|Hermann von Helmholtz|
Hermann von Helmholtz (1821-1894), Ice and Glaciers.
Vol. 30, pp. 211-223 of The Harvard Classics
There was a time when the snow fell and did not melt in summer. Then from the frozen north there descended huge masses of ice that covered northern Europe and most of North America. Glaciers reveal a new world to us.
(Helmholtz died Sept. 8, 1894.)
A Lecture Delivered at Frankfort-on-the-Main, and at Heidelberg, in February, 1865
(Translated by Edmund Atkinson)
THE WORLD of ice and of eternal snow, as unfolded to us on the summits of the neighbouring Alpine chain, so stern, so solitary, so dangerous, it may be, has yet its own peculiar charm. Not only does it enchain the attention of the natural philosopher, who finds in it the most wonderful disclosures as to the present and past history of the globe, but every summer it entices thousands of travellers of all conditions, who find there mental and bodily recreation. While some content themselves with admiring from afar the dazzling adornment which the pure, luminous masses of snowy peaks, interposed between the deeper blue of the sky and the succulent green of the meadows, lend to the landscape, others more boldly penetrate into the strange world, willingly subjecting themselves to the most extreme degrees of exertion and danger, if only they may fill themselves with the aspect of its sublimity.
I will not attempt what has so often been attempted in vain—to depict in words the beauty and magnificence of nature, whose aspect delights the Alpine traveller. I may well presume that it is known to most of you from your own observation; or, it is to be hoped, will be so. But I imagine that the delight and interest in the magnificence of those scenes will make you the more inclined to lend a willing ear to the remarkable results of modern investigations on the more prominent phenomena of the glacial world. There we see that minute peculiarities of ice, the mere mention of which might at other times be regarded as a scientific subtlety, are the causes of the most important changes in glaciers; shapeless masses of rock begin to relate their histories to the attentive observer, histories which often stretch far beyond the past of the human race into the obscurity of the primeval world; a peaceful, uniform, and beneficent sway of enormous natural forces, where at first sight only desert wastes are seen, either extended indefinitely in cheerless, desolate solitudes, or full of wild, threatening confusion—an arena of destructive forces. And thus I think I may promise that the study of the connection of those phenomena of which I can now only give you a very short outline, will not only afford you some prosaic instruction, but will make your pleasure in the magnificent scenes of the high mountains more vivid, your interest deeper, and your admiration more exalted.
Let me first of all recall to your remembrance the chief features of the external appearance of the snow fields and of the glaciers; and let me mention the accurate measurements which have contributed to supplement observation, before I pass to discuss the casual connection of those processes.
The higher we ascend the mountains the colder it becomes. Our atmosphere is like a warm covering spread over the earth; it is well-nigh entirely transparent for the luminous darting rays of the sun, and allows them to pass almost without appreciable change. But it is not equally penetrable by obscure heat rays, which, proceeding from heated terrestrial bodies, struggle to diffuse themselves into space. These are absorbed by atmospheric air, especially when it is moist; the mass of air is itself heated thereby, and only radiates slowly into space the heat which has been gained. The expenditure of heat is thus retarded as compared with the supply, and a certain store of heat is retained along the whole surface of the earth. But on high mountains the protective coating of the atmosphere is far thinner—the radiated heat of the ground can escape thence more freely into space; there, accordingly, the accumulated store of heat and the temperature are far smaller than at lower levels.
To this must be added another property of air which acts in the same direction. In a mass of air which expands, part of its store of heat disappears; it becomes cooler, if it cannot acquire fresh heat from without. Conversely, by renewed compression of the air, the same quantity of heat is reproduced which had disappeared during expansion. Thus, if for instance, south winds drive the warm air of the Mediterranean towards the north, and compel it to ascend along the great mountain wall of the Alps, where the air, in consequence of the diminished pressure, expands by about half its volume, it thereby becomes very greatly cooled—for a mean height of 11,000 feet, by from 18° to 30° C., according as it is moist or dry—and it thereby deposits the greater part of its moisture as rain or snow. If the same wind, passing over to the north side of the mountains as Fohn-wind, reaches the valley and plains, it again becomes condensed, and is again heated. Thus the same current of air which is warm in the plains, both on this side of the chain and on the other, is bitterly cold on the heights, and can there deposit snow, while in the plain we find it insupportably hot.
