Part 4 (1/2)

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 circ.u.mstances 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 a.s.signable 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 suns.h.i.+ne, 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 pa.s.sed 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.