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 -f 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 
