2/8 0?i a Method of examining 



apprehended : but as I cannoi obtain such precise informal* 

 tion as I could wish, rcspectvng the exactness with which 

 a bisection can be performed, or a length taken from the 

 scale of equal parts and laid upon the instrument, I will 

 not trouble vou with the deduction which I have made^ 

 It is understood, indeed, that Bird's method is now generally 

 laid aside, and that each artist employs one, which he con- 

 siders in many respects as peculiar to himself; but I pre* 

 sumed that there would still be such a connexion betwixt 

 Bird's method and those which have been snbstitu'cd in its 

 stead, as to render them in some degree liable to 'he same 

 errors to which it was subject; and the reports v> hich 1 have 

 uniformly received from persons, who 4iave had in oppor- 

 tunity of examining some of'the modern instrumt»nls, have 

 fully convinced me that my opmion w as just. But what- 

 ever may be the nature of the methods which are now in 

 use. or whatever their advantages over Bird's, F never could 

 persuade nn'self that it would be safe to trust to an instru- 

 ment, without a previous examination. To discover the 

 means of accomplishing this" object, is what I have for 

 some time been anxious to effect ; and though I fear my 

 endeavours have not been very successful, f will neverthe- 

 less take the liberty of presenting you with the result. 

 You are aware, I believe, that I use a circular instru- 



ment for observing both in altitude and azimuth, which 

 was made for me by Mr. Carey in the Strand ; that the ra- 

 dius of both the altitude and the azimuth circle is one foot, 

 and that each is divided into parts containing ten minutes. 

 The construction of this instrument does not difier ma- 

 terially from that of other similar instruments, with which 

 you are well acquainted, and I shall not therefore waste your 

 time by giving you a particular description of it. For the 

 purjiose of examining the divisions upon the two circles, I 

 procured an apparatus to be prepared by Mr. Carey, which 

 will be very easily explained. To the face of the rim which 

 surrounds the azimuth circle, and' with its left end close 

 to the stand which supports the micrometer on the east 

 side, an arc of brass, concentric with the circle itself, and 

 a little more than 90° in length, an inch in breadth, and 

 one eighth of an inch in thickness, is firndy fixed by 

 screws, so as to have the plane parallel to the plane* of the 

 circle, and 'a small portion of its lower surface resting upon 

 the extreme part of the rim. The screws pass through a 

 brass pre, which is fastened to this at right angles, and lies 

 with its broad side against (he face of the rim. Upon the 

 first-mentioned arc. a strong upright piece of brass, about 



six 



