STANDARD - Survey Instrument Antique Center!

142
THE GEODETIC LEVEL.
In response to a request of President T. C. Mendenhall to construct ir the
Worcester Polytechnic Institute a Precise Wye Level, the senior member of this firm
in 1896 designed and made the type shown on next page.
The leading features are great compactness, rigidity, simplicity of design, ease of
manipulation, and thorough adaptation of every part to its purpose. In order to lessen
the height above the tripod, the weight of the instrument and the surfaces exposed to
wind pressure, this particular form of cradle bar has been adopted ; and, while this
reduction might have been carried to a greater extent by placing tae vertical revolving
center inside the tripod head, it was thought not advisable, as sometimes it is desirable
to set the instrument on the leveling screws when detached from the tripod.
As will be seen, the improved tripod is of a very stiff form, which is necessary
where telescopes of great power and highly sensitive spirit levels are used. To eliminate
the effect of unequal expansion, the telescope collars are of smallest possible diameter
compatible with the diameter of the object glass, and at first were of hardened steel
resting on agates at point of contact in wyes. The striding level adopted in place of
the fixed level is of tubular form and has very short legs. To still further reduce the
effect of unequal expansion, the substructure, such as cradle bar, fixed bar and other
parts, below the agates at point of contact in wyes in this instrument, consisted of steel
and iron. Subsequently, however, as the danger of rusting in the field became very
apparent, the use of steel and iron was not thought to be as important in portable field
instruments when used on tripods as in the stationary astronomical instruments : there-
fore we are making the substructure of our customary hard gun-metal, and shall so
furnish them, unless ordered to be of steel, in which ca.se the instrument will have to
be specially made. (See below.) The same may be said of the hardened steel collars.
Unless ordered otherwise the collars will be made of hardest bell-metal.
All the parts that must be handled during a field operation are protected by a
shield of a non-heat-conducting material. By means of a micrometer screw the telescope
can be moved in the vertical plane around the center in the middle of the instrument
in order not to disturb the height of the intrament.
The striding level has a bubble tube reading to 3'' of arc and is provided with an
air chamber. Mounted above it is a mirror to enable the observer to read the bubble
without stepping aside. Provision is made to readily lock the striding level to the
cradle bar, to enable one to use the instrument like an ordinary Wye Level (with level
fixed to the telescope) ; and to carry the instrument on its tripod from station to station.
An arrangement is also provided by which the striding level upon reversing will always
find its proper plane on the collars, so as to require but very little attention on the part
of the operator. By means of a clamp screw opposite the micrometer screw the cradle
bar can be secured so that the instruments can be used for ordinary wye level work.
Two auxiliary levels placed at right angles serve to level up approximately ; after
which the final setting of the striding level is done by the micrometer screw attached
to the fixed bar. The vertical center is of hardened steel and runs in a socket of cast
iron. All the main parts are either cloth finished or japanned. The beautiful appearance
of the instrument does not depend upon the external finishing and polishing of
parts, but entirely upon the harmony, simplicity and excellence with which the essential
features of the instrument are designed.
For a detailed description of the instrument we refer to a paper read by Mr. David
Molitor before the Am. Soc. of C. E. (See Proceedings 1899-1900.)
The telescope is inverting, with an object glass of 1^"? focal length of 17",
and a power of 40 diameters. It is provided with the usual cross and stadia wires.
Instrument packs in pine wood box, which contains a sunshade, screw driver, adjusting
pin and gossamer bag.
Weight of instrument ....... 14 Ibs.
Weight of tripod 15 Ibs.
Gross weight of instrument packed securely for shipment in two boxes 75 Ibs.
Price as above, of brass and bell metal . . collars, ... $280.00
Price of instrument, substructure of steel, as above, extra, . . 25.00
Price of instrument, collars of telescope of hardened steel, extra, . 25.00
Code Word for Geodetic L,evi of brass and bell metal collars Artemisia