the Hintereis Glacier, 165 



■which occasioned it were warmer and the snow-line was 

 raised, the one effect neutralizing the other. When, how- 

 ever, the snow-line is lowered, the neve or alimentation area. 

 is increased and the tongue area or main ablation area is 

 decreased — at the snow-line these two areas overlap. But as 

 the total snow-fall in the alimentation area must be melted 

 in the ablation area, the glacier tongue ultimately advances 

 if the lowering o£ the snow-line persists for any length ot 

 time. In other words, although the precipitation has not 

 increased, the area over which snow falls rather than rain 

 has increased, and the glacier tongue advances until the 

 ablation area is large enough to get rid of the increased 

 quantity of snow flowing down from the neve. However, 

 the effect of the greater quantity of snow falling on the 

 increased neve area does not at once affect the glacier snout. 

 In fig. 8, at a distance of 2775 metres from the end of the 

 glacier, from 1901 forwards, the velocity of the ice was in- 

 creasing, but during the same time, the velocity on the point 

 of the ice, 396 metres from the end, was decreasing, and con- 

 tinued to do so until the commencement of 1903. A period 

 of cold during which the snow-line is lowered does not on 

 this account affect the ends of all glaciers at the same time. 

 The advance or decreased rate of retreat is more delayed the 

 longer the glacier is. Indeed, in the case of a long glacier 

 it may altogether exhaust itself before reaching the end. 



The sensitiveness of the ice movement to changes of 

 thickness and changes of slope render it probable that no 

 great error is generally likely to be introduced into calcu- 

 lations of viscosity by taking the conditions of slope, &c, 

 at any particular moment. We shall, therefore, consider 

 that the mean annual velocity we have used, instead of being- 

 decreased by 10*2 per cent, to get the winter velocity, should 

 be decreased by 11*2 per cent, so as to get the summer 

 velocity to compare with the summer slope measurements. 

 We did not previously recognize that the accumulated winter 

 snow made such a difference in the winter velocity of the 

 glacier tongue. It will be seen that the alterations in the 

 figures thus introduced are too small to render any correction 

 of the figure for viscosity necessary, since the actual summer 

 velocity is nearly equal to our theoretical winter velocity, 

 and the ice is probably dry and little affected by heat, even 

 in the summer. 



In some cases where the thickness of the ice has rapidly 

 increased in the lower portions of the neve, the ice comes 

 down the glacier in distinct waves or Schwellungswellen, 

 which travel three or four times as fast as the ice and become 



