finally (C) reversing the principle under letter A, the cap-cloud constantly forms on the summit of Mont Blanc, while it will obstinately refuse to appear on the Dome du Goûte or Aiguille Sans-nom, where the snow-fields are of greater extent, and the air must be moister, because lower. [Illustration: J. Ruskin. J. C. Armytage. 69. Aiguilles and their Friends.] [Illustration: FIG. 86.] § 5. The fact is, that the explanation given in that fourth paragraph can, in reality, account only for what may properly be termed "lee-side cloud," slightly noticed in the continuation of the same chapter, but deserving most attentive illustration, as one of the most beautiful phenomena of the Alps. 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 Fig. 86 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 lines enclose the space in which cloud forms densely, floating away beyond and above in irregular tongues and flakes. The second figure from the top in Plate 69 represents the actual aspect of it when in full development, with a strong south wind, in a clear day, on the Aiguille Dru, the sky being perfectly blue and lovely around. So far all is satisfactory. But the true helmet cloud will not allow itself to be thus explained away. The uppermost figure in Plate 69 represents the loveliest form of it, seen in that perfect arch, so far as I know, only over the highest piece of earth in Europe. § 6. Respecting which there are two mysteries:--First, why it should form only at a certain distance above the snow, showing blue sky between it and the summit. Secondly, why, so forming, it should always show as an arch, not as a concave cup. This last question puzzles me especially. For, if it be a true arch, and not a cup, it ought to show itself in certain positions of the spectator, or directions of the wind, like the ring of Saturn, as a mere line, or as a spot of cloud pausing over the hill-top. But I never saw it so. While, as above noticed, the lowest form of the helmet cloud is not white as of silver, but like Dolon's helmet of wolf-skin,--it is a gray, flaky veil, lapping itself over the shoulders of a more or less conical peak; and of this, also, I have no word to utter but the old one, "Electricity," and I might as well say nothing.