The lower temperature at greater heights, which is due to both these causes, is, as we know, very marked on the lower mountain chains of our neighbourhood. In central Europe it amounts to about 1° C. for an ascent of 480 feet; in winter it is less—1° for about 720 feet of ascent. In the Alps the differences of temperature at great heights are accordingly far more considerable, so that upon the higher parts of their peaks and slopes the snow which has fallen in winter no longer melts in summer. This line, above which snow covers the ground throughout the entire year, is well known as the snow line; on the northern side of the Alps it is about 8,000 feet high, on the southern side about 8,800 feet. Above the snow line it may on sunny days be very warm; the unrestrained radiation of the sun, increased by the light reflected from the snow, often becomes utterly unbearable, so that the tourist of sedentary habits, apart from the dazzling of his eyes, which he must protect by dark spectacles or by a veil, usually gets severely sunburnt in the face and hands, the result of which is an inflammatory swelling of the skin and great blisters on the surface. More pleasant testimonies to the power of the sunshine are the vivid colours and the powerful odour of the small Alpine flowers which bloom in the sheltered rocky clefts among the snow fields. Notwithstanding the powerful radiation of the sun the temperature of the air above the snow fields only rises to 5°, or at most 8°; this, however, is sufficient to melt a tolerable amount of the superficial layers of snow. But the warm hours and days are too short to overpower the great masses of snow which have fallen during colder times. Hence the height of the snow line does not depend merely on the temperature of the mountain slope, but also essentially on the amount of the yearly snowfall. It is lower, for instance, on the moist and warm south slope of the Himalayas than on the far colder but also far drier north slope of the same mountain. Corresponding to the moist climate of western Europe, the snowfall upon the Alps is very great, and hence the number and extent of their glaciers are comparatively considerable, so that few mountains of the earth can be compared with them in this respect. Such a development of the glacial world is, as far as we know, met with only on the Himalayas, favoured by the greater height; in Greenland and in Northern Norway, owing to the colder climate; in a few islands in Iceland; and in New Zealand, from the more abundant moisture.
Places above the snow line are thus characterized by the fact that the snow which in the course of the year falls on its surface does not quite melt away in summer, but remains to some extent. This snow, which one summer has left, is protected from the further action of the sun’s heat by the fresh quantities that fall upon it during the next autumn, winter, and spring. Of this new snow also next summer leaves some remains, and thus year by year fresh layers of snow are accumulated one above the other. In those places where such an accumulation of snow ends in a steep precipice, and its inner structure is thereby exposed, the regularly stratified yearly layers are easily recognised.
But it is clear that this accumulation of layer upon layer cannot go on indefinitely, for otherwise the height of the snow peak would continually increase year by year. But the more the snow is accumulated the steeper are the slopes, and the greater the weight which presses upon the lower and older layers and tries to displace them. Ultimately a state must be reached in which the snow slopes are too steep to allow fresh snow to rest upon them, and in which the burden which presses the lower layers downwards is so great that these can no longer retain their position on the sides of the mountain. Thus, part of the snow which had originally fallen on the higher regions of the mountain above the snow line, and had there been protected from melting, is compelled to leave its original position and seek a new one, which it of course finds only below the snow line on the lower slopes of the mountain, and especially in the valleys, where, however, being exposed to the influence of a warmer air, it ultimately melts and flows away as water. The descent of masses of snow from their original positions sometimes happens suddenly in avalanches, but it is usually very gradual in the form of glaciers.
Thus we must discriminate between two distinct parts of the ice fields; that is, first, the snow which originally fell—called firn in Switzerland—above the snow line, covering the slopes of the peaks as far as it can hang on to them, and filling up the upper wide kettle-shaped ends of the valleys forming widely extending fields of snow or firnmeere. Secondly, the glaciers, called in the Tyrol firner, which as prolongations of the snow fields often extend to a distance of from 4,000 to 5,000 feet below the snow line, and in which the loose snow of the snow fields is again found changed into transparent solid ice. Hence the name glacier, which is derived from the Latin, glacies; French, glace, glacier.
The outward appearance of glaciers is very characteristically described by comparing them, with Goethe, to currents of ice. They generally stretch from the snow fields along the depth of the valleys, filling them throughout their entire breadth, and often to a considerable height. They thus follow all the curvatures, windings, contractions, and enlargements of the valley. Two glaciers frequently meet the valleys of which unite. The two glacial current then join in one common principal current, filling up the valley common to them both. In some places these ice currents present a tolerably level and coherent surface, but they are usually traversed by crevasses, and both over the surface and through the crevasses countless small and large water rills ripple, which carry off the water formed by the melting of the ice. United, and forming a stream, they burst, through a vaulted and clear blue gateway of ice, out at the lower end of the larger glacier.
