Year: 1860

Were it not for the aerial envelope which sur
rounds our earth, all parts of its surface would
probably become as cold at 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 earths surface may be
illustrated by an experiment originally made by
Suasstre. This physicist lined a cubical wood
en- boz with blackened cork, and, after placing
within if a thermometer, 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. thermometer indicated a temperature
within the box above that of boiling water
The same experiment was repeated at the Cape
of Goea Hope, by Sir John Herschel, with
slmllarE result, which was rendered, however,
more Apressive by employing the heat thus ac-
cumuiSled in cooking the viands of a festive
dinner ahe explanation of the result thus pro-
lucea s not difficult, when we understandsthat
a bodytheated to different degrees of intensity
gives pf rays of different quality. Thus, if an
iron ball be suspended in free space, and heated
to emperature of boiling water, it emits
- s 10 no,ver
Which entirely intercepted eTsr
body is heated to higher degree, the penetrat-
ing power of the rays Increase, and finally, when
the temperature Of the ball reaches that of a
glowing white heat, it emts rays which readily
penetrate glass and other transparent substances


The heat which comes from the sun, consists
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 bad conduc..
tor OF heat its temperature is soon elevated, and
it in turn radiates heat but the rays which it
81 IL
which it receives. They are voluminous, and
have little penetrating power; 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 tempera-
ture 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 atmosphere which surrounds the
earth produces a similar effect 1t transmits
the rays from the sun and heats the earth be-
meath, which in its turn emits rays that do not
readily penetrate the air, but give rise to an ac-
cumuation of heat at the surface. The resist
ance of the transmission Of heat of low intensity
depends upon the quantity Of vapor contained
in the atmosphere, and perhaps also on the den
sity of the air. The radiation of the earth, there
fore, differs very much on different nights and
in different localities. In very dry places, for
example, in the African deserts and our own
western plains, the heat of the day is excessive,
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 600 between
the temperature of the day and that of the night.
Indeed, the air is SO permeable to heat, even of


iptensities, in this region, that a very re-


able difference was observed on some occa-


when the camp ground was chosen in a


between two steep hills The interradia-


etween the hills prevented in measure


sual diminution of temperature, and the


Aonlcter in such situation stood several


es higher than on the open plain.