paragraph of Professor Tyndall's 'Forms of Water,' and the
following seventh one, in which the phenomenon of transparent steam
becoming opaque is thus explained. "Every bit of steam shrinks,
when chilled, to a much more minute particle of water. The liquid
particles thus produced form a kind of water dust of exceeding
fineness, which floats in the air, and is called a cloud."
But the author does not tell us, in the first place, what is the
shape or nature of a 'bit of steam,' nor, in the second place, how
the contraction of the individual bits of steam is effected without
any diminution of the whole mass of them, but on the contrary,
during its steady _expansion_; in the third place he assumes that
the particles of water dust are solid, not vesicular, which is not
yet ascertained; in the fourth place, he does not tell us how their
number and size are related to the quantity of invisible moisture
in the air; in the fifth place, he does not tell us how cool
invisible moisture differs from hot invisible moisture; and in the
sixth, he does not tell us why the cool visible moisture stays
while the hot visible moisture melts away. So much for the present
state of 'scientific' information, or at least communicativeness,
on the first and simplest conditions of the problem before us!
In its wider range that problem embraces the total mystery of
volatile power in substance; and of the visible states consequent
on sudden--and presumably, therefore, imperfect--vaporization; as
the smoke of frankincense, or the sacred fume of modern devotion
which now fills the inhabited world, as that of the rose and violet
its deserts. What,--it would be useful to know, is the actual bulk
of an atom of orange perfume?--what of one of vaporized tobacco, or
gunpowder?--and where do _these_ artificial vapors fall back in
beneficent rain? or through what areas of atmosphere exist, as
invisible, though perhaps not innocuous, cloud?
All these questions were put, closely and precisely,
four-and-twenty years ago, in the 1st chapter of the 7th part of
'Modern Painters,' paragraphs 4 to 9, of which I can here allow
space only for the last, which expresses the final difficulties of
the matter better than anything said in this lecture:--
"But farther: these questions of volatility, and visibility, and
hue, are all complicated with those of shape. How is a cloud
outlined? Granted whatever you choose to ask, concerning its
material, or its aspect, its loftiness and luminousness,--how of
its limitation? What hews it into a heap, or spins it into a web?
Cold is usually shapeless, I suppose, extending over large spaces
equally, or with gradual diminution. You cannot have in the open
air, angles, and wedges, and coils, and cliffs, of cold. Yet the
vapor stops suddenly, sharp and steep as a rock, or thrusts itself
across the gates of heaven in likeness of a brazen bar; or braids
itself in and out, and across and across, like a tissue of
tapestry; or falls into ripples, like sand; or into waving shreds
and tongues, as fire. On what anvils and wheels is the vapor
pointed, twisted, hammered, whirled, as the potter's clay? By what
hands is the incense of the sea built up into domes of marble?"]
[Footnote 9: The opposed conditions of the higher and lower orders
of cloud, with the balanced intermediate one, are beautifully seen
on mountain summits of rock or earth. On snowy ones they are far
more complex: but on rock summits there are three distinct forms of
attached cloud in serene weather; the first that of cloud veil
laid over them, and _falling_ in folds through their ravines,
(the obliquely descending clouds of the entering chorus in
Aristophanes); secondly, the ascending cloud, which develops itself
loosely and independently as it rises, and does not attach itself
to the hill-side, while the falling veil cloud clings to it close
all the way down;--and lastly the throned cloud, which rests indeed
on the mountain summit, with its base, but rises high above into
the sky, continually changing its outlines, but holding its seat
perhaps all day long.
These three forms of cloud belong exclusively to calm weather;
attached drift cloud, (see Note 11) can only be formed in the
wind.]
[Footnote 10: 'Glaciers of the Alps,' page 10.--"Let a pound weight
be placed upon a cube of granite" (size of supposed cube not
mentioned), "the cube is flattened, though in an infinitesimal
degree. Let the weight be removed, the cube remains a little
flattened. Let us call the cube thus flattened No. 1. Starting with
No. 1 as a new mass, let the pound weight be laid upon it. We have
a more flattened mass, No. 2.... Apply this to squeezed rocks, to
those, for example, which form the base of an obelisk like the
Matterhorn,--the conclusion seems inevitable _that the mountain is
sinking by its own weight_," etc., etc. Similarly the Nelson statue
must be gradually flattening the Nelson column, and in time
Cleopatra's needle will be as flat as her pincushion?]
[Footnote 11: 'Glaciers of the Alps,' page 146.--"The sun was near
the western horizon, and I remained alone upon the Grat to see his
last beams illuminate the mountains, which, with one exception,
were without a trace of cloud.
"This exception was the Matterhorn, the appearance of which was
extremely instructive. The obelisk appeared to be divided in two
halves by a vertical line, drawn from its summit half-way down, to
the windward of which we had the bare cliffs of the mountain; and
to the left of it a cloud which appeared to cling tenaciously to
the rocks.
