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EXPLANATION OE THE KRAKEN. 91
produce a mirage ; this, the necessary obliquity, will be
maintained up to a certain height of the slope, and,
so far, the dark rock will be invisible, and its place
occupied by a bright reflecting surface. The light,
thus reflected, will be scarcely distinguishable from the
transmitted light of the horizon, and hence it appears
(unless carefully observed) that the bright part of the
rock or shore is transparent, or that the rock is cut off
from below: hence the gaping jaw.
This apparent uplifting of low islands and coasts,
and more particularly of the long promontory slopes
of islands or coasts, is a very common illusion, that
may be witnessed without going to Norway. The
Londoner may see it very strikingly displayed on any
fine summer afternoon, from the deck of a
steam-packet going to Margate, or the Nore, or Heme Bay,
when the sun has been shining brightly all day, on the
Essex coast at the mouth of the Thames. The Essex
coast is very low, and there are trees upon it; under
the conditions I have mentioned, the land becomes
invisible, and the trees appear suspended in the air.
Sometimes the lower half of the trees are also invisible,
and only their tops are seen, cut off from the earth, and
standing apparently unsupported considerably above the
line of the horizon. Any one who will carefully observe
these phenomena, on this or any similar coast, will, I
think, be satisfied that my explanation is correct; for
it only occurs under the conditions I have named, viz.,
when the land is warmer than the air over the sea:
it only extends to those parts seen at a great obliquity,
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