GLACIER, a name given to a mass of ice, having its origin in the hollows of mountains where perpetual snow accumulates, but which makes its way down towards the lower valleys, where it gradually melts, until it terminates exactly where the melting, due to the contact of the warmer air, earth, and rain of the valley, compensates for the bodily descent of the ice from the snow reservoirs of the higher mountains.
The diminution of temperature as we ascend the slopes of mountains, is indicated by successive zones of vegetation, and finally by the occurrence of perpetual snow (see GEOLOGY, p. 280). It was first shown by Baron Humboldt and Von Buch that the limit of perpetual snow depends principally on the temperature of the summer, and not upon that of the whole year.
A glacier usually protrudes iuto a valley far below the limit of perpetual snow, and terminates amidst a wilderness of stones borne down upon its surface and deposited by its fusion. This earthy and rocky rubbish is termed moraine matter, and has already been described (GEOLOGY, p. 281). Lying in front of the lower end of a glacier, it marks in a characteristic and certain manner the greatest limit of extension which the glacier has at any one time attained. Sometimes a glacier is seen to have withdrawn very far within its old limits, leaving a prodigious barren waste of stones in advance of it, which, being devoid of soil, nourishes not one blade of grass. At other times the glacier pushes forward its margin beyond the limit which it has ever reached (at least within the memory of man), tears up the ground with its icy ploughshare, and shoves for-ward the yielding turf in wrinkled folds, uprooting trees, moving vast rocks, and scattering the walls of dwelling-houses in fragments before its irresistible onward march.
The lower end of a glacier is usually steep,sometimes with a dome-shaped unbroken outline, more frequently broken up by intersecting cracks into prismatic masses which the continued action of the sun and rain sharpen into pyramids, often assuming (as in the glacier of Bossons at Chamouni) grotesque or beautiful forms. From a vault in the green-blue ice, more or less perfectly formed each sum-mer, the torrent issues which represents the natural drain-age of the valley, derived partly from land springs, partly from the fusion of the ice. The united or crevassed condi-tion of the glacier generally depends almost entirely on the slope of its bed. If it incline rapidly, numerous transverse fissures are formed from the imperfect yielding of the ice during its forced descent along its uneven channel. These cracks often extend for hundreds of yards, and may be hundreds of feet in depth ; but their greatest depth is not accurately known, since they are rarely quite vertical. In many cases, however, the crevasses are comparatively few in number, and the glacier may be readily traversed in all directions. This is especially the case if a glacier of con-siderable dimensions meets with any contraction in its course. The ice is embayed and compressed, and its slope lessens, just as in the case of a river when it nears a similar contraction preceding a fall. Such level and generally traversable spaces may be found about the middle regions of the Mer de Glace, the lower glacier of Grindelwald, the lower glacier of the Aar, and in many other cases. The last-named glacier is perhaps the most remarkably even and accessible of any in Switzerland. The slope of its surface is in many places only 3°. The Pasterzen glacier in Carinthia is even less inclined. It is in such portions of a glacier that we commonly find internal cascades, or " moulins." These arise from the surface water being collected into a considerable mass by a long course over its unbroken surface, and then precipitated with violence iuto the first fissure it meets with. The descending cascade keeps open its channel, which finally loses the form of a fissure, presenting that of an open shaft, often of immense depth.
Nearly connected in their origin with the internal cas-cades are the " gravel cones," occasionally seen on the sur-face of glaciers, which appear to be formed in this way. A considerable amount of earthy matter derived by the superficial water-runs from the moraine accumulates in heaps in the inequalities of the ice, or at the bottom of the "moulins," As the glacier surface wastes by the action of the sun and rain, these heaps are brought to the surface, or rather the general surface is depressed to their level. If the earthy mass be considerable, the ice beneath is protected from the radiation of the sun and from the violent washing of the rain ; it at length protrudes above the general level of the glacier, and finally forms a cone which appears to be entirely composed of gravel, but is in fact ice at the heart, with merely a protecting cover of earthy matter. These singular cones are very well seen on the glacier of the Aar, but on most others they are comparatively rare. The similar protective action of large stones detached from the moraines and lying on the surface of the ice often produces the striking phenomenon of "glacier tables." Stones of any considerable size almost invariably stand upon a slightly elevated pillar of ice; but when they are broad and flat they occasionally attain a height of 6 and even of 12 feet above the general level.
