atmospherical refraction at Port Bowen. 213 
respective observers, are given in the Tables attached to the 
corresponding observations for refraction, except those of 
Lieutenant Ross, the details of which, were unfortunately left 
on board the Fury at the time of her loss. 
While making the above mentioned observations for the 
zenith-distances of the boards, Captain Parry had occasion 
to notice, on the 28th of February, some anomalies which 
had never before occurred, and which were at first attributed 
to some slight and imperceptible change in the position of the 
repeating circle (see Table III.) On continuing the observa- 
tions, however, it soon appeared that the changes coincided 
nearly with particular times of the day, the greatest zenith 
distance always occurring when the thermometer stood the 
highest, and the weather was most calm. To clear the zenith 
distances of this effect of refraction, the repeating circle was 
carried up the hill, the object-glass of its telescope being 
placed in a notch cut in the board, as already described above 
in using the micrometer ; when by several days’ observations, 
continued from morning till night, it was found that the same 
phenomenon as before occurred, the zenith distance of the 
station below uniformly increasing from the morning till 
the afternoon, and again decreasing as the sun fell. Two 
sets of observations taken at the board after midnight, by 
means of a lamp viewed through the tube of the telescope, at 
the lower station, gave nearly a mean of all the other obser- 
vations. Thus it appeared that whether observed from the 
top or the bottom of a hill whose altitude was 4°i, an increase 
of zenith distance ( varying from 9 " to 17"), took place about 
the same hours, indicating a comparatively rare medium near 
the surface of the ground, and giving such a curvature to the 
visual ray, as to produce a similar effect at both stations. 
