where
evaporation proceeded fastest, that is at the height of my waist, little
wisps of mist would detach themselves from the side of the funnel of
clear air in which I stood, and they would, in a slow, graceful motion,
accelerated somewhat towards the last, describe a downward and inward
curve towards the lower part of my body before they dissolved. I thought
of that elusive and yet clearly defined layer of mist that forms in
the plane of contact between the cold air flowing from Mammoth Cave
in Kentucky and the ambient air of a sultry summer day. [Footnote: See
Burroughs' wonderful description of this phenomenon in "Riverby."]

On another of the rare occasions when the mists had formed in the
necessary density I went out again, put a stone in my pocket and took a
dog along. I approached a shallow mist pool with the greatest caution.
The dog crouched low, apparently thinking that I was stalking some game.
Then, when I had arrived within about ten or fifteen yards from the edge
of the pool, I took the stone from my pocket, showed it to the dog, and
threw it across the pool as fast and as far as I could. The dog dashed
in and tore through the sheet. Where the impact of his body came, the
mist bulged in, then broke. For a while there were two sheets, separated
by a more or less clearly defined, vertical layer of transparency
or maybe blackness rather. The two sheets were in violent commotion,
approaching, impinging upon each other, swinging back again to complete
separation, and so on. But the violence of the motion consisted by
no means in speed: it suggested a very much retarded rolling off of a
motion picture reel. There was at first an element of disillusion in the
impression. I felt tempted to shout and to spur the mist into greater
activity. On the surface, to both sides of the tear, waves ran out, and
at the edges of the pool they rose in that same leisurely, stately
way which struck me as one of the most characteristic features of that
November mist; and at last it seemed as if they reared and reached up,
very slowly as a dying man may stand up once more before he falls. And
only after an interval that seemed unconscionably long to me the whole
pool settled back to comparative smoothness, though without its definite
plane of demarcation now. Strange to say, the dog had actually started
something, a rabbit maybe or a jumping deer, and did not return.

When fogs spread, as a rule they do so in air already saturated