STANDARD - Survey Instrument Antique Center!

51
Thus, the telescope being leveled, the gradienter screw was turned through a
space of 11"T' 23 dm required the arc :
11 revolutions, = 3 9' 10"
20 divisions, =0 6 50
3 =010
17' 00"
The whole arc, . . . =3
Conversely, it was desired to turn off a vertical angle of 4 35"
Then we have
4 0' 0" = 13'"- 48 diT -.0
30 = 1 37 .0
50= 15 .0
40 = 2 .0
rev -
The space on the head of the screw= 16
diT -
2 .0
The engineer will bear in mind that the examples given are purposely given in
be mental ones.
detail : that in practice the operations may
It will be seen that the vertical gradienter can be used for a variety of purposes ;
measuring distances, grades, differences of levels, vertical angles, and is a useful
check against errors of rod or chain measurement.
Messrs. C. L. Berger & Sons have also applied the same principle to their horizontal
tangent screws. By graduating a silver head attached to these screws subdivisions
ot one minute of arc are readily made.
For constant use with these screws it is better to have a rod with two inovafclt
targets, or a rod painted with white and black squares as used in the coast survey.
Stadia Lines
The gradienter screw is so universal in its application and can be so readily used
for angular, distance or grade measures, that it will generally be found best to have
it upon transits designed for current work. There are some cases however where
stadia lines are more expeditious in use than the gradienter screw, and give quite
as exact results.
Stadia lines, for instance, where an instrument is to be used for distance measures
alone, commend themselves for their greater simplicity. For such work, non-adjustable
lines, in connection with an inverting eye-piece, give the best results. If the
lines are adjustable, in the field usage of an instrument they may alter their distance
apart and there is a ; rapidity of work with fixed lines, and a rod graduated for
telemetrical work, which is not reached in any other way.
These lines may be webs, or platinum, or they may be ruled on glass. The latter
are extremely accurate, but the use of them is necessarily limited in the telescopes
of field instruments for the following reasons : thin as the glass may be on
which the lines are ruled, and intercepting only a small amount of light, yet the film
of dampness and dirt soon collecting on it will intercept a great amount of light
which in time may become a very serious impediment in the use of the telescope.
Another objection to their general adoption consists in the fact that as the image
of an object is focussed in the plane of these glass-lines, a portion of the light of the
image will become reflected from the polished surfaces of this glass, causing at
times a disturbance in the clearness of vision. Besides, this glass-" micrometer,"
as placed in most telescopes, is very difficult of access and must needs be removed
for cleaning, thereby increasing the liability of becoming broken, or detached from
its mounting.
Section
showing the
Diaphragm
natural Size.