t we do not know, I have sometimes
indulged myself with conjectures on the causes of the phenomena above
stated. I will hazard them on paper, for your amusement, premising for
their foundation some principles believed to be true.

Air resting on a heated and reflecting surface, becomes warmer, rarer,
and lighter: it ascends therefore, and the circumjacent air, which is
colder and heavier, flows into its place, becomes warmed and lightened
in its turn, ascends, and is succeeded as that which went before. 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. by its reflected rays; 3. by contact with the heated surface.
A forest receiving the sun's rays, a part of them enters the intervals
between the trees, and their reflection upwards is intercepted by the
leaves and boughs. The rest fall on the trees, the leaves of which being
generally inclined towards the horizon, reflect the rays downwards. The
atmosphere here, then, receives little or no heat by reflection. Again,
these leaves having a power of keeping themselves cool by their own
transpiration, they impart no heat to the air by contact. Reflection
and contact, then, two of the three modes before-mentioned, of
communicating heat, are wanting here; and, of course, the air over
a country covered by forest must be colder than that over cultivated
grounds.

The sea being pellucid, the sun's rays penetrate it to a considerable
depth. Being also f