304 I. P. Kocx. 
At times one may, when marking off one's station by means of 
a cairn after the descent get the opportunity of measuring a small 
base on the ice, and from the termini of the latter to level the cairn 
of the station. In the few cases where we have been able to do 
that, we have generally not only managed to control the result of 
the barometric measuring of altitudes, but also to improve upon it. 
(As to the control of the altitude of the station by measuring 
the dip. of the sea horizon, see p. 308). 
As mentioned above BORGEN and COPELAND have prepared special 
auxiliary tables to be used at the computation of the distanees. The 
accuracy with which these tables have been prepared rests, however, 
on the proved wrong ideas of the accuracy of the whole distance 
measuring. The tables are therefore unnecessarily elaborate and un- 
practical in use. In the following I am going to give a simpler and 
quicker method of computation. 
The general formula of the difference of altitude between two 
points 
H = H, See. m 
can with a trifling modification be used for this computation. H, is 
here the altitude of the telescope above the level of the sea; H — 
the altitude of the coast point — is equal to zero. D is the distance, 
k the refraction constant, R the radius of curvature of the earth. If 
the zenith distance z is put = 90° +», where v is the depression 
angle, the formula may be altered to 
Dan vtr; Be IDE 
For the computation we made use of a four-placed table of the 
numeric values of ine fangeut function?) as well as the small table 
below of the term — К p22 *). This latter table is computed for the 
co-efficient of refraction k — 0.2 and for R — VMN, where M and N 
are the radius of curvature of the meridian and of the normal seg- 
ment to that in 78°30’. D is expressed in kilometres, the other two 
colums in metres. 
1) A table of this kind is found in: TH. ALBRECHT, Vierstellige Logarithmentafel, 
Leipzig, Verlag von Wilhelm Engelmann, 1894. 
2) It rather often happens when computing vertical angles that one has to pee the 
1+ 
correction of refraction and curvature of the earth — the term = 
corresponding to values of D which exceed 40km. The correction is then easily 
computed from the table, when it is borne in mind that it is proportionate to 
the square of the distance. 
