Currents of warm air affect the extent of the glaciers and influence also the line of perpetual snow, which is by no means at the same level, even in neighboring localities. The size of glaciers, of course, determines to a great degree the height at which they terminate, simply because a small mass of ice will melt more rapidly, and at a lower temperature, than a larger one. Thus the small glaciers, such as those of the Rothhorn or of Trift, above the Grimsel, terminate at a considerable height above the plain, while the Mer de Glace, fed from the great snow-caldrons of Mont Blanc, forces its way down to the bottom of the Valley of Chamouni, and the glacier of Grindelwald, constantly renewed from the deep reservoirs where the Jungfrau hoards her vast supplies of snow, descends to about four thousand feet above the sea level. But the glacier of the Aar, though also very large, comes to a pause at about six thousand feet above the level of the sea; for the south wind from the other side of the Alps, the warm sirocco of Italy, blows across it, and it consequently melts at a higher level than either the Mer de Glace or the Grindelwald. It is a curious fact that, in the Valley of Hassli, the temperature frequently rises instead of falling, as you ascend; at the Grimsel the temperature is at times higher than at Meyringen, below, where the warmer winds are not felt so directly. The glacier of Aletsch, on the southern slope of the Jungfrau, and into which many other glaciers enter, terminates also at a considerable height, because it turns into the Valley of the Rhone, through which the southern winds blow constantly. Under ordinary conditions, vegetation fades in these mountains at the height of six thousand feet, but, in consequence of prevailing winds and the sheltering influence of the mountain walls, there is no uniformity in the limit of perpetual snow and ice. Where currents of warm air are very constant, glaciers do not occur at all, even where other circumstances are favorable to their formation.