They might—and no doubt did—alternately increase and decrease within historical times. They nevertheless must be viewed as a bequest, a kind of heirloom coming from a prehistoric ancestry. They are the survival of a phenomenon which, in its former compass and intensity, is no longer compatible with the meteorological régime of Central Europe.
The temperature most suitable for the formation of ice in nature is the temperature which remains the most steadily around the freezing-point of water. Extremes of temperature are not favourable to the formation of snow, which is the form in which water generally passes into glacier ice.
It stands to reason that the oftener the atmosphere can be saturated with moisture in circumstances which allow a frequent discharge in the shape of snow falling upon surfaces that are iced—or such as will retain the snow, assuring the transformation of some of it, ultimately, into ice—the more will the thermometer readings show a temperature rising and falling only moderately above and below the freezing-point of natural water. There is no use in further emphasising this obvious truth.
Everybody will understand that moisture formed in hot tracts of the atmosphere has little chance of being converted into snow, and that, while a warm atmosphere may generate water—destructive of ice and snow surfaces—a very cold atmosphere cannot assist in glacier formation—on high land, at any rate—for want of vapours to condensate and precipitate, and for want of water masses to consolidate.
It follows that, within historical times, the Alpine glaciers have undergone variations according to changes in the quantity of moisture contained in the atmosphere, theirs being such altitudes and such climatic conditions as might allow the Centigrade thermometer to swing pretty steadily between 20 degrees above zero and 20 degrees under, all the year round and in the course of a day.
These conditions existed more fully in periods when the Alps were well wooded. Such a period pre-existed the first historical epoch of Switzerland. Under the Romans, say from 50 B.C. to 500 A.D., this first historical epoch was marked by the wholesale destruction of forests—the usual price to be paid for civilisation—and the glacier world retreated in a ratio commensurate with the process of denudation.
Then came the Early Middle Ages, which for about six or seven hundred years show a distinct retrogression in Swiss civilisation. The glaciers now regained some of the ground they had lost, because the wooded surface, which is the most favourable to the condensation of moisture, underwent a considerable increase.
In modern times the forest area has again undergone such shrinkage that it has reached the minimum when artificial means have to be devised for its preservation. Glaciers have gone back again.
We may therefore define glaciers as ice and snow reservoirs formed under prehistoric conditions which no longer exist. They are kept alive on a reduced scale, in a direct ratio to the moisture yielded by the atmosphere as often as it is conveniently a little above and a little below the freezing-point of natural water.
