METHOD OF EXAMINING ASTRONOMICAL INSTRUMENTS. 



369 



according to any method which has hitherto been made 

 public. I need not, however, remark upon the very great will scarcely 

 improbability, that the errour of examination should ever ev 

 attain, or approach, to its extreme limit, as this must be 

 sufficiently obvious to any person, who is in the least de- 

 gree conversant with the doctrine of chances; but it may be 

 proper to observe, that we have it jn our power (and in 

 this respect the examiner possesses a most important advan-and mayb» 

 tage over the divider of an instrument) to diminish its P r °- mfnished!" 

 bable amount, as much as we please, by bringing the 

 movable wires of the micrometer and microscope several 

 times to bisect their respective points in the measurement of 

 every arc, and taking a mean of the different readings off 

 for the true position of the wire at the real bisection of the 

 point. The wire-may be moved in this manner eight or ten 

 times at each point (if such a degree of caution should be" 

 thought necessary,) and the mean taken in little more than 

 a minute, so that the time of performing the work will not 

 be so much increased, as might perhaps have been appre- 

 hended; and when it is completed, we may reasonably pre- 

 sume, that the distance of every point from zero (while the 

 temperature of the circle continues uniform) will have been 

 determined with sufficient exactness for every practical pur- 

 pose. 



Of the time necessary for the examination a pretty cor- Time required 

 rect idea may be formed, by considering how rn^ny measure- for . tne exami * 

 ments are required, and allowing about a minute and a half 

 for each; i. e. a quarter of a minute for bringing the ex- 

 treme points of the arc to the micrometer and the micro- , 

 scope, and a minute and a quarter for making the several 

 bisections. Now, in dividing the whole circle into arcs of 

 15° each, it will appear, that forty- four measurements must 

 be performed ; and to examine every point in each arc of 

 15°, there will be 161 required, making in all 3908 mea- 

 surements ; and consequently the time urcessary for com- 

 pleting the whole work will be 5862 minutes, or about 98 

 hours. 



The time and labour required for this examination are, no it renders great 



doubt, very considerable; but it ought to be recollected, precision m an 

 7 J ° instrument un- 



that it will render any great degree of precision, in dividing necessary j 



Vol.XXYI. Supplement. Bb the 



