cular opening of about 3 inches diameter
and covered this at the bottom by a circular piece 1 inch thick and 5
inches in diameter, thus forming a socket in which our compass fitted
snugly. A hole 1 inch in diameter was drilled through the center of this
circular piece to receive the pivot pin of a tripod. Across each end of
the baseboard we secured a block 4 inches long, 2 inches wide and 1 inch
thick. A 1-inch sight hole was drilled through each block at its center.
A ring of cardboard, on which Uncle Ed marked with radial lines the 360
degrees of the circle, was placed over the compass socket, with the zero
and 180 degree marks pointing toward the sight blocks. The outer faces
of the end blocks were now wet with mucilage and a hair was stretched
vertically across the center of each sight hole. The hairs were then
adjusted by sighting through the holes and moving the nearer hair
sidewise until it was exactly in line with both the zero and the 180
degree marks on the cardboard. Then a hair was stretched horizontally
across the center of each sight hole. Great care was taken to place the
hairs at exactly the same height above the baseboard. To protect the
hairs after they were adjusted, they were covered with a piece of glass,
which was secured in place by tacks driven into the wood with their
heads projecting over the edges of the glass.

SPIRIT LEVELS.

From one of his pockets Uncle Ed produced two small bottles, the kind
used for holding homeopathic pills. These he filled nearly to the top
with water, corked them and wedged them into grooves cut lengthwise in
the baseboard at opposite sides of the cardboard ring. These grooves
were filled with putty, and to make sure that the bottles were level
with the baseboard the latter was floated on a bit of quiet water and
the bottles were pressed down at one end or the other until the bubble
within rested at the exact center.

THE TRIPOD.

[Illustration: Fig. 73. The Tripod Head.]

[Illustration: Fig. 74. The Tripod Leg.]

[Illustration: Fig. 75. The Surveying Instrument Complete.]

[Illustration: Fig. 76. The Protractor.]

The tripod head was formed of a wooden disk 5 inches in diameter, with a
wooden pin projecting from its center adapted to engage the hole in the
circular piece above referred to. To the bottom of the tripod head were
nailed three blocks 2 inches long and 1 inch square in cross-section.
The tripod legs were made of light strips of wood, 3/8 inch