GRAPHIC FIELD NOTES. 327

form, consisting of a board 15 inches square, into one edge of
which is set a narrow box containing a compass-needle 3 inches
in length. The table is supported by a tripod of light construc-
tion, without levelling apparatus, the level of the instrument being
effected by the legs of the tripod. The table is adjusted in azi-
muth or oriented by means of the compass-needle, movement in
azimuth being provided by simply turning the table on top of the
tripod head. There is no clamp to the azimuth movement, the
table being held in place simply by friction. The alidade consists
of a brass rule 12 inches long, with raised sights hinged to turn
down when not in use. Ordinary drawing-paper backed with
cloth is used for plane-table sheets, and is attached to the board
by thumb-tacks." *

The operation of traversing with this instrument is very simple.
At each station the table is oriented by bringing the compass-
needle to a mark on its short scale ; the area of the map is usually
too small to show any convergence of magnetic meridians, and if
the magnetic declination be constant, it follows that at each station
the position of the table is parallel to all those preceding it. Courses
sighted and drawn with the alidade, whether successive foresights
or alternating foresights and backsights, therefore depart from each
other with angles equal to those included by the directions on the
ground, and the lengths of the sights being laid off to scale, the
plat is a figure mathematically similar to the traverse on the ground.
