I have, during the summer solstice of 1884, carried out an experimental
investigation for the purpose of demonstrating the temperature of the
solar surface corresponding with the temperature transmitted to the sun
motor. Referring to the illustrations previously published, it will be
seen that the cylindrical heater of the sun motor, constructed solely for
the purpose of generating steam or expanding air, is not well adapted for
an exact determination of the amount of surface exposed to the action of
the reflected solar rays. It will be perceived on inspection that only
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.
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