due to the change of inclination, and to the longitudinal thrust endured by the glacier at the base of an ice-fall.

The lamination of a glacier is a peculiarly interesting case of cleavage. It is produced in the same manner as the lamination of slate rock, which is known, through the distortion of its fossils, to have suffered great pressure at right angles to the planes of cleavage.

IV.

HELMHOLTZ ON ICE AND GLACIERS.

SWITZERLAND has attractions for the scientific philosophers of Germany, and around the Titlis, Bunsen, Helmholtz, Kirchhoff, and Wiedemann are not unfamiliar names. Nor have their visits to the Alps been unproductive of results. Some time ago I was favoured by Professor Helmholtz with the First Part of his Popular Scientific Lectures.' It contains four of them—the first, 'On the Relation of the Natural Sciences to Science in general;' the second, 'On Goethe's Labours in Natural Science;' the third, On the Physiological Origin of Musical Harmony;' and the fourth, On Ice and Glaciers.' The lectures are in German, and it is much to be desired that some competent person should undertake their translation into English.'

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I turned with natural interest to the last-mentioned discourse, to see how my notions and experiments on the formation and motion of glaciers were

I have reason to believe that a translation of the two parts hitherto published will soon be forthcoming.-J. T., 1871.

regarded by so eminent a man. I will here endeavour to give a summary of the scientific portion of the lecture.

Professor Helmholtz refers the cold of the upper regions of the atmosphere to the causes generally assigned; but he adds a remark important at the present moment, when the origin of the hot wind called Föhn in Switzerland is the subject of so much discussion. This wind, as Helmholtz justly observes, may not only be a cold wind upon the mountain-summits, but a wet one, and it may deposit its moisture there. A wind thus dried upon the heights, and warmed by its subsequent fall into the valleys, would possess the heat and dryness of the Föhn. These qualities are, therefore, no proof that the origin of the Föhnwind is Sahara.

It will probably be remembered that I deduced the formation of glaciers, and their subsequent motion through valleys of varying width and flexure, from the fact that when two pieces of ice are pressed together they freeze together at their places of contact. This fact was first mentioned to me verbally by its discoverer, Faraday. Soon afterwards, and long before I had occasion to reflect upon its cause, the application of the fact to the formation and motion of glaciers flashed upon me. Snow was in the yard of the Royal Institution at the time; stuffing a quantity of it into a steel mould,

which I had previously employed to demonstrate the influence of pressure on magnetic phenomena, I squeezed the snow, and had the pleasure of seeing it turn out from the mould as a cylinder of trans

lucent ice. I immediately went to Faraday, and expressed the conviction that his little outlying experiment would be found to constitute the basis of a true theory of glaciers. It became subsequently known to me that the Messrs. Schlagintweit had

made a similar experiment with snow; but they did not connect with it the applications which suggested themselves to me, and which have since been developed into a theory of glacier-motion.

A section of the mould used in the experiment above referred to is given in the foregoing figure. A B is the solid base of the mould; C D E F a hollow cylinder let into the base; P is the solid plug used to compress the snow. When sufficiently squeezed, the bottom, A B, is removed, and the cylinder of ice is pushed out by the plug. The mould closely resembles one of those employed by Professor Helmholtz.

The subsequent development of the subject by the moulding of ice into various forms by pressure is too well known to need dwelling upon here. In applying these results to glaciers, I dwelt with especial emphasis upon the fact that while the power of being moulded by pressure belonged in an eminent degree to glacier ice, the power of yielding, by stretching, to a force of tension, was sensibly wanting. On this point Prof. Helmholtz speaks as follows: Tyndall in particular maintained, and proved by calculation and measurement, that the ice of a glacier does not stretch in the smallest degree when subjected to tension-that when sufficiently strained it always breaks;' and he adds, in another place, that the property thus revealed