_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