CLOUDS ASD RIVERS, ICE AND GLACIERS. 163 



in volume ; draw a saw across it to a depth of half an 

 inch or an inch; and strike a pointed pricker, not 

 thicker than a very small round file, into the groove ; 

 the substance will split from top to bottom with a clean 

 crystalline fracture. How is this brittleness to be 

 reconciled Avith the notion of viscosity ? 



410. We have, moreover, been upon the glacier and 

 have witnessed the birth of crevasses. We have seen 

 them beginning as narrow cracks suddenly formed, days 

 being required to open them a single inch. In many 

 glaciers fissures may be traced narrow and profound for 

 hundreds of yards through the ice. What does this 

 prove ? Did the ice possess even a very small modicum 

 of that power of stretching, which is characteristic of a 

 viscous substance, such crevasses could not be formed. 



411. Still it is undoubted that the glacier moves like 

 a viscous body. The centre flows past the sides, the top 

 fiows over the bottom, and the motion through a curved 

 valley corresponds to fluid motion. Mr. Mathews, Mr. 

 JTroude, and above all Signor Bianconi, have, more- 

 over, recently made experiments on ice which strikingly 

 illustrate the flexibility of the substance. These experi- 

 ments merit, and will doubtless receive, full attention 

 at a future time. 



§ 61. Regelation Theory. 



412. I will now describe to you an attempt that has 

 been made of late years to reconcile the brittleness of 



