and ofihe Altitude and Azimuth Instrument. 105 



in tlie construction of the instrument : but to hrmrr it to no- 

 thing must tor ever be beyond the skill of the artist, because 

 it lies in some measure in the nature of the materials. I will 

 now try to explain how this source of inaccuracy affects ob- 

 servation, and to this end will suppose that an observer, when 

 he bisects a star, moves the index with the tangent screw, so 

 as to advance it on the graduated limb according to the num- 

 l)ering of the degrees ; the telescope indeed will be properly 

 pointed, but the index will show too much. On reversiufr the 

 position of the instrument, the telescope will naturally relieve 

 itself from the friction at the centre, and take the position due 

 to the index, instead of retaining that which it had when tlie 

 star was bisected ; therefore the following motion of the tele- 

 scope will begin from a wrong point ; and if the screw is al- 

 ways turned in the same direction, will produce an error in 

 excess at eveiy pair of observations. As much may be said 

 respecting the motion of the level, which, on account of its 

 socket embracing the exterior of the axis, will meet with a 

 greater resistance than the telescope did. As fiir as these two 

 parts of repetition are concerned, it is clear that the habits of 

 an observer may make this source of enor either fixed or ac- 

 cidental, and by constantly turning the screv/s in a particular 

 way, he may have for a fixed error either their sum or their 

 tlifference. There is another motion equally connected with 

 the oj)eiation of repetition, and quite as liable to this kind of 

 error; namely, the general one, which carries round together 

 the circle, the telescojie and level. To folloM' up these three 

 sources of error, and show how in their different combinations a 

 series of repetitions would be af!ected, is a task which I dare not 

 attempt. Instead of which I will content myself with remarking, 

 tliat the complicated centre-work of this instrument, and the 

 diffe-rent motions having a tendency to drag each other, to- 



S ether with the change of jiosition Uiat takes place without the 

 ifferent steps being registered, subject it to greater errors, 

 when the graduation is tolerable, than those which it professes 

 to correct. To examine whetluT the screws, which govern 

 the tin-ee motions here treated of, give immediate motion to 

 the i-especlive parts which they act upon, is a good criterion, 

 but I think incapable of<letecting a quantity of error less than 

 two or three seconds; which (juanlity, if it recurs as fixed, 

 will not be divided in obtaining the general result, because it 

 aflects all the operations, and it is the accumulated error only 

 that is reduced. 



In the instrument A, the resistance of the centre-work is 



extremely small; the pivots of the transit axis merely resting 



on their angular supports, while the screw for slow motion 



acts iinincdialciv Uf)on the circle, carrving also the telescope 



Vol. GO. No. '}y2. .-fw-f. 182'J. ■ O which 



