38 
The Trigonometrical Survey of India. [No. 1, 
first instance adopted. The selection has been made to the exclusion 
of those values obtained from short sides. 
Deduction.—Doom Dangi 
O 
Senchal 
;i } /= 
Thakoorganj 7 / _ 
Senchal ‘ * 
Doom Dangi ) „ 
Tonglo y 
Thakoorganj 1 ~ 
Tonglo \ f: 
V 
5 
Senchal 
Tonglo 
O 
.07617. 
.07636. 
.07915. 
.07819. 
.06201. 
Tonglo 
Darjeeling 
.08043. 
Mean f—* 0744. = T -|. ¥ nearly. 
69. With this value of f the heights from Senchal and Tonglo 
were computed, and the mean of these values, as also the differences 
between each value and its mean, were next found. The heights were 
now corrected, in such wise, that when the heights deduced from 
Senchal are compared with the mean heights already mentioned, the 
greatest and — differences should be numerically equal. The 
same process being gone through at Tonglo, IT. S., there resulted 
the mean values of f which have been employed for that station and 
for Senchal. These values will be found recorded in the heights 
herein given, and it will also be found, that they have been employed 
for all heights of the /S^S-Himalayas observed at Senchal and Tonglo 
hill stations. 
70. It may be useful to remember, that if there be two points 
A and B observed from O, whose heights respectively are h a and lib 
determined by a certain value of f at O — f 0 . Also if d a equal 
corrected geodotic distance O to A, and db — OB. Then if f 0 vary, 
so that h a (the height of A computed from O) changes by + 8 a , and 
Jib by ± 8b, so will ± 
8 b db 2 
— oc —. Hence should the foregoing method 
d„ a 3 6 
for finding the value of f at plain stations in terms of the observed 
value at hill stations, be hereafter ever adopted, it will be found 
advantageous to construct a table of the squares of the distances in 
miles, for this purpose. 
71. The general principle of procedure is now apparent. But as 
