The main part of the instrument is the mirror, which should be about 4 in. square, set in a wood frame and swung on trunnions made of two square-head bolts, each 1/4 in. in diameter, and 1 in. long, which are firmly held to the frame with brass strips, 1/2 in. wide, and 3 in. long. The strips are drilled centrally to admit the bolts, and then drilled at each end for a screw to fasten them to the frame. This construction is clearly shown in Fig. 1.
Detail of the Parts for Making the Mirror and Sight Rod Which are Placed on a Base Set on a Tripod Top, the Whole being Adjusted to Reflect the Sun's Rays in Any Direction Desired (Fig. 1)
A hole is cut centrally through the backing of the frame and a small hole, not over 1/8 in. in diameter, is scratched through the silvering on the glass. If the trunnions are centered properly, the small hole should be exactly in line with them and in the center.
(Fig. 2)
A U-shaped support is made of wood strips, 3/8 in. thick and 1 in. wide, the length of the uprights being 3-1/2 in. and the crosspiece connecting their lower ends a trifle longer than the width of the frame. These are put together, as shown in Fig. 2, with small brackets at the corners. A slot, 1/2 in. deep and 1/4 in. wide, is cut into the upper end of each upright to receive the trunnions on the mirror frame. Nuts are turned on the bolt ends tightly, to clamp the standard tops against the brass strips on the mirror frame. The cross strip at the bottom is clamped to the base by means of a bolt, 1-1/2 in. long. The hole for this bolt should be exactly below the peephole in the mirror and run through one end of the baseboard, which is 3/4 in. thick, 2 in. wide and 10 in. long.
(Fig. 3)
At the opposite end of the base, place a sighting rod, which is made as follows: The rod is 1/2 in. in diameter and 8 in. long. The upper end is fitted with a piece of thick, white cardboard, cut 1/4 in. in diameter and having a projecting shank 1 in. long, as shown in Fig. 3. The rod is placed in a 1/2-in. hole bored in the end of the baseboard, as shown in Fig. 2. To keep the rod from slipping through the hole a setscrew is made of a small bolt with the nut set in the edge of the baseboard, as shown in Fig. 4.