The Storm-Cloud of the Nineteenth Century

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,

[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!]

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.

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.)

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.

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.

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

[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"]

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.

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."

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.

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.

[I trust that Dr. Rae will forgive my making the reader better aware of the real value of this communication by allowing him to see also the following passage from the kind private letter by which it was supplemented:—

"Many years in the Hudson's Bay Company's service, I and my men became educated for Arctic work, in which I was five different times employed, in two of which expeditions we lived wholly by our own hunting and fishing for twelve months, once in a stone house (very disagreeable), and another winter in a snow hut (better), without fire of any kind to warm us. On the first of these expeditions, 1846-7, my little party, there being no officer but myself, surveyed seven hundred miles of coast of Arctic America by a sledge journey, which Parry, Ross, Bach, and Lyon had failed to accomplish, costing the country about £70,000 or £80,000 at the lowest computation. The total expense of my little party, including my own pay, was under fourteen hundred pounds sterling.

"My Arctic work has been recognized by the award of the founder's gold medal of the Royal Geographical Society (before the completion of the whole of it)"]

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,

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.

Nor, so far as I can read, has any proper account yet been given of the balance, in serene air, of the warm air under the cold, in which the warm air is at once compressed by weight, and expanded by heat, and the cold air is thinned by its elevation, yet contracted by its cold. There is indeed no possibility of embracing the conditions in a single sentence, any more than in a single thought. But the practical balance is effected in calm air, so that its lower strata have no tendency to rise, like the air in a fire balloon, nor its higher strata to fall, unless they congeal into rain or snow.

I believe it will be an extreme benefit to my younger readers if I write for them a little 'Grammar of Ice and Air,' collecting the known facts on all these matters, and I am much minded to put by my ecclesiastical history for a while, in order to relate what is legible of the history of the visible Heaven.