On the surface of the ice there is a large quantity of blocks of stone, and of rocky débris,which at the lower end of the glacier are heaped up and form immense walls; these are called the lateral and terminal moraine of the glacier. Other heaps of rock, the central moraine, stretch along the surface of the glacier in the direction of its length, forming long regular dark lines. These always start from the places where two glacier streams coincide and unite. The central moraines are in such places to be regarded as the continuations of the united lateral moraines of the two glaciers.
The formation of the central moraine is well represented in the view below given of the Unteraar Glacier (FIG. 104). In the background are seen the two glacier currents emerging from different valleys; on the right from the Shreckhorn, and on the left from the Finsteraarhorn. From the place where they unite the rocky wall occupying the middle of the picture descends, constituting the central moraine. On the left are seen individual large masses of rock resting on pillars of ice, which are known as glacier tables.
To exemplify these circumstances still further, I lay before you in FIG. 105 a map of the Mer de Glace of Chamouni, copied from that of Forbes.
The Mer de Glace in size is well known as the largest glacier in Switzerland, although in length it is exceeded by the Aletsch Glacier. It is formed from the snow fields that cover the heights directly north of Mont Blanc, several of which, as the Grande Jorasse, the Aiguille Verte (a, FIGS. 105 and 106), the Aiguille du Géant (b), Aiguille du Midi (c), and the Aiguille du Dru (d), are only 2,000 to 3,000 feet below that king of the European mountains. The snow fields which lie on the slopes and in the basins between these mountains collect in three principal currents, the Glacier du Géant, Glacier de Léchaud, and Glacier du Talèfre, which, ultimately, united as represented in the map, form the Mer de Glace; this stretches as an ice current 2,600 to 3,000 feet in breadth down into the valley of Chamouni, where a powerful stream, the Arveyron, bursts from its lower end at k, and plunges into the Arve. The lowest precipice of the Mer de Glace, which is visible from the valley of Chamouni, and forms a large cascade of ice, is commonly called Glacier des Bois, from a small village which lies below.
Most of the visitors at Chamouni only set foot on the lowest part of the Mer de Glace from the inn at the Montanvert, and when they are free from giddiness cross the glacier at this place to the little house on the opposite side, the Chapeau (n). Although, as the map shows, only a comparatively very small portion of the glacier is thus seen and crossed, this way shows sufficiently the magnificent scenes, and also the difficulties of a glacier excursion. Bolder wanderers march upwards along the glacier to the Jardin, a rocky cliff clothed with some vegetation, which divides the glacial current of the Glacier du Talèfre into two branches; and bolder still they ascend yet higher, to the Col du Géant (11,000 feet above the sea), and down the Italian side to the valley of Aosta.
The surface of the Mer de Glace shows four of the rocky walls which we have designated as medical moraines. The first, nearest the side of the glacier, is formed where the two arms of the Glacier du Talèfre unite at the lower end of the Jardin; the second proceeds from the union of the glacier in question with the Glacier de Léchaud; the third, from the union of the last with the Glacier du Géant; and the fourth, finally, from the top of the rock ledge which stretches from the Aiguille du Géant towards the cascade (g) of the Glacier du Géant.
To give you an idea of the slope and the fall of the glacier, I have given in FIG. 106 a longitudinal section of it according to the levels and measurements taken by Forbes, with the view of the right bank of the glacier. The letters stand for the same objects as in FIG.105; p is the Aiguille de Léchaud, q the Aiguille Noire, r the Mont Tacul, f is the Col du Géant, the lowest point in the high wall of rock that surrounds the upper end of the snow fields which feed the Mer de Glace. The base line corresponds to a length of a little more than nine miles: on the right the heights above the sea are given in feet. The drawing shows very distinctly how small in most places is the fall of the glacier. Only an approximate estimate could be made of the depth, for hitherto nothing certain has been made out in reference to it. But that it is very deep is obvious from the following individual and accidental observations.
