C I R C L K. 





not, alter one half of the appirent error by /and its fel- 

 low, ami the other half bjr the slider* of the leel. The 

 now give azimuth* and right-ascension*, 



(that ii. when placed co 

 but without farther ariju 

 indirxctly to be 



To adjust it in thii 

 low : Point tl 

 which liivcct by the mid 



pr cling the niriididii,) 

 stance* arc 



it. 



retpect, we way proceed a* fol- 

 > any very diltant object, 

 .ital wire, and read off 

 by the microscope* thr apparent zcr.iih distance ; now 

 l in azimuth, and turniug the tele- 

 scope again upon the same object, bisect it as before, 

 and again read off the angle which they shew. One of 

 these will be an altitude, and the other a zenith distance. 

 On one being reduced to the same name as the other, 

 ran of them will be the true zenith distance or al- 

 titude respectively ; and while the telescope bisects the 

 object, tiie microscopes must be adjusted by their pro- 

 per screw*, so a< to shew that mean. This operation 

 may be repeated for obtaining a greater degree of accu- 

 racy ; but its final determination should be deduced from 

 observations upon many heavenly bodies, and the mirntc 

 error that may remain unadjusted had better be allowed 



The above are the grand adjustments of the circle be- 

 fore us j but there are many other* which, to describe 

 minutely, would require separate drawings. We do not, 

 however, see the necessity for this ; for the general read- 

 er will, by what has already been said, come at all he 

 can want a general knowledge of its construction, pro- 

 perties, and use ; while those persons, who possess such 

 an instrument, will, beside* the above, have their atten- 

 tion guided to the parts of action ; and the ingenious 

 artist, who may attempt the making such instruments, 

 will, by our description and figure, be enabled to pro- 

 ceed ; for his own resources ought to supply what may 

 be but partially represented. A brief enumeration, how- 

 ever, of some of the minor adjustments are here sub- 

 joined. 



The microscope* of both circles have an adjustment 

 for placing them in the exact diameters of their respec- 

 tive circle*, which adjustment also sets them at their 

 proper distance from the centres. By another adjust- 

 ment, the whole length of the microscopic tube is va- 

 ried, in order to make the magnified image of a divi- 

 sion exactly measure the scale of five revolutions of the 

 micrometer screw. A consequent adjustment of back- 

 ward or forward motion of the whole tube restore* dis- 

 tinct vision, which was necessarily lost by the former 

 operation. The five single minutes are indicated by as 

 many notches, seen at the side of the field of view of 

 the microscopes ; they are indented by the micrometer 

 crew itfelf, and are adjusted to their place by a screw 

 at the end of the box, opposite to the micrometer head, 

 acting upon the plate which contains them. By these 

 means, a revolution of the screw becomes the exact mea- 

 sure of * minute ; and it only requires the head of that 

 crew to be divided into GO equal parts, in order to give 

 seconds. This circular scale of second* ha* a motion 

 round the head, whereby it* zero may be brought to the 

 pointer, without moving the cross lines within. 



Euth the level* are comtructed without adjusting 

 scrcwr, the moveable divided indices answering the same 

 purpose ; for if the point of adjustment, as ascertained 

 by reversion, be within the range of the bubble, the 

 performance will be equally accurate, as it would have 

 been, had that point betrn in the middle of the tube. 



It may be remarked, that, after the principal adjust- 

 havc been carefully made, they should not be 



distuibcd on slight occasions. If, therefore, the hori- CircU. 

 zontal axil were occasionally found to be out of level, '*^~,~~" 

 to the amount of a second or two, it would be wrung 

 to adjust it by the screwy or its fellow, bccausi- 

 would affect the circle respecting iti reading n 



.1 small deviation would be best corrected by touch- 

 ing one or more of the feet Si" 



Of the instrument whu-h we are now about to dis- 

 miss, it has been said, that its principal telescope mag- 

 and that the angles are read to single se- 

 cond., ; and we may add, that the circles being divided 

 by au accurate engine, the work of which is estimated 

 to be within 5" of the truth, and the levels, by whic'ii 

 the verifications are made, being ground to a long scale, 

 so a* to be depended on to less than '2", reason :ibl 

 pcctations may be formed, that when used under fa- 

 vourable circumstances, two observations, in reverse po- 

 sitions with it, will give the place of an object in the 

 heavens, in declination within 5", and in right ascension 

 within 0.5" of time. Nor can much of this degree of 

 accuracy be abated, where, in surveying, land object* 

 are observed ; but much depends here upon whit kind 

 of objects prceent themselves for bisection. 



Small as the errors of this instrument, as just st 

 may appear, contrivances have beeu suggested and ap- 

 plied, which reduce them to almost nothing. To give 

 the horizontal circle this advantage, the top of the pe- 

 destal should be made double, and the upper half mov- 

 able in a/.imuth, carrying the instrument round with it, 

 but capable of being firmly secured in any given posi- 

 tion. By this contrivance, an angle in azimuth may be 

 measured on as many different parts of the circle as may 

 be thought necessary. This, in truth, gives to this in- 

 strument a property, but little inferior to the far-famed 

 method of repetition. 



A contrivance, which furnishes the upper circle with 

 a similar advantage, was suggested by the ingenuity of 

 our present astronomer-royal, as stated in his paper up- 

 on the declination of some of the fixed stars, (]', 

 Trans, for 1806.) This is effected, by giving the read- 

 ing microscopes a motion of about 60 concentric with 

 the circle : for the sun and moon, which are continually 

 changing their meridian altitudes, no such contrivance is 

 necessary, except indeed for the former, at the points 

 of his greatest declination ; but for the fixed stars, which, 

 without it, would always have their places given by the 

 same division, the revolving microscopes are of the great- 

 est value. By changing the places of these as often as 

 we think necessary, the errors of division may bo redu- 

 ced to a quantity too small to deserve notice. In ill ca- 

 ses, the angle, through which the microscopes ;.re 

 ed from their horizontal position, must be applied to 

 the observations as an error of collunation. 



\Ve may observe, that this construction of the astro- 

 nomical circle seems peculiarly adapted to the conveni- 

 ence of a scientific gentleman when travelling. Its strong 

 and compact figure renders it safe and commodious in 

 carriage. All its verifications may be made in a short 

 space of time by land objects ; and its being applicable 

 to all the purposes of astronomy and geodesy, renders 

 it, in our estimation, superior to the repeating circle, 

 which is, for similar purposes, its only rival. 



SZCT. II. General Mil/toil of making Obtcrralions with 

 Astronomical ( V.-c/c.v. 



IF the instrument turn freely in azimut!; 'ike- 



wise a divided azimuth circle, this circle should first be 

 placed in a horizontal position, a verification nut always 



