21 a Observations to determine the amount of 
coincided with the edge of the board, when seen through the 
upper telescope ; the repeating circle was also raised or 
lowered until the same coincidence obtained, when looking 
through its telescope. The accuracy of the position thus ob- 
tained was finally verified by observing the setting of the 
star, through each telescope, when it was found to disappear 
to both observers at the same instant. 
For the altitude of the board, with respect to the lower tele- 
scope used by Captain Parry, a short staff, exactly equal in 
length to the measured distance between the telescopes, was 
fixed vertically above the board, and the zenith distance of its 
well defined top observed by the repeating circle in its former 
place. And as a confirmation of the results thus obtained, 
the method described above, as adopted by Lieutenat Foster, 
by means of the micrometer, was also resorted to ; a mean of 
the two methods (which differed 2",8), being used in the 
computation of the refractions. Lieutenant Ross's zenith 
distances were obtained by a repeating circle, placed on the 
same cask which held the telescopes he employed, the angular 
distance between each of these, and that of the circle ( when 
directed to the board), being determined by repeated obser- 
vations with the micrometer, fixed upon the respective 
boards in the manner already described. In some instances, 
Lieutenant Ross observed the re-appearence of a Aquilae 
under the board, thus obtaining an observation at another 
altitude. The corresponding zenith distance of that part of 
the board was determined by measuring with the micro- 
meter, the angle subtended by the board at the place of 
observation. 
The zenith distances of the boards, as obtained by the 
