lo Dec, 1910.] 



Irrigation. 



765 



comprehensible, that it is of no importance what the height of the instru- 

 ment may be. The difference in readings will be the same in any case. 

 Then, again, the shifting of the level from one point to another has been 

 a stumbling-block, which the following example is designed to explain. 



19. METHOD OF TAKING A SERIES OF LEVELS. 



The etched line represents the earth's surface, as would be seen by an 

 observer located in a railway or other cutting. The datum line or line of 

 reference is made 100 feet below the surface at A. With the instrument 

 set up at X between the points A and B and carefully levelled the dotted 

 horizontal lines represent the lines of sight, parallel with the datum line, 

 consequently if a reading of the staff at A were 2 ft. and at B 5 ft., B is 

 lower than A by 3 ft., and the surface of the earth at that point is 

 100 ft. — 3 ft. = 97 ft. above datum. The instrument is then moved 

 forward to y, and a reading is taken at B on exactly the same spot as the 

 previous reading and is 2 ft. ; the reading on C is then taken and found 

 to be 6 ft., consequently C is 4 ft. lower than B and the surface at C will 

 be 97 ft. — 4 ft. = 93 ft. above datum. Move the instrument forward 

 again to z and the readings on C and D are respectively 7 ft. and 4 ft., 

 consequently D is 3 ft. higher than C and the surface at D will be 

 93 ft. -I- 3 ft. = 96 ft. From the drawing it is now obvious that C is 

 7 ft. and D is 4 ft. lower than A, and that the system may continue to 

 infinity. The vertical lines shown intermediate to the points A B C D may 

 be any distance apart, according to irregularities of natural surface, 

 hut are generally i chain in flat country, and when levels are required 

 at these points, or any other points in the same series, according to the 

 degree of minuteness with which it is required to determine the surface, 

 they are all entered in the " Intermediate" column of the level book as 

 shown in the specimen given below, and their respective R.L.'s ascertained 

 by successively adding " Ri.ses " and deducting " Falls" from the original 

 '•' Reduced Level " 100.00. 



