atmospherical refraction at Port Bowen. 211 
The latitude, 73 0 13' 39 ff ,4 N.* used in these computations, 
is the result of 91 sets of observations on Polaris, at different 
horary distances from the north and south meridians, by 
Captain Parry and Lieutenant Foster ; employing Dr. 
Young's Table of Atmospherical Refractions, published at 
the end of the Nautical Almanack for each year. 
As soon as the sun afforded sufficient light for obtaining 
the apparent altitudes of the boards from the respective tele- 
scopes, observations were commenced for that purpose. The 
circle used by Lieutenant Foster afforded the direct means 
of doing this, for the upper telescope, by which the zenith 
distance of the edge of the board at the spot where the star 
set, was at once obtained by observation. The angular dis- 
tance between this telescope and the lower one, as seen from 
the board, was determined by means of a double wire micro- 
meter, attached to one of Dollond's achromatic telescopes 
4 6 inches focal length, the object-glass of which was let into 
the board, so as to make its centre exactly coincide with that 
part behind which the star set. 
The telescopes employed by Captain Parry and Lieutenant 
Ross, not being attached to an instrument calculated for mea- 
suring zenith distances, required some further contrivance to 
obtain the altitudes of the boards with respect to them. In 
order to place the repeating circle precisely at the same alti- 
tude with Captain Parry's upper telescope, a levelling staff 
was fixed into the ground, half way between the place of 
observation and the board. This being adjusted by sliding 
up or down till a fine brass point on its upper end exactly 
• The elements of this result, are given in the Appendix to Capt. Parry’s 
Narrative of the Third Voyage for the Discovery of a North West Passage into 
the Pacific Ocean. 