The superficial waste of a glacier is thus a very important phenomenon. Owing to it the body of the ice has its vertical thickness rapidly diminished during the heats of summer, and, as we have already intimated, the lower end of a glacier has its position determined by the amount of this waste. Suppose a glacier to move along its bed at the rate of 300 feet per annum, and imagine (merely for the sake of illustration) its yearly superficial waste to be 20 feet; then the thickness of the glacier will diminish by 20 feet for every 300 feet of its length, or at the rate of 360 feet per mile, so that the longitudinal section of a glacier has the form of a wedge; and however enormous its original thickness, after a certain course we must at length come to the thin end of the wedge, and that the more rapidly as the causes of melting increase towards the lower extremity. These causes are indeed so various that it is difficult to estimate them with accuracy. We have (1) the direct solar heat, (2) the contact of warm air, and (3) the washing of rain. All these causes act on the surface and produce the " ablation " of the surface. Besides these, the ice of the glacier wastes somewhat beneath by the contact of the soil and the washing of the inferior streams. This may be called its " subsidence." Further, the natural slope of the rocky bed of the glacier causes any point of the surface to stand absolutely lower each day in con-sequence of the progressive motion. These three causes united produce the " geometrical depression " of the sur-face. Principal J. D. Forbes showed how the several effects may usually be distinguished by observation. During the height of summer, near the Montanvert, he found the daily average ablation to be 3-62 inches, the daily subsidence to be 1-63 inches. Seven-tenths of the geo-metrical depression are due therefore to the former cause, and three-tenths to the latter. This is a very large amount, and it is certain that during the colder period of the year, and whilst the glacier is covered with snow, the subsidence is not only suspended, but the glacier recruits in thickness a portion of its waste during the seasons of summer and autumn. To this subject we shall again return.
The middle region of the great glaciers of the Alps extends from the level of about 6000 to 8000 feet above the sea. The inclination is usually there most moderatesay from 2|° to 6°. But this is not invariably the case. Beyond 8000 feet we reach the snow-line. The snow-line is a fact as definite on the surface of a glacier as on that of a moun-tain, only in the former case it occurs at a somewhat lower level. It cannot be too distinctly understood that the fresh snow annually disappears from the glacier proper. Where it ceases entirely to melt, it of course becomes in-corporated with the glacier. We have therefore arrived at the region where the glacier/o«»s; everywhere below it only wastes. This snowy region of the glacier is called in French neve, in German firn. As we ascend the glacier it passes gradually from the state of ice to the state of snow. The superficial layers are more snowy and white, in fact nearly pure snow ; the deeper ones have more colour and consist-ence, and break on the large scale into vast fragments, which at Chamouni are called seracs. The névé moves, as the glacier proper does, and it is fissured by the inequalities of the ground over which it passes. These fissures are less regular than those of the lower glacier. They are often much wider, in fact of stupendous dimensions, and, being often covered with treacherous snowy roofs, constitute one of the chief dangers of glacier travelling. The constitution of the névé may be well studied on the Glacier du Géant, a tributary of the Mer de Glace. The mountain-clefts in which large glaciers lie usually expand in their higher portions (in conformity with the ordinary structure of valleys) into extensive basins in which snow is perpetual, and which therefore contain the névé, the true origin and material of the glacier, which is literally the overflow of these snowy reservoirs. The amount of overflow, or the dis-charge of the glacierupon which depends the extent of its prolongation into the lower valleysdepends in its turn on the extent of the névé or collecting reservoir. Glaciers with small reservoirs of necessity perish soon. Their thickness being small, the wedge of the glacier soon thins out. They are common in confined cirques of the higher mountains. Such are the glaciers of the second order described by De Saussure. Their slope is often very greatfrom 20° to 40°.
