The Republican. [volume] (Oakland, Md.) 1877-current, 1897-11-25

but as it is necessary to correct every
possible error in the adjustren OF
the instrument, the horizontal plate
and telescope are revOlved horizon
tally for an angle of one hundred and
| eighty degrees, or a semi-circle, and q
sight taken again upon the rear tar
get, after which the telescope is once
more revOlved and a second point
taken upon the forward hub previ-
ously driven. By this operation the
north end of the transit has been
revolved to the south for the second
sight. If in the first case there were
an error ip the laveliag of tic plates
and the west side was slightly higher
by the revolving of the plate one
hundred and eighty degrees the error
would be found on the east side
Again, if the left standard supporting
the axis of the telescope is slightly
higher, when revolved it will come
upon the right side and divide the
error. SO also is the case with the
cross wires of the telescope. If the
wires cause the point to appear to
the west on the first sight, by the
revolution of the telescope, first hori-
zontally with its supporting plate
and then vertically upon its axis, it
will be thrown equally to the east
upon the second sight being taken
forward. The two points having
been taken upon the forward hub as
described, their distance apart is
measured and a small brass nail driv-
en- half way between, dividing 80 far
as possible all errors This becomes
the third point i'll the line. The
transit is moved to this forward point
and Il back sight taken to the second
point etc.
In was found that even a small rod
hung in a universal Joint, called a
! gimbal, was not sufficiently accurate
as a target for this work, and so tri-
pods were made for the front and rear
rodmen. To these tripods were at
tached a small round piece of wood
six inches long and about the size of
a lead pencil and painted white
These little sights are fitted into a
small piece of metal which can be
moved from side to side along bolt
in the top of the tripod ring; from
the bottom of the metal slides is sus-
pended by white cord brass plumb
bob weighing two pounds. 1t will be
seen that this makes perfect target
The standard is rigid, while the great
weight Of the plumb as compared
with the wooden sight, keeps it abso-
iutely straight, and unaffected bv
wind, while the adjustment of the
slide and string attached to the plumb
allow ready and accurate centreing
of the plumb and target above over
the brass nails These targets on
first thought seem small. but so DOW
erful is the glass of the transit that
they can be seen and sights taken,
where the ground is open, for a dis
tance of half mile, and within dis
tance of one thousand feet the cord
of the plum-bob is used for the sight,
making it very accurate.
To run a perfectly straight line over
a rough country is the hardest work
that can be done. An error of but
one minute Of arc, the smallest angle
that can be measured by the ordinary
transit, will cause a divergence of a
foot and a half per mile in the line,
from a true meridian.
Here wish again to call attention
to the numerous changes mentioned
above. to which the c0mpass is bIp
and to the fact that it will not give
the same course for two consecutive
hours. owing to the daily chance or
from one mile to the next. in running
north, from the change due to geo
graphical position. and these chances
to~atber NAV easiJv amonn+ +o
WE'VE lh'nu'eo OF elShteeh leeI per
mile, without including local attrac-
tions, which are usually unknown
ouan+itia. On +bo otba,. hand +ho
wv