Survey of Northeast Greenland. 307 
Ai 1008 ла, м = 106.3 tan и’ = 0.0195, 
H,=1008m; D,—52 km; D estimated at 70 km; Refr.and curv.oftheearth 4<77 = 308 
9.1316 7,08. -,:D = 67 - ; — — — 4x70.6— 282 
H5—1290 -; D;—66.9- ; 
D = D, = 66.9 km. 
That the first estimated value of 70 km is too great immediately 
appears from the fact that D, becomes smaller than 70 i. e. 68 km. 
The value of D looked for must therefore also necessarily be smaller 
than 68 km. As the differences between the D’s become smaller and 
smaller, according as the computation progresses, the value looked 
for must, however, be greater than 68 + 2 = 66km. Therefore it 
is only natural that in the second experiment one should try the 
_ value 67 as shown above. 
That the two results 66.8 km and 66.9 km differ by 0.1 is due 
to the imperfection of the engineer’s scale. But this little inaccuracy 
is of course without any practical importance whatsoever. 
Before leaving the chapter on the measuring of distances by means 
of the vertical base, I shall mention the importance of performing special 
levellings for the determination of the dip of the sea horizon. 
During a coast survey, where one has nearly always the oppor- 
tunity of seeing the sea horizon, one ought not to omit levelling the 
latter, both at the beginning and the termination of the survey per- 
formed from a station and, if possible, in two directions. The levelling 
at the sea horizon ought to be performed both with “telescope right” 
and “telescope left”, and by reading the level. In this manner one 
gets the means of computing ‚the co-efficient of refraction or of 
eliminating the latter from the computation of the distance. 
One gets: 
h tan? a 
— R —— 
_ 2(1+k) 
where h is the altitude of the telescope above the level of the sea, 
and « is the dip of the sea horizon. 
lf the altitude of the telescope has, in another way, been deter- 
mined at H,, one must get В = H, or 
1 ee tae 
АН ВЕ 
Ву inserting in the equation mentioned оп р. 304 
1—k,, 
Dtanv = H, + OR De 
this term is rounded off into 
Dino eS pat 
1H, 