"In reality, however, there was no clinging; the condensed vapor
incessantly got away, but it was ever renewed, and thus a river of
cloud had been sent from the mountain over the valley of Aosta. The
wind, in fact, blew lightly up the valley of St. Nicholas, charged
with moisture, and when the air that held it _rubbed against the
cold cone_ of the Matterhorn, the vapor was chilled and
precipitated in his lee."
It is not explained, why the wind was not chilled by rubbing
against any of the neighboring mountains, nor why the cone of the
Matterhorn, mostly of rock, should be colder than cones of snow.
The phenomenon was first described by De Saussure, who gives the
same explanation as Tyndall; 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 storm-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 book
of the Iliad,--(I came on the passage in verifying Mr. Hill's
quotation from the 5th.)
"[Greek: hama de nephos eipeto pezôn.
Hôs d' hot' apo skopiês eiden nephos aipolos anêr
Erchomenon kata ponton hypo Zephyroio iôês,
Tô de t', aneuthen eonti, melanteron, êute pissa
Phainet', ion kata ponton, agei de te lailapa pollên;
Rhigêsen te idôn, hypo te speos êlase mêla;
Toiai ham Aiantessin arêithoôn aizêôn
Dêion es polemon pykinai kinynto phalanges
Kyaneai,]"
I give Chapman's version--noting only that his _breath_ of
Zephyrus, ought to have been 'cry' or 'roar' of Zephyrus, the
blackness of the cloud being as much connected with the wildness of
the wind as, in the formerly quoted passage, its brightness with
calm of air.
"Behind them hid the ground
A cloud of foot, that seemed to smoke. And as a Goatherd spies
On some hill top, out of the sea a rainy vapor rise,
Driven by the breath of Zephyrus, which though far off he rests,
Comes on as black as pitch, and brings a tempest in his breast
Whereat he, frighted, drives his herds apace into a den;
So, darkening earth, with swords and shields, showed these with
all their men."
I add here Chapman's version of the other passage, which is
extremely beautiful and close to the text, while Pope's is
hopelessly erroneous.
"Their ground they still made good,
And in their silence and set powers, like fair still clouds they stood,
With which Jove crowns the tops of hills in any quiet day
When Boreas, and the ruder winds that use to drive away
Air's _dusky vapors_, being _loose_, in many a whistling gale,
Are pleasingly bound up and calm, and not a breath exhale."]
[Footnote 14: 'Reflected.'--The reader must be warned in this place
of the difference implied by my use of the word 'cast' in page 11,
and 'reflected' here: that is to say, between light or color which
an object possesses, whatever the angle it is seen at, and the
light which it reverberates at one angle only. The Alps, under the
rose[A] of sunset, are exactly of the same color whether you see
them from Berne or Schaffhausen. But the gilding to our eyes of a
burnished cloud depends, I believe, at least for a measure of its
luster, upon the angle at which the rays incident upon it are
reflected to the eye, just as much as the glittering of the sea
beneath it--or the sparkling of the windows of the houses on the
shore.
Previously, at page 10, in calling the molecules of transparent
atmospheric 'absolutely' unreflective of light, I mean, in like
manner, unreflective from their _surfaces_. Their blue color seen
against a dark ground is indeed a kind of reflection, but one of
which I do not understand the nature. It is seen most simply in
wood smoke, blue against trees, brown against clear light; but in
both cases the color is communicated to (or left in) the
_transmitted_ rays.
So also the green of the sky (p. 13) is said to be given by
transmitted light, yellow rays passing through blue air: much yet
remains to be known respecting translucent colors of this kind;
only let them always be clearly distinguished in our minds from the
firmly possessed color of opaque substances, like grass or
malachite.
[Footnote A: In speaking, at p. 11 of the first lecture, of the
limits of depth in the rose-color cast on snow, I ought to have
noted the greater strength of the tint possible under the light of
the tropics. The following passage, in Mr. Cunningham's 'Natural
History of the Strait of Magellan,' is to me of the greatest
interest, because of the beautiful effect described as seen on the
occasion of his visit to "the small town of Santa Rosa," (near
Valparaiso.) "The day, though clear, had not been sunny, so that,
although the snowy heights of the Andes had been distinctly visible
throughout the greater part of our journey, they had not been
illuminated by the rays of the sun. But now, as we turned the
corner of a street, the chain of the Cordillera suddenly burst on
our gaze in such a blaze of splendor that it almost seemed as if
the windows of heaven had been opened for a moment, permitting a
flood of _crimson_ light to stream forth upon the snow. The sight
was so unexpected, and so transcendently magnificent, that a
breathless silence fell upon us for a few moments, while even the
driver stopped his horses. This deep red glow lasted for three or
four minutes, and then rapidly faded into that lovely rosy hue so
characteristic of snow at sunset among the Alps."]]
[Footnote 15: Diffraction.--Since these passages were written, I
have been led, in conversation with a scientific friend, to doubt
my statement that the colored portions of the lighted clouds were
brighter than the white ones. He was convinced that the resolution
of the rays would diminish their power, and in _thinking_ over the
matter, I am disposed to agree with him, although my impression at
the time has been always that the diffracted colors rose out of the
white, as a rainbow does out of the gray. But whatever the facts
may be, in this respect the statement in the text of the
impossibility of representing diffracted color in painting is
equally true. It may be that the resolved hues are darker than the
white, as colored panes in a window are darker than the colorless
glass, but all are alike in a key which no artifice of painting can
approach.
