116 
KIETLEY F. MATHER 
This method is frequently employed by ingenious surveyors 
to good advantage in determining distances without the use of 
the stadia rod. Any two points, one above the other, at known 
distances apart suffice; two flags at a measured interval, the 
crown plate and girths (commonly eight feet apart) of a stand- 
ard derrick, the eaves and lower copings of a church tower, are 
listed merely as suggestions. If the location of such a target is 
plotted, the surveyor may ‘‘shoot himself in,” with a fair degree 
of accuracy, at any point from which it is visible. 
5. If alidade is equipped as in 3, an alternative method which 
may be used is as follows: Place middle cross hair on top of 
rod; read and record Stebinger, denoting the record as A. Turn 
down until middle hair intersects the lowest visible primary 
division; read and record Stebinger (record B), Turn down 
until top hair rests on top of rod; read and record Stebinger 
(record C). Compute distance by the formula 
B 
200 r 
C-A 
B-A 
in which D represents the distance, r the length of the rod above 
lowest visible primary division, and A, B, and C, respectively 
the three readings of the Stebinger drum. For example: a 
13-foot rod is entirely visible. The middle hair on top of rod 
gives a Stebinger reading of 31; middle hair on bottom of rod 
gives a reading of 54; top hair on top of rod gives a reading of 78. 
Distance = 200 x 13 x 
78 - 31 
54 - 31 
5300 feet. 
The formula may be more easily recalled if one has grasped 
the principle upon which it is based. The Stebinger difference, 
C-A, is theoretically a constant, the measure of the angle between 
the rays converging from the top and middle cross hairs to the 
focus of the telescope. If the rod at the distant point were of 
sufficient length, the intercept subtended by this angle could be 
read and, multiplied by 200, would give the distance to the rod. 
That is, if i be taken to mean the length in feet of that hypothet- 
ical intercept, 
