Survey of Northeast Greenland. 161 
line of vision ASB corresponds to an upward mirage, and such a 
one must always be found, when a levelling at the sea horizon 
тен 90 
In the case of z < 90°, the presumption on which the formula 
MEME Е 
ske (HAVE 
was based fails altogetlrer. In these cases it can only be used for 
the purpose of computing k by insisting on the very plausible claim 
that dk should not change signs with z — 90°; in other words, in 
the case of z < 90° the formula must be 
IL; ‚ 12 90° В 
Ww 
With a telescope altitude as small as that of the Observatory, 
13.89 m, there might be a question of applying a correction for the 
tidal movement on the levellings at the horizon. From tidal obser- 
vations at Maroussia it appeared that the difference between the 
flow and ebb may here be estimated at about 1 m, and also that the 
times of high water at Maroussia and Danmarks Havn were the 
same”). It would consequently be possible to apply a rather uncertain 
correction for the tidal movement. 
a gh toh ei, | : 
From 1—k = — | op We get for the angle of depression 
ЕЕ. 
290° = ш |/ ee В 
from which in its turn 42 = wl а 
As the telescope altitude in the Observatory corresponds to the 
mean water level at Danmarks Havn for 1907, dh as a maximum 
may be put at !!vm. Suppose that we further put k = 0.584 (the 
ee — about 6”. 
In the face of the uncertainty of the observations at the sea 
horizon there will evidently be no reason to apply such an im- 
material and uncertain correction, which may furthermore not be 
supposed to influence the determination of the mean value of k, but 
only of the single values. 
In the table below the zenith distances measured in the direc- 
tions I, II and III are given. Like the preceding one this table is 
divided by means of horizontal lines into three sections, corresponding 
mean value of II) we get dz — w и 
1) A. TROLLE: Hydrographical observations from the Danmark-Expedition, Medd. 
om Grønland XLI, p. 417. 
uk: 
