Survey of Northeast Greenland. 197 
particular the existence of mirages or Föhn clouds. Temperature, 
pressure of the air, velocity and direction of the wind, should be 
regularly recorded. During ascensions of mountains one ought in 
particular to direct one’s attention towards changes in the velocity 
and direction of the wind according to the altitude; f. i. it very often 
happens in East Greenland, that there is calm or an easterly wind 
at the level of the sea, whereas at an altitude of a few hundred 
metres one may rather suddenly get a fresh westerly breeze and at 
the same time a higher temperature. Here one passes a stratification 
which may have a disturbing effect on the refraction in the lines of 
vision traversing it; consequently one ought to know approximately 
the corresponding altitude. Finally, at all the stations, from where 
the sea horizon is visible, one ought to level the latter at the com- 
mencement and conclusion of the observations. In so far as the 
altitude of the station is well defined, one gets through the latter a 
good basis for control, as to whether the value of the co-efficient of 
refraction used is fairly suited for sights at lower lying points. In 
case the altitude of the station is only verified in a defective manner, 
the levelling at the sea horizon may serve to control or according 
to the circumstances to correct the altitude of the station. (As to 
this see further chapt. Ш. 
In the course of survey work performed in Greenland, one has 
often used the value k — 0.2, and generally there will hardly be 
any reason to choose a different value. In cases where low sights 
across water are the principal thing, there may, however, in North 
Greenland be occasion to use a considerably higher value, according 
to the circumstances as much as 0.6. 
In practical navigation the knowledge of the angle of depression 
at the sea horizon plays a considerable part. The constant generally 
used for the computation of the angle of depression is badly adapted 
for the ice-filled part of the Greenland Sea. As we are here only 
dealing with low lines of vision, we may use the value k — 0,584. 
From 
2(1-k) _ 
z - 90° = w V a R Vh 
we then get the angle of depression 
d = z-90° = 744Vh) | 
where d is expressed in seconds, h in metres. 
') In “Formeln und Hülftafeln” ALBRECHT gives for k = 0.13 
290° — 107.7 Vh. 
