166 I. P. Косн. 
tion the mean value to Cairn X and to the “Skerry”, k* and ka 
must, as mentioned above, be considered very uncertain. The best 
plan will consequently be to look at once for a control of the deter- 
minations made. The basis of the continued discussion will then 
appear, of its own accord, as the result of this research. 
For the co-efficient of refraction at a given point we have the 
following term"): 
= b Phen 9. 
x = 0.000293 >< 760  T <XGXRXE, where 
j 08 
mi Sb za ay 
о Ge 
9 2h 
ar 1 + 0.00265 cos 29 — R 
In this term b is the pressure of the air in millimetres, reduced 
to 0°, T the absolute temperature — 273° + Р, 9 the acceleration 
of gravity for the place in question, G the acceleration of gravity 
in 45° latitude at the level of the sea, R the radius of curvature of 
the ellipsoid, r the variation of temperature per metre FR e the 
pressure of the aqueous vapour in millimetres, ф the geographical 
latitude and h the altitude above the level of the sea. 
In the case of о = 76°46’ a oscillates between 1.0024 (h — 0) 
and 1.0023 (h — 429m), consequently in the case of computation 
with a four-placed table г. 
When computing the factor F we are confronted with а diffi- 
culty, in that our knowledge of the pressure of the aqueous vapour 
is rather uncertain. As far as the Observatory is concerned, the 
pressure may be computed from the percentage of moisture; but as 
the term for x is here to be used to compute a mean value of the 
co-efficient of refraction corresponding to the whole of the line of 
vision, one ought to know the mean value of e corresponding to 
the whole line. The relative moisture at Danmarks Havn varies 
between 40/0 and 100 ®/o; in the case of the months June to October, 
which here may be taken into consideration, all of the mean values 
of the months are somewhere about 70—80°/o. Above water the 
percentage of moisture is probably somewhat greater. The variation 
of moisture with the altitude is immaterial to this research. If we 
becomes — 1.002 in all instances. 
1) Е. В. HELMERT: Die mathematischen und physicalischen Theorien der höheren 
Geodæsi, II Teil, p. 577. 
