Survey of Northeast Greenland. 169 
estimate of the ,,vertical movement” would therefore result in the 
latter being considered as strongest and most frequently occurring 
in the case of objects farther away, which also appears from the 
observation journal. 
From the observation journal it appears that out of thirty-two 
observation series the designation 0 (absolute rest over all objects) 
was only given to six, whereas in the case of twenty-six the designa- 
tion 0—3 was used, generally varied in the case of the different 
objects. 
From what has been indicated it appears that the co-efficient 
of refraction, k, is generally subject to sudden variations, which may 
become very considerable. On a few occasions, when the zenith 
distance at the control turned out to be perceptible, a new observa- 
tion was made by means of the slow moving screw, and so the 
vertical movement was measured. On the strength of this it becomes 
possible to compute the corresponding change in k. As an example I 
shall here give three instances of very considerable variations of k 
within a period of about '/2 minute. 
Vertical movement. 
: | Intensity | measured | 
Levelling at . I, AK 
| estimated | at | 
Cairn IX — Sept. 14th 07 at 11210... | 1 5.5 | 0.130 
Cairn X — Oct. 213407 at 8250..... | 3 i Ge MOSS 
Cairn У — June 15% 08 at 10p10.. | 3 | 27” | 0.106 
The causes of the vertical movement will be mentioned later 
on; here attention is only to be directed to the fact, that in the face 
of such extensive and momentary changes in k, one cannot in the 
single cases expect any agreement between the values of k and x. 
On the other hand one may, when a great number of nearly simul- 
taneous values of k and x are forthcoming, count on a reasonable 
agreement between the means. 
As k must be considered a mean value of the co-efficients of 
the refraction in the line of vision in question, the values of b, - and 
T, which are necessary for the computation of x, must also be deter- 
mined as mean values of this line. In the cases where simulta- 
neously with the levellings kite- and balloon-ascensions have been 
made, it has been possible to form a fairly reliable estimate of these 
quantities corresponding to the lines of vision at the cairns IX and X. 
The manner of proceeding in such cases has been as follows: 
By means of the temperature of the air at the Observatory as 
