On the tops of the highest mountains, even in very fine weather, the wind often blows with great force; and the north wind, supposed to be the mountaineer's best friend, is sometimes his enemy. It not unfrequently happens that a gale renders the passage of some exposed slope or ridge too dangerous, or the intense cold produces frost-bites, so that an expedition has to be abandoned when success is within reach, which naturally is very annoying. Professor Bonney, speaking of such a gale which he experienced in 1864, says,-- "The cold was something horrible; the wind seemed to blow not round, but through me, freezing my very marrow, and making my teeth chatter like castanets; and if I stopped for a moment, I shook as if in an ague-fit. It whisked up the small spiculæ of frozen snow, and dashed them against my face with such violence that it was hardly possible to look to windward. Thin sheets of ice as large as my hand were whirled along the surface of the glacier like paper.... When these gales are raging, the drifted snow is blown far to leeward of the peaks in long streamers like delicate cirrus-clouds; and on such occasions the mountain is said by the guides _fumer sa pipe_ (to smoke his pipe). This Mont Blanc was doing to some purpose the day that we were upon him." It is a curious fact that these gales are often confined to the crests of the mountains, so that the wind may be raging among the peaks while a few hundred feet lower down there is comparative calm. The chief of the prevailing winds in the Alps is the Föhn. This is a hot blast from the south which probably comes from the African deserts. On its approach the air becomes close and stifling, the sky, at first of unusual clearness, gradually thickens to a muddy and murky hue, animals become restless and disquieted by the unnatural dryness of the hot blast which now comes sweeping over the hills. In some villages, it is said, all the fires are extinguished when this wind begins to blow, for fear lest some chance spark should fall on the dry wooden roofs and set the whole place in a blaze. Still the Föhn is not altogether an "ill wind that blows nobody any good," for under its warm touch the winter snows melt away with marvellous rapidity. In the valley of Grindelwald it causes a snow-bed two feet thick to disappear in about a couple of hours, and produces in twenty-four hours a greater effect than the sun does in fifteen days. There is a Swiss proverb which rather profanely says: "If the Föhn does not blow, the golden sun and the good God can do nothing with the snow." In summer-time, however, the south wind is never welcome, for the vapour which it brings from the Italian plains is condensed by the snows of the Alps, and streams down in torrents of rain.