LEVELLING AND TOPOGRAPHY. 57 



Starting-point. Call this point A and write its eleva- 

 tion in the elevation column opposite station A, (See 

 Fig. II, and the notes to accompany the figure, both 

 being used to illustrate level practice.) Set the level 

 up midway between this point and the next point B 

 whose elevation is to be obtained. Have the rodman 

 hold his rod at Ay taking care to hold it vertically, take 

 the reading of the rod and enter it in the column of 

 Back Sights opposite station A, In the example it is 

 5.10. Add the back sight to the elevation of the point 

 A and we have the elevation of the line of sight through 

 the instrument or the ** height of instrument, " as it is 

 called, and indicated on the notes as H.I. This is 

 15.10. Enter it in the H.I. column opposite station 



A . Next take a sight on B, called a foresight, and 

 enter the reading in the F.S. column opposite station 



B. This in the example is 2.80. Subtract this read- 

 ing from 15.10, the height of the instrument, and write 

 the difference, 12.30, in the elevation column opposite 

 station B, This is the height of B with reference to 

 the starting-point A . If the elevation of other points 

 is desired before a change of instrument is made, take 

 as many foresights as wanted and obtain the elevation 

 of the points in the same way as just described. Next 

 change the instrument to some point beyond By and 

 take a back sight on station B, Add this reading to 

 the elevation of B to obtain the height of the instru- 

 ment in its new position. Enter the sum in the H.I. 

 column opposite B, In the example the back sight is 

 3.70, elevation 12.30, H.L 16.00. Take a foresight 

 on C and subtract the reading from 16.00, the H.L, and 

 obtain 13.80, the elevation of C Remove the instru- 



