58 Scientific Intelligence. 
cier when shortening, loses also in its other dimensions. There 
is internal ablation as well as external-—interstitial fusion taking 
place between the grains of the lacier and ablation along the 
walls of crevasses; and this action, besides being one of the means 
by which the thinning takes places, causes loss of movement, the 
amount of which diminishes upward along the glacier. More- 
over, the greater the thickness the less propor —— is there of 
this internal ablation. Internal ablation tends then to exagger- 
ate the small variations in the rate of flow, and pr mi — 
effects in the inferior part of the glacier, although smal 
The variations in the amount of sno Ws, which cause ‘bd pare 
thidknoss of the glacier at its source, depend on long terms of 
periodicity. The precipitation at Geneva was below the normal 
shear from 1835 to 1841 inclusive ; pho it, 1842 to 1857; 
ear the normal in 1858 to 1861; below it in 1862 to 187 a3 above 
since 1877. It is seen that there is an approximate parallelism 
between these changes and the variations in the glaciers of the 
Alps. The effects are not immediate, as should be expected, but 
t is seldom that. all the glaciers of the Alps, without any 
exception, lengthen or shorten together. In this — the only 
case of simultaneous elongation took place in the year 1817 and 
1818, and the only one of general retreat in 1872 to 1874. Dur- 
a 
ing the years 1812 to 1836, most of the glaciers underwen 
a 
shortening which included the above-mentioned years, 1872--1874, 
mmenced in Mont Blane about 1854; in the Upper Grindel- 
wald, in 1855; the Giétroz, in 1855; the Rhone e, in 1857 
Aletsch, i in 1860; the Gorner, in 1870; the Fiesch, in 1870: the 
Unteraar, in 1871; and it ended with the Bossons about 1875; 
with the Bois, i in 1879; the Trient, in 1879 ; the Giétroz, in 1880. 
The facts seem to show that it took twenty years after the cycle 
of retreat began for the time of simultaneous retreat to arrive. 
Hugi, besides giving the above explanations, illustrates them by 
reference to the same class of re though of less range of years. 
ith regard to the coming o of the conditions of the Glacial pe- 
with a series of moist and mild winters and mois d cold sum- 
ers, as was long since suggested by Professor Guyot. The in- 
mentioned effec 
But M. Forel ‘aida: while these effects would follow in moun- 
tain regions like the Alps, other regions, as the Vosges, the Cé- 
