MANIPULATION OF THE TELESCOPIC ALIDADE 
141 
Fiducial edge. While it is not essential for the fiducial edge 
to be more than approximately parallel with the line of sight, it 
is important that this edge be straight. Draw a line against 
the straight edge and turn the alidade end for end. If the 
straight edge coincides perfectly with the test line, the require- 
ments are satisfied. 
CARE OF THE ALIDADE 
Like all instruments of precision the alidade must be handled 
with extreme care. Its metal parts are composed almost exclu- 
sively of brass, bronze, and ^^red metal,” materials which are 
not notably resistant to abrasion. Precautions should con- 
stantly be taken, therefore, to keep the bearings free from gritty 
particles. The instrument, for example, should never be placed 
on the ground or laid on a rock pile. If for any reason it must 
be removed from the plane table, return it to its leather case and 
close the case, tightly before depositing it anywhere. The 
bottom must be kept clean. The instrument in its case should 
at all times be protected against jar and shock. The habit of 
dropping the alidade to the floor of an automobile to be shaken 
around in transit with hammers, specimens, tire tools and other 
impedimenta is indefensible. Treat it with at least as much 
consideration as one gives to lunch-kit and thermos bottles. 
Once or twice a month, bearings, clamps and screws should 
be wiped clean with a cloth dampened in a light oil such as 
^^3-in-l.” The springs which play against the bearing studs on 
the opposite sides from the vernier and tangent screws should 
be removed from their housings, wiped clean, stretched a little 
and replaced. 
If the Stebinger drum is used in determining the elevations, 
the tangent screw must be treated with special care. Experience 
indicates that very trivial and unobtrusive things may change 
the relation of the screw to the arc sufficiently to make a Stebinger 
table no longer applicable and necessitate the construction of a 
new one. If possible, the gradienter screw should be entirely 
withdrawn every week or two and wiped absolutely clean with 
the oily cloth. The bearing plate stud against which the point 
