is twice that of the secant of 17 deg. 12’, viz., 61 deg. 28’, determines the minimum atmospheric absorption at New York.  The result of observations conducted during a series of years shows that the maximum solar intensity at 17 deg. 12’ reaches 66.2 deg.  F., while at a zenith distance of 61 deg. 28’ it is 52.5 deg.  F.; hence, minimum atmospheric absorption at New York, during the summer solstice,

13.7
is 66.2 deg.-52.5 deg. = 13.7 deg.  F., or ------ = 0.207 of the sun’s
66.2

radiant energy where the rays enter the terrestrial atmosphere.

[Illustration:  CAPTAIN ERICSSON’S SOLAR PYROMETER, ERECTED AT NEW YORK, 1884.]

In order to determine the loss of energy attending the reflection of the rays by the diagonal mirrors, I have constructed a special apparatus, which, by means of a parallactic mechanism, faces the sun at right angles during observations.  It consists principally of two small mirrors, manufactured of the same materials as the reflector, placed diagonally at 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 deg. 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 1,000 squared to 6 squared = 28,000 to 1; hence the comet was subjected to a temperature of 28,000 x 180 deg./3 = 1,680,000 deg., an intensity exactly “2,000 times greater than that of red-hot iron” at a temperature of 840 deg..  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