METHOD *OF EXAMINING ASTRONOMICAL INSTRUMENTS. 361 



sure upon the abilities and integrity of the artist. It is in 

 vain that we observe with so much nicety, and read off with 

 so much precision, if we are still uncertain, whether there 

 may not be an errour in the instrument itself of much 

 greater magnitude, than those which we^are endeavouring 

 to prevent ; and that our best instruments must be liable to 

 such errours, no person can possibly doubt, who has paid 

 due attention to the sources whence they may arise. I Errours in 



i • » , T ,., L a Bird's method 



have estimated, as accurately as 1 could, the amount to may accumu . 

 which they may accumulate in Bird's method of dividing by late more than 

 continual bisections ; and have satisfied myself, that they are p 0se( i. 

 much more considerable, than is generally apprehended: 

 but as I cannot obtain such precise information as I could 

 wish, respecting the exactness with whicli a bisection can be 

 performed, or a length taken from the scale of equal parts 

 and laid upon the instrument, I will not trouble you 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 considers in many respects as 

 peculiar to himself: but I presumed, that there would still Other methods 

 be such a connection betwixt Bird's method and those which liable to the^ 

 have been substituted in its stead, as to render them in some san > e - 

 degree liable to the same errours to which it was subject; 

 and the reports which I have uniformly received from per- 

 sons, who have had an opportunity of examining some of 

 the modern instruments, have fully convinced me, that my 

 opinion was just. But whatever may be the nature of the 

 methods which are now in use, or whatever their advantages 

 over Bird's, I never could persuade myself, that it would be 

 safe to trust to an instrument, without a previous examina- 

 tion. 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, 

 I will nevertheless take the liberty of presenting you with 

 the result. 



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



ment for observing both in altitnde and azimuth, which was a ^muth circle 



w of one foot 



made for me by Mr. Cary in the Strand; that the radius of radius. 



both the altitude and the azimuth circle is one foot, and that 

 #ach is divided into parts containing ten miuutes. The con- 

 struction 



