CIRCLE. 



TABLE III. 



The Numbers in this Table are always additive. 



513 



SECT. IV. On the Use of the Repealing Circle as a 

 Meridian Circle, when its repeating Property is 

 dispensed with, but the Errors of Division equally 

 done away. 



As the French astronomers have never used this instru- 

 ment but in the manner above mentioned, it is not per- 

 haps generally known, that it may be employed as a meri- 

 dian instrument to determine the declinations of stars, 

 without recurring to its property of repeating ; and we 

 are permitted, on the authority of the astronomer r >yal, 

 to state it as his opinion, that, when so employed, it is 

 the most perfvct poruble instrument of its size hitlierto 

 constructed. When placed in the plane of the meridian, 

 and there suffered to remain till a series of observations 

 on many stars is completed, it will very nearly resemble, 

 in it? mode of action, the mural circle lately erected by 

 Mr Troughton at Greenwich. Indeed, the only differ- 

 ence will be, that not being ecurc-d on a wall, the level 

 must be examined at each observation ; whereas ir ap- 

 pears, by a paper in the volume of the P/iifo>op/ncal 

 Ti'/iiixuclioni for 1813, that the Greenwich instrument 

 is so securely fixed, as to need neither l*-vel nor plumb- 

 line. It oii)/ht also to be it marked, that the small 

 instrument has an advantage, which ihe mural circle does 

 not possess, of having its divided surface turned either to 

 the east or west, so as to give either zenith distances or 

 polar distances in the manner which we shall now describe. 



To illustrate the me'.hod which we propose to recom- 

 mend, we shall take such acase as is most likely to occur to 

 an ingenious astron-.-mer who may possess sui:h an instru- 

 ment, and be inclined to avail himself of the powers it 

 possesses. We shall suppose his object is to determine the 

 north polar distance, or declination of a certain number 



VOL. TI. TAUT II. 



of stars, if the result is intended to be of public utility; Circle, 

 but few stars should be chosen, and those should be se- *"" *"Y"* 

 lected, whose places there is reason to suppose may not 

 be accurately known ; or the observer may attempt a se- 

 ries of circumpolar stars, with a view to determine the 

 quantity of refraction best adapted to his own observa- 

 tory. Perhaps it may admit of doubt, whether so small 

 an instrument can be sufficiently accurate for so nice a 

 purpose ; but the method of observation is in eddi case 

 the same. First, If zenith distances be required, (the 

 co. latitude of the place being supposed known by previous 

 observations of the pole star taken by repetition,) the 

 method of proceeding is as follows : Clamp the back te- 

 lescope firmly in any position, and observe a series of 

 stars one night with the face of the instrument towards 

 the east, observe the same series the next night with the 

 face of the instrument towards the west, repeat this ope- 

 ration alternately, till a considerable number of observa- 

 tions are obtained, the means of the opposite readings 

 will be the required zenith distances subject to the er- 

 ror of division ; and as each star will have been observed 

 on eight different arcs, this error will certainly be ex- 

 tremely email. To reduce it, however, to within still nar- 

 rower limits, let the level be again moved to some new 

 position, and another series be observed, the mean of the 

 two will now approach nearer the truth, and this process 

 of continually changing the telescope with its level should 

 be persevered in, till satisfactory results are obtained. 

 The principal error to be apprehended, is from the alter- 

 ation which the bubble of the level may be liable to from 

 change of temperature. Perhaps a judicious adaptation 

 of a plumblinc might for this reason be a great improve- 

 ment to this instrument. If the pole star be observed 

 both below and above the pole in every series, then the 

 position of the polar point may be found, and the north 

 polar distances will in that case be obtained by direct 

 measurement from the pole. The reversion of the instru- 

 ment, if this method be adopted, will have no other effect 

 than to bring- a new set of divisions into the series, and 

 will be equivalent to a change of position of the level, 

 and rather inferior in point of convenience. An intelli- 

 gent astronomer will choose between these methods, and 

 combine them according to circum.-tances, as it is almost 

 impossible in practical astronomy to devise any general 

 rule, that will not in many cases admit of considerable 

 modification. Both these methods suppose, that the obser- 

 ver derives his results trom his own observations alone ; 

 but we conceive that, for instruments -f small dimensions 

 it would practically be much more advisable for an ob- 

 lerver, to avail h'nmelf of the Catalogue of Stais given 

 in tin.- toll jwing pjge. Indted, one of the principal in- 

 tention, of tins catalogue, is to afford to astronomers a 

 certain number of standani stars, to enable them to de- 

 t i mine tat polar dibtanccb of other stars by comparison, 

 in the same mdnncr as Dr Masktlyne's Catalogue of the 

 Right AiCrnsions is universally tak n an a standard of 

 companion, when the right a?c- 1141011 of other celestial 

 objecia is required. We avail ourselves of this early 

 opportunity ot giving this catalogue to our readers. It 

 is deduced trom nearly two thousand observations, made 

 with the most accurate instrument hiih.no construct- 

 ed. 



If this method be adopted, the instrument should at 

 first be so place d, as to mark nearly z^ro when the tele- 

 scope points to the pole, a certain number of the Green- 

 wich stars should be observed in each aeries, and from 

 these standard stars the position of the instrument should 

 be deduced, or, in other words, the error of collimaticra 

 should be found. 



3T 