The ice of the glacier proper has a very peculiar struc-ture, quite distinct from the stratification of the snow on the névé (the relics of its mode of deposit), and one which requires special notice. When we examine the appearance of the ice in the wall of an ordinary crevasse (especially if it be tolerably near the side of the glacier) we are struck with the beautiful vertically laminated structure (first observed by Principal Forbes) which it commonly presents, resembling delicately veined marble (especially the variety called in Italy cipollino), in shades varying from bluish-green, through green, to white. When we trace the direction of the planes constituting the laminated structure, by observing them on the surface of the glacier (where they are usually well seen after rain, or in the channels of superficial water-runs), we find that where best developed (or not very far from the sides of the glacier) these laminoe are nearly parallel to the sides, but rather incline from the shore to the centre of the ice stream as we follow the declivity of the glacier.
Forbes found that certain superficial discolorations in the form of excessively elongated hyperbolas are due to the recurrence (at intervals of some hundred feet along the course of the glacier) of portions of ice in which the veined structure is more energetically developed than elsewhere, and where, by the decomposition of the softer laminae, portions of sand and dirt become entangled in the superficial ice, and give rise to the phenomena of " dirt bands," which thus at a distance display (though in a manner requiring some attention to discover) the exact course of this singular structure on the surface of the glacier. Fig. 1 displays the superficial form of the dirt bands, and the course of the structural laminae projected horizontally. Fig. 2 shows an ideal transverse section of the glacier, and fig. 3 another vertical section parallel to its length. These three sections in rectangular planes will serve to give a correct idea of the course of this remarkable structure within the ice, but a more popular conception will be formed of it from the imaginary sections of a canal-shaped glacier in slopes forwards until at the lower termination it has a very slight dip inwards, or indeed may be reversed and fall out-wards and forwards. The general form of a structural lamina of a glacier rudely resembles that of a spoon.
This structure and the accompanying dirt bands have been recognized by different observers in almost all glaciers, including those of Norway and of India. The interval between the dirt bands has been shown in the case of the Mer de Glace (and therefore probably in other cases) to coincide with annual rate of progression, and in the higher parts of the glacier (towards the névé) to be accompanied by wrinkles or inequalities of the surface which are well marked by the snow lying in them during the period of its partial disappearance.
The Motion of Glaciers and its Causes.There is some-thing about a glacier which almost inevitably conveys to the mind the idea of a stream. This may be traced in the descriptions of unscientific tourists, of poets, and of some of those who have addressed themselves more seriously to the question of the real nature of these bodies. To the latter class of observers belong Captain Basil Hall and Monseigneur Rendu, bishop of Annecy, who h,ad much more than hinted at the possibility of a true mechanical connexion between the descent of a glacier and that of a mountain torrent, or of a stream of lava. But until the actual conditions of motion were reduced to rule, it was impossible to know how far the analogy was real.
The most characteristic and remarkable feature of gla-ciers is their motion downwards from the névé towards the lower valley. The explanation of it is by far the most important application of mechanical physics connected with the subject. The prin-cipal theories to account for the progressive motion of glaciers which were prevalent pre-vious to 1842 may be briefly characterized as De Saussure's and De Charpentier's, though each had been maintained long before by the earlier Swiss writers. The first may be called the gravitation theory, the latter the dilatation theory. Both suppose that the motion of the ice takes place by its sliding bodily over its rocky bed, but they differ as to the force which urges it over the obstacles opposed by friction and the irregularities of the surface on which it moves.
