1844.] MOTION OF A SEMIFLUID. 35 



by the weight of the ice itself, appears to explain the chief 

 facts of glacier-movement, viz. (1.) That it is more rapid at 

 the centre than at the sides. (2.) For the most part, most 

 rapid near the lower extremity of glaciers, but varying rather 

 with the transverse section than the length. (3.) That it is 

 more rapid in summer than in winter, in hot than in cold 

 weather, and especially more rapid after rain, and less rapid 

 in sudden frosts. (4.) It is farther in conformity with what we 

 know of the plasticity of semisolids generally, especially near 

 their point of fusion. Many examples will occur to every one 

 of what they have observed of the plasticity of hard bodies 

 such as sealing-wax, for example exposed for a long time to 

 a temperature far below their melting heat, and which have 

 moulded themselves to the form of the surfaces on which they 

 rest. (5.) When the ice is very highly fissured, it yields 

 sensibly to the pressure of the hand, having a slight determi- 

 nate play, like some kinds of limestone, well-known for this 

 quality of flexibility. (6.) I have formerly endeavoured to 

 shew how such a condition of semirigidity, combined with the 

 determined movements of the glacier, accounts for the remark- 

 a,ble veined structure which pervades it.* 



VI. FIFTH LETTER on GLACIERS, addressed to the 

 Right Honourable EARL CATHCART.t 



Motion of the Mer de Glace in the year 1842-3 Winter Movement The Veined 

 Structure reproduced at the foot of an Ice Cascade Cuts the Medial Moraines 

 Wrinkles in the Ice of the Glacier du Geant and that of Grindelwald, probably 

 corresponding to the position of the Dirt-Bands An Ancient Moraine at 



Chamouni. 



ROME, 29th January 1844. 



My Lord In reply to your kind letter of the 14th Decem- 

 ber last, requesting me to communicate to the Royal Society 

 any observations upon glaciers which I was enabled to make 



* [See page 23.] 



f Edinburgh New Philosophical Journal, July 1844. [Read to the Royal 

 Society of Edinburgh.] 



