_Atmospheric Absorption._--The principal object of conducting the investigation during the summer solstice has been the facilities afforded for determining atmospheric absorption, the sun's zenith distance at noon being only 17° 12' at New York. The retardation of the sun's rays in passing through a clear atmosphere obviously depends on the depth penetrated; hence--neglecting the curvature of the atmospheric limit--the retardation will be as the secants of the zenith distances. Accordingly, an observation of the temperature produced by solar radiation at a zenith distance whose secant is _twice_ that of the secant of 17° 12', viz., 61° 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° 12' reaches 66.2° F., while at a zenith distance of 61° 28' it is 52.5° F.; hence, minimum atmospheric absorption at New York, during the summer solstice, 13.7 is 66.2°-52.5° = 13.7° 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._