TELESCOPE. 



any perlon tolerably acquainted witli the mecliaiiifin of 

 rack-work. Tlie quick motion in azimuth is given by 

 Aiding the lower end of the tube gently along the bar on 

 wliich it reils, or by moving the whole frame, which 

 moves on caftors ; but the (low motion is produced by the 

 fcrew at D. It is fcarcely neccfTary to add, that the eye of 

 the obferver is applied to ti>e Jide of the tube near its 

 mouth, when the finder has pointed the tube properly to its 

 objed. This Hand was contrived by fir William Herfchel, 

 whofc experience in the uie of vai-ious Hands directed him 

 to prefer one that is not hable to propagate vibratory 

 motion to the large fpeculum, and that has a point of fup- 

 port near the upper extremity of the tube. We have, how- 

 ever, feen a fix-feet refleftor very fteadily fupported on a 

 frame fimilar to that exhibited in Jig. 2 ; and the com- 

 pofer of the prefent article has a Caffegrainian telefcopc, 

 with a tliree-feet tube, fixed between the cheeks of one of the 

 doors of his obfervatory, which turns round witli the 

 moveable dome in azimuth, and which elevates in altitude 

 on two pivots relling in the notches of a pair of brafs plates 

 let into the faid cheeks ; which mode of mounting is not 

 only convenient for celeftial obfervations, but is remarkably 

 free from tremors, which advantage may be owing partly 

 to fteadinefs of pofitiori, and partly to the mouth of the 

 tube being nearly two feet advanced into the c^en air. If the 

 dome had not a remarkably eafy motion on tliree loofe ebony 

 balls, placed at equal diftanccs, this mode of mounting a 

 large telefcope would not afford a flow adjuftment for 

 motion in a/.imutli, which it now does with facility. 



It is always interefting to a nian of fcience to know by 

 what progrefTive fteps a great undertaking is accomplithed, 

 as well as to learn under what imprefTion the original 

 idea was entertained of forming the plan of operations. 

 When fir William Herfchel, who was brought up a 

 mufician, refided as organift at Bath, the natural bent 

 of his mind led him to cultivate the pleafing fcience of 

 optics, and to ftudy the theory of mechanics fo far as 

 to enable him to amufe himfelf with attempts to con- 

 ftruft a refledling telefcope : his fnccefs, in an undertaking 

 of confiderable difficulty, increafed with his endeavours to 

 attain fome degree of excellence ; and though at firll he 

 was fatisfied to pick a tolerable fpeculum out of fome dozens 

 at which he had laboured ; yet, feeling that his experience 

 began to give him facilities both in the contrivance and 

 execution of his manipulations, he proceeded by degrees to 

 conftrucl ipecula of ieven, ten, and even twenty feet focus 

 of the Newtonian form, to the number of more than 400, 

 befides feveral of the Gregorian kind : but as yet he was un- 

 acquainted with any certain praftical method of giving 

 a parabohc curve to the face of his metal ; on which 

 account he fele6ted, by trial, fuch fpecula for ufe as he 

 found moll perfeft in figure, and repolifhed the remainder, 

 in all thefe operations there was much room for experi- 

 mental obfervatioii, and the time was not expended in 

 vain. To a mind like Herfchel's, even a failure roufed 

 a feeling for a new cnterprife ; and it was no fmall ftep 

 towards advancement, to have perceived the caufe of un- 

 luccefsful mealures : the objcft being attainable, the means 

 were to be found by (kilful perfeverance. Nor were the 

 labours of our optician to be confined to the formation of 

 a fpeculum ; his mechanical flvill was dircfted to the con- 

 trivance and execution of various ftands for telefcopes of 

 an unufual length ; and in the year 1778, he produced that 

 which is now ufually applied to the Newtonian telefcope, 

 and which we have Juft defcribed as reprefcnted bv 

 P/d/^XXX. /^. 3. By the year 17S1, Herfchel (beft 

 known by this title at that period) felt fuch confidence in 

 Vol. XXXV. 



