41 



oftentimes very troublesome, and, it is to be feared, sometimes 

 cannot be performed with the desired accurac3^ The plumb 

 line passing near the plane of the circle*, no screen can be 

 used. Hence even a slight agitation of the air occasions a 

 slow motion in the plumb line ; and the observer, without 

 much tedious precaution, may be deceived as to this essen- 

 tial adjustment. From hence, doubtless, have arisen greater 

 discordances in the observations than would have otherwise 

 taken place. This imperfection arises principally from the 

 situation of the plumb line, it would not be difficult to give 

 it another situation in which it might be safely screened from 

 the agitation of the air, and were the instrument Avithin a 

 convenient distance from the maker, I should endeavour to 

 have this alteration effected. 



Notwithstanding this imperfection, I think I may safely 

 pronounce that as far as errors of observation are concerned 

 a-^mean of 10 observations (five the face of the circle being 

 east, and five the face being west) will give the zenith dis- 

 tance exact to much less than one second, and that a mean of 

 20 observations, according to a very high degree of probabi- 

 lity cannot induce an error of nearly half a second as far as 

 errors of observation are concerned. 



* An adjvistment of the microscopes was intended by means of the plumb line, and 

 four gold dots placed on the limb of the circle. This made it necessary that the plumb 

 line should pass very near the plane of the circle. But the method of observing thence 

 resultuig I found inferior both in accuracy and conveniencs to that in which the plumb 

 line is only used for the adjustment of the vertical axis. 



VOL. XII. 11 



