ph,
that is, an instrument for sending signals with flashes of sunlight.
Although our wigwagging system was good enough for our requirements, yet
we thought it would be more scientific to use the sun instrument, and
besides, the latter could be used for signaling many miles.

THE SINGLE MIRROR INSTRUMENT.

[Illustration: Fig. 153. Trunnion for Mirror.]

[Illustration: Fig. 154. The Single Mirror Instrument.]

The first thing we did was to procure a small mirror about 4 inches
square, mounted in a wooden frame. Then we got a pair of small square
head bolts about 1/4 of an inch in diameter and 1 inch long, also two
strips of brass 1/2 inch wide and 3 inches long. In the center of each
brass strip we drilled a hole just large enough to admit the shank of
one of the bolts, and then the strips were fastened with screws tight
against opposite edges of the mirror frame, with the heads pressed
against the frame and the shanks sticking out at each side, as shown in
Fig. 153. These projecting shanks served as "trunnions" (that is,
pivots) for the mirror to turn on when it was mounted in place. After
the trunnions had been set in place we made a peep hole in the center of
the mirror by cutting out a piece of the wooden back of the frame and
scratching away the silver from the back of the glass. Only a very small
hole was required, about 1/8 inch in diameter. Great care was taken to
have the unsilvered spot exactly on a line with the trunnions and just
half-way between them. This done, we took two sticks of 3/8-inch wood, 1
inch wide and 3-1/2 inches long. In the upper end of each stick a slot
was cut 1/2 inch deep and 1/4 inch wide. Into these slots the trunnions
of the mirror were placed, and then the nuts were screwed tightly on,
clamping the sticks against the sides of the mirror. The sticks were now
connected by nailing a 1/2-inch strip at the bottom, and braced by a
couple of corner pieces. This formed a swiveled frame for the mirror,
which was clamped to the base of the instrument by means of a bolt 1-1/2
inches long. The bolt passed through the bottom board of the frame,
squarely under the peep hole of the mirror and through the baseboard of
the instrument near one end. The baseboard was 2 inches wide, 10 inches
long and 3/4 inch thick.

THE SIGHT ROD.

[Illustration: Fig. 155. The Sight Rod.]

[Illustration: Fig. 156. Nut Set in Baseboard.]

At the end opposite to where the mirror frame was swiveled we mounted