MERIDIONAL OBSERVATIONS. 



215 



The Error of Collimation of the center wire was obtained from observations 

 of Polaris on side wires and of the Meridian Mark on the center wire as 

 follows, the sign being considered positive when it implies an additive cor- 

 rection to the observed time of transits of equatorial stars with the lamp 

 West : 



1874. 



The value 9" '72 (+ for lamp West) has been used throughout the 

 reductions. 



The Azimuth Error depends generally upon the observation of a close 

 circumpolar star ; occasionally upon the Meridian Mark. This was placed 

 upon a small brick pier on the ridge of a slight eminence to the north, un- 

 fortunately, owing to the nature of the ground, only 1,229 feet from the 

 transit instrument. The mark itself was a copper plate perforated with a 

 horizontal line of circular holes, sensibly equidistant, 1 875 inches apart, 

 from center to center, every fifth hole being somewhat elongated in a vertical 

 direction. In the field of view of the instrument, therefore, the distance 

 between the holes from center to center subtended 26" 23. The mark was 

 observed by daylight, and was rendered sufficiently distinct by placing a 

 pasteboard diaphragm before the object-glass, having a vertical slit of an 

 inch wide. The value of the intervals on the copper plate were also found 

 from numerous observations of the position of side wires at the times when 

 the center wire was usually observed. From these are obtained the following 

 results : 



