CAUSE OF GLACIAL MOTION. Many theories have been advanced to explain why the ice of glaciers, which is apparently so very brittle, should flow like a plastic substance. There are two questions to be answered—namely, What is the force causing the ice to move, and what property of ice enables it to move as it does? There is a general unanimity at present in the belief that the weight of the ice itself is the only force causing the motion, but there is not so much unanimity in answering the second ques tion. Three explanations still hold their ground: First, according to Forbes and Rendu, the ice. in spite of the fact that it is very brittle to any rapid change of shape, is truly plastic to slow changes; just as shoemaker's wax will break under a sharp blow, but will allow a bullet by its own weight to sink slowly through it. This has been abundantly proved by the experiments of Pfaff, Andrews, Main, McConnell, and Kidd. Experiments on single ice-crystal show that the crystal is plastic in planes at right angles to the optic axis. This is the plastic or viscous theory. Second, Tyndall considered ice to be devoid of true plasticity, but thought that in a glacier it is continually shattered and refrozen. He was
led to this idea by the fact which Faraday dis covered, that two pieces of ice when brought into contact will freeze together. He showed by many experiments that ice could be crushed and forced through curved tubes and come out clear ice, the fragments having entirely coalesced. This is the fracture and regelation theory. Third, James Thomson discovered that the freezing-point of ice is lowered .0075° C. from an increase of one at mosphere of pressure, and applied this fact to the explanation of glacial motion. He supposed that at any point, where, by the movement of the ice, the pressure becomes a little greater than the average, the freezing-point will be lowered and a small amount of ice melted; the pressure being thus relieved, the ice will move slightly, and the pressure will be transferred to other points; the water thus formed will be squeezed through crevices in the ice to other points where the pressure is less, and will there freeze. A continuation of this process will result in the gen eral progression of the ice down its valley. This is the pressure and regelation theory.