THEODOLITK. 



^iaccil on it, aiul one of tiic four fafteiiing fircws may be 

 feen at B, at the jiinAion of the bracinj^ bars. Upon this 

 llool another, but larger, oftagonal board of mahogany, 

 U E, r' lU, whicli has a ring or circular curb on its plane, 

 about half an inch from tiie fides of the odagon : tliis upper 

 octagonal board may be fixed to, or rcleafed from, the top of 

 the Hand by four vertical fcrcws, wliich penetrate both boards, 

 but which allow one to flide over the other, fo as to he either 

 concentric or excentric, as the adjullmont of a plumb-line to 

 the mark, or hole under the inilrument may require, for the 

 exaft place of the Itation, over wliich the centre of the 

 iuflrument mult be exaftly fixed. Tlie four horizontal 

 fcrews of adjullment, of which three are feen at F, F, and V, 

 carried by the board D E, are fo contrived as to effeft this 

 adjuftment by prefTnre againft the edge of the (land, after 

 whicli the two otlagonal boards are made fad logetlier by 

 the faid four verticil! fcrews not fhewn. The upper odta- 

 gonal D E has an open conical focket of brafs, three inches 

 in diameter, in its centre. Next above the board D E, thus 

 adjufted and fccurcd, comes the third board, which is cir- 

 cular, and which forms the bafis of the inilrument. In the 

 centre of this bafe another brafs conical focket, three inches 

 and a quarter in diameter, is made faft, and flips over 

 the fmaller conical focket of the board D E of adjuftment 

 for central pofition, fo that the centre of the inilrument 

 being concentric with this board partakes of the adjuftment, 

 while the plumb-line defcends through both fockets down 

 towai-ds the ground. The large mahogany circle G H, of 

 more than three feet in diameter, is uipportcd by feveral 

 pillars connefted with the circular board, which we have 

 called the bafe of the inilrument, and forms with tliem a 

 haluftrade, that proteftslhe inilrument, as feen in the figure. 

 A brafs circle of three feet diameter, within the haluf- 

 trade, is attached by ten ftrong conical radii to the hu'ge 

 vertical hollow axis, formed into the fruftum of a cone, of 

 twenty-four inches in height above the metallic v/hcel, which 

 we fhaU in future denominate the graduated circle, when 

 confidered with its radii and hollow axis attached to it. 

 This axis, by way of diftinftion, may be called the exterior 

 axis ; it has a collar of eaft-lteel driven faft into the cavity of 

 its inferior or thicker end, and a plate of bell-metal, with a 

 lloping edge, kirmounts the fuperior end, which plate may 

 be raifed or lowered by means of five fcrews afting verti- 

 cally. The inftrument ftands on three fliort feet near 

 D and E, and at an equidiftant point not feen behind, which 

 feet are firmly united together, at the place where they 

 branch off, by a circular ftrong plate of bell-metal, upon 

 which is carried an attached vertical cone of metal fmaller 

 than the former one, and as it fills the cavity of the other, 

 we will call it the interior conical axis ; the exterior one being 

 moveable round the interior one without the leaft perceptible 

 liberty, beyond wliat is neceflary for rotatory motion. On 

 the vertex of this interior axis is inferted a caft-fteel pivot with 

 floping cheeks, which, entering the central hole of the bell- 

 metal plate, exaftly fits its cheeks there, while the bell- 

 metal bafe of the interior axis fits the caft^-fteel collar in- 

 lerted into the lower extremity of the exterior conical axis. 

