STANDARD - Survey Instrument Antique Center!

44
line of collimation of the telescope ; and the letters denoting the cardinal points,
East and West, must be transposed; i. e., when the letter N is towards the North,
the letter W should be towards the East. Of course the needle indicates magnetic
north, and in the case of instruments unprovided with means of setting off the local
variation of the needle, all the readings of the needle must be corrected for this
local deviation.
If the transit is provided with a variation plate and it is desired to set off the variation
by means of the horizontal plate closer than half and quarter degrees say to
minutes this can be done on the horizontal circle. The verniers of the Transit are
set at zero of the graduation and clamped. The zero of the variation ring must also
be made to coincide with the stationary pointer. The needle is then released and
when at rest the zeros of the variation ring must be made to coincide with the needle
ends by the lower tangent screw. To set off the variation, the vernier plate clamp is
released and the vernier plate turned so that the telescope (pointing Korth) moves
toward the actual East if the variation is West, and toward actual West if the variation
is East, until the vernier reads the desired declination on the horizontal circle.
The zero of the variation ring is now brought to coincide with the needle ends and the
telescope will be pointing to the true North.
If great accuracy is not desired, the variation may be set off directly on the variation
ring in the following manner : If the variation is East, move the zero point of the
shifting compass ring the amount of variation for the locality toward the astronomical
East (or toward the transposed W. point in the bottom of the compass box). Then
clamp shifting ring. When the N. point of needle reads on the graduated ring
the telescope will then be pointing to the true North. If the declination is West, turn
the shifting ring the proper amount toward the astronomical West, or toward E.
point in bottom of compass box.
The variation plate of our Transits with yoke standard frame has a rack and pinion
motion with a capstan-headed nut for clamping when in position. To operate it,
slightly unscrew the capstan -h jaded nut at side of milled head, set off the variation,
and then again clamp this nut tightly.
Spirit-Levels.
The spirit levels, as regards their sensitiveness, should be in strict keeping with
the optical power, and the graduations of the instrument, but the quality should be
of the best. A level-bubble should move uniformly over the same distance, when
the telescope is made to point on two objects alternately, differing slightly in alti-
tude, by the leveling screws alone. In change of temperature the bubble should
lengthen symmetrically from the center and no matter what its ; length, it should
move quickly, without any of the hitching, which is caused usually by a little d-'rt
introduced when it is filled.
Of the three levels attached to the complete transit, the telescope level is the
most sensitive. It should be sensitive enough for ordinary leveling, such as good
railroad work. The level in front, or at right angles to the standards, should be
sensitive enough to make a line plumb by it to any height ; while the third lev** 1
on the standard is used in leveling up the instrument, and to establish the 7***
point for the vernier correctly when vertical angles must be measured.
The test of the fitness of the various levels for the capacity of the instrument
should lie in this : that after carefully bi-secting an object in the field of view, in
such a position of the instrument that all the levels can be read, and then slightly
deranging them all with the leveling screws, the bi-section will be accurately made
after restoring the levels to the exact position they before occupied, by the leveling
screws a^one.
Leveling Screws.
Messrs. C. L. Berger & Sons usually cut their leveling screws with 32 threads to an
inch provide the usual four screws in opposing pairs. The plates once set firmly
apart by tightening two of these screws on the same side, the leveling of the instrument
is easily accomplished by turning the two screws of an opposing pair so that
both thumbs shall move toward each other (when the bubble will go toward the
right), or both thumbs away from each other, when the bubble will move toward
the left. Instruments intended for i. triangulation, e., reading to 10" or less, should
however be supported on three, instead of upon four screws. In this case the instrument
is rapidly leveled by bringing one level parallel to two of the screws, the
other level will now be at right angles to it. Level both levels at the same time
by turning one of the screws to which the first level is parallel and the screw
which is at right angles to this level. Of course the instrument may now be
reversed to guard against non-adjustment of the levels.