At the end of a vertical rock wall of the Tacul, the edge of the Glacier du Géant is pushed forth, forming an ice wall 140 feet in height. This would give the depth of one of the upper arms of the glacier at the edge. In the middle and after the union of the three glaciers the depth must be far greater. Somewhat below the junction Tyndall and Hirst sounded a moulin, that is, a cavity through which the surface glacier waters escape, to a depth of 160 feet; the guides alleged that they had sounded a similar aperture to a depth of 350 feet, and had found no bottom. From the usually deep trough—shaped or gorge—like form of the bottom of the valleys, which is constructed solely of rock walls, it seems improbable that for a breadth of 3,000 feet the mean depth should only be 350 feet; moreover, from the manner in which ice moves, there must necessarily be a very thick coherent mass beneath the crevassed part.
To render these magnitudes more intelligible by reference to more familiar objects, imagine the valley of Heidelberg filled with ice up to the Molkencur, or higher, so that the whole town, with all its steeples and the castle, is buried deeply beneath it; if, further, you imagine this mass of ice, gradually extending in height, continued from the mouth of the valley up to Neckargemünd, that would about correspond to the lower united ice current of the Mer de Glace. Or, instead of the Rhine and the Nahe at Bingen, suppose two ice currents united which fill the Rhine valley to its upper border as far as we can see from the river, and then the united currents stretching downwards to beyond Asmannshausen and Burg Rheinstein; such a current would also about correspond to the size of the Mer de Glace.
FIG. 107, which is a view of the magnificent Gorner Glacier seen from below, also gives an idea of the size of the masses of ice of the larger glaciers.
The surface of most glaciers is dirty, from the numerous pebbles and sand which lie upon it, and which are heaped together the more the ice under them and among them melts away. The ice of the surface has been partially destroyed and rendered crumbly. In the depths of the crevasses ice is seen of a purity and clearness with which nothing that we are acquainted with on the plains can be compared. From its purity it shows a splendid blue, like that of the sky, only with a greenish hue. Crevasses in which pure ice is visible in the interior occur of all sizes; in the beginning they form slight cracks in which a knife can scarcely be inserted; becoming gradually enlarged to chasms, hundreds or even thousands, of feet in length, and twenty, fifty, and as much as a hundred feet in breadth, while some of them are immeasurably deep. Their vertical dark blue walls of crystal ice, glistening with moisture from the trickling water, form one of the most splendid spectacles which nature can present to us; but, at the same time, a spectacle strongly impregnated with the excitement of danger, and only enjoyable by the traveller who feels perfectly free from the slightest tendency to giddiness. The tourist must know how, with the aid of well-nailed shoes and a pointed Alpenstock, to stand even on slippery ice, and at the edge of a vertical precipice the foot of which is lost in the darkness of night, and at an unknown depth. Such crevasses cannot always be evaded in crossing the glacier; at the lower part of the Mer de Glace, for instance, where it is usually crossed by travellers, we are compelled to travel along some extent of precipitous banks of ice which are occasionally only four to six feet in breadth, and on each side of which is such a blue abyss. Many a traveller, who has crept along the steep rocky slopes without fear, there feels his heart sink, and cannot turn his eyes from the yawning chasm, for he must first carefully select every step for his feet. And yet these blue chasms, which lie open and exposed in the daylight, are by no means the worst dangers of the glacier; though, indeed, we are so organised that a danger which we perceive, and which therefore we can safely avoid, frightens us far more than one which we know to exist, but which is veiled from our eyes. So also it is with glacier chasms. In the lower part of the glacier they yawn before us, threatening death and destruction, and lead us, timidly collecting all our presence of mind, to shrink from them; thus accidents seldom occur. On the upper part of the glacier, on the contrary, the surface is covered with snow; this, when it falls thickly, soon arches over the narrower crevasses of a breadth of from four to eight feet, and forms bridges which quite conceal the crevasse, so that the traveller only sees a beautiful plane snow surface before him. If the snow bridges are thick enough, they will support a man; but they are not always so, and these are the places where men, and even chamois, are so often lost. These dangers may readily be guarded against if two or three men are roped together at intervals of ten or twelve feet. If then one of them falls into a crevasse, the two others can hold him, and draw him out again.
