A scale graduated in hundredths of a unit is shown in figure 1111, view A, and a direct vernier for reading it to thousandths of a unit.
CIRCLES
AND VERNIERS. The horizontal and vertical circles
and their verniers are the parts of the engineer'stransitbywhich the values of
horizontal and vertical angles are determined. A stadia arc is also included
with the vertical circle on some transits.
The
horizontal circle and verniers of the transit that are issued to SEABEE units
are graduated to give least readings of either 1 min or 20 sec of arc. The
horizontal circle is mounted on the lower plate. It is graduated to 15 min for
the 20sec transit (fig. 118) and 30 min for the 1min transit (fig. 119).
The plates are numbered from 0 o to 360 o , starting with a common point and
running both ways around the circle. Two double verniers, known as the A and B
verniers, are mounted on the upper plate with their indexes at circle readings
180 o apart. A double vernier is one that can be read in both directions from
the index line. The verniers reduce the circle graduations to the final reading
of either 20 sec or 1 min.
Figure 119.Horizontal
scales, 1minute transit.
The
A vernier is used when the telescope is in its normal position, and the B
vernier is used when the telescope is plunged.
The
VERTICAL CIRCLE of the transit (fig. 1110) is fixed to the horizontal axis so
it will rotate with the telescope. The vertical circle normally is graduated to
30� with 10 o numbering. Each quadrant is numbered from 0 o to 90 o ; the 00
graduations define a horizontal plane, and the 90 o graduations lie in the
vertical plane of the instrument. The double vernier used with the circle is
attached to the left standard of the transit, and its least reading is 1�. The
left half of the double vernier is used for reading angles of depression, and
the right half of this vernier is used for reading angles of elevation. Care
must be taken to read the vernier in the direction that applies to the angle
observed.
In
addition to the vernier, the vertical circle may have an H and V (or HOR and
VERT) series of graduations, called the STADIA ARC (fig. 1110). The H scale is
adjusted to read 100 when the line of sight is level, and the graduations
decrease in both directions from the level line. The other scale, V, is
graduated with 50 at level, to 10 as the telescope is depressed, and to 90 as
it is elevated.
Figure 1110.Vertical
circle with verniers, scales, and stadia arc.
The
VERNIER, or vernier scale, is an auxiliary device by which a uniformly
graduated main scale can be accurately read to a fractional part of a division.
Both scales may be straight as on a leveling rod or curved as on the circles of
a transit. The vernier is uniformly divided, but each division is either
slightly smaller (direct vernier) or slightly larger (retrograde vernier) than
a division of the main scale (fig. 1111). The amount a vernier division
differs from a division of the main scale determines the smallest reading of
the scale that can be made with the particular vernier. This smallest reading
is called the LEAST COUNT of the vernier. It is determined by dividing the
value of the smallest division on the scale by the number of divisions on the
vernier.
1.Direct Vernier
.
A scale graduated in hundredths of a unit is shown in figure 1111, view A, and
a direct vernier for reading it to thousandths of a unit. The length of 10
divisions on the vernier is equal to the length of 9 divisions on the main
scale. The index, or zero of the vernier, is set at 0.340 unit. If the vernier
were moved 0.001 unit toward the 0.400 reading, the Number 1 graduation of the
vernier shown in figure 1111, view A, would coincide with 0.35 on the scale,
and the index would be at 0.341 unit. The vernier, moved to where graduation
Number 7 coincides with 0.41 on the scale, is shown in figure 1111, view B. In
this position, the correct scale reading is 0.347 unit (0.340 + 0.007). The
index with the zero can be seen to point to this reading. Retrograde Vernier.
A retrograde vernier on which each division is 0.001 unit longer than the 0.01
unit divisions on the main scale is shown in figure 1111, view C. The length
of the 10 divisions on the vernier equals the length of the 11 divisions of the
scale. The retrograde vernier extends from the index, backward along the scale.
Figure 1111, view D, shows a scale reading of 0.347 unit, as read with the
retrograde vernier.
2.Vernier for Circles.

Views
E and F of figure 1111 represent part of the horizontal circle of a transit
and the direct vernier for reading the circle. The main circle graduations are
numbered both clockwise and counterclockwise. A double vernier that extends to
the right and to the left of the index makes it possible to read the main
circle in either direction. The vernier to the left of the index is used for
reading clockwise angles, and the vernier to the right of the index is used for
reading
Figure 1111.Types of
verniers.
counterclockwise
angles. The slope of the numerals in the vernier to be used corresponds to the
slope of the numerals in the circle being read. Care must be taken to use the
correct vernier. In figure 1111, view E, the circle is graduated to half
degrees, or 30 min. On this vernier, 30 divisions are equal in length to 29
divisions on the circle. The least reading of this vernier is 30 min divided by
30 divisions, or 1 min. The index (fig. 1111, view E) is seen to lie between
342 o 30� and 343 o . In the left vernier, graduation Number 5 is seen to coincide
with a circle graduation. Then, the clockwise reading of this circle is 342 o 30�
plus 05�, or 342 o 35�. When the right vernier is used in the same way, the
counterclockwise reading of the circle is 17 o 00� plus 25�, or 17 o 25�. In figure
1111, view F, the circle is graduated in 15min divisions and each half of the
double vernier contains 45 divisions. The least reading on this vernier is 20
sec. The clockwise reading of the circle and vernier is 351 o 30�plus 05�40"
or 351 o 35�40". The counterclockwise reading is 8 o 15� plus 9�20", or
8 o 24�20".