STANDARD - Survey Instrument Antique Center!

57
Adjustment of the Improved Transverse Striding Level
resting- on special collars for the Engineers' and Surveyors'
Transits, sizes No. 1, 2 and 11.
This striding level differs from the one formerly made by us, and referred to in the
preceding article, in several respects. It permits of a longer spirit-level in the same
length of outer tube so important in this case on account of the short available distance
between the collars; and is simpler in design, so that after an adjustment has
been once properly performed, barring accidents, it hardly ever needs to be repeated
again.
To make a readjustment: first find out whether the level needs a lateral adjustment
by placing it on its collars with instrument leveled up, and fastening it by the
milled headed nut to the horizontal axis, place the bubble in the center of its tube by
the leveling screws, and then elevate and depress the telescope 10 to 20 from 0; note
carefully the amount of displacement of bubble, also which one of the two capstanheaded
screws (one white and one red, to readily distinguish one from the other) must
be moved one-half the error; move screw outwardly if bubble must be moved away from
this screw, and inwardly if bubble must be raised towards it to make the level tube
laterally parallel to the transverse axis of the telescope. Then move the other adjusting
screw an equal amount, but in the opposite direction to retain the same height of
leg, and repeat this adjustment until one-half the error is corrected. Before operating
any of the adjusting screws first remove the level from the collars.
This accomplished, now make the longitudinal adjustment. Clamp the telescope
in the horizontal position and reverse the level on its collars; note whether the
leg with the screws should be raised or lowered to bring the bubble to the center of its
tube, and then turn each screw, alternately, an amount equal to one-quarter the error
(inwardly if the leg is to be raised, or outwardly if leg is to be lowered) in order to
eliminate one-half the error of the level-tube sought to be corrected. Then again reverse
the level on its collars, and repeat this adjustment if necessary. If the level has
been considerably out of adjustment it will be well to again verify the lateral adjustment
in the manner prescribed for it above, and also to repeat the longitudinal adjustment
by again following the method just explained until completed. Remember, that
if the leg has to be raised, both screws must be moved inwardly towards the telescope,
and that if it has to be lowered, both screws must be moved outwardly in equal amounts.
This method has been prescribed by us as being the most simple to follow. One somewhat
versed in making adjustments probably may succeed in making it quicker (if the
level is considerably out) by noting mentally the amount of displacement of bubble for
the longitudinal and lateral adjustments, and then correcting same simultaneously, or
nearly so, with one of the screws alone, by moving it in the desired direction. To adjust
the height of standards see page 54-
The Adjustments of the Telescope's Axis of Revolution in Transit-
Theodolites by means of the Transverse Striding-Level resting
on points of contact in the Wyes.
The striding-level of the finest class of instruments, such as Transit-Theodolites
used for triangulation wholly, and Mining Transit used mainly for very steep sighting,
will rest directly upon the cylindrical pivots of the transverse axis, at the circle of
contact in the wyes.
Whenever exact vertical motion is desired, the striding-level in such instruments
should be depended upon to the exclusion of the front plate-level. The latter is then
entirely subordinate to the striding-level and should be depended upon merely for leveling
the instrument approximately. The plate-levels are also useful in indicating
quickly any large disturbances of position. When the objects sighted at do not differ
much in altitude, the front plate-level is, in these instruments, sufficiently sensitive to
give satisfactory results without using the striding-level.
The adjustment of the transverse axis of revolution. The striding-level
having been carefully adjusted, level up the instrument generally with the plate-levels,
put the striding-level in position and bring its bubble to the center of its graduation
by means of the leveling screws, then turn the instrument 180 on its vertical axis and
note whether the bubble of the striding-level remains in the center of the graduation.
If it does, the adjustment is correct. If it does not, correct one-half the error by means
of the leveling screws, and the other half by means of the wye adjustment of the
standard. Repeat the process until the adjustment is correct. Observe also, in adjusting
the wye adjustment of the standards, that it will be best performed and more
lasting when the last turns of the lower capstan-headed screw are always applied in
an upward direction.