STANDARD - Survey Instrument Antique Center!

49
In practically applying this table, it is preferable to take the mean of several
readings of the rod in each position of the gradienter screw.*
Thus, with the target near the foot of the rod, and then moved to correspond to
two revolutions of the gradienter screw, three readings hi each position were a
follows :
Means,
I.
Altitude 18 2V
ft
0.625
0.627
0.625
II.
. . . 0.626 3.378
0.626
Difference, 2.752
Factor for direct distance for 18 = 94.80 For Horizontal Distance=90.17
" " 1S30'= 94.51 " " "
=89.63
Differences, . . . =0.29 0.54
Therefore, the factor, for 18 20' will be for the direct distance 94.80, ?3 of 0.28
-a
94.61, and for the horizontal distance, 90.17 2/3 of 0.54=89.81.
Then we have, 2.752 X 94.01 = 260.37 = the direct distance.
2.752 X 89.81 = 247.15 = the horizontal distance.
This direct distance being the distance from the position of the foot of the rod
or the lower target to the center of the horizontal axis of the telescope,! and the
horizontal distance, the one usually desired, that distance reduced to a level line.
The mean value of two revolutions of the Gradienter Screw in arc. is 34' 23*.
Hence the value in arc of one of its smallest divisions on the head is 20" nearly.
Vertical angles therefore may be laid off with facility when they are confined to
the range ot' the screw.
To insure at all times accurate results, the telescope axis should revolve free, but without any looseness in
the bearings. The engineer should examine these bearings from time to time, and, if necessary, fresh and pure
watch oil must be applied.
To make a measurement with a micrometer screw, its graduated head should be set back slightly, then bring
it up to the readings in the same direction in which the measurement must be effected.
* Should the engineer desire the direct distance between the foot of the rod, and tk* Joint *vrr mkick t*t
tivmb-iob is suspended, it may be found by the following formula.
X = V