148 I. P. Kocu. 
Reciprocal, simultaneous zenith distances. 
The Observatory — Cairn X. 
The one of the two points must be the Observatory; as the second 
point we chose Cairn X, which lay at an altitude of 426 metres 
above sea level. This corresponded very well with the normal posi- 
tion of the inversion of temperature. The distance of Cairn X from 
the Observatory was 7!/2 km. 
With the view of enabling us to make exact observations from 
Cairn X to the Observatory there was in the neighbourhood of the 
latter set up an ordinary 
hour glass signal (see Fig. 7), 
the position of which in 
relation to the telescope of 
the Observatory vas deter- 
mined by the following 
elements: 
Horizontal distance from 
the centre of the Observa- 
tory to the cross of the 
signal 15.9 m. 
Zenith distance of the 
cross from the Observatory 
23722: 
Angle X - Observatory- 
Cross 50°11'5. 
In order to be able to 
point Cairn X sharply from 
the Observatory it was 
Fig. 7. provided with a special 
sighting mark, in that a 
stanchion to which was attached a wooden disc was run through it 
(Fig. 8). 
The expedition only having at its disposal one large universal 
instrument, one of the small instruments had to be used for the 
observations from Cairn X. The small instrument naturally could not 
furnish the same degree of accuracy as the larger one, which was 
besides firmly fitted into a granite pillar. In order to remedy this draw- 
back a particularly solid tripod, which was firmly planted into the soil 
at Cairn X was made for the small instrument (see chapter VI, Fig. 88). 
Further the little instrument was, during the observations, shielded 
against the rays of the sun by means of an umbrella. Finally the 
observations at Cairn X were begun five minutes before the cor- 
