SURVEYING 51 



reading is called a back sight (B. S.), and if added to the 

 elevation of the bench mark will give the elevation of the 

 instrument, or the height of instrument (H. I.), as generally 

 designated. The first B. S. thus obtained is entered in the 

 notes in the second column, opposite the B. M. elevation 

 in the first. This B. S. plus the elevation of the B. M. is 

 entered in the third column under the head of height of 

 instrument, or H. I. 



Thus if the elevation of the B. M. be assumed as 10.00 

 feet, and the B. S. reading of the instrument on this point 

 be 6.50 feet, the H. I. will be 16.50 feet. 



Now if the instrument be turned so as to extend the line 

 of sight in the direction of the first point in the line of levels 

 (Sta. A) and a reading be taken in the same way, the reading 

 on the rod will be the distance of the elevation of this point 

 below the line of sight. The reading is called a fore sight 

 (F. S.), and is entered in the fourth column opposite Station 

 A., on which the reading was taken. If this fore sight read- 

 ing be subtracted from the elevation of the line of sight 

 (H. I.), the elevation of Station A will be obtained. For 

 instance, suppose the F. S. reading thus obtained is 4.10 

 feet, then H. I., 16.50 feet, minus the F. S., 4.10 feet, equals 

 12.40 feet, the elevation of Station A, which is entered in the 

 proper column opposite Station A. 



To continue the line of levels, the instrument is moved to 

 a position midway between Station A, and Station B, 

 and, after the instrument is leveled, a B. S. reading is made 

 on Station A. This reading added to the elevation of 

 Station A gives a new H. I., from which the F. S. reading on 

 Station B is subtracted to obtain the elevation of Station B. 



Thus the process is continued until the elevations of all 

 the points in the line of levels are obtained. It is easy to see 

 how additional readings may be taken with the same height of 



