122 
KIRTLEY F. MATHER 
rod, with level sight. The vertical distance can be determined 
later by addition or subtraction. 
2. The step method.^^ The same principle may be extended to 
cover a much larger range of circumstances. Suppose the rod is 
so far below the elevation of the alidade that with level sight all 
three hairs project rays slightly above the top of the rod. Note 
where the middle hair intersects any fixed object in the field — a 
point on a nearby tree, or a certain rock on the distant hill-side. 
Turn down the instrument until the top hair intersects the 
Fig. 8. Stadia Rod in Field of View 
WITH Level Telescope at Dis- 
tance OF 1050 Feet 
The base of the 13 foot rod is 14.8 feet 
below the elevation of the alidade. 
Fig. 9. Stadia Rod in Field of View 
WITH Level Telescope at Dis- 
tance OF 1960 Feet 
The base of the rod is 8.0 feet above 
the elevation of the alidade. 
now appears to be 1/100 the 
same object. The bottom hair 
horizontal distance, previously determined by the stadia inter- 
cept on the rod, below the point where the middle hair had for- 
merly been. If the bottom hair now cuts the rod, read its inter- 
Douglas, E. M., The stadia and stadia surveying. ‘Engineering News, vol. 
63, pp. 483-484, 1910. Meyer, A. F., The ‘‘interval” method of determining 
elevations in stadia surveys. Engineering News, vol. 64, pp. 231-232, 1910. 
Edgerton, H. H., Jr., Modern methods of economical railway location. 
Engineering and Contracting, vol. 41, pp. 229-232, 1914. 
