MANIPULATION OF THE TELESCOPIC ALIDADE 
111 
the customary scales represents an area on the ground, 30 to 60 
feet in diameter. 
Lines representing bearings should be drawn with the chiseled 
edge of a 9-H pencil, being careful always to hold the pencil 
at the same angle and to see that the contact of rule and paper 
is perfect. By placing the ruler edge in a position tangential to 
the tiny circle formed by needle-hole or dot, the line when drawn 
should exactly cut the center of the point.’’ If the distant 
station is subsequently to be occupied and orientation there is 
to be by back-sight, the fore-sight line should be the full length 
of the alidade base; if not, the fore-sight line may be short, cover- 
ing only the estimated position of the point. It is better not to 
draw lines through the dot or needle-hole representing the oc- 
cupied station; break the line for a fraction of an inch on either 
side. 
To measure distance 
Distances are directly determined with the telescopic alidade 
by means of the stadia hairs and rod. Stadia work depends 
upon the hypothesis that the sizes of objects required to produce 
an image of fixed size in the telescope are directly proportional 
to their distances from the point over which the telescope is set. 
This hypothesis is not rigidly correct, but the theoretical error 
is small and the practical error negligible. The limits of the 
image in the telescope are fixed by the parallel stadia hairs in 
the reticle. The object most convenient to use is a graduated 
rod. In the alidades commonly used by the geologist, the stadia 
hairs are so adjusted that the ratio 'between the distance from 
the telescope to the rod and the distance intercepted on the rod 
by the upper and lower hairs, when the rod is held at right angles 
to the line of sight and to the hairs, is as 100 to 1. If the rod is 
100 feet from the instrument, the outer hairs appear to subtend 
1 foot upon it; if 1200 feet distant, 12 feet of the rod will appear 
between them (see figure 3) . Moreover, the middle cross hair in 
the reticle is placed as nearly as possible equidistant from the 
outer two. Therefore, the distance subtended on the rod by 
the middle hair and either outer hair is 1/200 the distance of the 
