STANDARD - Survey Instrument Antique Center!

139
C L. BERGER & SONS' ENGINEERS' PRECISE LEVEL.
Patented. {For cut see page 141.}
With micrometer screw for close setting the spirit level.
For use in cities in establishing benches, etc., also for all work requiring speed and the highest
degree of accuracy in spirit leveling.
It is a well-known fact that, satisfactory as it may be on account of its great simplicity
and compactness, the ordinary wye level (pp. 134, 135) will fail in degree of accuracy
or in rapidity of manipulation when the closest results are required. It often
happens when precise work is required, the time spent in leveling up and keeping the
level bubble of an ordinary good wye level in the center of its graduation by means of
the four leveling screws is often very considerable and, when the course is over
swampy or frozen ground, the vexation attending the work is apt to be great, and the
results vitiated by the numerous readjustments required to keep the bubble in its
place. This manipulating of the leveling screws is very apt to lead to a change in the
height of the telescope, varying in magnitude according to the style of the instrument.
(It is here to be noted that this change in the height of the telescope is less in our
levels, or transits with leveling attachments, than is the case with the instruments
of other makes).
To aid the Engineer in the prosecution of exact work, avoiding the errors caused
by the readjustments above referred to, we have designed and are prepared to furnish
the instrument shown on page 141.
By referring to the cuts it will be seen that this instrument is mounted on three
leveling screws, and that the center about which the instrument revolves is unusually
long and unyielding. Two small spirit levels attached to arms extending from what
we may call the cross-bar (since the center of the instrument is permanently secured
to it as in the ordinary style of levels) serve to put the center in a vertical position,
thus securing at once a nearly horizontal position to the cross-bar. These small levels
are adjusted the same as the ordinary piaie levels of a transit.
At the eye end this cross-bar carries a micrometer screw by which the telescope
and its level can be raised or lowered at will independently of the leveling screws. A
strong spiral spring on the same side holds the wye-bar down upon the micrometer
screw. This arrangement provides a most delicate motion up and down, and enables
one to set the bubble accurately at every sight and in a very much better manner than
can be done by the leveling screws alone. The head of the micrometer screws is divided
into one hundred parts, and as a rule its pitch will be such that 250 to 252 parts
of revolution of the screw will make a change of one foot in the reading of the rod held
at a point 100 feet away from the center of the instrument. It may be seen that the
instrument can be very advantageously used for making grade measurements. The
graduated disc, when reading zero on the index-bar, brings the instrument at once
within one or two divisions of its normal position.
The di-c can also be readily turned
on its hub by taking hold of the mille'l head (the disc is held on its arbor simply by
friction), so that, for convenience, a reading may always start from zero, though the
cross-bar be not leveled up. This instrument, as above stated, is provided with three
leveling screws, which give a firm support on the tripod, and allow a closer setting of
the bubble when the instrument is run as an ordinary wye level, without making use
of the micrometer. (Seep. 44.)
The Chief Feature of the Instrument, however, consists in the fact that the
pivots * ou which the wye bar can be raised or lowered, are in the middle of the instrument
and within a fraction of an inch of the plane of the line of collimation, thus securing
to the telescope a motion in altitude free from any change in height of the line
of collimation, though the telescope were to move throughout the entire range of the
micrometer screw during an extended leveling operation. As 9 rule, the working range
of the micrometer will be limited to a few revolutions each way from its normal position
in order to keep the instrument as compact as possible. The instrument is also
arranged so that, whenever desirable, it may be used as an ordinary wye level. For
this purpose, it is provided, at the object end of the cross-bar, opposite the micrometer
screw, with a milled-head screw and check nut, by means of which, and by the micrometer
screw, whep set at zero (see cul), the wye bar may be set exactly at right angles
to the vertical center. However, for the fine settings of the bubble in bench leveling
or pointing of the telescope, etc., the micrometer screw should be used exclusively.
A clamp and tangent screw motion is also provided and so arranged, that it can
be readily reached from the eye end of the telescope. The cross and wye-bars are
cast hollow and the former fits inside the latter.
*NOTE. It will be noticed that in instruments of a similar character, having pivot screws acting in and below
the wye opposite the micrometer screw, as for instance, in the TJ. S Coast Survey geoaesic leveiSj designed
after Stampfer (see Reoort 1879), any motion ot the telescope in altitude will also change its height. By an
injudicious use of the micrometer screw our own hvdrographic wye level (see page 1043, catalogues 1888-1891).
partook of this same error, and this together with the marked wear on the collars due to this same motion led
us to the abandonment of it. We note, however, that other firms who are in the habit of copyh^'our style*
aed patterns have since brought it out as a detail of a precise level.