If the
heated surface be circular, the air flows to it from every quarter,
like the rays of a circle to its centre. If it be a zone of determinate
breadth and indefinite length, the air will flow from each side
perpendicularly on it. If the currents of air flowing from opposite
sides, be of equal force, they will meet in equilibrio, at a line
drawn longitudinally through the middle of the zone. If one current be
stronger than the other, the stronger one will force back the line of
equilibrium, towards the further edge of the zone, or even beyond it:
the motion it has acquired causing it to overshoot the zone, as the
motion acquired by a pendulum in its descent, causes it to vibrate
beyond the point of its lowest descent.
Earth, exposed naked to the sun's rays, absorbs a good portion of them;
but, being an opaque body, those rays penetrate to a small depth only.
Its surface, by this accumulation of absorbed rays, becomes considerably
heated. The residue of the rays are reflected into the air resting on
that surface. This air, then, is warmed, 1. by the direct rays of the
sun; 2.
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