CIRCLE. 



485 



Circlf. 



Circle 

 made by 

 Trouglron 

 for Count 

 Bruhl. 



Ramvleu'* 

 equatorial. 



Trough- 

 ton'* equa- 

 torial. 



Circle 

 made for 

 Mr Pond 

 by Trough 



tun. 



Circle 

 made 

 for Mr 



bridjT- by 



Trougl.. 



ton. 



merit, in whatever manner its vertical axis be placed, in- 

 dicates, by a double observation, the angle which the ob- 

 j-ct make with the axis, round v.-liich the whole instru- 

 ment has revolved n> pn^ing from one position to the 

 Other. For let Pp be the axis, Tx the 

 telescope in one pusitton ; it i evidi nt ; hat 

 in turning the instrument half rou.id. ty 

 will then be the position of the telescope, 

 Px being equal to P y. The arc xy 

 which the telescope passes through to 

 regain its former position, is the quan- 

 tity really given by the instrument; and if the axis Pp 

 be vertical, half this quantity is ihe true zenith distance 

 of the object. Now the intention of Mr Trnnghton's 

 verification is to injure a vertical position to the axis P p. 



For instruments which rest on movcablo pillars, and 

 turn freely in azimuth, this method is much to be pre- 

 ferred ; but it is not without a considerable defect, for 

 if by any derangement in the plumb-line apparatus, the 

 error in collimation be changed, it cannot be restored 

 with certainty to its former position, s.> that some:iines 

 a very valuable series of observations may be lost, for 

 want of a corresponding one to compare with it. 



It was in 1792, after having made several smaller ones, 

 that Tronghton constructed a circle for Count Bruhl. 

 The observations made with this instrument were very 

 few, but exceeded in accuracy any that had been made 

 before. The diameter of this circle was only two feet, 

 b'jt we are strongly inclined to beli-. ve that it exceeded 

 in accuracy the hve feet circle of Piazzi. 



The equatorial instrument constructed by Ramsden 

 for Sir George Shuckburg, properly belongs to the class 

 of astronomical circles Jt will b" more fully noticed 

 under the article OBSERVATORY. Few observations have 

 been, we believe, ever mad^ with it. Iii'leed it is a 

 waste of so very capital an instrument to place it in the 

 equatorial position, because meridian observations will 

 always celeris paribus be better ; and in practical astro- 

 nomy, a s'cond be*t method is absolutely of no use. 

 This valuabl. ind superb instrument has lately been 

 presented to the Royal Observatory of Greenwich by the 

 Hon Mr J<Tikinson, and is intended to be used both in 

 the vertical and inclined position; and as an altitude and 

 azimuth instrument, there can be no doubt but it will 

 prove a most valuable acquisition. 



An equatorial instrument, by Troughton, of which 

 the vertical circle was 3 feet in diameter, was erected at 

 Armagh ; and Dr Hamilton'.! observations made with it 

 were inserted with Mr Pond's.in theTran.actionsof 1806. 



Several other circles were made by Troughton of dif- 

 ferent dimensions, varying considerably in the subordi- 

 nate contrivances, but much resembling each other in 

 their general principle. Mr Wollaslon describes a very 

 perfect instrument, by Troughton, of thi- kind, (n.,t 

 very unlike the one hereafter treated at,) in the Appen- 

 dix to Ins Fasfictilux; and gives some excellent directions 

 to less experienced observers, which we shall avail our- 

 selve^ of before we conclude this subject. 



Mr Pond's catalogue of stars, which first demonstrated 

 the defect of the Greenwich quadrant, was deduced from 

 observations made with a circle constructed by Trough- 

 ton, SO ii.ches in H'ameter, =md is minutely described in 

 the Philosophical Transactions. It is exactly similar to 

 a s ; alter one nuw possessed by General Brisbane, ai.d of 

 which we have g veil a repres-. .tition in P!,,tr XLV . and 

 a description in the articl-- ASTR INOMY, vol. ii. p. 727. 



