and from whom, in the first volume of
'Modern Painters,' I adopted it without sufficient examination.
Afterwards I re-examined it, and showed its fallacy, with respect
to the cap or helmet cloud, in the fifth volume of 'Modern
Painters,' page 124, in the terms given in the subjoined note,[A]
but I still retained the explanation of Saussure for the lee-side
cloud, engraving in plate 69 the modes of its occurrence on the
Aiguille Dru, of which the most ordinary one was afterwards
represented by Tyndall in his 'Glaciers of the Alps,' under the
title of 'Banner-cloud.' Its less imaginative title, in 'Modern
Painters,' of 'Lee-side cloud,' is more comprehensive, for this
cloud forms often under the brows of far-terraced precipices, where
it has no resemblance to a banner. No true explanation of it has
ever yet been given; for the first condition of the problem has
hitherto been unobserved,--namely, that such cloud is constant in
certain states of weather, under precipitous rocks;--but never
developed with distinctness by domes of snow.

[Illustration]

But my former expansion of Saussure's theory is at least closer to
the facts than Professor Tyndall's "rubbing against the rocks," and
I therefore allow room for it here, with its illustrative wood-cut.

"When a moist wind blows in clear weather over a cold summit, it
has not time to get chilled as it approaches the rock, and
therefore the air remains clear, and the sky bright on the windward
side; but under the lee of the peak, there is partly a back eddy,
and partly still air; and in that lull and eddy the wind gets time
to be chilled by the rock, and the cloud appears, as a boiling mass
of white vapor, rising continually with the return current to the
upper edge of the mountain, where it is caught by the straight wind
and partly torn, partly melted away in broken fragments.

"In the accompanying figure, the dark mass represents the mountain
peak, the arrow the main direction of the wind, the curved lines
show the directions of such current and its concentration, and the
dotted line encloses the space in which cloud forms densely,
floating away beyond and above in irregular tongues and flakes."

[Footnote A: "But both Saussure and I ought to have known,--we did
know, but did not think of it,--that the covering or cap-cloud
forms on hot summits as well as cold ones;--that the red and bare
rocks of Mont Pilate, hotter, certainly, after a day's sunshine
than the cold st