UEXKIIAL METHODS <:' KXCAVA i K>N 



35 



tally and at right angles to the vertical renter plane of the 

 tunnel the distance designated on the horizontal lines of the 

 drawing, he will have located points on the profile of the sec- 

 tion, or in other words have established the limits of excava- 

 tion. 



In the excavation of the Croton Aqueduct for the water 

 supply of New York city, an instrument called a polar pro- 

 tractor was used for determining the location of the sectional 



FlO. 17. Polar Protractor tt>r Poterminlng Profile of KxcTtsl CltMi Beetlon. 



profile. This instrument consists of a circular disk graduated 

 to degrees, and mounted on a tripod in such u manner that 

 it may be leveled up, and also have a vertical motion and a 

 motion about the vertical axis. The construction is shown 

 clearly by Fig. 17. In use the device is mounted with its 

 center at the axis of tli- tunnel. A light wooden measuring- 

 rod ta|tcnii'4 to a point. >ln>d with brass and graduated to feet 

 and hundivdths of a foot, lies UJNUI the wooden arm or rest. 

 which revolves upon the face of the disk, and slides out to 



