t. A substantial frame was then made of 3/4-inch stuff
1-1/2 inches wide. The frame was square, with an opening that measured 6
inches each way, into which the slats were fitted. Before nailing the
frame together we drilled holes in the side pieces for the trunnions of
the slats to turn in. These holes were just 1-3/4 inches apart. After
the slats had been set in place, the frame was fastened together and
then nailed to a baseboard, which was fastened by a bolt to the tripod.
The shutter was operated by a key something like a telegraph key. It was
made of a narrow stick of wood hinged at one end to the lower strip of
the shutter frame, and a spool sawed in two was fastened to the other
end to serve as a handle for the key. A string connected the key with
the connecting rod. The slats were kept closed by a spring, which was
fastened at one end to the connecting rod and at the other to the top of
the frame. At first we used a rubber band for this purpose, but it soon
wore out, so we then made a spiral spring out of stiff spring brass wire
by wrapping it around a pencil. When the key was pressed down the slats
would be turned open, as shown in Fig. 159; but as soon as the key was
released the spring would pull them back again.

FOCUSING THE INSTRUMENT.

[Illustration: Fig. 159. The Heliograph in Operation.]

We were now ready to commence operations with our instruments. The
heliograph was set up on the ledge at the top of the cliff. First the
disk was turned down, uncovering the point of the sighting rod. Then
Bill sighted through the unsilvered spot in the mirror and shifted the
rod up and down until the tip end came squarely in line with the door of
our straw hut, where Jack was seated, notebook in hand, to take down our
message. Reddy stood by him with his wigwag flag to answer back. When
the instrument was properly sighted the shutter was set up directly in
front of it and the sighting disk turned up to cover the point of the
sighting rod. Then came the rather troublesome task of focusing the
mirror. The mirror reflected a square panel of light, in the center of
which there was a small shadow spot made by the unsilvered peep hole.
The object was to get this shadow to fall on the center of the sighting
disk. We knew that then the mirror would reflect the sunlight squarely
on the straw hut. We found it quite easy to direct this shadow spot to
the disk by holding a sheet of paper in front of the mirror six or eight
in