STANDARD - Survey Instrument Antique Center!

68
From the -(- sign of the " difference" of declination, we see that the decimation is
of the same name as the latitude, whence the correction is an increment, and accordingly
the -f- sign as suffixed. This sign belongs to the refraction.
When the object is in the meridian, refraction a.ffects declination by its full
amount; but, if both the observer and the object were in the plane of the equator,
refraction would have no effect on the object with regard to refraction ; whence, between
these limits, only a part of refraction is effective in changing the declination.
Just what portion is effective, is shown by table II. of this paper.
Thus, in the given Lat. 44, and for, say 4 hours from noon, the percentage of re-
fraction to be is .74 of that
applied corresponding to the altitude of the object at the
time of observation. The sign ^ to be used must be determined, as above, by considering
whether the sun is going north or south at the time.
This part of the reduction of declination cannot, of course, be made till the altitude
is found at the time of observation.
To Find the Latitude.
Having prepared the declination for the day, as above, level up the transit carefully.
Level the main telescope, observing that the vertical arc reads zero, arid set
the polar axis to a vertical position by means of the solar telescope level.
These points being attained, set the main telescope, pointing south. Then for a
declination >
south [
the south end of the te1680^ elevate |
6 tin . the vertical arc in-
dicates the declination thus found.
Then, having turned the solar telescope into a vertical plane parallel with that
containing the optical axis of the main telescope, level it carefully and clamp it.
A few minutes before the time of the sun's culmination, bring the telescope into
with
1 '
the vertical plane passing through the observer and the sun, and "find the sun
the solar telescope. This is readily done by varying the altitude when the sun's
image will appear on the diagonal eye-piece.
Having "found the sun," bisect his image with the vertical wire, by varying the
azimuth with the tangent screw of the transit plate, or with that of the outer center;
and, simultaneously, follow him in altitude the horizontal wire bisecting the image
till it ceases to rise, then clamp and read the vertical arc. This reading should
be the sum of the co-latitude and refraction, the refraction being that due to the meridian
altitude of the sun, which is the algebraic sum of declination and co-latitude. From
this reading the latitude is readily deduced. With the latitude and declination
known, we are prepared
To Find the Meridian.
(a.) As for finding latitude, level up the instrument carefully, the vernier of plate
lamped, reading zero.
(b.) Point the telescope to the sun to find his altitude for the refraction. This
can be found with sufficient accuracy by turning the telescope, till the shadow of a
pencil held across the end, or till the shadow of the screws on the side, are parallel
with the tube.
(c.) The refraction corresponding to this altitude must be multiplied by the corresponding
coefficient, for the time from noon and the latitude, and applied to the
declination, as per instructions above, for the corrected declination.
(d.) Point the telescope to the south, tne south end for decli
^evithi'g )
south |
'
nation, till the vertical arc reads the corrected declination, and clamp the vertical
arc.
(e.) The main telescope being dipped to the corrected declination, level the solar
telescope by means of its level, being careful to do so when it is in a vertical plane
parallel with that containing the optical axis of the main telescope, for only when it
is in this plane can the declination be properly set off.
(f.) Elevate the south end of the main telescope to the co-latitude, by means of
the vertical arc, and turn the telescope approximately into the meridian, by means
of tb magnetic needle.
(g.) " Find the sun " with the solar telescope. This is done by turning the whole instrument
in azimuth, on its outer center, simultaneously with a motion of the solar telescope
in right ascension, till the sun's image is seen in the eye-end of solar telescope.
Bisect the image, as nearly as may be, by the two motions above named clamp
and complete the bisection, by both wires, or by the wires forming a square, by means
of the transit's lower tangent screw, and by that of the solar telescope. If the image
of the sun should be so large that it cannot all be seen from one position of the eye.