﻿488 M. A. Heim on Glaciers, 



The mean annual rate of motion of the Aletsch Glacier is 

 about 40 metres. Measured from the foot of the Dreieckhorn 

 and Faulberg (where the glacier, properly speaking, begins) 

 onwards, it is 17000 metres long. We will now, as regards 

 alteration in volume, consider the glacier below the transverse 

 section situated at the above-designated place, until it is all 

 melted, and thus our transverse section, slowly travelling with 



the ice, has arrived at the lower end. For this about = 425 



years are necessary. The volume of our glacier-tongue may be 

 estimated as about 6150 milliou cubic metres; of course the 

 number is not certain to 100 millions more or less. This mass 

 of ice, together with its increase through the growth of its gra- 

 nules, actually turns to water in 425 years by ablation. The 

 melting of the glacier beneath through the warmth of the ground 

 is doubtful — at any rate, vanishingly small. The vertical sink- 

 ing of the surface through melting away amounts to an average 

 of about 3 metres yearly : it was measured by the constantly in- 

 creasing protrusion of stakes which had been driven into the 

 surface, or by the relative elevation of parts artificially sheltered 

 from ablation. The surface of ablation in the first of the 425 

 years, measured on the Federal atlas, is 25,000,000 square 

 metres. In the second, the transverse section forming the upper 

 boundary of our glacier-mass has moved 40 metres down the 

 valley ; here the glacier is 1600 metres wide ; thus the surface of 

 ablation is, for the second year, 40 x 1600 = 64000 square metres 

 less than that of the first. Just so in the following years it be- 

 comes continually less the further our transverse section travels. 

 At the end of the 425 years it reaches 0. The surface on which, 

 during the 425 years, the vertical ablation of about 3 metres 

 yearly has operated, and effectually melted the mass of the gla- 

 cier, is the sum of all these 425 unequal surfaces belonging to 

 the individual years. If the surface of the glacier were exactly 

 rectangular, as broad below as above, this sum would be 425 

 times half the surface of the first year, consequently 



=425 x 25000000 x J = 5312500000 



square metres. Were it exactly a triangle (of which the base, 

 the greatest breadth, =1800 metres, and the height the length 

 = 17000), the sum in question would be 



or =2175000 square metres. In reality it lies between; we 

 will take the mean— namely, 3740 million square metres. To 

 mark on the map the situation of the upper limit for each year, 



