Surveying - Carpenter's and Builder's Math, Plans, and Specifications - page 216

90

90

0

Figure 6-9 Two typical compass boxes. The compass circles
are graduated to one-half degrees and numbered in quadrants.
The variation plate is provided so that the magnetic declina-
tion may be set accurately. The cardinal points shift with the
graduated circle. (Courtesy C. L. Berger & Sons, Inc.)

is strongly magnetized. Its north end is distinguished by color or
ornamentation, and its balance is regulated by a small coil of fine
wire wound around one arm that can be shifted. The limb that is
formed by the edge of the sides of the box is divided into 360 ◦ with
half degrees shown. They are numbered from two zeros marked
at the ends of a diameter to 90 ◦ right and left. The bottom of the
box is marked with two rectangular diameters corresponding to the
graduations 0 ◦ and 90 ◦

of the vernier and two other diameters at

45 ◦ to the first.
The forward end of the diameter marked 0 ◦ is designated by
the letter N, and the rear end is designated by the letter S, cor-
responding to north and south. The ends of the transverse di-
ameter marked 90 ◦ are designated by the letters E on the left
and W on the right, corresponding to east and west. Note that
this designation is the reverse of the standard mariner's compass.
Since the telescope is fixed to sight from south to north, the com-
pass indicates the direction of the sighting. When set to an ordi-
nary surveyor's compass, the forward end of the frame carries a
vernier and a tangent screw to read fractions smaller than one-half
degree.