SURVEYING 47 



instruments. There is more than one method of making 

 certain adjustments, but only one will be explained here. 



As has been stated, there are three elements in a level 

 which should be kept in proper relation: namely, the vertical 

 axis, or the line about which the instrument can be rotated ; 

 the bubble axis, which is a level line; and the line of sight. 

 The last two must be parallel, and the first perpendicular to 

 both. If the line of sight be inclined upward, it is obvious 

 that all rod readings will be too great, and the error will be 

 proportional to the distance of the rod from the level. If the 

 line of sight be inclined downward, all readings will be too 

 small. If the length of sights, or the distance between the 

 level and the stations, be equal in making front and back 

 sights, the error in each case will be the same, and the rela- 

 tive elevation of the stations will be obtained without error. 

 For this reason it is desirable to make fore sight and back 

 sight distances equal. 



The adjustment making the vertical axis of the level and 

 the bubble axis perpendicular, is a matter of convenience, 

 for this will cause the line of sight to describe a plane con- 

 taining all the level lines through the instrument. This 

 means that it will not be necessary to change or "level up" 

 the instrument in sighting in different directions. 



First Adjustment To make the vertical axis of instru- 

 ment perpendicular to the bubble axis: 



Adjust the bubble tube to the vertical axis as follows: 

 Level up the instrument, bringing the bubble to the center 

 of the tube, turn the telescope through 180 degrees, 

 and, if the bubble changes position, raise or lower the 

 adjustable end of the tube until the bubble is brought half 

 way back to its former position. Level the instrument 

 again and repeat the operation; and if the bubble moves in 

 the tube, make further adjustments. Continue this process 



