Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/648

Rh G3- produce large fissures in the ice, and the consequent sliding ' of one detached part over another, but rather the effect of a general bruise over a consider-.1ble space of the yielding body. According to this view, the delicate veins seen in the glacier, often less than a quarter of an inch wide, have thzir course parallel to the direction of the sliding effort of one portion of the ice over another. Aniongst other proofs of this fundamental conception that the veined structure is l the external symbol of this forced internal motion of a body c miparatively solid, Forbes cited a striking instance from ; the glicier of L9. Brenva, on the south side of Mount Blane. I In this case the ice of the glacier_. forcibly pressed against the naked rocky face of an opposing hill is turned into a new direction; and in thus shoving and squeezing past a I prominence of rock, he observed developed in the ice a ' “ veined structure ” so beautiful that “ it was impossible to I resist the wish to carr_y off slabs, and to perpetuate it by hand specimens.” This perfectly developed structure was l visible opposite the promontory which held the glacier in check, and past which it struggled, leaving a portion of its i ic: completely embayed in a recess of the shore behind it. Starting from this point as an origin, the veined lamintc extended backwards and upwards into the glacier, but did not spread laterally into the cmbayed ice. They could, however, be traced from the shore to some distance from the promontory into the icy mass. The direction of lamin- ation exactly coincided with that in which the ice must have moved if it was shoved past the promontory at all. That it did so move was made the subject of direct proof, by ﬁxing two marks on the ice opposite the promontory, one on the nearer, the other on the farther side of the belt I of ice which had the lamination best developed. The ﬁrst mark was 50 feet from the shore, and moved at the rate of 4'9 inches daily ; the other mark was 170 feet further off, and moved almost three times faster, or 14:‘? inches daily. Throughout this breadth of 170 feet there was not a single I longitudinal crevasse which might have facilitated the dif- ferential motion. A parallelogram of compact ice, only 170 feet wide, was therefore moving in such a manner that, whilst one of its sides advanced only a foot, the other advanced a yard. No solid body, at least no rigid soli.l body, can advance in such a manner; Forbes therefore concluded that glacier-ice is plastic, that the veined structure is unquestionably the result of the struggle between the rigidity of the ice and the quasi-fluid character of the motion impressed upon it, and that this follows, not only from the direction of the larninae, but from their becoming distinct exactly in proportion to their nearness to the point where the bruise is necessarily strongest. The subsequent experiments of Sorby on the cleavage structure of rocks - proved that it has arisen as the result of intense lateral compression, and could be imitated in many artificial sub- stances. Tyndall obtained it even in beeswax, the analog -' between which and the veined structure of ice is very close. Though Forbes termed his expression of the laws of glacier motion the “ viscous” or “plastic the0ry,”it was rather a state- ment of fact than an explanation of the physical processes , conzerncd in the descent of glaciers. Against his views it was of course objected that ice is by its nature a brittle solid, and not sensibly possessed of any viscous or plastic quality. But he cogently replied that the qualities of solid bodies of vast size, and acted on by stupendous and long—continued forces, cannot be estimated from experiments on a small scale, especially if short and violent ; that sealing-wax, pitch, and other similar bodies mould themselves, with time, to the surfaces on which they lie, even at atmospheric temperatures, and whilst they maintain, at the same time, the quality of excessive brittleness under a blow or a rapid change of form ; that even ice does not pass at once, and per saltum, from the solid to the liquid state, but absorbs its latent heat through- G L A C1 E R out a certain small mnge of tcnipemture (between 28°'4 and 32° of Fahrenheit), which is precisely that to which the ice of glaciers is actually exposed; that, after all, a glacier is not a crystalline solid, like ice, tranquilly frozen in a mould, but possesses a peculiar fissured and laminated structure, through which water enters (at least for a great part of tln- year) into its intrinsic composition. lle in-sistcd that the quasi-ﬂuid or viscous motion of the ice of glaciers is not a theory but a fact. A substance which is seen to pour itself out of a large basin through a narrow outlet without losing its continuit '; the different parts of which, from top to bottom, and from side to centre, possess distinct though related velocities; which moves over slopes inconsistent with the friction between its surface and the ground on which it rests; which surmounts obstacles, and even if cleft into two streams by a ‘projecting rock, instead of being thereby anchored as a solid would necessarily be, reunites its streams below, and retains no trace of the fissure, leaving the rock an islet in the icy flood,'~a substance which moves in such a fashion cannot, Forbes maintained, in any true sense of the word, be termed a rigid solid, but must be granted to be ductile, viscous, plastic, or semiﬂuid, or to possess qualities reprc.scntcd by any of these terms which we may choose to adopt as least shocking to our ordinary conception of the brittleness of ice. The problem of the cause of glat-ier—motion cannot yet be con.-idcrerl to be satisfactorily solved. One of the most im- portant contributions to the solution of this question was made by Pr-.»fcssor James Thomson when he predicted that the freezing point of ice must be lowered by pressure, and when he sought by means of this property to explain tln plastic or viscous behaviour of glaciers contended for by Forbes. This prediction was experimentally verified by his brother, Sir WV. Thomson. Tyndall subscrpicntly to F orbcs's work brought forward an explanation termed the “ pressure or fracture and rcgclation theory.” Some experi- ments of I-'a1'a(lay in 1850 had shown that two pieces of ice with moistened surfaces would if in contact adhere, owing to the freezing of the thin ﬁlm of vater between them, while at a lower temperature than 32°, and with consequently dry surfaces, no adhesion took place. The freezing w-.i.~ obtained even under warm water. Starting from those observations Tyndall was lezl to make experiments on the effects of compression upon ice, and found that a quantity of pounded ice could be mouhled into a compact l1omo— geneous mass. This property possessed by ice of reuniting by pressure after fracture was termed regelation, and v."-as applied by Tyndall in explanation of the motion of glaciers. He maintained that the ice of a glacier is a solid brittle substance, and that its descent down a valley is due to constant rupture produced by the effects of gravitation and to the consequent sliding forward of the mass in which the surfaces of fracture speedily reunite. He pointed more particularly to the ice-falls of glaciers where the ice in pass- ing over a steep descent and undergoing great tension does not yield as a viscous body, but is fractured as a solid. More recently Canon Moscly investigated the physics of glaciers, especially by determining the shearing force of the ice. Ile found that in a glacier of such a uniform section and slope, moving at such a uniform rate, as the Mer dc Glace at Les Ponts, the aggregate resistance offered by the ice to its descent is about 3:1 times greater than the force of gravitation. He therefore concluded it to be physically impossible that a glaciercould slide down its valley byits own weight, and consequently that the gravitation or fracture and rcgelation theory could not be maintained. The slow descent of sheet lead on a roof of moderate inclination, and its ability even to draw out from the rafters the nails with which it had been fastened, led him to propound another theory of glacier-motion, viz., that it is due to expansion