬ 
cupied eighteen large plates* in the fecond part of vol. 
lxxxv. of the Philolophical Tranfaftions, for their expla¬ 
nation ; but deferibe fo much of the ir.drument, and of 
its appendages, as are ufeful in making adtual obferva- 
tions. The belt view for general reprefentation of the 
Herfchelian telefcope, is that which has been given in 
plate xxiv. of the volume juft named, which therefore 
we have copied into our Plate IX. annexed, with fome 
flight alterations, arifing out of fubfequent improvements 
or curtailments of unnecefl'ary appendages. This view, 
taken from a flation to the fouth-wefl of the eredftion, re- 
prefents the telefcope elevated in the meridian line, and 
affords the means of feeing the front parts of the inflru- 
ment, and of its numerous appendages ; but does not 
allow the mechanifm that fupports the inferior end of the 
tube, and that gives motion in fome of the adjudments, 
to be explained by a reference to their parts, and therefore 
mud be comprehended from a verbal defeription. The 
foundation on which the frame-work of theforty-feet tele¬ 
fcope is erected, confids of two concentric circles of brick¬ 
work, one 42, and the other 21, feet in diameter, both 
funk feet under ground, and taperingfrom the breadth 
of a ft. 3 in. below, to 1 ft. 2 in. above, where they are 
capped with paving (tones of iaj inches wide, and 3 thick. 
In the centre of thefe circles is fixed fad into the ground, 
by brick-work and oppodte braces of wood, a vertical 
beam, as a centre of motion,, round which the whole' 
drudture may have a circular motion in azimuth, the 
plane of the outer circle being made perfectly level. The 
platform that connects the different parts of the-frame- 
work below, has three principal horizontal beams lyinfj 
parallel to each other, and three ‘others lying parallel, 
eroding the fil'd at right angles, befides various bracing- 
beams, that tie the whole compactly together by iron 
bolts pa ding through the places of crofting. In our draw¬ 
ing, the outer circle of brick-work and mafonry is de¬ 
noted by the letters AB, ,and the circumference of the 
platform of wood by CD. Under each oppodte end of the 
fix main beams is dxed a roller, of fix inches in diameter 
and eight long, having each a drong iron frame bolted 
into the end of its relpeCiive beam; fo that the outer 
circle has twelve rollers : but thefe were not fufficient to 
bear the whole at 21 feet from the centre of motion ; 
therefore eight more rollers, nearly equididant, were 
fixed to drong parts of the platform, fo as to be borne by 
the inner circle of 21 feet’diameter ; and thus the whole 
platform, with its fuperdrucfure, is capable of making a 
I c s. 
revolution, when fufneient force is applied round the 
central vertical beam, that enters a hole at the jundtion 
of the two central main beams, and that afeends but a 
little way out of the ground. Six out of the twelve 
rollers of the outer circle are feen at a, b, c, See. between 
the britk-work and the circular edge of the' platform ; 
and the red may be imagined, not only on the remainder 
of this circle, but alfo on the inner circle, which is con¬ 
cealed. In thefe rollers it is of great importance that 
the axes of motion all point towards the central beam 
round which they carry the platform, and alfo that their 
diameters and frames be precifely of like dimenfions, 
otherwife they will not bear alike on the balls of ma¬ 
fonry. 
At twelve feet didance from, and all round, this mov¬ 
able platform, are dxed fad into the ground eight equj- 
didant pods, to an oppofite pair of which the ends of a 
long pliable rope are hooked, that give the motion in 
azimuth ; which rope, being conducted over two feparate 
pulleys, fixed upon the platform, at oppofite fides of the 
centre, has its ends turned in the diredtion of tangents 
that point in oppofite directions to their refpedlive pods. 
The middle part <pf the rope is made to pafs round one of 
the fpokes of a large wheel, carried by the platform, be¬ 
fore it winds round the axle, fo as to coil-up both ends of 
the rope equally ; which rope therefore pulls by both 
tangental ends alike, fo as to apply an equal force at each 
oppofite pulley, while the refidance of the pods produces 
the requiiite motion, without a drain on the centre. This 
mechanifm gives the operator a great mechanical advan¬ 
tage. That part of the platform D, which connedts the 
extreme ends of the three longitudinal beams, over the 
rollers at < 7 , e, f, is made drong, and is the fupport for a 
pair of double ladders, that are feen afeending to the fum- 
mit of the whole frame-work, one on- each fide of the 
large tube E ;.and at C is another fnnilar fupport for two 
other double ladders, which, afeending in like manner, 
meet the former ones, and crofs into them in fttch a way, 
as to admit of being bolted together at the points of 
eroding. Thefe ladders are propped by other diorter 
ladders, as feen in the figure ; and fome upright malts, of 
which one is feen erected over the roller at b, afeend in like 
manner, and afford the' means of obtaining horizontal 
braces at different heights, all round the frame, except 
where the elevated end E of the telefcope requires an 
opening to be left between the front ladders for its dif¬ 
ferent degrees of elevation. The tranfverfe beam FG, 
which lies horizontally over the erodings of the double 
ladders, and is bolted to them, receives the hooks of the 
diderent pulleys, at the fame time that it conne&s and 
braces together all the ladders at their upper extremities. 
Thefe ladders are each 49 feet 2 inches long, fo that the 
hei'ght'-of the tranfverfe beam FG mud be 3/49 X 49 — 
20 x 20 = 45 nearly, and will therefore admit of the long 
tube of 40 feet in length to be railed into-a vertical po- 
fition under it. Below the mouth of the large tube, a 
gallery HI, with its attached brackets K and L, reds 
upon the (lopes of the interior halves of the double lad¬ 
ders, at K and L, refpectively; and'may be made to dide 
up or down, into any date of elevation, by two fydems of 
pulleys, and ropes going round the blocks hooked at the 
jundtion of each pair of ladders to the tranfverfe beam 
FG, as may be feen in the figure; and, when this galr 
lery is lowered to the landing of the flight of deps M, a 
party may be admitted into it to gratify their curiofity, 
the door being 13 feet 6 inches by 6 feet inch, and 
palifaded on the front, as well as partly at both ends. 
The hafes or diding parts of the brackets are prevented 
from flipping afide by lateral rollers of brafs, adting again ft 
the Itraight fides of the middle pole of each double lad¬ 
der ; while other rollers of the fame metal, adting under 
them, diminifh their fridtion, when drawn up or let down 
by the pulleys. In the framing of thefe brackets, it was 
necedary to introduce contrivances for allowing fome de- 
4 ' viation 
