Page:The American Cyclopædia (1879) Volume VII.djvu/846

 830 GLACIER portions were impelled by different degrees of force, arising from the expansion of the water congealing in all the fissures and capillary tubes of the ice into which it found its way. The facts developed by Forbes that the motion was greatest in the warm summer weather, when the temperature did not descend below the freezing point, and that it did not cease when the ice was no longer liquefied in the cold of winter demanded some new explanation. With the other phenomena they were regarded by him as sufficient to establish the fact that ice in large bodies is not a brittle solid, but that it possesses, particularly when saturated with water, so much plasticity, that with time it can yield to a stupendous and steadily exerted force, and move somewhat like a body of vis- cous pitch or lava, which, while it appears brit- tle when suddenly struck, can yet mould itself in the mass to the surface upon which it rests. By this theory, which was generally received even by those who first opposed it, all the diffi- culties attending the explanation of the move- ment disappear. It was confirmed by a sim- ple experiment made by Mr. Christie, secretary to the royal society. He filled with water a 10-inch hollow shell of iron, the shell itself being besides 1 in. thick, and exposed this to severe cold. As the water expanded in freez- ing, a cylinder of ice was pushed up through the fuse hole, and it continued to increase in length as the water continued to freeze. As the outer portions of the water must have been first converted into ice, it is plain that it was this so-called solid material which was forced through the narrow aperture and made to as- sume the form of a cylinder of its diameter. But the peculiar nature of this quality of mo- bility belonging to ice has been more perfectly explained, together with some of the other phenomena of glaciers, by the researches of Tyndall and Huxley, an account of which is published in the "Philosophical Magazine," vol. xv. (4th series), 1858. The property of particles of ice when exposed to higher tem- peratures than the freezing point to adhere, and under pressure to unite in one mass, was observed by Faraday, and was afterward made the subject of various experiments by Tyndall and Huxley. They found that compact trans- parent ice might be crushed to fragments, and these be made by a hydraulic press to assume in a few seconds the shape of any mould, re- covering in their new form perfect solidity and transparency. A straight bar of ice was bent into a semicircular form by using a succession of four moulds of gradually increasing curva- ture. As the prism conformed itself to these, cracks were produced, and crackling sounds were emitted, reminding one of those which are so often noticed among the phenomena of glaciers. By reference to this before unob- served property of ice the movement and un- broken continuity of glaciers and their branches are now explained. The glaciers from their very source present a series of changes of structure, which have been critically observed and traced, and in some instances illustra- ted and explained by experiments on a small scale with other materials. The snowy region known by the French term neve is formed of dry and granular snow, which extends for miles, sometimes broken by immense chasms, and at others presenting no irregularities of surface such as are common to the glacier be- low, no streams, crevices, moraines, or cones. The snow lies in strata, which reach to great depth, each representing the accumulation of a single year, the lowest the most dense and ap- proaching the blue color of ice. These bodies move onward to form the glacier proper ; and as they pass into this, their material assumes more and more the character of compact ice. But a remarkable and peculiar feature is the veined or laminated structure, real or only ap- , parent, which it assumes. This is noticed in the walls of the fissures, and is also displayed upon the surface of the glacier itself, when this has been wasted by rain. Thin laminae of transparent blue ice alternate with others of white porous ice, and standing together in a vertical position the edges of the former pro- ject a little above those of the latter, which more readily melt, and thus a ribbed appear- ance is produced. The direction of the lami- nea is across the fissures, and as observed by Tyndall and Huxley these are produced at right angles to the direction of greatest ten- sion. They find an analogy between the lami- nation of the ice and the slaty cleavage of the clays and slates, both which they refer to pressure causing the development of divisional planes in lines approximately at right angles to the direction of pressure. Hence the ob- liquity of the lamination to the sides of the glacier as the lines extend from the margin to- ward the middle and down its course; and the deviation directly across the glacier, or at right angles with this and parallel with its axis, as the form of its bed or other causes produce a pressure in the one case exerted lon- gitudinally and in the other laterally with the line of the glacier. By submitting plastic ma- terials, as wax, to pressure, and observing the laminated structure these assumed, these in- vestigators were led to this explanation of the phenomenon as developed in glacier ice ; but others, as Prof. Forbes, describe the white ice as produced merely by lines of cavities or of air bubbles in the blue ice itself, the result, according to the observation of Prof. James Thompson, of partial liquefaction induced by pressure ; and Prof. William Thomson at- tempts to prove " that the first effect of pres- sure not equal in all directions on a mass of snow ought to be, according to the theory, to convert it into a stratified mass of layers of alternately clear and vesicular ice, perpendicu- lar to the direction of maximum pressure." But the complete explanation of this structure will require experimental researches upon ice which have not yet been made. Another in- r