gannett.] GEOGRAPHIC TABLES AND FORMULAS. 243 
Table 24. — For obtaining differences of altitude for any minute up to 15 degrees, and for 
any distance. 
[Prepared by Arthur P. Davis.] 
EXPLANATION OF TABLE. 
The left-hand column is the minutes of the vertical angle, the 
degrees being denoted by the large number at top of page. The bold- 
face figures at top of column is the distance in miles. Numbers in the 
body of the table denote the difference of elevation corresponding to 
the angle on the left and the distance at top. The correction for 
curvature, refraction, and height of instrument is always plus; it there- 
fore increases the difference of level for angles of elevation, and is 
subtracted from the difference of level for angles of depression. 
Example. — Required the difference of altitude corresponding to a 
vertical angle of -4- 9° 18' at a distance of 3.628 miles. On page 253 
the tabular number corresponding to 9° 18' and — 
Feet. 
A distance of 3 miles is 2, 594 
For a distance of 6 miles is 5, 188 — for 0.6 is therefore 519 
For a distance of 2 miles is 1,729 — for 0.02 is therefore 17 
For a distance of 8 miles is 6,917 — for 0.008 is therefore ,. 7 
Correction for curvature, refraction, and height of instrument for 3.6 miles is + . 12 
Total difference of altitude + 3, 149 
