STANDARD - Survey Instrument Antique Center!

16
owing to the difference existing in the expansion and contraction of glass and brass
at diiferent temperatures, a spirit level so mounted may sometimes become loose, involving
inaccuracy and unreliability of adjustment. Upon finding that the adjustment
of a spirit level in an even temperature is not as stable as desirable, the level
fastenings, tube, screws, etc. should be examined, to see if any of them are loose. If
the trouble is in the screws, tighten them up; but if the spirit level can be shifted in
its tube by a touch of the finger, take it apart ; soften the plaster of paris in water,
and remove it with a sharp pointed stick of wood. Cautiously move the spirit level
with your finger, at first only a trifle to and fro, increasing the length of stroke little
by little, until it can be safely taken out without breaking; clean thoroughly. Cut
pieces of white paper, of the width of the radius of the tube, and somewhat shorter
than the length of the spirit level, but longer than the opening in the brass tube, and
insert these of sufficient quantity at the bottom of the brass tube, to fill up the space
intervening between the glass and the brass tube. The uppermost layer of paper
should, however, be so wide, as to envelope the spirit level up to the opening in the
brass tube. Now insert the spirit level, taking care not to touch the glass ends that
are sealed up, and place the division or other marks, indicating wh ire the level has
been ground to a true curvature, uppermost in the brass tube. The level must be
pushed in with sufficient friction to prevent slipping in the tube, yet not so tight as
to cause a crack at a subsequent low temperature, as brass will contract more than
glass. No part of the spirit level should touch any part of the metal tube. Now
prepare some plaster of paris with water, of the consistency of paste, and pour in at
each end enough to fill up the space between the end-pieces and the glass, stirring
it sufficiently to make a perfect contact by it and the glass and the brass, but leaving
the spirit level ends exposed. Now put the level together, and adjust as described
elsewhere.
There are other causes, such as centers and flanges that have been bent by falls,
etc., or that have been worn out unequal expansion or contraction in different tem-
peratures of the metals employed in the construction of an instrument, or a nonsymmetrical
lenghtening or shortening of the air-bubble at different temperatures all
of which, singly or combined, tend to impair the adjustment of spirit levels on
Instruments. Of these we will not speak here, as it requires a most thorough mechanician
and instrument-maker to trace the cause to its proper source.
Being assured that the level is mounted as explained above, our advice is, not to
meddle too frequently with the adjustment of a spirit level. Though it may appear
to be out one day. it may be in perfect adjustment other days. It is the function of
a spirit level to indicate the changes taking place in an instrument, so that the
engineer may make proper allowance and apply his corrections, as the character of
his work may require. The finer an instrument, the more sensitive the spirit levels
must be, in order to admit of corrections to arrive at closer results. As a rule, a
spirit level that does not indicate changes taking place in an instrument, is too insensitive
for the character of the instrument, and in many cases entirely unfit fof
reasonably good work.
Replacing Broken Cross-Wires.
The cross-lines in our telescopes are bonajtde spider webs (except where plati
num wires have been specially ordered). In case they should be broken, they may
be restored in the following manner : clean the reticule frame of all foreign matter ;
put it on a sheet of white paper with the cuts on its surface uppermost. Prepare a
little shellac by dissolving it in the best alcohol and waiting until it is of the consistency
of oil. From the spider's cocoon, (those from a small black wood-spider
preferred), which the engineer has prudently secured at some previous time, select
two or three webs, each about two inches long and of the same appearance. Attach
each end of these webs to a bit of paper or wood to act as weights, and immerse
them in water for five or ten minutes. Remove one web from the water, and very
gently pass it between the fore-finger and thumb nails, holding it vertically to remove
any particles of moisture or dirt. Stretch the web carefully over two of the
opposite cuts in the reticule frame. Fasten one end by a drop of the shellac, let
fall gently from a bit of pointed wood or the blade of a penknife. Wait a moment
lor this drop of shellac to harden. See that the web is stretched tight across the