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. 
B.S. 
F.S. 
Remarks 
9.52' 
4.45' 
B.S. onto B.M. of previous 
survey. 
10.12' 
3.27' 
8.56' 
1.01' 
7.40' 
5.71' 
3.65' 
8.62' 
F.S. to pond level required. 
39.25' 
23.06' 
23.06' 
16.19' 
Pond is above B. M. 
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- 
