The weight of the glacier, as a whole, is about the same all the year round. If, therefore, pressure, resulting from that weight, be the all-controlling agency, its progress should be uniform daring the whole year, or even greatest in winter, which is by no means the case. By a series of experiments, I have ascertained that the onward movement, whatever be its annual average, is accelerated in spring and early summer. The average annual advance of the glacier being, at a given point, at the rate of about two hundred feet, its average summer advance, at the same point, will be at a rate of two hundred and fifty feet, while its average rate of movement in winter will be about one hundred and fifty feet. This can be accounted for only by the increased pressure due to the large accession of water trickling in spring and early summer into the interior through the net-work of capillary fissures pervading the whole mass. The unusually large infiltration of water at that season is owing to the melting of the winter snow. Careful experiments made on the glacier of the Aar, respecting the water thus accumulating on the surface, penetrating its mass, and finally discharged in part at its lower extremity, fully confirm this view. Here, then, is a powerful cause of pressure and consequent motion, quite distinct from the permanent weight of the mass itself, since it operates only at certain seasons of the year. In midwinter, when the infiltration is reduced to a minimum, the motion is least. The water thus introduced into the glacier acts, as we have seen above, in various ways: by its weight, by loosening the particles of snow through which it trickles, and by freezing and consequent expansion, at least within the limits and during the season at which the temperature of the glacier sinks below 32 deg. Fahrenheit. The simple fact, that in the spring the glacier swells on an average to about five feet more than its usual level, shows how important this infiltration must be. I can therefore only wonder that other glacialists have given so little weight to this fact. It is admitted by all, that the waste of a glacier at its surface, in consequence of evaporation and melting, amounts to about nine or ten feet in a year. At this rate of diminution, a glacier, even one thousand feet in thickness, could not advance during a single century without being exhausted. The water supplied by infiltration no doubt repairs the loss to a great degree. Indeed, the lower part of the glacier must be chiefly maintained from this source, since the annual increase from the fresh accumulations of snow is felt only above the snow-line, below which the yearly snow melts away and disappears. In a complete theory of the glaciers, the effect of so great an accession of plastic material cannot be overlooked.