I have stated, in my work upon glaciers, that my infiltration-experiments were chiefly made at night; and I chose that time, because I knew the glacier would most readily admit an additional supply of liquid from without when the water formed during the day at its surface and rushing over it in myriad rills had ceased to flow. While we admit a number of causes as affecting the motion of a glacier, namely, the natural tendency of heavy bodies to slide down a sloping surface, the pressure to which the mass is subjected forcing it onward, the infiltration of moisture, its freezing and consequent expansion,--we must also remember that these various causes, by which the accumulated masses of snow and ice are brought down from higher to lower levels, are not all acting at all times with the same intensity, nor is their action always the same at every point of the moving mass. While the bulk of snow and ice moves from higher to lower levels, the whole mass of the snow, in consequence of its own downward tendency, is also under a strong vertical pressure, arising from its own incumbent weight, and that pressure is, of course, greater at its bottom than at its centre or surface. It is therefore plain, that, inasmuch as the snow can be compressed by its own weight, it will be more compact at the bottom of such an accumulation than at its surface, this cause acting most powerfully at the upper part of a glacier, where the snow has not yet been transformed into a more solid icy mass. To these two agencies, the downward tendency and the vertical pressure, must be added the pressure from behind, which is most-effective where the mass is largest and the amount of motion in a given time greatest. In the glacier, the mass is, of course, largest in the centre, where the trough which holds it is deepest, and least on the margins, where the trough slopes upward and becomes more shallow. Consequently, the middle of a glacier always advances more rapidly than the sides. Were the slope of the ground over which it passes, combined with the pressure to which the mass is subjected, the whole secret of the onward progress of a glacier, it is evident that the rate of advance would be gradually accelerated, reaching its maximum at its lower extremity, and losing its impetus by degrees on the higher levels nearer the point where the descent begins. This, however, is not the case. The glacier of the Aar, for instance, is about ten miles in length; its rate of annual motion is greatest near the point of junction of the two great branches by which it is formed, diminishing farther down, and reaching a minimum at its lower extremity. But in the upper regions, near their origin, the progress of these branches is again gradually less. Let us see whether the next cause of displacement, the infiltration of moisture, may not in some measure explain this retardation, at least of the lower part of the glacier. This agency, like that of the compression of the snow by its own weight