STANDARD - Survey Instrument Antique Center!

C.
1-28
BEKGEU & SONS.
Monitor Type Dumpy Level.
Additional Information pertaining to its mechanical Construction.
From the illustrations, pages 127, 129, it will be seen that the mechanical parts of
this Dumpy Level are few and can easily be made to be mechanically correct, and that
there are no working strains whatever in the metal to exert an undue influence upon the
adjustments with changes of temperature.
The telescope barrel and both uprights are cast in one piece of hard composition
metal; and in order to arrive at a high degree of accuracy the barrel is bored out to be
truly cylindrical, a condition never attained by the use of drawn tube. (For reasons
mentioned later both ends of the outside tube are slightly larger in diameter, forming
collars turned truly concentric to the bore, serving in principle the same object as collars
of a Wye Level telescope.) This being accomplished, the bottom surface of the uprights
is turned truly parallel to the bore. The strongly-ribbed cross-bar and instrument center
are cast in one piece of hard bell-metal. At the time when the center about which the
instrument revolves is fitted to its socket, the resting-places for the uprights are also turned
off so as to be truly at right angles to it, from which follows that the geometrical axis of
the telescope barrel when latter is placed upon them must also be at right angles to the
center. The level casing, too, is a casting. The spirit-level itself is fastened into this cas-
ing by a superior method to preclude any strain, so that its true form may "be preserved,
The focusing slide is the only tube made of brass, turned and closely fitted in the lathe.
In adjusting this instrument in the shop it is treated like a Wye Level. The collars
at the end of the telescope barrel serve to adjust the cross-wires for collimation by revolving
the telescope in wyes. When this is accomplished the telescope barrel with its attached
level is then firmly screwed to the cross-bar. The next step in the shop is to adjust the spiritlevel
to the line of collimation as described elsewhere. This being done, the adjuster
in the shop now proves whether the geometric and optical axes of the telescope are really
in the horizontal plane by revolving the instrument 180 on its center; should the bubble
remain in the middle of its tube it must be assumed that it is; however, if not, he touches
one of the uprights off until it does. As a rule the mechanical work is so correct that
the geometric and optic axes coincide within a few division marks of the graduated level,
requiring but a very few strokes with a fine file for final adjustment.
From the foregoing it will be seen that a leveling instrument so constructed, barring
severe accidents, must hold its adjustments for years, and that all subsequent verifications
of the line of collimation in the field by the two peg method must be made by the Engineer
by slightly moving the cross-wires, and that the adjustment of the spirit-level is
to be made in the customary manner by simply turning the instrument 180 on its center.
An instrument so constructed needs but little care and therefore is better adapted to
rough usage (to which it is subject at times) since its simplicity ensures greater freedom
from derangement.
The Dumpy Level described above must stand as an example of good practice.
Many Engineers prefer it to an ordinary Wye Level. The prevailing mistrust can
generally be traced to the use of cheap commercial Dumpy Levels. The above information
as to the method of construction in the shop has been given at great detail tr eho\v
that this instrument may well rank with the best wye levels.
(For Price of this Instrument and Extras see page 126.)
Berber's Collapsible Dust Guard
This feature as shown below is furnished with the 14, lo and 1 7>2 inch 3Ioiiitor
Type Dumpy Levels and protects the telescope's focussing slides almost completely
from grit and water within their entire range.
TELESCOPE BARREL-
C.L.BE.RGERiSON3 BOSTON MA5S.U.5.A.
EXTENDED CONTRACTED