sunlit realm forced its way into the
circumpolar darkness.

The sun's heat does not directly disturb the atmosphere; if we could
take the solid sphere of the world away, leaving the air, the rays
would go straight through, and there would be no winds produced. This
is due to the fact that the air permits the direct rays of heat, such
as come from the sun, to pass through it with very slight resistance.
In an aerial globe such as we have imagined, the rays impinging upon
its surface would be slightly thrown out of their path as they are in
passing through a lens, but they would journey on in space without in
any considerable measure warming the mass. Coming, however, upon the
solid earth, the heat rays warm the materials on which they are
arrested, bringing them to a higher temperature than the air. Then
these heated materials radiate the energy into the air; it happens,
however, that this radiant heat can not journey back into space as
easily as it came in; therefore the particles of air next the surface
acquire a relatively high temperature. Thus a thermometer next the
ground may rise to over a hundred degrees Fahrenheit, while at the
same time the fleecy clouds which we may observe floating at the
height of five or six miles above the surface are composed of frozen
water.

The effect of the heated air which acquires its temperature by
radiation from the earth's surface is to produce the winds. This it
brings about in a very simple manner, though the details of the
process have a certain complication. The best illustration of the mode
in which the winds are produced is obtained by watching what takes
place about an ordinary fire at the bottom of a chimney. As soon as
the fire is lit, we observe that the air about it, so far as it is
heated, tends upward, drawing the smoke with it. If the air in the
chimney be cold, it may not draw well at first; but in a few minutes
the draught is established, or, in other words, the heated lower air
breaks its way up the shaft, gradually pushing the cooler matter out
at the top. In still air we may observe the column from the flue
extending about the chimney-top, sometimes to the height of a hundred
feet or more before it is broken to pieces. It is well here to note
the fact that the energy of the draught in a chimney is, with a given
heat of fire and amount of air which is permitted to enter the shaft,
directly proportionate to the height; thus in very tall flues, between
two and