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90 THROUGH NORWAY WITII A KNAPSACK.
horizontally at the bottom of a glass vessel—such as
an aquarium tank—and viewed tlu’ough the sides, an
explanation of the mirage of the desert is at once
exhibited: the black surface disappears, and a mirror
takes its place; such a mirror as the thirsty traveller
sees upon the distant sands, and mistakes for a sheet
of water. The hot sand rarefies the film of air in
contact with it; the spectator’s head is immersed in a
denser stratum of air, and looking from that, very
obliquely to the rarer film upon the sand, he sees the
mirror just as you may see it on the air-film of the
blackened plate; but he sees it only afar off, near
the horizon, and not at his feet: and as he advances,
the bright illusion advances also; the reason of this
being, that the difference is so small between the density
of the film upon the sands and the stratum
enveloping his head, that a very great obliquity is necessary
for this total reflection to take place. Many other
explanations of the mirage have been given, but this
I believe to be the true one. The common explanation
that it is reflection from vapour will not bear
examination.
The reader, however, may still be at a loss to see how
this bears upon our sea-serpent and his uplifted jaw. It
is thus : let us suppose one of these island rocks to have
a sloping shore, or that there is a reef of low rocks
close to it; these, being heated, will be covered with
a film of rarefied air clinging to them for a while before
Ö O
ascending. Such rocks, or sloping coast, when near
the horizon, will be seen at an obliquity sufficient to
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