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.
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