A thunder-storm is always a grand spectacle. Among mountains such storms are more frequent than on the plains, and also, as might be expected, far more magnificent, especially at night. Flashes, or rather sheets, of unutterable brilliancy light up the sky; distant chains of mountains are revealed for a moment, only to be instantly eclipsed by the pall of night. Says Professor Bonney,-- "No words can adequately express the awful grandeur of these tempests when they burst among the mountains. I have often been out in them,--in fact, far more frequently than was pleasant; but perhaps the grandest of all was one that welcomed me for the first time to Chamouni. As we entered the valley, and caught sight of the white pinnacles of the _glacier des Bossons_, a dark cloud came rolling up rapidly from the west. Beneath it, just where two tall peaks towered up, the sky glowed like a sheet of red-hot copper, and a lurid mist spread over the neighbouring hills, wrapping them, as it seemed, in a robe of flame. Onward rolled the cloud; the lightning began to play; down the valley rushed a squall of wind, driving the dust high in air before it, and followed by a torrent of rain. Flash succeeded flash almost incessantly,--now darting from cloud to cloud; now dividing itself into a number of separate streaks of fire, and dancing all over the sky; now streaming down upon the crags, and at times even leaping up from some lofty peak into the air. The colours were often most beautiful, and bright beyond description." [Illustration: A STORM ON THE LAKE OF THUN. AFTER TURNER.] The mountain traveller, when caught in a thunder-storm, undergoes a strange experience, not unattended with danger. One observer[12] thus describes his sensations:-- [12] Mr. R. S. Watson, in "The Alpine Journal," vol. i., p. 143.