his improved methods of proceeding, that he crcdled a llaud 

 for a thirty-feet i-ofliding metal of 36 inches aperture, and 

 fucceeded in calling it ; but to hib "mortification the metal 

 cracked in the coohng. The difappoiutment attending tluK 

 accident mult have been fewere, but did not damp ilie .irdour 

 of the mechanicul adventurer, in which light, no doubt, the 

 enterprifing contriver was now viewed. A fecond molting 

 of the fame metal was immediately determined upon, and 

 a furnace was conllruded for the purpofe, which unfor- 

 tunately gave way, and the liquid metal blew up the pave- 

 ment. The mortification conlequent on this fecond acci- 

 dent only ))lungcd our adventurer the dec-per, tliat he might 

 rife the higher in his next attempt. DunW an intervaFof 

 fome refpite from optical and mechanical labours, the 

 aftronomer, however, wae not alkxp ; and »vhile obfervations 

 were making on the rotations of the planets, with tele- 

 fcopes of the Newtonian form, of 7, 10, and 20 feet focil 

 length, the little planet, at firll fuppofed to be a comet, 

 from its having a vifible magnified difc, was difcovered. 

 This lucky event rekindled the optician's ardour, and at 

 the fame time introduced him to the notice of his majelly ; 

 who, by his liberal patronage, promoted the views of this 

 amateur inftrument-makcr, and afforded facility to his future 

 operations. 



In the year 1782, a good twenty-feet refleftor was 

 finiflied with a large aperture, and mounted on the Her- 

 fchelian (land for admitting of front obfnval'wm , for which 

 it is found very ufeful. The forty-feet telefcope, or 

 mafter-piece of mechanifm, which is more immrdiately the 

 objeft of our examination, was begun at Clay-Hall at the 

 latter end of the year 1785, when, through the mcduition 

 of the prefident of the Royal Society, the fupport of regal 

 munificence had been gracioudy promifcd ; and, when tjie 

 various portions of the bulky llrudlure, which employed 

 forty workmen of different denominations, had been re- 

 moved to Slough, near Windfor, the foundation was begun, 

 which was to be the (cite for the largeft telefcope that had 

 ever been pointed to tiie heavenly regions. We will not 

 detain our readei- by delcribing the details of mafonry, car- 

 pentry, and fmith's work, which have occupied eighteen 

 large plates, in the fecond part of vol. Ixxxv. of the Phi- 

 lofophical Tranla6tions, for their explanation, but defcribe 

 fo much of the inftrument, and of its appendages, as are 

 ufeful in making aClual obfervations. 



The beil view for general reprefentation of the Hcrfchc- 

 lian telefcope, is that which has been given in plate 24. 

 of the volume jull named, which therefore we have copied 

 into our Plate XXXI. of yljlronomical Injlrumcnts, with 

 fome flight alterations arifing out of fubfequent improve- 

 ments or curt.iilments of unneceffary appendages. Thi» 

 view, taken from a Ration to the fouth-wefl of the creftion, 

 reprefents the telefcope elevated in the meridian line, and 

 affords the means of feeing the front parts of the inftru- 

 ment, and of its numerous appendages ; but does not allow 

 the mechanifm that fupports the inferior end of the tube, 

 and tiiat gives motion in fome of the adjuflments, to be ex- 

 plained by a reference to their parts, and therefore mull be 

 comprehended from a verbal defcription. The foundati>.in on 

 which the frame-work of the forty-feet telefcope is erected, 

 confifls of two concentric circles of brick-work, orx 42, 

 and the other 21 feet in di.imeter, both funk 2^ feet 

 under ground, and tapering from the breadth of 2 ft. 3 in. 

 below, to I ft. 2 in. above, where they arc capped with 

 paving-ftoncs of 125: inches wide, and 3 thick. In the 

 centre of thcfe circles, is fixed fall into the ground by 

 brick-work, and oppofite braces of wood, a vertical beam, 

 as a centre of motion, round which the whole (Irudurc 

 N n may 



