236 Professor Forbes on the Leading Phenomena of Glaciers. 



a glacier is in conformity to the second principle stated above, 

 that the retardation due to friction will be more completely 

 distributed over the whole section in proportion as the matter 

 is less yielding. 



Thirdly^ The chief variation of velocity is near the sides. 



Fourthly^ We have found a most remarkable confirmation 

 of Dubuat's principle, that the amount of lateral retardation 

 depends upon the actual velocity of the stream under expe- 

 riment ; whether we consider different points of the glacier, or 

 the same point at different times. * 



Fifthly., The glacier, we have seen, like a stream, has its 

 still pools and its rapids. Where it is embayed by rocks, it 

 accumulates — its declivity diminishes, and its velocity at the 

 same time ; — when it passes down a steep, or issues by a narrow 

 outlet, its velocity increases. 



The central velocities of the lower, middle, and higher re- 

 gions of the Mer de Glace are — 



1.398 .574 .925 



And if we divide the length of the glacier into three parts, we 

 shall find something like these numbers for its declivity f — 

 15° 4|° 8° 



Lastly. When the semifluid ice inclines to solidity during a 

 frost, its motion is checked ; if its fluidity is increased by a 

 thaw, the motion is instantly accelerated. Its motion is greater 

 in summer than in winter, because the fluidity is more com- 

 plete at the former than at the latter time. The motion 

 does not cease in winter, because the winter's cold penetrates 

 the ice as it does the ground, only to a limited extent. It is 

 greater in hot weather than in cold, because the sun's heat 

 affords water to saturate the crevices : but the proportion of 

 velocity does not follow the proportion of heat, because any 



* Une chose etonnante, c'est que ni la grandeur du lit, ni celle de la pente 

 n'influent en rien sur le rapport des diflFerentes vitesses dout nous parlons, tant 

 que lea vitesses moyennes restent les memes ou celle de la surface est constante. — 

 J)uBUAT, Art. 65. 



t These numbers do not express the actual slopes at the points where the 

 velocities were measured, but the slope of the inferior, middle, and superior 

 regions of the glacier. 