 This mode of centering allows the wheel to be taken off 

 and put on without injury, and is alfo free from the objedlion 

 that applies to thofe large inllruments that have the 

 fuperior end of the vertical axis fupported in a frame that 

 is liable to alter in its dimenfions by expofure to the fun ; 

 of which imperfeftion, as we have noticed in our article 

 Circle, Piazzi had great reafon to complain. Befides, 

 this kind of centre -work allows of carriage from one place 

 to another, without any danger of injury being done to the 

 inftrument when properly packed. There are two achro- 



matic tclefcopes with double objcft-glalicsof each thirty-fi>; 

 inches focus, with eye-pieces of different powers both for 

 ere6t and inverted pofitions. One of thcfe telefcope» lies 

 acrofs the body of the inftrument, with the ends feen be- 

 tween the oppofitc pillars of the baluftrade, the ufe of which 

 is to watch the pofition of the inftrument during the time 

 of an oblervation being made ; and, therefore, it requires 

 but little elevation in altitude : the other is mounted, 

 cxaftly like a tranfit-inftrument, over the top of the ex- 

 terior vertical axis, and has a lemi-circle attached to the 

 extreme end of its horizontal axis of motion, of fix inches 

 radius, and graduated for ftiewing angles of altitude or of 

 deprcflion. The Y's in which the pivots of the upper 

 telefcope move, are fupported by the horizontal bar I K, 

 which is braced by the ladder-pieces attached to the 

 thick part of the exterior conical axis, and made faft 

 to the top of this axis by its focket, as feen in the 

 fedlion in Ji^. 4 ; which feftion exhibits moreover the 

 internal fittings of both the internal and external axes at 

 their fuperior ends. This tipper telefcope has a fpirit- 

 level witli tlie ufual adjuftments at the Y's, and at the bar 

 of fufpenfion for the horizontal pofition ; and, as it will 

 reverfe in the Y's, and has moveable wires in the focus of 

 the eye-piece, it may alfo be adjufted by a horizontal 

 mark for colhmation, and for taking cxaci altitudes, (as 

 well as for taking minute angles of elevation withoat the 

 femi-circle by the motion of the micrometrical wires,) when 

 the level is applied to a rod on the Jicic of the tube, as is the 

 cafe in our drawing. When thefe adjuftments are made, 

 by dividing the errors between the proper fcrews, as ufual, 

 the level is hung to the crofs-bar I K to watch its pofition 

 while this telefcope is ufed, aud when both tclefcopes con- 

 tinue to bifeft the fame diilant mark during an obfervation, 

 with the bubble of the level in the middle, it is a proof 

 that the inftrument keeps its pofition. When a ftar or 

 other objetl is viewed by night, the illuminating lamp K 

 throws light into the axis of the telefcope, which has a dia- 

 gonal perforated refledlor, as is ufual in tranfit tclefcopes. 

 A fyftem of darkening prifms is alfo applied to the fame 

 end of the axis to regulate the quantity of light that ftiall 

 come to the eye. All thefe adjuftments and appendages 

 have been minutely explained under our articles Cikcle and 

 TRAHSlT-Iii/lruiTKnt, and therefore need not be detailed in 

 this article ; but it may be proper to obferve, tliat the 

 femi-circle has a moveable clamping-piece, bearing the fteel 

 arbor of a vertical fcrew, the lower end of which falls on 

 a poliftied piece of fteel on the plane of the horizontal bar 

 I K, the ufe of which clamp and fcrew is n >t only to give 

 a flow motion in altitude, but to allow the preponderating 

 eye-end of the telefcope to reft fteady thereon, while the 

 obfervation has been read and repeated. The obfervation 

 in altitude is read by the compound microfcope at I, which 

 is nine inches long, and which, by means of its micrometricafl 

 fcrew, reads the divifions of the femi-circle to the accuracy 

 of 5", when an allowance of iz" is made for excentricity. 



When this theodolite was firft brought into ufe, it was 

 found that the fcrew L, with an ivory thumb-piece, moved 

 the circle in azimuth by jerks, on which account the appa- 

 ratus for (low motion, feen in Jig. 2, was fubftituted, in 

 which two crown-wheels and a Hooke's joint are in- 

 troduced to give motion to the tangent-fcrew ; which addi- 

 tion not only remedied the jerks, but allowed the obfervcr 

 to reach the handle while his eye remained at th ■ upper tele- 

 fcope. The large brafs horizontal circle is divided into 

 quarters of a degree, and the fubdivifioiis are made by the 

 vertical micrometrical microfcope.i, the divided heads of 

 which read exaft fecon^s, when properly adjufted for zero, 



for 



