right angles to each other; a thermometer being applied between the two,
whose stem points toward the sun. The direct solar rays entering through
perforations of an appropriate shade, and reflected by the inclined
mirrors, act simultaneously on opposite sides of the bulb. The mean result
of repeated trials, all differing but slightly, show that the energy of
the direct solar rays acting on the polygonal reflector is reduced 0.235
before reaching the heater.

In accordance with the previous article, the investigation has been based
on the assumption that _the temperatures produced by radiant heat at given
distances from its source are inversely as the diffusion of the rays at
those distances. In other words, the temperature produced by solar
radiation is as the density of the rays._

It will be remembered that Sir Isaac Newton, in estimating the temperature
to which the comet of 1680 was subjected when nearest to the sun, based
his calculations on the result of his practical observations that the
maximum temperature produced by solar radiation was one-third of that of
boiling water. Modern research shows that the observer of 1680 underrated
solar intensity only 5° for the latitude of London. The distance of the
comet from the center of the sun being to the distance of the earth from
the same as 6 to 1,000, the author of the "Principia" asserted that the
density of the rays was as 1,000² to 6² = 28,000 to 1; hence the comet was
subjected to a temperature of 28,000 × 180°/3 = 1,680,000°, an intensity
exactly "2,000 times greater than that of red-hot iron" at a temperature
of 840°. The distance of the comet from the solar surface being equal to
one-third of the sun's radius, it will be seen that, in accordance with
the Newtonian doctrine, the temperature to which it was subjected
indicated a solar intensity of