STANDARD - Survey Instrument Antique Center!

53
ness that may exist in the leveling screws; but to secure the necessary stability of the
instrument to the tripod, the clamp-nut should be well fastened to the sliding piece.
To operate the shifting center, both the spiral-spring and the clamp-nut must be releas?d
slightly from their hold upon the tripod and the sliding piece, when the instrument
can be moved over the given point on the ground. This device adds about 2 Ibs.
to the weight of the tripod.
Arrangement for Offsetting at Bight Angles.
The most common off-set with the transit is one at 90 to the line of sight.
Several methods have been proposed for doing this without disturbing the telescope.
Messrs. C. L. Berger & Sons have a very neat one ; it consists in simply perforating
the horizontal axis, so that by drawing the head back fifteen or twenty
inches from one end of the axis, the eye may be placed so that the eye, the horizontal
axis of the telescope, and a rod set beyond, may be readily placed in the
same straight line, at right angles to the line of sight of the telescope, no matter
at what altitude the telescope may be pointing.
In off-setting by the arrangement proposed above, the rod is made plumb by
lining it with the plumb-line ot the instrument itself. The advantage of this method
is, that it holds equally well for any inclination of the telescope. The disadvantage
is, that the engineer is obliged to leave the eye-end of. the telescope at each off-set
made
Setting Up.
In setting up a transit, push the iron shoe of one leg firmly into the ground, by
pressing on the other two legs near the tripod head. Having secured a firm foundation
for this leg, separate the other two legs, at the same time drawing the tripod
head toward you. Then set the two remaining legs in the same manner as the first
one. If the ground is pretty level, merely noticing that the tripod feet are equidistant,
r ill insure that no unsightly appearance will be ^iven to the leveling screws.
If the ground is uneven, however, nothing but practice can produce a graceful
position of the instrument. The plumb-bob attached to the instrument should
swing within say half an inch of the point on the ground, and the plate on which the
leveling screws rest, if possible, should be approximately horizontal, when this
stage is completed.
Now with the level screws not tightened up, after leveling approximately,
bring the plumb-bob exactly over the point on the ground, by moving the body of
the instrument on its shifting head. Then complete the leveling of the instrument,
and it is ready for work.
The Adjusting of the Transit
If the instrument is out of adjustment generally, the engineer will find it profitable
to follow the makers in not completing each single adjustment at once, but
rather bring the whole instrument to a nice adjustment by repeating the whole
series.
After setting up, bring the two small levels each parallel to a line joining two
of the opposing leveling screws. Bring both bubbles to the center of the level tubes,
by means of the leveling screws. In doing this, place the two thumbs on the inner
edges of the two leveling screws, parallel to the bubbles, and the fore fingers ot
each hand on the outer edge. Turn the leveling screws so that both thumbs move
inwards or both outwards. In the former case the bubble will move toward the
right, in the latter case toward the left.
Now turn the instrument 180 hi azimuth. If the small levels still hare their
bubbles in the center of their tubes, these levels are adjusted, and the circles are
respectively as nearly horizontal and vertical as the maker intended them to be.
If the bubbles, however, are not in the center of their tubes, then bring hem
half way back by means of the leveling screws, and the remaining half by means
of the adjusting screw at the end of each of the level tubes.
It may be necessary to repeat this adjustment several times, but when made, the
instrument once leveled will have its small levels in the center of their tubes through
an entire rotation of the circle.