130 
KIRTLEY F. MATHER 
wire above base of rod by adding one-half the expressed stadia distance 
(in feet subtended) to the reading of the lower wire. 
Example: If the half wires subtend 7.2 on rod, the distance would 
be 7.2 X 2 = 14.4 (1440 feet). If the lower wire cuts the rod 8.7 feet 
above its base, the computed middle wire reading would be 8.7 4- 7.2 
= 15.9 feet above base of rod. Then compute as before. 
The advantages of the stadia arc are readily apparent. The 
use of stadia tables, slide rules, or diagrams is entirely obviated, 
nor is there any vernier to be read. The accuracy of results is 
identical with that obtained from formula or table computa- 
tions; in fact differences in elevation may be read more closely 
than is possible where vertical angles are determined only to the 
nearest minute. Moreover the simplicity of the process elim- 
inates many of the chances of error which are incidental to the 
use of other methods and gives final results in minimum time. 
The use of the arc is, however, limited to sights which involve 
the reading of the stadia rod, and for most shots’^ it holds the 
rodmah on the station longer than is necessary with certain other 
methods. 
' If it is desired to use the Beaman stadia arc principle with 
an instrument not regularly equipped for such work,, the ordi- 
nary vernier arc may be used by reference to the following table, 
which is also of use in checking the action of the Beaman scale. 
It is computed from the formula: — vertical distance = i sine 
of twice the vertical angle, and gives values by which the Bea- 
man intervals can be translated into angular valuations and 
vice versa. 
S. Stebinger gradienter drum. The accuracy of a sensitive 
bubble vial in the striding level is greater than that implied by 
the reading of the vertical angle only to even minutes. The 
fine adjustment tangent screw is so threaded^^ that a complete 
Topographic Instructions of the U. S. Geol. Survey, Washington, Gov. 
Printing Office, 1918, pp. 131-2. 
The intention of the makers commonly is to calculate the pitch of the screw 
and the length of the clamp arm so that one complete revolution of the screw 
head moves the line of sight 1 foot vertically at a horizontal distance of 100 feet, 
but this ratio may not be safely depended upon except as a broad approximation. 
