SNOW, PERENNIAL, or l'Eltl'ETUAL. In the article CLIMATE (coL 968-70), a general view of this subject has been given, together with particulars of the elevation of the snow-line under different latitudes and in different localities, both as indicated by theory and shown by observation. We are now enabled to enter in a more precise manner into the consideration of the varying circumstances which govern the position of the snow-line, and also into the special history of perpetual snow as it exists about and upon the greatest and highest mountains of the globe. For the materials of these we are chiefly indebted to Dr. Joseph D. Hooker, who, in the Appendix to his ' Himalayan Journals,' has treated the subject with views at once more general and more philosophical, perhaps, than have been applied to it since the original investigations of Wahlenberg, Von Buch, and Humboldt ; and has largely illustrated them by the facts of the dis tribution of snow in Nepaul, Tibet, and the Himalaya, which are stated in their appropriate places in his 'Journals ' themselves.
It must be premised that three things are frequently confounded together in popular science and in books of travels. First, the elevation in the atmosphere, or above the general surface of the earth, as defined by the level of the sea, of the temperature of 32° Fahr., or the freezing point, for each latitude, as affected by the distribution of land and sea, from which, in union with the solar temperature, results the temperature of each isothermal line at the surface.
Secondly, the position of the inferior limit of the beds of perpetual snow—the actual snow-line in this sense—on mountainous elevations, as dependent on the cause just assigned, in conjunction with local causes on the great scale, and as existing in fact at that particular elevation for each region, below which all the snow that falls melts. This may be considerably below or considerably above the line of 32°.
Thirdly, the altitude at which much snow is perennial, in the ordinary sense of the word—its permanence arising from local and from temporary (though recurrent) meteorological eauses—maskiug, by its extension downwards, the true limit of perpetual snow as defined above. It is requisite, however, for the due apprehension of
the subject, that the distinction between these three things should be carefully preserved in the mind.
Dr. Hooker has stated his belief (` Him. Journ.,' p. 394), "that the limit of perpetual snow is laid down too low in all moun tain regions, and that accumulations in hollows, and the descent of glacial ice, mask the phenomenon more effectually than is generally allowed." He defines the limit, " in general terms only, as being that where the accumulations are very great, and whence they are con tinuous upwards, on gentle slopes. All perpetual snow, however," he continues, " becomes ice, and, as such, obeys the laws of glacial motion ; whence it follows, that the lower edge of a suow-bed placed on a slope is, in one sense, the termination of a glacier, and indicates a position below that where all the snow that falls melts. . . . It is im possible to define the limit required with any approach to accuracy. Steep and broken surfaces, with favourable exposures to the sun or moist winds, are bare much above places where snow lies throughout the year; but the occurrence of a gentle slope, free of snow, and covered with plants, cannot but indicate a point below that of per petual snow." A careful examination of those great beds of snow in the Alps, from whose position the mean lower level of perpetual snow, in that latitude, has been deduced, convinced Dr. Hooker that they are winter accumulations—of the kind alluded to above, due mainly to eddies of wind—of far more snow than can be melted in the follow ing summer, being hence perennial in the ordinary sense of the word only. It follows that the true limit of perpetual snow is much higher in the Alps than it is usually supposed to be. He proceeds to show that the altitude of the limit in the Alps has been stated by Professor James Forbes more than 1000 feet in defect, the Jardin, on the Mer de Glace, at the elevation of 9500 feet, being evidently below the limit, to which, however, Professor Forbes had assigned the elevation of 8500 feet only.