The experiment proved that the currents alternated, and,
as I fancied, regularly; and in order to determine, if possible, the
law of this alternation, I observed with my watch the exact duration
of each current. For twenty-two seconds the flame of the _bougie_ was
blown away from the entrance, so strongly as to assume a horizontal
position, and almost to leave the wick: then the current ceased, and
the flame rose with a stately air to a vertical position, moving down
again steadily till it became once more horizontal, but now pointing
in towards the cave. This change occupied in all four seconds; and the
current inwards lasted--like the outward current--twenty-two seconds,
and then the whole phenomenon was repeated. The currents kept such
good time, that when I stood beyond their reach, and turned my back, I
was enabled to announce each change with perfect precision. On one
occasion, the flame performed its semicircle in a horizontal instead
of a vertical plane, moving round the wick in the shape of a
pea-flower. The day was very still, so that no external winds could
have anything to do with this singular alternation; and, indeed, the
pit was so completely sheltered by its shape, that a storm might have
raged outside without producing any perceptible effect below.
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