In some places the crevasses may be entered, especially at the lower end of a glacier. In the well-known glaciers of Grindelwald, Rosenlaui, and other places, this is facilitated by cutting steps and arranging wooden planks. Then any one who does not fear the perpetually trickling water may explore these crevasses, and admire the wonderfully transparent and pure crystal walls of these caverns. The beautiful blue colour which they exhibit is the natural colour of perfectly pure water; liquid water as well as ice is blue, though to an extremely small extent, so that the colour is only visible in layers of from ten to twelve feet in thickness. The water of the Lake of Geneva and of the Lago di Garda exhibits the same splendid colour as ice.
The glaciers are not everywhere crevassed; in places where the ice meets with an obstacle, and in the middle of great glacier currents the motion of which is uniform, the surface is perfectly coherent.
FIG. 108 represents one of the more level parts of the Mer de Glace at the Montanvert, the little house of which is seen in the background. The Gries Glacier, where it forms the height of the pass from the Upper Rhone valley to the Tosa valley, may even be crossed on horseback. We find the greatest disturbance of the surface of the glacier in those places where it passes from a slightly inclined part of its bed to one where the slope is steeper. The ice is there torn in all directions into a quantity of detached blocks, which by melting are usually changed into wonderfully shaped sharp ridges and pyramids, and from time to time fall into the interjacent crevasses with a loud rumbling noise. Seen from a distance such a place appears like a wild frozen waterfall, and is therefore called a cascade; such a cascade is seen in the Glacier du Talèfre at 1, another is seen in the Glacier du Géant at g. FIG. 110, while a third forms the lower end of the Mer de Glace. The latter, already mentioned a the Glacier des Bois, which rises directly from the trough of the valley at Chamouni to a height of 1,700 feet, the height of the Konigstuhl at Heidelberg, affords at all times a chief object of admiration to the Chamouni tourist. FIG. 109 represents a view of its fantastically rent blocks of ice.
We have hitherto compared the glacier with a current as regards its outer form and appearance. This similarity, however, is not merely an external one: the ice of the glacier does, indeed, move forwards like the water of a stream, only more slowly. That this must be the case follows from the considerations by which I have endeavoured to explain the origin of a glacier. For as the ice is being constantly diminished at the lower end by melting, it would entirely disappear if fresh ice did not continually press forward from above, which, again, is made up by the snowfalls on the mountain tops.
But by careful ocular observation we may convince ourselves that the glacier does actually move. For the inhabitants of the valleys, who have the glaciers constantly before their eyes, often cross them, and in so doing make use of the larger blocks of stone as sign posts—detect this motion by the fact that their guide posts gradually descend in the course of each year. And as the yearly displacement of the lower half of the Mer de Glace at Chamouni amounts to no less than from 400 to 600 feet, you can readily conceive that such displacements must ultimately be observed, notwithstanding the slow rate at which they take place, and in spite of the chaotic confusion of crevasses and rocks which the glacier exhibits.
Besides rocks and stones, other objects which have accidentally alighted upon the glacier are dragged along. In 1788 the celebrated Genevese Saussure, together with his son and a company of guides and porters, spent sixteen days on the Col du Géant. On descending the rocks at the side of the cascade of the Glacier du Géant, they left behind them a wooden ladder. This was at the foot of the Aiguille Noire, where the fourth band of the Mer de Glace begins; this line thus marks at the same time the direction in which ice travels from this point. In the year 1832, that is, forty-four years after, fragments of this ladder were found by Forbes and other travellers not far below the junction of the three glaciers of the Mer de Glace, in the same line (at s, FIG. 110), from which it results that these parts of the glacier must on the average have each year descended 375 feet.
In the year 1827 Hugi had built a hut on the central moraine of the Unteraar Glacier for the purpose of making observations; the exact position of this hut was determined by himself and afterwards by Agassiz, and they found that each year it had moved downwards. Fourteen years later, in the year 1841, it was 4,884 feet lower, so that every year it had on the average moved through 349 feet. Agassiz afterwards found that his own hut, which he had erected on the same glacier, had moved to a somewhat smaller extent. For these observations a long time was necessary. But if the motion of the glacier be observed by means of accurate measuring instruments, such as theodolites, it is not necessary to wait for years to observe that ice moves—a single day is sufficient.
Such observations have in recent times been made by several observers, especially by Forbes and by Tyndall. They show that in summer the middle of the Mer de Glace moves through twenty inches a day, while towards the lower terminal cascade the motion amounts to as much as thirty-five inches in a day. In winter the velocity is only about half as great. At the edges and in the lower layers of the glacier, as in a flow of water, it is considerably smaller than in the centre of the surface.