MANIPULATION OF THE TELESCOPIC ALIDADE 
133 
necessary always to begin an angle reading with the tangent 
screw in approximately the same position as that from which 
the determination of the Stebinger factors was made. This 
position, generally about a quarter turn of the screw after it 
first ^ Takes hold,” should be indicated by a mark on the cellu- 
loid or steel index. After each reading the tangent screw should 
be withdrawn to that position, ready for the next reading. 
In practice, then, the first reading is made with the Stebinger 
drum somewhere near the predetermined starting point and 
with the cross-hair on the distant object, if it is higher than the 
instrument or with the telescope level if the sight is a ^^down 
shot.” The reading, a figure between 0 and 100, is recorded in 
the proper column of the note book. The telescope is then 
turned down by means of the tangent screw to position for the 
second reading. As the Stebinger drum revolves the total 
number of revolutions should be counted. The count may be 
verified by the graduations on the index bar if present and is 
set down at the left of the two digits which indicate the Stebinger 
division, beneath the index. For instance, after completing 8 
revolutions, the Stebinger drum is brought to rest at 67; the 
second reading is therefore recorded in the appropriate column 
as 867. With an alidade which “reads down,” as is the more 
common arrangement, the smaller of the two Stebinger readings 
will be in the Sight” column if the target is higher than the 
instrument, and in the Level” column if lower than the instru- 
ment. The difference of the two readings expresses the size of 
the vertical angle in terms of Stebinger divisions. From the 
Stebinger tables prepared for the individual instrument the 
corresponding Stebinger factor” is selected. This factor multi- 
plied by the apparent distance gives the difference of elevation 
of target and plane table. 
The preparation of the Stebinger tables is essentially the 
determination of the value of Stebinger units in terms of circular 
measure. Withdraw the micrometer screw to the position from 
which determinations of vertical angles will be started. Set the 
drum on an even division and read the vertical arc; turn the 
drum through 100 divisions and read the arc again. Turn 
