like a Judge of the Vehmgericht Tribunal,—I leave these questions with you, and pass on.
Admitting degrees of darkness, we have next to ask what color, from sunshine can the white cloud receive, and what the black?
You won't expect me to tell you all that, or even the little that is accurately known about that, in a quarter of an hour; yet note these main facts on the matter.
On any pure white, and practically opaque, cloud, or thing like a cloud, as an Alp, or Milan Cathedral, you can have cast by rising or setting sunlight, any tints of amber, orange, or moderately deep rose—you can't have lemon yellows, or any kind of green except in negative hue by opposition; and though by stormlight you may sometimes get the reds cast very deep, beyond a certain limit you cannot go,—the Alps are never vermilion color, nor flamingo color, nor canary color; nor did you ever see a full scarlet cumulus of thundercloud.
On opaque white vapor, then, remember, you can get a glow or a blush of color, never a flame of it.
But when the cloud is transparent as well as pure, and can be filled with light through all the body of it, you then can have by the light reflected14 from its atoms any force conceivable by human mind of the entire group of the golden and ruby colors, from intensely burnished gold color, through a scarlet for whose brightness there are no words, into any depth and any hue of Tyrian crimson and Byzantine purple. These with full blue breathed between them at the zenith, and green blue nearer the horizon, form the scales and chords of color possible to the morning and evening sky in pure and fine weather; the keynote of the opposition being vermilion against green blue, both of equal tone, and at such a height and acme of brilliancy that you cannot see the line where their edges pass into each other.
No colors that can be fixed in earth can ever represent to you the luster of these cloudy ones. But the actual tints may be shown you in a lower key, and to a certain extent their power and relation to each other.
I have painted the diagram here shown you with colors prepared for me lately by Messrs. Newman, which I find brilliant to the height that pigments can be; and the ready kindness of Mr. Wilson Barrett enables me to show you their effect by a white light as pure as that of the day. The diagram is enlarged from my careful sketch of the sunset of 1st October, 1868, at Abbeville, which was a beautiful example of what, in fine weather about to pass into storm, a sunset could then be, in the districts of Kent and Picardy unaffected by smoke. In reality, the ruby and vermilion clouds were, by myriads, more numerous than I have had time to paint: but the general character of their grouping is well enough expressed. All the illumined clouds are high in the air, and nearly motionless; beneath them, electric storm-cloud rises in a threatening cumulus on the right, and drifts in dark flakes across the horizon, casting from its broken masses radiating shadows on the upper clouds. These shadows are traced, in the first place by making the misty blue of the open sky more transparent, and therefore darker; and secondly, by entirely intercepting the sunbeams on the bars of cloud, which, within the shadowed spaces, show dark on the blue instead of light.
But, mind, all that is done by reflected light—and in that light you never get a green ray from the reflecting cloud; there is no such thing in nature as a green lighted cloud relieved from a red sky,—the cloud is always red, and the sky green, and green, observe, by transmitted, not reflected light.
But now note, there is another kind of cloud, pure white, and exquisitely delicate; which acts not by reflecting, nor by refracting, but, as it is now called, diffracting, the sun's rays. The particles of this cloud are said—with what truth I know not15—to send the sunbeams round them instead of through them; somehow or other, at any rate, they resolve them into their prismatic elements; and then you have literally a kaleidoscope in the sky, with every color of the prism in absolute purity; but above all in force, now, the ruby red and the green,—with purple, and violet-blue, in a virtual equality, more definite than that of the rainbow. The red in the rainbow is mostly brick red, the violet, though beautiful, often lost at the edge; but in the prismatic cloud the violet, the green, and the ruby are all more lovely than in any precious stones, and they are varied as in a bird's breast, changing their places, depths, and extent at every instant.
The main cause of this change being, that the prismatic cloud itself is always in rapid, and generally in fluctuating motion. "A light veil of clouds had drawn itself," says Professor Tyndall, in describing his solitary ascent of Monte Rosa, "between me and the sun, and this was flooded with the most brilliant dyes. Orange, red, green, blue—all the hues produced by diffraction—were exhibited in the utmost splendor.
"Three times during my ascent (the short ascent of the last peak) similar veils drew themselves across the sun, and at each passage the splendid phenomena were renewed. There seemed a tendency to form circular zones of color round the sun; but the clouds were not sufficiently uniform to permit of this, and they were consequently broken into spaces, each steeped with the color due to the condition of the cloud at the place."
Three times, you observe, the veil passed, and three times another came, or the first faded and another formed; and so it is always, as far as I have registered prismatic cloud: and the most beautiful colors I ever saw were on those that flew fastest.
This second diagram is enlarged admirably by Mr. Arthur Severn from my sketch of the sky in the afternoon of the 6th of August, 1880, at Brantwood, two hours before sunset. You are looking west by north, straight towards the sun, and nearly straight towards the wind. From the west the wind blows fiercely towards you out of the blue sky. Under the blue space is a flattened dome of earth-cloud clinging to, and altogether masking the form of, the mountain, known as the Old Man of Coniston.
The top of that dome of cloud is two thousand eight hundred feet above the sea, the mountain two thousand six hundred, the cloud lying two hundred feet deep on it. Behind it, westward and seaward, all's clear; but when the wind out of that blue clearness comes over the ridge of the earth-cloud, at that moment and that line, its own moisture congeals into these white—I believe, ice-clouds; threads, and meshes, and tresses, and tapestries, flying, failing, melting, reappearing; spinning and unspinning themselves, coiling and uncoiling, winding and unwinding, faster than eye or thought can follow: and through all their dazzling maze of frosty filaments shines a painted window in palpitation; its pulses of color interwoven in motion, intermittent in fire,—emerald and ruby and pale purple and violet melting into a blue that is not of the sky, but of the sunbeam;—purer than the crystal, softer than the rainbow, and brighter than the snow.
But you must please here observe that while my first diagram did with some adequateness represent to you the color facts there spoken of, the present diagram can only explain, not reproduce them. The bright reflected colors of clouds can be represented in painting, because they are relieved against darker colors, or, in many cases, are dark colors, the vermilion and ruby clouds being often much darker than the green or blue sky beyond them. But in the case of the phenomena now under your attention, the colors are all brighter than pure white,—the entire body of the cloud in which they show themselves being white by transmitted light, so that I can only show you what the colors are, and where they are,—but leaving them dark on the white ground. Only artificial, and very high illumination would give the real effect of them,—painting cannot.
Enough, however, is here done to fix in your minds the distinction between those two species of cloud,—one, either stationary,16 or slow in motion, reflecting unresolved light; the other, fast-flying, and transmitting resolved light. What difference is there in the nature of the atoms, between those two kinds of clouds? I leave the question with you for to-day, merely hinting to you my suspicion that the prismatic cloud is of finely-comminuted water, or ice,17 instead of aqueous vapor; but the only clue I have to this idea is in the purity of the rainbow formed in frost mist, lying close to water surfaces. Such mist, however, only becomes prismatic as common rain does,
'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
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
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.
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
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
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
'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
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.
'Finely comminuted water or
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,
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),
"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
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
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
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.