The following quotation from De Saussure explains his views with, his usual precision :" These frozen masses, carried along by the slope of the bed on which they rest, disengaged by the water (arising from their fusion owing to the natural heat of the earth) from the adhesion which they might otherwise contract to the bottomsometimes even elevated by the watermust gradually slide and descend along the declivity of the valleys or mountain slopes (croupes) which they cover. It is this slow but continual sliding of the icy masses (des glaces) on their inclined bases which carries them down into the lower valleys, and which replenishes continually the stock of ice in valleys warm enough to produce large trees and rich harvests." Very sufficient objections have been urged against this theory. It is evident that De Saussure considered a glacier as an accumulation of icy fragments, instead of a great and con-tinuous mass, throughout which the fissures and "crevasses" bear a small proportion to the solid portion ; and that he has attributed to the subglacial water a kind and amount of action for which there exists no sufficient or even probable evidence. The main objection, however, is this, that a sliding motion of the kind supposed, if it commence, must be accelerated by gravity, and the glacier must slide from its bed in an avalanche. The small slope of most glacier-valleys, and the extreme irregularity of their bounding walls, are also great objections to this hypothesis.
The dilatation theory ingeniously meets the difficulty of the want of a sufficient moving power to drag or shove a glacier over its bed, by calling in the well-known force with which water expands on its conversion into ice. The glacier being traversed by innumerable capillary fissures, and being in summer saturated with water in all its parts, it was natural to invoke the freezing action of the night to convert this water into ice, and by the amount of its expansion to urge the glacier onwards in the direction of its greatest slope. In answer to this, it is sufficient to observe, in the first place, that during the height of summer the portions of those glaciers which move fastest are never reduced below the freezing point, and that, even in the most favourable cases of nocturnal radiation producing congelation at the surface, it cannot (by well-known laws of conduction) pene-trate above a few inches into the interior of the glacier. Again, the ascertained laws of glacier-motion are (as will be immediately seen) entirely adverse to this theory, as it is always accelerated by hot weather and retarded by cold, yet does not cease even in the depths of winter.
It is singular how slow observers were to perceive the importance to the solution of the problem of glacier-motion of ascertaining with geometrical precision the amount of motion of the ice, not only from year to year, but from day to day, in summer and winter, whether constant or variable at the same point, whether continuous or by starts; if variable, on what circumstances it depended, and in what manner it was affected at different points of the length and breadth of a glacier.
This method of studying the question was taken up by Forbes. His observations were commenced on the Mer de Glace of Chamouni, in June 1842. Between the 26th and 27th of that month the motion of the ice opposite a point called the " Angle " was found, by means of a theodolite, to be 16-5 inches in 26 hours ; between the 27th and 28th, 17-4 inches in 25|- hours; and from about 6 A.M. to 6 P.M. on the 28th the motion was 9'5 inches, or 17-5 inches in 24 hours ; whilst the proportional motion during even an hour and a half was observed. No doubt could therefore remain that the motion of the ice is continuous and toler-ably uniformin short, that it does not move by jerks. He also ascertained about the same time that the motion of the ice is greatest towards the centre of a glacier and slower at the sides, contrary to an opinion then maintained on high authority. He next found that the rate of motion varied at different points of the length of the same glacier, being on the whole greatest where the inclination of its surface is greatest. As the season advanced, he observed notable changes in the rate of motion of the same part of the ice, and connected it by a very striking direct relation with the temperature of the air. These facts were established during the summer of 1842, and promptly published. By means of occasional observations during the following winter and spring by his guide, Auguste Balmat of Chamouni, and by a more full comparison of the entire motion of a glacier for twelve months with its motion during the hot season of the year, another generally received error was rectified : the motion of the glacier continues even in winter, and it has a very perceptible ratio to the summer motion. Last of all, it was found that the surface of a glacier moves faster than the ice nearer the bottom or bed.
