MANIPULATION OF THE TELESCOPIC ALIDADE 
105 
lug which fits into a circular hole bored in the base plate near 
its margin. The centers of these holes are in a line parallel to 
the fiducial edge, and the short bars are of equal length. There- 
fore the removable straight-edge, when put in place, is parallel 
to the fiducial edge and may be shifted close to it or out away 
from it without losing its parallelism. In ^Tining-in’^ a distant 
station, all the time involved in getting the fiducial edge exactly 
on the occupied station point is saved. The line of sight is 
mechanically shifted in parallel position to the proper place on 
the map. Similar holes may be bored near the left side of the 
alidade base, and the same straight-edge attached there for work 
close to the left margin of the table, but if that change is made, 
the table must be re-oriented because of the probable change in 
the relation between the line of sight and the ruler edge. The 
parallel rule, when detached, may be conveniently carried in a 
pocket sewn across one edge of the alidade case. 
The complete instrument weighs about 3 pounds, stands 
between 3 and 4 inches in height, and is 10 to 12 inches in length. 
It is fitted into a sole-leather case with shoulder strap for carry- 
ing in the field. 
MANIPULATION OF THE ALIDADE 
To determine hearing 
In traversing with plane table and alidade, the location of an 
unplotted point is determined by its direction and distance from 
another point which has previously been plotted on the plane 
table sheet. Instead of determining this direction in terms of a 
compass or angle reading, to be later drawn on the map in the 
office, as is done with transit or theodolite, the determination 
and the plotting of the bearing to the distant point are accom- 
plished by a single operation. This makes necessary the accu- 
rate orientation of the plane table sheet so that the lines drawn 
on it will, whenever the plane table is set up for use in the field, 
be parallel to the position which they occupied at every preced- 
ing set-up. 
