1862.] 
37 
G7. Process of estimating terrestrial refraction .-—Let A, B, C, 
D, (vide figure) be plain stations, T and S stations on the Sub- 
Himalayas, and I. to IV. inaccessible points on tlie range of perpetual 
snows. Let tbe values of f at T and S equal respectively/* and/b 
We may deduce from these, two trustworthy values of tbe heights 
of I. and II. Calling this mean height of I = I m , and re¬ 
membering that we have elevation (E) at C of I, as also the 
contained arc for C I — (c) given, it is clear that the values of f at 
C, corresponding to I m may be found. Let this value —f Pro¬ 
ceeding in the same manner we shall find f G ==—- ^ ^ f n 
n 
Similarly /h &c., may be obtained, and with/^, /* D &c., may be com¬ 
puted IIl m , IV m &c., from which again in turn may be found the 
values of f for the other plain stations from which III, IV &c , have 
been observed. By this process the computed values of f are deter¬ 
mined nearly in terms of ft and fi, errors of observation not being 
taken into account. It remains to mention how f s and f t were 
obtained. 
68. The computations originate from Senchal and Tonglo hill 
stations, at which stations, the following mean value of f was in the 
