STANDARD - Survey Instrument Antique Center!

46
Obviously then, the number of whole revolutions made by the screw, in turning
the telescope through a vertical arc, can be ascertained from this scale. The clamp
arm of the telescope has its clamping screw just above the horizontal axis, in the
usual manner. When this screw is free, the telescope may be revolved ; but when
it is clamped, the telescope can only be moved by the gradienter screw, which thus
takes the place of the ordinary vertical tangent screw. The screw is cut with such
a value of a single revolution, as to cause the horizontal cross-line of the telescope
to move over a space of of a jjg foot, placed at a distance of 100 feet, when the
screw is turned through one of the smallest spaces on its graduated head and
;
since there are fifty such spaces on the head, it follows that one revolution of the
screw is equivalent to ^ of a foot, at a distance of 100 feet. The numbered graduations
on the screw head are then each equivalent to ^ of a foot in 100 feet and ; two
entire revolutions of the screw would be twice 55, or 1 foot to the 100. It is readily
seen that grades can be established with great rapidity with this screw. It is only
necessary after setting the gradienter screw to zero, and leveling and clamping the
telescope, to move it up or down as many spaces of the head ofthe gradienter screw
as there are hundredths of feet to the hundred, in the grade to be established.
Thus, to establish a grade of l ft
85, the screw head is turned through three whole
50, and through three of the numbered
spaces of the scale, which corresponds to l. ft
divisions, and five of the shortest ones to make up the entire reading of 1." 85.
For measuring distances this screw takes the place of stadia lines, and is more
convenient ; since for any approximately horizontal distance, the space on an ordinary
leveling rod expressed in hundredths of feet, included in two revolutions of
the screw, will be the number of feet the level rod is distant from the center of the
instrument. Thus the difference between two readings of the level rod was 2" . 965
when the telescope was moved in altitude through two revolutions of the screw.
The rod therefore was distant 296.5 feet.
It is unnecessary even that a leveling rod be used. A ranging pole or walking
stick, or any arbitrary length which can afterwards be measured, will suffice. Thus
a stick, which was afterwards measured and found to be 3". 38 long, was found to
be subtended by 3^ revolutions of the screw at an unknown distance.
In this case the distance was
i|| X 100 = 213 .9 feet.
1 58
In case, however, the distance to be measured is not approximately in the same
level plane with the transit telescope, it is necessary to compute the distance, from
the readings of the rod. In taking such readings at an altitude, it is customary to
incline the rod towards the telescope, and by trial find the least space subtended by
two stadia lines. A skilful rod-man will plumb a rod more readily than he can
incline it at the proper angle, and a reading of the plumb rod can be taken with
jrreater accuracy, and in less time than with the inclined rod ; but it ordinarily
involves some additional computing to reduce such vertical readings to horizontal