For the rest, the phenomena of diffraction are not yet arranged
systematically enough to be usefully discussed; some of them
involving the resolution of the light, and others merely its
intensification. My attention was first drawn to them near St.
Laurent, on the Jura mountains, by the vivid reflection, (so it
seemed), of the image of the sun from a particular point of a cloud
in the west, after the sun itself was beneath the horizon: but in
this image there were no prismatic colors, neither is the
constantly seen metamorphosis of pine forests into silver filigree
on ridges behind which the sun is rising or setting, accompanied
with any prismatic hue; the trees become luminous, but not
iridescent: on the other hand, in his great account of his ascent
of Mont Blanc with Mr. Huxley, Professor Tyndall thus describes the
sun's remarkable behavior on that occasion:--"As we attained the
brow which forms the entrance to the Grand Plateau, he _hung his
disk upon a spike of rock_ to our left, and, surrounded by a glory
of interference spectra of the most gorgeous colors, blazed down
upon us." ('Glaciers of the Alps,' p. 76.)
Nothing irritates me more, myself, than having the color of my own
descriptions of phenomena in anywise attributed by the reader to
accidental states either of my mind or body;--but I cannot, for
once, forbear at least the innocent question to Professor Tyndall,
whether the extreme beauty of these 'interference spectra' may not
have been partly owing to the extreme _sobriety_ of the observer?
no refreshment, it appears, having been attainable the night before
at the Grands Mulets, except the beverage diluted with dirty snow,
of which I have elsewhere quoted the Professor's pensive
report,--"my memory of that tea is not pleasant."]
[Footnote 16: 'Either stationary or slow in motion, reflecting
unresolved light.'
The rate of motion is of course not 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 definite
observations which he has had the kindness to set down with their
dates for me, in the following letter:--
"4, ADDISON GARDENS, KENSINGTON, _4th Feb., 1884._
DEAR SIR,--I have looked up my old journal of thirty years ago,
written in pencil because it was impossible to keep ink unfrozen in
the snow-hut in which I passed the winter of 1853-4, at Repulse
Bay, on the Arctic Circle.[A]
On the 1st of February, 1854, I find the following:--
'A beautiful appearance of some cirrus clouds near the sun, the
central part of the cloud being of a fine pink or red, then green,
and pink fringe. This continued for about a quarter of an hour. The
same was observed on the 27th of the month, but not so bright.
Distance of clouds from sun, from 3° to 6°.'
On the 1st February the temperature was 38° below zero, and on the
27th February 26° below.
'On the 23d and 30th (of March) the same splendid appearance of
clouds as mentioned in last month's journal was observed. On the
first of these days, about 10.30 a.m., it was extremely beautiful.
The clouds were about 8° or 10° from the sun, below him and
slightly to the eastward,--having a green fringe all round, then
pink; the center part at first green, and then pink or red.'
The temperature was 21° below zero, Fahrenheit.
There may have been other colors--blue, perhaps--but I merely noted
the most prominent; and what I call green may have been bluish,
although I do not mention this last color in my notes.
From the lowness of the temperature at the time, the clouds _must_
have been frozen moisture.
The phenomenon is by no means common, even in the Arctic zone.
The second beautiful cloud-picture shown this afternoon brought so
visibly to my memory the appearance seen by me as above described,
that I could not avoid remarking upon it.
Believe me very truly yours,
JOHN RAE." (M.D., F.R.S.)
Now this letter enables me to leave the elements of your problem
for you in very clear terms.
Your sky--altogether--may be composed of one or more of four
things:--
Molecules of water in warm weather.
Molecules of ice in cold weather.
Molecules of water-vapor in warm weather.
Molecules of ice-vapor in cold weather.
But of the size, distances, or modes of attraction between these
different kinds of particles, I find no definite information
anywhere, except the somewhat vague statement by Sir William
Thomson, that "if a drop of water could be magnified so as to be as
large as the earth, and have a diameter of eight thousand miles,
then a molecule of this water in it would appear _somewhat larger
than a shot_." (What kind of shot?) "_and somewhat smaller than a
cricket-ball_"!
And as I finally review the common accounts given of cloud
formation, I find it quite hopeless for the general reader to deal
with the quantity of points which have to be kept in mind and
severally valued, before he can account for any given phenomena. I
have myself, in many of the passages of 'Modern Painters' before
referred to, conceived of cloud too narrowly as always produced by
_cold_, whereas the temperature of a cloud must continually, like
that of our visible breath in frosty weather, or of the visible
current of steam, or the smoking of a warm lake surface under
sudden frost, be above that of the surrounding atmosphere; and yet
I never remember entering a cloud without being chilled by it, and
the darkness of the plague-wind, unless in electric states of the
air, is always accompanied by deadly chill.