Year: 1860

Were it not for the aerial envelope which
surrounds our earth, all parts of its surface
would probably become as cold as night, by
radiation into space, as the polar regions are
during six months absence of the sun. The
mode in which the atmosphere retains the heat
and increases the temperature of the earth's
surface may be illustrated by an experiment
originally made by Saussure. This physicist
lined a cubical wooden box with blackened
cork, and, after placing within it therm ONE


ter, closely covered it with a top of two panes
of glass, separated from each other by thin
stratum of air. When this box was exposed
to the perpendicular rays of the sun, the ther-
mometer indicated temperature within the


box above that of boiling water. The same ex-
pertinent was repeated at theCape of GoodHope,
by Sir John Herschel. with similar result,


which was rendered, however, more impressive
by employing the heat thus accumulated in
CO oking the viands OF festive dinner. The
explanation of the result thus produced is not
difficult, when we understand that body heat
ed to different degrees of intensity gives off
rays of different quality. Thus, if an iron-ball


be suspended in free space, and heated to the


temperature of boiling water, it emits rays of


dark heat, of little penetrating power, which
are Intercepted by glass. As the body is heat
ed to higher degree, the penetrating power


of the rays increases, and, finally, when the


temperature of the ball reaches that of glow


ing white heat, it emits rays which readily


penetrate glass and other transparent substan-


ces. The heat which comes from the sun, con-


sists principally of rays of high intensity and


great penetrating power. They readily pass


through glass, are absorbed by the blackened
surface of the cork, and as this substance is a


bad conductor of heat, its temperature is soon
elevated, and ii in turn radiates heat but the
rays which it gives off are of different char


acter from those which it receives. They are


nonuminous, and have little penetrating pow


er; they cannot pass through the glass, and
are retained within the box, and thus give rise


to the accumulation of the heat. The limit of
the increase of temperature will be attained
when the radiation from the cork is of such an


intensity that it can pass through the glass,
and the cooling from this source becomes just
equal to the heating from the sun. The at
mosphere which surrounds the earth produces
a similar effect. 1t transmits the rays from the
sun, and heats the earth beneath, which in it's
turn emits rays that do not readily penetrate
the air, but give rise to an accumulation of
heat at the surface. The resistance of the
transmission of heat of low intensity depends
upon the quantity of vapor contained in the
atmosphere, and perhaps also on the density
of the air. The radiation of the earth, there
fore, differs very much on different nights and
in different localities. In very dry places, as,
for example, in the African deserts and our
own western plains, the heat of the day is ex-
cessiye, and the night commensurably cool.


Colonel Emory states, in his Report of the


Mexican Boundary Survey, that, in some
cases, on the arid plains, there was difference
of go degrees between the temperature of the
day and that of the night. Indeed, the air is
SO permeable to heat, even of low intensities.
in this region, that a very remarkable differ
ence was observed on some occasions when the
camp-ground was chosen in a gorge between
two steep hills. Tie inter radiation between
the hills prevented in measure the usual
dimnution of temperature, and the thermo-


meter in such situation stood several degrees
higher than on the open plain.-Prof. Henry.