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.
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