124 
KIRTLEY F. MATHER 
error involved in the placing of one hair in the position previ- 
ously occupied by another. Moreover, there is no simple way 
of correcting the error resulting from the inclination of sight to 
the rod when the intercept is read. The ^^step’’ method should 
A 
ii> 
IS 
V 
E 
D 
C 
B 
A 
Fig. 10. The Step Method 
Circle A encloses the field of view as observed through the level telescope. 
Circle B is the field after the telescope has been raised so that the bottom cross 
hair occupies the position occupied in A bj^ the middle hair. C and D represent 
the second and third steps, in each of which the bottom cross hair rests in the 
position previously occupied by the top hair. In D, the third step, the top hair 
cuts the rod 0.5 feet above its base. The rod intercept, 16 feet, is indicated 
in E. The base of the rod is therefore 3 X 16 — 0.5 = 47.5 feet above the 
instrument. 
never be used when more than 6 steps are necessary, nor to 
determine the elevation of a turning-point or set-up. It is well 
fitted to serve as a check upon the more accurate methods next 
described, when there is need for especial care to guard against 
error. 
