S8 



ENGINEERING FOR LAND DRAINAGE. 



ment to a point beyond C, and obtain the elevation of 

 D in the same way. The points upon which two read- 

 ings are taken are called turning- points. All others 

 except bench-marks are called intermediates. Pegs 

 should be driven upon which to make turning-points. 

 These are frequently called **hubs '' in practice. By 

 carefully observing the figure here given, and compar- 

 ing it with its accompanying notes, the routine of simple 

 levelling can be clearly comprehended by the reader. 

 The column of elevations shows at a glance the com- 

 parative height of every point taken with reference to 

 the datum used. 



It will now be understood how levelling is simply 

 finding how much higher or lower one point is than 

 another. To insure correct results the instrument 

 should be in good adjustment, rod readings should be 

 taken correctly and entered accurately in the notes, the 

 book work should be correctly carried out, and not the 

 least irhportant matter to be observed is that the in- 

 strument should be set about equally distant from both 

 turning-points. 



The notes may be proved, first by reviewing carefully 

 all of the additions and subtractions, and second by 

 adding the column of foresights and the column of back 

 sights. Take the difference of these sums, and if it 

 equals the difference of the elevations of the points com- 

 pared, the work on the book is correct. 



LEVEL NOTES TO ACCOMPANY PIG. H. 



Sta. 



BS. 



S.IO 

 3.70 



s 40 



H.L 



15.10 

 16.00 

 19.20 



P.S. 



Elev. 



A 



B 



C 



D ^. 



' 2 ! 80 ' 

 2. 20 

 4.25 



10.00 

 12.30 

 13-80 

 14.9s 









