STANDARD - Survey Instrument Antique Center!

63
THE DUMPY LEVEL.
(For description and cuts see pages 123 to 129.)
1
.
Adjusting
Two-Peg Method.
A theoretically perfect dumpy level has the same points established that are
mentioned under the head of wye level; but since its construction differs from the
wye level, the methods of adjustment are not so convenient, resembling closely the
adjustment of the transit telescope and its attached level. After attaching the sunshade
remove parallax by pointing the telescope to the sky, and turn the eye-piece un-
til the cross hairs are sharp and distinct. Since the eye itself may have slightly
accommodated itself to the eye-piece, test the adjustment by looking with the unaided
eye at some distant point, and while still looking, bring the eye-piece of the telescope
before the eye. If the cross hairs are sharp at the first glance, the adjustment is made.
Now focus in the usual manner upon any object, bringing the cross hairs and image to
a sharp focus by the rack-work alone. A point should remain bi-sected when the eye
is moved from one side of the eye-piece to the other.
To place the horizontal cross wire at right angles to the vertical center bi-sect
some well-defined object such as a chimney top, top of roof, fence-rail or window-sill
(the best views are against the sky for a background) and move the telescope on its
vertical center. If the horizontal wire bi-sects the point throughout its entire field of
view it is adjusted.
If it does not slightly loosen the four capstan-headed adjusting screws (in the
inverting telescope those nearest the eye-piece or the further set from the eye when
the telescope shows objects erect) and turn the wire diaphragm until the selected
point remains bi-sected when the telescope is moved in azimuth throughout the entire
field of view.
To adjust the level, bring the level over two of its foot screws, and bring the
bubble to the middle of its tube by means of the foot screws alone. Revolve the
instrument 180 in azimuth, and if the bubble remains in the middle it is adjusted,
if it does not, then correct half its deviation by the capstan-headed adjusting screw
of the spirit level, and the remaining half by the two foot screws. Repeat the operation
over the other two screws, until the instrument may be revolved in any position,
and the level bubble will remain in the middle of its tube.
To adjust the horizontal wire so that the line of sight will be parallel to
the spirit level, one method is to use a sheet of water, or where that is not available,
two stakes which are driven with their surfaces in the same level plane.
To make the adjustment with the stakes, set up the level half way between two
points lyin? very nearly in a horizontal line, and say 300 feet apart. Drive a stake
at one of these points, place the rod on it and take a reading, first bringing the bubble
to the middle of its tube. Point the telescope in the opposite direction, again bring
the bubble to the middle of its tube, and drive a second stake at the second point
selected until the rod held upon the second stake gives the same reading as when
held upon the first stake. The tops of these two stakes now lie in the same Level line.
Take up the level and set it outside in line as near as it can be focussed on the first
stake and level up. Now read the rod upon the first stake, and then upon the second. If
the two readings agree, and the bubble is in the middle of its tube, the collimation is correct.
If the two readings do not agree, change the horizontal wire* to read the same
on the distant rod by means of the capstan-headed screws near the eye-piece in the
inverting telescope and furthest from the eye-piece in the erecting telescope. Refocus
on the nearest rod, take a reading, then focus on the distant rod and again by means of
the capstan-headed adjusting screws, make the horizontal wire read the same. Repeat
this operation until both rods read the same, with the bubble in the middle of its tube.
The telescope and uprights are in a single casting, which is finished and fitted to
the level bar, so that the line of collimation may be permanently parallel to it.
The dumpy level will then be in adjustment, since the adjustment of its vertical
cross line is of no importance.
attached level of
Adjustment of the Dumpy Level 1 and
Transit Telescope.
Two-Peg Method.
The following method is simple, direct, and geometrically accurate, requiring no
approximate measurements from a peg to the center of lens, no trial setting of the tele-
scope, no trials to drive a peg just enough and not too far, and no auxiliaries except
level-rod and tape or chain.
Contributed by Prof. R. Fletcher, Thayer School Dartmouth College.
* See page 58.