These and some minor laws of motion, being undoubted expressions of the way in which glaciers move, were formu-lated by Forbes in an approximate theory : " A glacier is an imperfect fluid or a viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts." The analogy subsisting between the motion of a glacier and that of a river (which is a viscous fluid,were it not so, its motion would be widely different) will be best perceived by stating more precisely its laws of motion.
1. Each portion of a glacier moves, not indeed with a constant velocity, bnt in a continuous manner, and not by sudden sub-sidences with intervals of repose. This, of course, is characteristic also of a river.
2. The ice in the middle part of the glacier moves much faster than that near the sides or banks ; also the surface moves faster than the bottom. Both these facts obtain in the motion of a river in consequence of the friction of the fluid on its banks, and in con-sequence also of that internal friction of the fluid which constitutes its viscosity.
Thus, at four stations of the Mer de Glace, distant respectively
from the west shore of the glacier 100 230 405 365 yds.,
the relative velocities were 1-000 1-302 1-356 T367.
3. The variation of velocity (as in a river) is most rapid near the sides, whilst the middle parts move nearly uniformly. This and the preceding laws are also fully brought out by the subsequent experiments of M. Agassiz on the glacier of the Aar, and of MM. Schlagintweit on the Pasterzen glacier.
4. The variation of velocity of a glacier from the sides to the middle is nearly in proportion to the absolute velocity of the glacier,whether that absolute velocity change in the same place in consequence of change of season, or between one point and another of the length of the same glacier, depending on its declivity. See (5) and (6) below.
5. The glacier, like a stream, has its pools and its rapids. Where it is embayed by rocks it accumulates, its declivity in-creases, and its velocity at the same time. When it passes down a steep, issuing by a narrow outlet, its velocity increases. Thus the approximate declivities of the inferior, middle, and superior regions of the Mer de Glace (taken in the direction of its length) are 15° 4£° 8° and the relative velocities are as the numbers .. 1-398 -574 -825.
6. A fact not less important than any of the preceding is that increased temperature of the air favours the motion of the ice, and generally whatever tends to increase the proportion of the watery to the solid constituents of a glacier, as mild rains, and especially the thawing of the superficial snow in spring. The velocity does not, however, descend to zero even in the depth of winter. Indeed, in the lower and most accessible portions of the Mer de Glace (or Glacier des Bois) and the Glacier des Bossons, the ratio of the winter to the summer motion is almost exactly 1:2. On en- deavouring to establish a relation between the velocity of the glacier and the temperature of the ambient air, we find that those diminish together almost regularly down to the freezing-point, below which the velocity seems to remain constant.
Any mechanical theory of glaciers must be more or less imperfect which does not explain the remarkable veined or ribboned structure of the ice, with its peculiar course through the interior of the glacier, as above described. According to Forbes the fundamental idea is that the veined or ribboned structure of the ice is the result of internal forces, by which one portion of ice is dragged past another in a manner so gradual as not necessarily to produce large fissures in the ice, and the consequent sliding of one detached part over another, hut rather the effect of a general bruise over a considerable 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 their course parallel to the direction of the sliding effort of one portion of the ice over another. Amongst other proofs of this fundamental conception that the veined structure is the external symbol of this forced internal motion of a body comparatively solid, Forbes cited a striking instance from the glacier of La Brenva, on the south side of Mount Blanc. 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 prominence of rock, he observed developed in the ice a " veined structure " so beautiful that " it was impossible to resist the wish to carry off slabs, and to perpetuate it by hand specimens." This perfectly developed structure was visible opposite the promontory which held the glacier in check, and past which it struggled, leaving a portion of its ice completely embayed in a recess of the shore behind it. Starting from this point as an origin, the veined laminae extended backwards and upwards into the glacier, but did not spread laterally into the embayed 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 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 14-2 inches daily. Throughout this breadth of 170 feet there was not a single 