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case of tlie Rhone glacier, in 1855 or 1856. In Scandinavia, 
it appeared to have varied from one glacier to another, but it 
has now become general. M. Ny strom was so kind as to 
consult the illustrious Swedish traveller, M. ISTordenskiold, 
upon this subject, who told him that the glaciers of Spitzbergen 
had undergone a similar diminution during the last few years. 
In August, 1880, I laid a communication on this subject 
before the Scientific Congress at Rheims. In the discussion 
which arose upon the matter, several people cited facts in 
support of those which I had pointed out ; and one of those 
who took part in the debate, and who had been several times 
to Greenland, had observed that the glaciers of that country 
had also retreated considerably. 
It would be interesting to know what has been the condi- 
tion of the Asiatic and American glaciers, and of those of the 
southern hemisphere, during the same periods. But in any 
case the retreat of the four great European groups of glaciers, 
those of the Pyrenees, the Alps, the Caucasus, and Scandinavia, 
as well as those of Greenland and Spitzbergen, is a fact of 
great importance in physical geography, since it involves all 
those of the northern hemisphere over an extent of 100° of 
longitude. It is well worth while to pay attention to it, and 
to watch the progress of the phenomenon. 
It is perhaps difficult at the moment to indicate the cause 
of this retreat, for, from the meteorological point of view, the 
last few years have not been very different from those which 
preceded them. Moreover, the advance or the retreat of a 
glacier depends upon several factors ; first of all, upon the heat 
and humidity of the summer, and then upon the frequency and 
intensity of warm winds. As regards heat and wind, the 
consequences are indirect, but they are less so with respect 
to humidity. In fact, as M. Forel and I have shown by 
experiments made in 1870 upon the Rhone glacier, the 
aqueous vapour of the atmosphere condenses upon the ice, 
as it condenses in winter upon the glass windows of our rooms, 
and this water of condensation considerably augments the 
volume of the water that issues from the glacier; now this 
condensation is accompanied by an evolution of latent heat, 
which contributes greatly to the melting of the ice. But 
the extent of a glacier depends not only upon the forces 
which tend to destroy it, but also upon those which tend to 
form it, that is to say, upon the accumulation of snow in the 
basin of reception which is the origin of the glacier. These 
snows, which by degrees become converted into ice, are in 
considerable quantities, and represent the falls which have 
taken place during several years. This mass moves slowly, 
and it is not until the lapse of a great many years that the ice, 
