155. Out of a piece of heavy white
cardboard we cut a round disk about 1/4 inch in diameter, with a shank 1
inch long sticking out at one side. This was fastened with a single tack
to the flattened end of the rod in such a position that the point lay
exactly against the center of the disk. The disk could then be turned up
or down, to cover or uncover the point of the rod, as desired. The rod was
fitted snugly into a hole in the baseboard, and could be raised or lowered
to any extent desired, but we had to provide some sort of an arrangement
for making it stay where it was put. A small hole was drilled from the
edge of the baseboard through to the hole in which the rod was fitted. A
square socket was chiseled out around the small hole to receive a nut. The
nut was firmly wedged in and held in place by driving in nails along the
edges. A bolt or machine screw was threaded through the nut, so that its
inner end pressed against the sighting rod. By tightening this screw the
rod could be secured at any height desired.
The instrument was mounted on a tripod similar to the one used for our
surveying instrument. To this it was attached by means of a bolt, which
passed through the center of the baseboard and the tripod head.
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