394 



Prof. Tyndall on Ice and Glaciers. 



the Fohn are therefore no proof that its origin is Sahara, and not 

 the Atlantic Ocean. 



It will probably be remembered that I deduced the formation 

 of glaciers, and their subsequent motion through valleys of vary- 

 ing width and flexure, from the fact that when two pieces of ice 

 are pressed together they freeze together at their places of con- 

 tact. This fact was first mentioned to me verbally by its dis- 

 coverer, Mr. 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 one of the steel moulds 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 translucent ice. I 

 immediately joined Mr. 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 ap- 

 plications 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 fig. 1. AB 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 pres- 

 sure is too well known to need 

 dwelling upon here. In apply- 

 ing these results to glaciers, 

 I dwelt with especial emphasis 

 upon the fact that while the 

 power of being moulded by pres- 

 sure belonged in an eminent 

 degree to ice, the power of yield- 

 ing by stretching to a force of 

 tension was wanting. On this 

 point Prof. Helmholtz speaks 



Fig. 1, 



