STANDARD - Survey Instrument Antique Center!

look around it by moving the eye around it in such a manner as to see the entire circumference,
and bring the cross-wires on the four sides of the image normal to their
respective sides, by means of the motion hi azimuth and the motion of the solar
telescope, as above described. This being attained, the optical axis of the main telescope
should be in the astronomical meridian. Refer to an azimuth mark, and repeat
the operation. The above is called a direct observation.
(h.) To make a reverse observation. Having made the direct observation, turn the
whole instrument 180 in azimuth, and set the co-latitude off on the opposite side
of the vertical arc. Also turn the solar telescope 180 , and proceed as before. The
object of repetition is to eliminate personal non-precision and nossible errors in
manipulation, while the object of reversing is to eliminate any possible remaining
errors of adjustment of the instrument. The prudent surveyor will not trust his work
without such verification, and he will take the mean of both observations.
Remarks.
(1.) To unscrew the solar attachment from the packing-piece in the box, first
release the clamp and tangent screw, and then turn carefully the milled-edged disk or
base plate a few turns to the left. To screw the solar attachment to the instrument,
turn this milled-edged disk from left to right around the screw on top of the main
telescope without revolving the solar attachment. To insure a perfect contact of
screw-shoulder against the flange, on which depends the permanency of the adjustment
of the polar axis to the main telescope, it is necessary that these parts be free
from dust, grit, or dirt of any kind.
(2.) The auxiliary or latitude level, if one is ordered, attaches in the same manner
to the end of the cross axis on the side of the vertical circle.
(3.) The latitude level is used to facilitate during repeated observations the resetting
of the polar axis to the co-latitude, assuming that the polar axis has been previously
set to be at right angles to the main telescope by its milled capstan-headed
screws and the solar level, the polar axis being placed in its position for an observation
with more facility and precision with this level than by reading the vertical arc.
(6.) The latitude having been found for the initial point of a survey, it may be
found for other points within moderate limits by allowing 92 chains of northing or
southing for 1' of latitude.
(7.) The object of bringing the main telescope into the meridian by means of
motion on the outer spindle is to have the zero line of the horizontal plate hi the
meridian, so that the azimuth or bearing of lines can be referred to that line.
(8.) If for any cause one is obliged to work with an uncertain latitude, it is better
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to so with the sun as far from the meridian as practicable, for the following
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reasons :
It is only when the sun is in the pole of the meridian that it has its maximum
efficiency in pointing out the direction of the meridian.
Hence a large hour-angle, and a small declination, are conducive to the elimination
of errors resulting from an incorrect latitude.
Indeed, with the sun precisely in the pole of the meridian, the meridian is determined
independently of latitude.
(9.) In making the several adjustments, or rather in verifying them, the student
should have a true meridian established by some other means than by the " solar
transit," as from the North Star, by some of the methods given in works on surveying.
He should compare the results of his observations with this meridian at different
times in the day, and under different states of the atmosphere, till he has learned any
peculiarity of the instrument and the utmost precision obtainable with it, as well as the
ordinary limit of non-precision,
NOTE. The great utility of this auxiliary, or level attachment, is seen in the setting ofgrades. Two of
these levels being applied to the telescope of a pivot-levelling instrument one on each side or one on each
nd of the cross-axis of a transit telescope, and one of them being adjusted to the up, the other to the down
, the engineer may work in either direction on his grade with the same facility that he would on a level