Their immobility is only apparent, they move and advance without ceasing. Careful investigation has ascertained the rate of motion of a glacier to be as much as two feet in twenty-four hours; but it is a curious fact that the whole stream does not move at the same rate; the centre moves quicker than the sides and drags them after it. Agassiz, the Swiss naturalist, began in 1842 a series of careful observations on the Aar glacier, taking up his abode in a little hut constructed on purpose, and called the “_Hôtel des Neuchâtelois_.” Men mocked at him when he set up his stakes on the glacier to discover the rate of the invisible motion, but he persisted in his minute, painstaking labors, futile and inconsequential as they seemed to the unscientific mind, till he plucked out from every glacier in Switzerland the heart of its mysterious movement. He held that the differences in speed between different and sundry parts of the same glacier were the results of unequal density and of unequal declivity. Savants differ as to the causes which set the glacier in motion. Schaelzer holds that its expansion arises from thaw; Professor Hugi is of the same opinion, that the glacier, like an enormous sponge filled with aqueous particles, expands and grows larger when it freezes. Of all the theories that of De Saussure is the most generally accepted. He attributes the forward movement of a glacier to gravitation,--that is to say, to the pressure of the superior masses on the inferior. Certain naturalists affirm that the glaciers add to their power by their own cold, and that in time, without the intervention of some new natural phenomenon, they will eventually extend themselves downward into the valleys that lie on the next level beneath, overcoming vegetation and destroying life. There must be a limit somewhere to the increase of the ice, and it is almost certain that these limits have been attained during the centuries that the present physical formation of Switzerland is known to have existed. As a whole, the contest between heat and cold ought to be set down as producing equal effects.