One f the most perKcc iir-tnimt-nls ot the kind is 

 that in the pos-ession of Stfuren Groombi idge, E:>q of 

 Biackheath. It is a transit circle i laceii betw<-< . t wu piers, 

 made according to tire idea suggested by Mr Wollaston. 



The four microscopes are placed in the horizontal dia- 

 meter, two on each side, the instrument being on both 

 sides divided. From observations made with this instru- 

 ment, a very valuable paper on astronomical refractions 

 has been already communicated to the public. 



The largest astronomical circle that has been yet con- 

 structed, is that belonging to the observatory of Dublin. 

 The vertical circle is eight feet diameter. It was begun 

 by Ramsden, and finished by his successor Mr B -rge. 

 It was erected about the year 1807. We have not seen 

 any printed description of it, nor have many observations 

 made with it yet been published ; but we understand that 

 the learned astronomer, Dr Brinkley, to whose care it is 

 now entrusted, is perfectly satisfied with its performance; 

 and it is with this instrument that he has been able to in- 

 fer the apparent annual parallax of Lyrae, which he esti- 

 mates to be between two or three seconds. 



To the above, we may add a very beautiful two feet 

 circle, in the possession likewise of General Brisbane, 

 and which was actually the model of the six feet mural 

 circle erected last summer at Greenwich. It differs in 

 principle from all those above mentioned ; and though it 

 is not in our power at present to describe very minutely 

 the mechanism oi either of these instruments, of which 

 no account has yet been published, we are nevertheless 

 enabled to explain some of the more striking peculiari- 

 ties in their con-truction. 



The Greenwich mural circle was placed on its stone 

 pier in the month of May 1812, and the observations for 

 publication commenced the llth of June. In the last 

 volume of the Transactions for the present year (1813), 

 the astronomer-royal has communicated his observations 

 of the summer and winter solstice of 1812, and likewise 

 a catalogue of some of the principal fixed stars, deduced 

 from nearly 2000 observations. From what little is said 

 of the construction of the instrument, it may be collect- 

 ed, that no use whatever is made either of level or plumb- 

 line, but that the position of the polar point on the in- 

 strument is found by assiduous observation of the pole- 

 star; and that the position of thit point on the in^tru- 

 ment is inferred, by comparing the observed places with 

 the real pl.tces of a certain number of standard stars, pre- 

 viously determined by the instrument itself. By this 

 method the astronomer-royal observes, that the position 

 of his instrument may always be known to within a tenth 

 of a second, a degree of precision which no level or 

 plumb line can ever he expected to obtain. We shall 

 have occasion to insert this valuable catalogue, and to re- 

 commend the adoption of a method somewhat similar, 

 when we shall have to explain the different uses of Bor- 

 da's repeating circle, which we have hitherto omitted to 

 notice, certainly not from its inferior importance, but 

 because we have confined ourselves chiefly to the instru- 

 ments of our own country. And as we propose to give 

 a very minute account of the construction of this instru- 

 ment, we shall only at present remark, that in France it 

 seems to have superseded the use of every other astrono- 

 mical instrument, for measuring either the zenith distan- 

 ces of heavenly bodies, or the angular distances of ter~ 

 restrij ones. 



This brief introduction embraces all the principal as- 

 tronomical circles which we know of in fixed observa- 

 tori's-. ft is, however, our intention to give, under the 

 article OBSERVATORY, a more detailed account of some 

 of them, particularly Mr Groombndge's transit circle, 

 and the mural circle now in use at Greenwich. For 

 we mean here to confine our descriptions, &c. to the- 

 thiee (list-net classes of portable circl s, nam-ly, the /Ze- 

 focling Circle, the Altitude and Azimuth Circle, and the- 

 Repeating Circle; and beginning with the reflecting cir^ 



Circle. 



Circle 

 made for 

 the Dublin 

 observa- 

 tory by 

 Ramideu. 



Circle 

 made for 

 General 

 Brisbane 

 by Trough- 

 torn 



The grand: 

 mural cir- 

 cle made 

 for the ob- 

 servatory 

 of Green- 

 wich by 

 Trough- 

 ton.. 



