orm-wind which sweeps to them from the Alps,
nevertheless have been renowned for their helmet of cloud, ever
since the Romans watched the cloven summit, gray against the south,
from the ramparts of Vindonissa, giving it the name from which the
good Catholics of Lucerne have warped out their favorite piece of
terrific sacred biography. And both my master and I should also
have reflected that if our theory about its formation had been
generally true, the helmet cloud ought to form on every cold
summit, at the approach of rain, in approximating proportions to
the bulk of the glaciers; which is so far from being the case that
not only (A) the cap-cloud may often be seen on lower summits of
grass or rock, while the higher ones are splendidly clear (which
may be accounted for by supposing the wind containing the moisture
not to have risen so high); but (B) the cap-cloud always shows a
preference for hills of a conical form, such as the Mole or Niesen,
which can have very little power in chilling the air, even
supposing they were cold themselves; while it will entirely refuse
to form huge masses of mountain, which, supposing them of chilly
temperament, must have discomforted the atmosphere in their
neighborhood for leagues."]]

[Footnote 12: See below, on the different uses of the word
'reflection,' note 14, and note that throughout this lecture I use
the words 'aqueous molecules,' alike of water liquid or vaporized,
not knowing under what conditions or at what temperatures
water-dust becomes water-gas; and still less, supposing pure
water-gas blue, and pure air blue, what are the changes in either
which make them what sailors call "dirty "; but it is one of the
worst omissions of the previous lecture, that I have not stated
among the characters of the plague-cloud that it is _always_
dirty,[A] and _never blue under any conditions_, neither when deep
in the distance, nor when in the electric states which produce
sulphurous blues in natural cloud. But see the next note.

[Footnote A: In my final collation of the lectures given at Oxford
last year on the Art of England, I shall have occasion to take
notice of the effect of this character of plague-cloud on our
younger painters, who have perhaps never in their lives seen a
_clean_ sky!]]

[Footnote 13: Black clouds.--For the sudden and extreme local
blackness of thundercloud, see Turner's drawing of Winchelsea,
(England series), and compare Homer, of the Ajaces, in the 4th b