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 solid 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 laminae, 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 analogy 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 theory, "it was rathera state-ment of fact than an explanation of the physical processes concerned 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-out a certain small range of temperature (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 the year) into its intrinsic composition. He insisted 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 continuity; 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 represented 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 glacier-motion cannot yet ba considered to be satisfactorily solved. One of the most im-portant contributions to the solution of this question was made by Professor 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 the plastic or viscous behaviour of glaciers contended for by Forbes. This prediction was experimentally verified by his brother, Sir W. Thomson. Tyndall subsequently to Forbes's work brought forward an explanation termed the " pressure or fracture and regelation theory." Some experi-ments of Faraday 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 water between them, while at a lower temperature than 32°, and with consequently dry surfaces, no adhesion took place. The freezing was obtained even under warm water. Starting from those observations Tyndall was led to make experiments on the effects of compression upon ice, and found that a quantity of pounded ice could be moulded into a compact homo-geneous mass. This property possessed by ice of reuniting by pressure after fracture was termed regelation, and was 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 Mosely investigated the physics of glaciers, especially by determining the shearing force of the ice. He found that in a glacier of such a uniform section and slope, moving at such a uniform rate, as the Mer de Glace at Les Ponts, the aggregate resistance offered by the ice to its descent is about 34 times greater than the force of gravitation. He therefore concluded it to be physically impossible that a glacier could slide down its valley by its own weight, and consequently that the gravitation or fracture and regelation 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 and contraction caused by changes of solar heat. He con-tended that the ice, like the lead, is expanded by heat, and that, as it cannot on expansion move up the valley without overcoming the resistance of gravitation as well as of friction, it necessarily moves chiefly downward, in which direction gravitation co-operates. Contraction on the other hand must also tend to send the ice downward, for a larger part will move with the force of gravitation than against it. Dr Croll, objecting to Canon Mosely's views that no observed alterna-tions of glacier temperature warrant the conclusion that the ice can be impelled downward by that cause, has proposed yet another explanation. He regards the motion of the ice of a glacier as molecular, resulting from the very conduc-tion of heat through the mass of the glacier. He contends that from the thermal conditions of glacier-ice its molecules will melt before their temperature can be raised. Any given molecule on melting will transmit its extra heat or part of it to the next molecule, which in turn may melt, and thus a wave of thaw will travel through the ice. But as each molecule loses its heat again it freezes, and in the act of solidification exerts an enormous pressure on the walls of the interstice into which while fluid it entered. Hence in proportion to the amount of heat received by it the ice is subjected to great molecular pressure. As the glacier cannot expand laterally on account of the walls of its channel, and as gravitation opposes its expansion up the valley, it necessarily finds relief by a downward move-mentthe direction in which gravitation co-operates.
See De Saussure's Voyages dans les Alpes, § 535; De Charpentier, Essai sur les Glaciers, 1841; Agassiz, Études sur les Glaciers, 1840, Système Glaciaire, 1847 ; L'Abbé Rendu, " Théorie des Glaciers do la Savoie," in Mem. Acad. Savoie, x., 1841, translated by G. Forbes and published 1875 ; J. D. Forbes, Travels in the Alps, 1843, Norway and its Glaciers, 1853, and Occasional Papers on Glaciers, 1859 ; Tyndall's Glaciers of the Alps, 1857 ; Mousson's Gletscher der Jetztzeit, 1854 ; Mosely, Proc. Hoy. Soc, 1869 ; Crcll, Climate and Time, 1875 ; J. Thomson, Proc. Hoy. Soc., 1856-7.
The following are synonyms in different languages and dialects: French, glacier ; German, gletscher ; Italian, ghiacciaia ; Tyrolese, fern ; in Carinthia, kàss ; in the Yalais, biegno ; in part of Italy, vedrette- ; in Piedmont, ruize ; in the Pyrenees, serneille ; in Norway, iisbrm or iisbrede ; in Lapland, geikna or jegna ; in Iceland, joJeuil or fall-jiikull.