Improved Agricultural Drainage Level. 



171 



for the purpose of ascertaining whether 

 draining be practicable or not, the Level 

 Book need only contain the two columns, 

 third and fourth, the difference of whose 

 sums will be the difference of level of the 

 two places. No distances need be mea- 

 sured. If, after this trial is made, draining 

 is ascertained to be practicable, determine 

 the best position and direction for the 

 projected drain, then begin at the outlet 

 and take a new set of observations, pro- 



ceeding as in the preceding example. 



It is recommended that the levelling 

 staff be divided into inches and numbered 

 as such ; it is more convenient for cal- 

 culation. The final results can readily be 

 reduced to feet if necessary. 



To adjust the line of sight, or Collirnation 

 of the Telescope. — The line of collirnation 

 is a line passing through the centre of 

 the object glass and the middle of the 

 horizontal line near the eye end. 



This line of sight should be parallel to 

 the horizon, when the spirit bubble rests 

 in the centre of the opening. To ascertain 

 this, select a piece of ground as nearly level 

 as you can find, drive in two short stakes 

 even with the surface, at the distance of 

 about 60 yards from each other. 



Place the level exactly in the middle 

 between the two B. C, adjust it carefully, 

 and read off the staff at both the stations, ^ 

 as directed in the first example of level- 

 ling. Suppose the reading at B, 60 at C, 

 58J inches, the difference is lj^ inches 

 which C is higher than B. Remove the 

 level to B, and place it so that the object 

 end is directly over the stake, point the 

 telescope to the staff on C, and adjust the 

 spirit bubble carefully to the centre of the CP 

 opening, then measure the height of centre 

 of object glass above B, which suppose 

 58 inches ; read off the staff on C, which, as C is higher than B 

 by 1J inches, if the collirnation is correct, will be 56^ inches. If 

 is more, the line of sight points too high, if less, too 

 If too high, unscrew the lower screw c of the level, and 



the reading 



low. 



