THE MECHANICS OF GLACUiRS 927 



making a higher angle with the surface, tends more directly to 

 increase the thickness. This increases the velocity, causing a 

 further departure from the equilibrium surface, and the glacier 

 grows in length with a small increase of thickness in the reser- 

 voir, and with a great increase of thickness at its lower end. 

 The ice is rapidly carried off until the upper part of the dissipa- 

 tor becoming thinner, there is a gradual diminution in velocity, 

 and the end of the dissipator having advanced to warmer 

 regions the melting is faster and the glacier begins to recede. 

 This makes it evident that under certain conditions, such as a 

 bed of increasing slope, a glacier, when once the equilibrium 

 between its flow and melting is destroyed, might advance with 

 strides entirely out of proportion to its usual motion, as has fre- 

 quently happened with the Vernagt glacier. 



A glacier having the equilibrium form corresponding to the 

 climatic conditions prevailing at the time, would lose this form 

 and respond immediately, by a change in length, to the slightest 

 climatic change ; whereas, a glacier widely removed from its 

 equilibrium form could not respond by a change in length until 

 the effects of the climatic change had accumulated sufficiently 

 to bring the glacier back to, and carry it beyond, its equilibrium 

 form. Two glaciers, in general similar, but differing in their 

 exposure or slope, might be very differently removed from their 

 respective equilibrium forms, and would therefore respond at 

 different times to a given climatic change ; indeed, one of them 

 might be so far removed from its equilibrium form that a cli- 

 matic change lasting for several years might not be long enough 

 to reverse the condition of retreat or advance in which it 

 happened to be. 



Smaller glaciers in general respond more quickly to climatic 

 variations, and we can see why this should be true ; for, a change 

 in the amount of snow-fall or of melting, would produce the 

 same change in the actual thickness of two glaciers of different 

 sizes, but subject otherwise to the same conditions. The smal- 

 ler glacier would, however, experience a greater relative change 

 in thickness, resulting in a greater relative change m flow ; it 



