part of the bottom of the cylindrical heater of the motor is acted upon by
the reflected rays, and that their density diminishes _gradually_ toward
the sides of the vessel; also that owing to the imperfections of the
surface of the reflecting plates the exact course of the terminal rays
cannot be defined. Consequently, the most important point in the
investigation, namely, the area acted upon by the reflected radiant heat,
cannot be accurately determined. I have accordingly constructed an
instrument of large dimensions, a polygonal reflector (see Fig. 1),
composed of a series of inclined mirrors, and provided with a central
heater of conical form, acted upon by the reflected radiation in such a
manner that each point of its surface receives an equal amount of radiant
heat in a given time. The said reflector is contained within two regular
polygonal planes twelve inches apart, each having ninety-six sides, the
perimeter of the upper plane corresponding with a circle of eight feet
diameter, that of the lower plane being six feet. The corresponding sides
of these planes are connected by flat taper mirrors composed of thin glass
silvered on the outside. When the reflector faces the sun at right angles,
each mirror intercepts a pencil of rays of 32.61 square inches section,
hence the entire reflecting surface receives the radiant heat of an
annular sunbeam of 32.61 × 96 = 3,130 square inches section. It should be
observed that the area thus stated is 0.011 less than the total
foreshortened superficies of the ninety-six mirrors if sufficiently wide
to come in perfect contact at the vertices. Fig. 2 represents a transverse
section of the instrument as it appears when facing the sun; the direct
and reflected rays being indicated by dotted lines. The reflector and
conical heater are sustained by a flat hub and eight radial spokes bent
upward toward the ends at an angle of 45°. The hub and spokes are
supported by a vertical pivot, by means of which the operator is enabled
to follow the diurnal motion of the sun, while a horizontal axle, secured
to the upper end of the pivot, and held by appropriate bearings under the