MANIPULATION OF THE TELESCOPIC ALIDADE 103 
*ally expressed by the change which takes place in the inclination of the 
axis when the bubble moves over a single space. Thus in a 1 -minute 
level for a displacement of one division the inclination changes 1 min- 
ute, and in a 20-second level it changes 20 seconds.'^ 
Ordinarily, the alidades in use by geologists are fitted with 
60-second levels, but it is preferable where possible to use 
a 20-second level, a change which at low cost adds greatly to the 
possible accuracy of the work. 
The telescope and axis-sleeve are carried on standards about 
two inches high to permit a reasonable vertical swing of the 
telescope. The transverse axis projects beyond the bearings 
which cap these standards and is gripped on the right by the 
vertical clamp. This is tightened or loosened by the knurled 
screw, set close to the top of the right hand standard. When 
loosened, .the telescope swings freely through an arc of about 
45 degrees in the plane at right angles to the transverse axis. 
When tightened, the swing of the telescope is limited by the play 
of the clamp arm, a downward extension of the clamp proper. 
In one style of miniature alidade, this arm is held by a horizontal 
spring against the point of a horizontal tangent screw working 
through the lower part of the telescope standard. In another 
model this arm is expanded, and itself carries the tangent screw 
and horizontal spring bearing against a stud fixed to the inside’ 
surface of the right hand standard. In either, the rotation of 
the tangent screw" causes the telescope to be slowly elevated or 
depressed. ‘ The screw is therefore spoken of sometimes as the 
fine adjustment or micrometer screw. A graduated drum may 
be attached to this screw, as in figure 1, which as explained later 
may be used in the measurement of distances or the determina- 
tion of vertical angles. When thus equipped the tangent screw 
is known as the gradienter screw. Or if the special gradienter 
drum provided with a celluloid index, as suggested by Eugene 
Stebinger, United States Geological Survey, is attached, it is 
frequently referred to as the Stebinger drum and screw. 
^ J. C. Tracy, loc. cit., p. 544. 
