THE LEVEL 



91 



a point whose elevation is known or assumed, the reading 

 that is obtained is called a (+) or backsight. Similarly, 

 a reading on a point ahead or unknown is called a ( ) or 

 foresight. A point occupied by the rod in this way, but 

 not recorded or used further, is called a turning-point. 

 When two points have been connected by a series of read- 

 ings of this kind, the sum of the backsights minus the sum 

 of the foresights gives the difference in elevation. If the 

 backsights are greater, the second point is the higher of the 

 two. If the foresights are greater, it is the lower. A brief 

 set of notes is given and worked out illustrating this 

 matter. Work of this kind is called differential levelling. 



When levelling is employed to get the elevation of a 

 large number of points in a region, several or many fore- 

 sights may be taken from one position of the instrument. 

 It is customary then to note the height of instrument, and 

 the elevation of any point observed will be that height 

 less the foresight to the point. 



A benchmark is a point whose elevation has been deter- 

 mined and which is marked and left for reference. It is 

 noted B. M. in level notes. 



The following set of notes illustrates those commonly 

 kept in running profiles of a road or railway. The form 

 may be easily modified for any other class of work. 



Summary. Levelling is comparatively simple work. 

 Even though a level is somewhat out of adjustment, accu- 



