CAUSE OF GLACIER MOTION. 251 



The following is the theory which was advanced in 

 1 Climate and Time ' to account for regulation : — 



The freezing-point of water and the melting-point of 

 ice, as Professor Tyndall remarks, touch each other, as 

 it were, at this temperature 32° F. At a hair's-breadth 

 lower water freezes; at a hair's-breadth higher ice melts. 

 Now, if we wish, for example, to freeze water already 

 just about the freezing-point, or to melt a piece of ice 

 just about the melting-point, we can do this either by 

 a change of temperature or by a change of the melting 

 point. But it will be always much easier to effect this 

 by the former than by the latter means. Take the 

 case already referred to, of the two pieces of ice sus- 

 pended in an atmosphere above the melting-point. 

 The pieces at their surfaces are in a melting condition, 

 and are surrounded by a thin film of water just an 

 infinitesimal degree above the freezing-point. The 

 film has on the one side solid ice at the freezing-point, 

 and on the other a warm atmosphere considerably 

 above the freezing-point. The tendency of the ice is 

 to lower the temperature of the film, while that of the 

 air is to raise its temperature. When the two pieces 

 are brought into contact the two films unite and form 

 one film separating the two pieces of ice. This film is 

 not like the former in contact with ice on the one side 

 and warm air on the other. It is surrounded on both 

 sides by solid ice. The tendency of the ice, of course, 

 is to lower the film to the same temperature as the ice 

 itself, and thus to produce solidification. It is evident 

 that the film must either melt the ice or the ice must 

 freeze the film, if the two are to assume the same 

 temperature. But the power of the ice to produce 

 solidification, owing to its greater mass, is enormously 

 greater than the power of the film to produce fluidity, 

 consequently regelation is the result. 



