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rivers on the continent, a mist or fog began to appear as soon as
the temperature of the air was diminished from 3 to 6 degrees
below that of the water. This will depend upon the previous
moisture or dryness of the air, and partly on the current of the
wind; but a fog is seldom seen on these lakes, until the
difference of the temperature is more than 12 degrees.
On the disappearance of the sun in a clear evening, a mist is
sometimes observed over a piece of moist ground; where it seems
to be formed, and for some time kept afloat, by a kind of
contention between the heated surface of the earth below it, and
the colder atmosphere above; but the earth not continuing to
afford the necessary supply of heat, the conflict ceases; and the
vapour settles on the grass in the shape of dew. When walking
along the side of a hill sloping gently towards the west, on the
early part of a clear dewy morning, it is interesting to observe
the brilliant circle of light, formed round the head of the
spectator's shadow, by the reflection of the solar rays from that
part of each globule of dew which is directly opposite to the
sun.
One fruitful source of the fog so much complained of in the
metropolis is smoke, which the cold air above deprives of its
caloric before its contents are sufficiently dissipated in the
atmosphere; so that the inhabitants of large towns are enveloped
in clouds of their own creating, and obliged to burn
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