Apams.—To calculate Distances by Reciprocal Vertical Angles. 187 
In order to compute this correction by the above rules, the distance 
between the stations is required to be known; but as in all cases where this 
method is used the distance between the stations is not known, we must 
proceed as follows :— 
With the observed vertical angles, as they stand in the field-book, 
compute the distance between the stations; and with this approximate dis- 
tance, compute the eye and object correction. Then, with the corrected 
angles, again compute the distance, and in most cases no further calcula- 
tion will be required; but in cases where the second calculation gives a 
result differing greatly from the first approximation, it may be advisable to 
repeat the calculation. 
Instead, however, of neglecting the eye and object correction altogether, 
in calculating the first approximation, it will be sometimes advantageous to 
ascertain the correction roughly, and take it into account. This may be 
done as follows :— 
As 1 inch subtends 1" at 26044 links or 8} miles nearly, we can easily 
ascertain the angle subtended by any number of inches, at any number of 
miles distance, by the following rule :— 
Multiply the inches by 81 and divide the product by the number of miles, 
ihe quotient will be the number of seconds subtended. The distance in 
miles can generally be estimated to within 10 per cent. or so, and calcu- 
lating the first approximate pedo ia this way will often save time. 
Exa 
Bryant's Hill to ahadi s Hill. Elev. r " 18” 
Barker's Hill to Bryant's Hill. Dep. 2’ 50" 2 
Bryant's Hill to Barker's Hill. Height of eye ao Gy 
3s object = 0 0 
Ix, 
Eye exceeds object 3 1 = 87 
Distance, say 10 miles 10) 120 
12:0" 
Ew. Ix. 
Barker's Hill to Bryant's Hill. Height of eyo = a 
te object = T 
— HE Ix. 
Eye exceeds object = 1 9 = £^ 
10 ) 68 
6:8 
Bryant 1° 14 13” + 12' = 1° 14 25" 
ee o D Elev., D M trat. LE i 
Barker's io Bryant's Hill. Dep., 
8 182 = 498"2 
3771 
49820 
34874 
8487 
To compute the eye and object corrections 149 
