OPERATIONS IN THE PENINSULA. 355 



Means of the Zenith Distances taken on the right and 

 left Arcs, corrected for refraction, equation of the 

 sectorial tube, and the mean runs of the Micrometer, 



Previous to this arranQ-ement of the zenith dis- 

 taiices It may he proper to say a few words on the 

 different corrections here mentioned. 



The refraction is liad from the tables of mean re- 

 fraction, and no notice taken of the barometer or 

 thermometer, or of the heights of the stations above 

 tlie level of the sea, considering it doubtful what cor- 

 rections to apply until observations are made, and 

 tables of refraction constructed, for this climate, and 

 for different elevations, 



The corrections for the micrometer were deter* 

 mined by taking the runs between every dot on the 

 arc when the mean temperature was 74°, it having 

 been discovered upon more minute attention, that 

 one degree on the limb was more than 3^00 divisions 

 marked seconds on the micrometer ; and the average 

 of all the results gave 36()4. Therefore one minute 

 counted by that scale required a deduction of 0".066 

 to give its true measure from the nearestMot. In all 

 these observations two thermometers were used, one 

 opposite the upper axis, the other opposite the arc, 

 and the experiments for ascertaining the runs were 

 made when the thermometers stood at the same 

 deo-ree. 



& 



This error in the scale of the micrometer has doubt- 

 less arisen in a great measure from the unequal expan- 

 sion of the sectorial tube and the frame which carries 

 it, whereby the point of the screw does not coincide 

 with the centre of the steel plate against which it 

 presses, and in consequence causes a greater equation 

 than what would arise simply from the expansion of 



Aa 2 



