60 AQUEOUS AGENCIES. 



theodolite. The rate of the center is often many times greater than 

 that of the margins. 



2. The Velocity of the Surface is greater than that of the Bottom. 

 — This law of currents, which is the necessary result of friction on the 

 bed, is more difficult to prove in the case of glaciers, because it is dif- 

 ficult to get a vertical section. The necessary observation was, how- 

 ever, successfully accomplished by Prof. Tyndall in 1857. We have 

 already said (page 56) that glaciers conform to large but not to small 

 inequalities of their channels : a glacier, therefore, passing by a narrow 



side -ravine will expose its whole 

 thickness on the side. Prof. Tyn- 

 dall, having found such a side ex- 

 posure more than 140 feet vertical, 

 placed three pegs in a vertical line, 

 one near the top, one near the mid- 

 dle and one at the bottom (Fig. 50, 

 ale). The vertical line became 

 more and more inclined daily. The daily motion at top was six inches, 

 in the middle 4*5 inches, and at the bottom 2'5 inches. Thus, glaciers, 

 like rivers, slide on their beds and banks, producing erosion ; but, also, 

 the several layers, both horizontal and vertical, slide on each other. 



3. The Velocity increases with the filope. — Fig. 51 represents the 

 surface-slope of the glacier Du Giant, G ; the Mer de Glace, M ; and 

 the glacier De Bois, B ; and their daily motion. The increase of ve- 

 locity with the slope is evident. 



4. The Velocity increases with the Fluidity. — The daily motion of 

 glaciers is greater in summer, when the ice is rapidly melting, than in 

 winter ; and in mid-day than at night. 



G 



18 IN M ■ ^^ 



5 JL " 



Fig. 51. 



5. Tlie Velocity increases with the Depth. — In the Alps, where the 

 thickness is 200 to 300 feet, the mean daily motion is one to three 

 feet ; but in Greenland, where the thickness is 2,000 to 3,000 feet, the 

 daily motion, in spite of the much lower temperature, is in some cases 

 60 feet* or even 99 feet.f The Muir glacier in Alaska moves 70 feet 

 a day (Wright). 



6. Fluid Currents conform to the Irregularities of their Channel. — 

 Glaciers, like water-currents, conform to the inequalities of the bottom 



* Ilelland, Journal of Geological Society, vol. xxxiii, p. 142 et seq, 

 f Science, vol. xi, p. 259, 1888. 



