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| 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.
Fig. 104
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.
Fig. 105
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.
Fig. 106
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.
Fig. 107
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
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.
Fig. 109
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.
Fig. 110
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.
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