ending flashes from
the cliff to the island, but we couldn't make it work very well sending
messages from the island to the cliff, because we had to face almost due
north, and then the sun was nearly always at our backs and couldn't
shine squarely on the mirror. This led to our building a double mirrored
heliograph the following summer. To begin with, we built an instrument
which was the exact duplicate of our first heliograph; then, in
addition, to fit in the socket of the sighting rod, we rigged up a
second mirror, which was mounted in exactly the same way as the first.
The second mirror was called the station mirror, and differed from the
other, or sun mirror, in having a small patch of white paper pasted at
the center instead of a peep hole. When using this instrument, we set it
up so that the station mirror faced the ledge, then by sighting through
the hole in the sun mirror at the reflection in the station mirror we
could see just what was in focus. The station mirror had to be moved
until the patch at its center hid the ledge from view. After that the
sun mirror was shifted until the shadow spot fell on the white patch of
the station mirror. When once the station mirror was focused, it could
be clamped tightly in place by screwing up the trunnion and swivel nuts.
But the sun mirror had to be constantly shifted to keep the shadow on
the patch. Another way of focusing the mirrors was to stand behind the
instrument with the head close to the station mirror, shift the sun
mirror until the entire station mirror was reflected in it, with the
white patch squarely over the unsilvered spot; then still looking at the
sun mirror, the station mirror was shifted until the reflection of the
distant station was brought squarely in line with the unsilvered spot on
the mirror. The station mirror was now firmly bolted and the sun mirror
adjusted until the shadow spot fell on the paper patch.

CHAPTER XIV.

ICE BOATS, SLEDGES AND TOBOGGANS.

As our vacation was drawing to a close, we began to make plans for the
Christmas holidays. Our previous Christmas vacation had been so
completely taken up with preparations for the trip to Willow Clump
Island that we had had no time for the trip itself. We resolved this
time to have everything ready beforehand, so that we could spend the
entire two weeks in solid pleasure. Our skate sails and snow shoes were
stored in the attic, ready for use. If we were to make a trip in the
snow we