132 
KIRTLEY F. MATHER 
ing the tangent screw is graduated into 100 divisions, so broadly 
spaced that the drum may be read accurately by estimation to 
0.2 division, and so quickly legible that there is marked saving 
of time and increased safeguard against error in observation 
when it is used in preference to the vertical arc. It is in reality 
simply another method of reading the vertical angle, denoting 
the angle by an arbitrary unit instead of by degrees and minutes. 
The value of that unit in length of chord at known distances 
may be expressed in tables similar to those provided for com- 
putation from vertical arc readings. 
In most instruments a clock-wise rotation of the Stebinger 
drum depresses the objective end of the telescope by pressing 
against a little stud fixed to the inside surface of the right hand 
standard. A counter-clock-wise rotation permits a spiral spring 
to expand against the opposite side of the stud and thus to raise 
the objective end of the telescope. Experience indicates that it 
is unsafe to trust the spring to act with uniform regularity and 
smoothness. It is therefore necessary in using the Stebinger 
method always to read vertical distances in one direction — 
usually downwardly — the direction in which the telescope is 
moved by clock-wise rotation of the drum. If the station is 
higher than the telescope, the first reading is taken with the 
horizontal cross-hair cutting the target; the telescope is then 
turned down to the level position for the second reading. If 
the station is lower than the instrument, the telescope is leveled 
for the first reading of the Stebinger drum and then turned down 
till the cross-hair cuts the target for the second reading. 
The fine adjustment screw to which the Stebinger drum is 
attached is a tangent screw; that is, its motion is tangential to 
the arc described by the arm of the clamp of the telescope axis. 
Therefore, a revolution of the screw, when it is near one of its 
limits of motion will elevate or depress the telescope through an 
arc slightly different from that resulting from an equal turn of 
the screw when it is midway between its limits. Therefore it is 
In some instruments the screw is fixed to the telescope stanclaid and the 
stud is attached to the arm of the clamp of the telescope axis; when so attached 
the direction of movement to be used in reading the gradienter is upward. 
