252 ON GLACIERS IN GENERAL. 



of a river in consequence of the friction of the fluid on its banks, 

 and in consequence^ also of that internal friction of the fluid 

 which constitutes its viscosity. 



Thus, in the case of a glacier, at four stations of the Mer 

 de Glace, distant respectively from the west shore of the glacier 



100 230 305 365 yds., 

 the relative velocities were . . I'OOO 1'332 1-356 1'367. 



3. The variation of velocity (as in a river) is most rapid 

 near the sides, whilst the middle parts move nearly uniformly. 

 This and the preceding laws are also fully brought out by the 

 subsequent experiments of M. Agassiz on the Glacier of the Aar, 

 and of MM. Schlagintweit on the Pasterzen Glacier.* 



4. The variation of velocity of a glacier from the sides to 

 the middle, varies nearly in proportion to the absolute velocity 

 of the glacier, whether that absolute velocity change in the same 

 place in consequence of change of season, or between one point 

 and another of the length of the same glacier, depending on its 

 declivity. (See (5) and (6) below.) These facts, clearly brought 

 out in my observations of 1842, present a striking analogy to the 

 phenomena of rivers, as observed by Dubuat.f 



5. The glacier, like a stream, has its pools and its rapids. 

 Where it is embayed by rocks it accumulates, its declivity 

 decreases,! and its velocity at the same time. When it passes 

 down a steep, issuing by a narrow outlet, its velocity increases. 

 Thus the approximate declivities of the inferior, middle, and 

 superior region of the Mer de Glace (taken in the direction of 



its length) are 15 4 8 



and the relative velocities are as the 



numbers 1-398 -574 -925 



* See Mousson, Gletscher der Jetzt-zeit, p. 126. 



f Traite cC Hydraulique, arts. 37, 49, 65. 



\ [In the Encyclopaedia, increases, apparently by mistake.] 



The absolute velocity of a glacier depends upon so many circumstances 

 besides its declivity, that this law must not be sought to be verified, except under 

 like conditions. The breadth and depth of a glacier (as of a river) no doubt mate- 

 rially affect its rate of motion, and its elevation has a not less important influence. 

 Small lofty glaciers of the second order move slowly over steep inclinations. See 

 Philosophical Transactions, 1846, p. 177. [Reprinted in the present volume.] 



