a
sight rod, which was merely a round stick of wood 1/2 inch in diameter
and about 8 inches long. We cut the stick from one of the rounds of an
old broken chair. The upper end of the rod was whittled to a point and
one side was flattened as shown in Fig. 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.

THE SCREEN.

[Illustration: Fig. 157. Section through Shutter.]

[Illustration: Fig. 158. General View of Screen.]

The screen, or shutter, of the heliograph was mounted on a separate
tripod, so as to prevent shaking the mirror when it was operated. It was
made something like a window shutter. We cut out two slats, each 2-1/2
inches wide and 6 inches long. They were made of hardwood 3/8 inch
thick. The upper and lower edges were tapered down to a thickness of
3/16 inch. Light nails were driven into the slats at the ends, and the
nail heads were then filed off so that the projecting ends formed
trunnions for the slats to turn on. The slats were linked to a
connecting rod with double point tacks. A small double point tack was
driven into the upper edge of each slat about 1/2 inch from the right
hand end. Then through each of these tacks we hooked a second double
point tack and drove it into the rod. The tacks on the rod were placed
just 2 inches apar