114 
KIRTLEY F. MATHER 
Long sights. Occasionally, it is necessary to measure distances 
greater than 200 times the rod length with an alidade equipped 
only with three stadia hairs. Several different methods of pro- 
cedure are available; the choice of the one to use depends upon 
the equipment of the instrument, the geographic environment, 
and the custom of the individual surveyors. Some of the meth- 
ods are in effect schemes to provide a longer rod. Others de- 
pend upon trigonometric principles. The method numbered 
5 is probably best suited to the more common conditions met 
in geologic mapping. 
1. Rotate the telescope 90 degrees in its sleeve so that the 
stadia hairs are vertical instead of horizontal. Signal rodman 
to mark his station with a flag or to select a certain sapling or 
post for his station. Place one hair on the station and signal 
rodman to move at right angles to the line of sight until the rod, 
held vertically, is in line with the other stadia hair. The rodman 
will then measure the distance horizontally on the ground from 
his second position to his first, using the rod as a measuring 
stick, and report the result to the instrument man. Distances 
of 4000 to 7000 feet may be determined with a fair degree of 
accuracy by this method. It may be used to advantage only 
when the two men are able to communicate freely by signals 
such as the two-arm semaphore code. 
2. Rotate the telescope as before. Signal rodman to hold rod 
horizontally and to be prepared to move laterally so that the 
base of the rod will occupy the point now occupied by its top. 
Intersect base of rod with one stadia hair. Signal rodman to 
move over in the desired direction a sufficient number of rod- 
lengths so that the other stadia hair will finally intersect the rod. 
Read intersection; add the number of feet indicated by the 
length of the rod multiplied by the number of moves; multiply 
by 100 or 200 depending upon which hairs were used. 
3. If instrument is equipped with a Stebinger gradienter drum 
attached to the fine adjustment screw, proceed as follows: Place 
bottom hair on lowest visible primary division of the rod; read 
and record Stebinger. Turn Stebinger drum until middle hair 
rests on the top of the rod; read and record Stebinger. Take 
