396 Prof. Tyndall on Ice and Glaciers, 



nature, had been referred to regelation long before I occupied 

 myself with the cause of regelation itself. This latter question 

 is not once referred to in the memoir in which the regelation 

 theory was first developed*. The inquiries, though related, were 

 different. In referring the motion of glaciers to a fact experi- 

 mentally demonstrated, I referred it to its proximate cause. To 

 refer that cause to its physical antecedents formed the subject of 

 a distinct inquiry, in which, because of my belief in the substan- 

 tial correctness of Mr. Faraday's explanation, I took compara- 

 tively little part. 



Five persons, however, mingled more or less in the inquiry — 

 viz. Professor Faraday, Principal Forbes, Professor James Thom- 

 son, Professor William Thomson, and myself t. Professor James 

 Thomson explained regelation by reference td an important de- 

 duction, first drawn by him J, and almost simultaneously by Pro- 

 fessor Clausius§, from the mechanical theory of heat. He had 

 shown it to be a consequence of this theory that the freezing- 

 point of water must be lowered by pressure ; that is to say, water 

 when subjected to pressure will remain liquid at a temperature 

 below that at which it would freeze if the pressure were removed. 

 This theoretic deduction w r as confirmed in a remarkable manner 

 by the experiments of his brother, Professor William Thomson ||. 

 Regelation, according to Professor Thomson's theory, was thus 

 accounted for : — " When two pieces of ice are pressed together, 

 or laid the one upon the other, their compressed parts liquefy. 

 The water thus produced has rendered latent a portion of the heat 

 of the surrounding ice, and must therefore be lower than 0° C. 

 in temperature. On escaping from the pressure this water 

 rcfreezes and cements the pieces of ice together." I always ad- 

 mitted that this explanation dealt with a "true cause." But 

 considering the infinitesimal magnitude of the pressure sufficient 

 to produce regelation, in common with Professor Faraday and 

 Principal Forbes I deemed the cause an insufficient one. Pro- 

 fessor James Thomson, moreover, grounded upon the foregoing 

 theory of regelation a theory of glacier-motion, in which he 

 ascribed the changes of form which a glacier undergoes to the 

 incessant liquefaction of the ice at places where the pressure is 

 intense, and the refreezing, in other positions, of the water thus 

 produced^. I endeavoured to show at the time that this theory 

 was inapplicable to the facts, Professor Helmholtz has recently 



* Phil. Trans, vol. cxlvii. p. 327. 



t Proc. Roy. Soc. vol. ix. p. 141 ; and vol. x. p. 152. Phil. Mag. S. 4. 

 yol. xvi. pp. 347 & 544 ; and vol. xvii. p. 1G2. 

 X Proc. Roy. Soc. Edinb. February 1850. 



I Pogg. Ann. vol. lxxxi. p. 168. || Phil. Mag. August 1850. 



*T Proc. Roy. Soc. vol. viii. p. 455. 



