rature produced by solar radiation at a zenith
distance whose secant 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 aut