essentially connected with the
method of illumination; their connection, in this instance, needs
explanation of some points which could not be dealt with in the
time of a single lecture.

It is before said, with reserve only, that "a cloud is where it is
seen, and is not where it is not seen." But thirty years ago, in
'Modern Painters,' I pointed out (see the paragraph quoted in note
8th), the extreme difficulty of arriving at the cause of cloud
outline, or explaining how, if we admitted at any given moment the
atmospheric moisture to be generally diffused, it could be chilled
by formal _chills_ into formal clouds. How, for instance, in the
upper cirri, a thousand little chills, alternating with a thousand
little warmths, could stand still as a thousand little feathers.

But the first step to any elucidation of the matter is in the
firmly fixing in our minds the difference between windless clouds,
unaffected by any conceivable local accident, and windy clouds,
affected by some change in their circumstances as they move.

In the sunset at Abbeville, represented in my first diagram, the
air is absolutely calm at the ground surface, and the motion of its
upper currents extremely slow. There is no local reason assignable
for the presence of the cirri above, or of the thundercloud below.
There is no conceivable cause either in the geology, or the moral
character, of the two sides of the town of Abbeville, to explain
why there should be decorative fresco on the sky over the southern
suburb, and a muttering heap of gloom and danger over the northern.
The electric cloud is as calm in motion as the harmless one; it
changes its forms, indeed; but imperceptibly; and, so far as can
be discerned, only at its own will is exalted, and with its own
consent abased.

But in my second diagram are shown forms of vapor sustaining at
every instant all kinds of varying local influences; beneath,
fastened down by mountain attraction, above, flung afar by
distracting winds; here, spread abroad into blanched sheets beneath
the sunshine, and presently gathered into strands of coiled cordage
in the shade. Their total existence is in metamorphosis, and their
every aspect a surprise, or a deceit.]

[Footnote 17: 'Finely comminuted water or _ice_.'

My impression that these clouds were glacial was at once confirmed
by a member of my audience, Dr. John Rae, in conversation after the
lecture, in which he communicated to me the perfectly defini