TELESCOPE^ 



may have a circular motion in a/imiitli, the plnne of the 

 outer circle bcinpr made perfeAly level. The platform that 

 ronneAs the diffen-nt parts of the frame-work below, has 

 three principal horizontal beams lying parallel to each otht r, 

 and three others lying paralli-l, crolTing tlie fir(t at risjht an- 

 gles, befidcs various Ijracing-beams, that tie the wholi com- 

 paftly together, bv iron bolts palling through the places of 

 croflitig. In our drawing, the outer circle of brick-work 

 and mafonry is denoted by the letters A B, and the circum- 

 ference of the platform of wood by C D : undr-r each op- 

 pofite end of the fix main beams is fixed a roller, of fix 

 inches m diameter, and eight long, having each a ftrong iron 

 frame bolted into the end of its refpeftive beam ; fo that 

 tlte outer circle has twelve rollers: but thefe were not fuffi- 

 ciest to bear the wholi- at 21 feet from the centre of motion ; 

 therefore eight more rollers, nearly equidiflant, were fixed 

 to flrong parts of the platform, fo as to be borne by the 

 inner circle of 2 I feet diameter ; and thus the whole platform, 

 with its fuperftrudlure, is capable of making a revolution, 

 when fufficient force is applied round the central vertica'l 

 beam, that enters a hole at the junction of the two central 

 main beams, and that afcends but a little way out of the ground. 

 Six out of the twelve rollers of the outer circle are feen be- 

 tween A B, the brick-work, and C D, the circular edge of 

 the platform, and the rell may be imagined, not only on the 

 remainder of this circle, but alfo on the inner circle, which 

 is concealed. 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 bafis of mafonry. At twelve feet 

 diftance from, and all round this moveable platform, are 

 fixed faft into the ground eight equidiflant polls, to an oppo- 

 iite pair of which the ends of a long pliable rope are hooked, 

 that give the motion in azimuth ; which rope, being condufted 

 over tVTo feparate puUies, fixed upon the platform, at oppofite 

 fides of the centre, has its ends turned in the direftion of 

 tangents, that point in oppofite direftions to their refpeiSlive 

 polls. The middle part of the rope is made to pafs round 

 one of the fpokes of a large wheel, carried by the platform, 

 before 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 refillance of the polls produces the 

 requifite motion, without a flrain on die centre. This 

 mechanifm gives the operator a great mechanical advan- 

 tage. That part of the platform C, which connefts the 

 extreme ends of the three longitudinal beams, over the 

 rollers at A, is made ftrong, and is the fupport for a pair of 

 double ladders, that are fee.n afeending to the fummit of the 

 whole frame-work, one on each fide of the large tube E ; 

 and at D is another fimilar fupport for two other double 

 ladders, which, afeending in like manner, meet the former 

 ones, and crofs into them in fuch a way, as to admit of 

 being bolted together at the points of croffing. Thefe 

 ladders are propped by other {horter ladders, as feen in the 

 figure, and fome upright mafts, of which one is feen eretled 

 over the roller at B, afcend 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 tele- 

 fcope requires an opening to be left between the front ladders 

 for Its different degrees of elevation. The tranfverfe beam 

 F G, which lies horizontally over the croffings of the double 

 ladders, and is bolted to them, receives the hooks of the dif- 

 ferent pulhes, which we fiiall (hortly have occafion to de- 

 fcribe, at the fame time that it conneas and braces together 

 all the ladders at their upper extremities. Thefe ladders are 



each 49 feet 2 inches Jong, fo that liie height of the iralif* 



verfe beam F G muft be ■^''49 x 49 — 20 x 20 = 45' 

 r.carly, and will therefore admit of the long tube, of 40 feet 

 in length, to be raifed into a vertical pofition und-^r it. Be- 

 low the mouth of the large tube, a gallery H I, with its at- 

 tachr^d brackets K and L, reds upon the (!op s of the in- 

 terior halves of the double ladders, at K and L r- f ftively, 

 and may be made to flide up or down, into any (late of eleva- 

 tion, by two fyftems of pullies, and ropes goi:.ii; round the 

 blocks hooked at th- junftion of each pair of ladd. rs, to tli-- 

 tranfverfebeam F G, as may be feen in the figure ; and whea 

 tliis gallery is lowered to the landing of the pair of fteps M,. 

 a party may be admitted into it to gratify their curiofity, the 

 floor being 13 feet 6 inches by 6 feet i^ inch, and palifaded 

 on the front, as well as partly at both ends. The bafes, or 

 Aiding parts of the brackets, are prevented from flipping 

 afide by lateral rollers of brafs, afting againft the (Iraight 

 fides of the middle pole of each double ladder, while other 

 rollers of the fame metal, acting under them, diminifli ih^'.y 

 friftion, when drawn up or let down by the pullies. In tb.'i 

 framing of thefe brackets, it was necelfary to introduce con- 

 trivances for allowing fome deviation of the gallery from a.i 

 exatl level, in cafe one of the brackets was elevated by its. 

 pulley failer than the other ; which contrivances are not 

 eafily defcribed without a reference to the drawings of the 

 feparate parts in the original account, or without infpeftion 

 of the parts themfelves. 



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 wo'jld 

 have exceeded an iron one in weight by at leaft 3000 lbs. 

 The fheets were firft; put together by a kind of feaming, that 

 requires no rivets ; and when the fides of the iron platform 

 were cut ftraight, it was lifted by proper tackle into a 

 hollow gutter, and then brought gradually, by vario-.s 

 tools, into a cylindrical form. Various hoops r.re fixed 

 within the tube, and longitudinal bai'S of iron, connefting 

 fome of them, were attached to the two ends of t!-.c 

 tube, by way of bracing the Iheets, and keeping the fiiape 

 perfeft when the pullies are applied to give the neceffary 

 elevation at the upper end, and that the fpeculum might be 

 kept fecure in its bed at the lower end. The hoop by which 

 the upper end of the tube is fufpended is eight inches broad, 

 and thicker than the refl ; and the fyftem of three puDies, 

 feen at N, with each a double block, has a correiponding fet 

 at O, hooked to the tranfverfe pole G F ; and the bars to 

 which the blocks are hooked are fo bent, that the moving 

 ropes wiil not come in contaft ; nor will the elevated 

 tube have its vertical motion difturbed by the tackle, either 

 in afeending or defceniing, which was. an important precau- 

 tion. The lower end of the tube is firmly fupported on 

 rollers, that are capable or being mo\'ed forwards or back- 

 wards by a double rack, moved by wheels and pinio:?s 

 at R, which we fltall not attempt to defcribe minutely ; 

 but the v:fe of which every mechanic ■(vill comprehend '.vith- 

 out particular explanation. Originally there were feveral 

 appendages near the mouth of the tube Aiding by pullies, or 

 fixed to the tube, for the purpofe of regulating the fiuceps 

 taken by this inttrum.er.t ; but as the twenty-feet refleftor is 

 now ufed for this purpofe, they are taken off, and have bean 

 omitted in our drawing. By an adjuftment at the lower 

 extremity of the tube, the fpeculum is turned to a fmail 

 inclination, fo that the line of collimation is not coinci- 

 dent with the longitudinal axis of the tube, but croffes 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 intercepts the incident rays, and the ebfcrva- 



tion 



