STANDARD - Survey Instrument Antique Center!

196
Different Types of Vertical Arcs and Circles
for Mine Transits, etc.
The regular arcs and vertical circles shown in the Engineer's and Surveyor's Tran-
sits No, Ib, page 153, No. If, etc., commend themselves for their simplicity of style,
accuracy of graduation and ease of reading. The latter 'feature is particularly well
attained in the above instruments where the double verniers are situated between the legs of the
standard, where they are well protected from injury and can be read simultaneously with the level
attached below.
In mines and underground work, where often the Transit must be set up in cramped places
and on stages erected in shafts, the difficulty of reading the vertical verniers without stepping aside,
or without shifting the horizontal plate, becomes apparent. To improve these conditions and in
order to obtain compactness the vertical arc in the older types of instruments, used extensively in
the coal mines of Pennsylvania, is permanently screwed to the side of the standard. It is of larger
diameter, and has a movable vernier arm. In other types the verniers are placed at the sides, as
exemplified in No. 6d, page 193, etc., or the graduations are placed on the edge of the vertical circle,
which latter type embodies, however, a great deal of mechanical refinement.
All of these types have advantages and disadvantages, and therefore should be chosen simply
with a view to attain highest efficiency of an instrument intended for special work. It will hardly
be commendable to put the most refined style of vertical circle (requiring a more careful treatment
not to speak of its attendant greater cost to make and keep in repair) upon an instrument
intended for the more ordinary purposes, while in changed conditions all these refinements may
become necessary to obtain maximum efficiency under trying circumstances.
To enable to make the proper selection for the various instruments the different styles are
given below.
The Pennsylvania Arc.
This arc is of larger diameter than
usual but is not strictly of high accuracy.
It differs from the regular arc
in that it is permanently screwed
to the standard, and that its movable
vernier arm can be readily set at zero
and clamped to the cross axis with the
telescope in any position, enabling to(
read vertical angles, plus or minus
from the horizontal plane to about 50
to 60 as well as to read also vertical
angles between any two points, when
by clamping the vernier arm at for
first point and then pointing the telescope
at second point the angle can be
read from of graduation.
Code Word.
Eleusine.
This arc can be attached, without extra cost, to any Transit of size and style No.
1 and No. 2, in place of a regular arc enumerated with instrument. Made to order only.
Movable Arc.
(Patented.)
This is an improved form over
the Pennsylvania arc in so far as it
is of more accurate design though
similar in manipulation. The arc
is screwed to a hub which is revolvable
on the cross axis of the telescope
and is so arranged that the
hub can bt readily clamped with
telescope in any position and without
straining the arc. During non-use the arc can be clamped to the vernier frame and therefore,
unlike the regular arc, never projects above the standards when the telescope is in reversed position.
The vernier can be set to read zero by means of a tangent screw.
The mechanical arrangement of the various parts of this device is, however, more complicated,
as will be seen from the cut, and on this account it is not only more expensive to make but is mon,
liable to derangement as compared with the Pennsylvania arc or the regular arc (page 153) fixed
to the cross axis of telescope. Made to Order only.
Price extra (above price enumerated for regular arc)