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."
[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."
[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.
[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!
[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.)
"ἅμα δὲ νέφος εἴπετο πεζῶν.
Ὡς δ' ὅτ' ἀπὸ σκοπιῆς εἶδεν νέφος ἀιπόλος ἀνὴρ
Ἐρχόμενον κατὰ πόντον ὑπὸ Ζεφύροιο ἰωῆς,
Τῶ δέ τ', ἄνευθεν ἔοντι, μελάντερον, ἠύτε πίσσα
Φαίνετ', ἰὸν κατὰ πόντον, ἄγει δέ τε λάιλαπα πολλήν‧
Ῥιγησέν τε ἰδὼν, ὑπό τε σπέος ἤλασε μῆλα‧
Τοῖαι ἅμ Αἰάντεσσιν ἀρηϊθόων αἰζηῶν
Δήϊον ἐσ πόλεμον πυκιναὶ κίνυντο φάλαγγες
Κυάνεαι,"
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."