480 STATE BOARD OF AGRICULTURE. 



(d) If the figures on the leveling rod do not appear suflSeiently clear 

 to be read by the person in charge of the level then the rod man must 

 raise or lower the target as directed by signs from the person in charge 

 of the level until the horizontal bisecting line of the target lies exactly 

 behind the horizontal cross-hair of the telescope, as seen through the 

 telescope. 



The rod man should now carefully tighten the set screw of the target, 

 and then read to the level man the height at which the horizontal bi- 

 secting line of the target crosses the face of the rod. This height the 

 level-man should carefully record. 



This reading is called the dacJc-sight. Backsight is the name always 

 given to the reading taken at a point whose elevation is known or as- 

 sumed and is always taken to determine the "height of the instrument." 



(e) Let us suppose the reading just taken to be 4,95 ft. This means 

 that the instrument is 4.95 ft. higher than the point at which the rod 

 was held. Let us suppose also that the height of the point at which 

 the rod was held is known to be 11.35 ft. above datum. If now we add 

 4.95 ft. to 11.35 ft. we have 16.30 ft. as the height of the instrument 

 above datum. 



3. The height or elevation of mother points loithin range of the level 

 are determined. 



(a) The rod-man carries the rod to and holds it upon one of the 

 points whose height is sought. 



(b) The telescope is turned toward the rod in its new position and 

 focussed to bring out most clearly the figures on the face of the rod. 



(c) The reading is taken and recorded as in 2 (c) above. This read- 

 ing is called a fore-sight. Fore-sight is the name given to the read- 

 ing taken at a point whose elevation is to be determined. 



(d) Let us suppose that this fore-sight reading is 4.22 ft. It means 

 that the point at which the rod was held and whose elevations is sought 

 is 4.22 ft. lower than the instrument — i.22 ft. nearer the datum plane 

 than the instrument. 



If then we subtract 4.22 ft. (fore-sight) from 16.30 ft. (the height of 

 the instrument) we get 12.08 ft. as the elevation of the new point. 



In like manner the rod should be placed at other points within the 

 range of the instrument, and fore-sight readings taken. In each case 

 subtracting its fore-sight reading from the height of the instrument 

 gives the elevation of the point. I^et us suppose two other fore-sight 

 readings are taken and that these two readings are 3.75 ft. and 3.06 

 ft. respectively. 



RECORDS. 



Every reading should be accurately recorded in its proper place in 

 a table provided for the purpose. If it is desired merely to find the 

 elevation of several points, the form of table given below will serve the 

 purpose. 



