THE LEVEL 89 



adjusting screws. When the two axes are in the same 

 plane, the bubble will remain in the center while the 

 telescope is revolving. 



(c.) Make the level tube parallel to the line of sight. 

 This may be done in two ways. The first or indirect 

 method is as follows : 



Clamp the instrument over a pair of levelling screws ; 

 then bring the bubble to the center of the tube, lift the tele- 

 scope out of the wyes, turn it end for end, and set it down 

 in the wyes again. The eye end now is where the objective 

 was originally. This operation must be performed with 

 the greatest care, as the slightest jar of the instrument will 

 vitiate the result. If the bubble returns to the center of the 

 tube the axis of the tube is in the correct position. If it does 

 not return to the center, the end of the tube provided with 

 the vertical adjustment should be moved until the bubble 

 moves half-way back to the center. This test must be 

 repeated to make sure that the movement is due to defec- 

 tive adjustment and not to the jarring of the instrument. 



For the second, the direct or peg adjustment, select the 

 points A and B, say 200 feet apart. The distance need not 

 be measured. Set up the level close to A so that when the 

 rod is held upon it the eyepiece of the telescope will swing 

 within about half an inch of its face. Bring the bubble to 

 the middle of the tube and looking through the telescope 

 wrong end to, put a pencil mark on the rod at the center 

 of the small field of view. Note the rod reading thus ob- 

 tained. Then turn the telescope toward B and take a rod 

 reading in the usual way, making sure that the bubble is 

 in the middle of the tube. The difference between these 

 two rod readings is the difference in elevation of the two 

 points + or the error of adjustment. Next take the 

 level to B and repeat the above operation. The result here 

 gained is the difference in elevation or + the error 

 of adjustment, and the mean of the two results is the differ- 

 ence of elevation between points A and B. Now, knowing 

 the difference between A and B and the height of the in- 

 strument above B, the rod reading at A which will bring 

 the target on the same level as the instrument may be com- 

 puted. With the horizontal cross-hair on the target, the 



