Use of Drainage Levels 471 



and there will be errors in deciding when it is set 

 exactly, whereas the water-level can never be in error, 

 and automatically adjusts itself at once, the only 

 chances for error being in taking the sights. Other 

 forms of drainage levels are represented in Fig. 153. 



LEVELING A FIELD 



If the field has but small fall, and is quite flat and 

 even, so that the inexperienced eye fails to detect the 

 direction of greatest slope, it will usually be safest to 

 check it into squares of 50 or 100 feet, driving short 

 stakes at the several corners, whose elevations may 

 then be determined. To do the leveling, set the 

 instrument at a, Fig. 155, midway between stations 

 1-1 and 1-2, having first provided a notebook, ruled 

 as indicated in the table below. Turning the level 

 first upon 1-1, its distance below the instrument is 

 read on the target -rod held upon that stake, and 

 the result, 4 feet, is recorded in the table in the 

 column headed "back-sight." The instrument is next 

 directed to 1-2 and its distance below the level found 

 to be 3.8 feet, which shows that its elevation must be 



4 ft. 3.8 ft. =.2 ft. 



above that of station 1-1. This reading of the target- 

 rod is entered in the column headed "fore -sight." In 

 the column headed " Elevation " the first station is 

 given arbitrarily a value of 10 feet, as is customary 

 to avoid minus signs, and on the same plan station 



