SURVEYING 



43 



by an instrument. A leveling rod which has a sliding disk 



or target which may be raised or lowered until the center lies 



in the line indicated by the leveling instrument, is called a 



target rod. A rod which may be read from 



a distance or from the leveling instrument 



is a speaking rod. 



Leveling rods are graduated to feet, and 



tenths and hundredths of a foot. In work 



requiring extreme care, the target may be so 



made as to be read to one-thousandths of a foot. 

 Bench marks . are permanent objects 



whose elevations are known or assumed, 



and which may be used as reference marks 



fcr the elevation of other points. 



The Plumb Line. The plumb line is per- 



haps the simplest and most generally used 



of the leveling instruments. Even the most 

 expensive instruments use the 

 plumb line to locate the instru- 

 ment directly over a given point. \ n g 'fods^' th e ev one 

 Provisional levels may be taken ISnSJJSni rod! 

 by means of a combination of the Ne\7 n York S ; and 

 plumb line and steel carpenter's is n a e S p e n ak J 

 square, and the difference in * 



the elevation of points not far 

 apart may be thus obtained. This instrument 

 may be used not only in laying drains but 

 also in road construction to determine the grade 

 of the road and the slope to the side ditches. 



The U Tube or Water Level. This instru- 

 ment depends upon the principle that a liquid " seeks its 

 level." It consists in two glass tubes fastened vertically 

 about three feet apart on a suitable arm and connected with 



