West 17 15 16 17-1/4 15 22-1/4 13 23-1/4 12 23-1/4
East 3 12-1/2 4 15-1/4 5 15-1/2 7 18-1/4 9 19-1/2
[Sidenote: WESTERN HALF MOVES QUICKEST.]
The table explains itself. We see that while stake 17, which stands
_west_ of the centre, moves 15 inches, stake 3, which stands an equal
distance _east_ of the centre, moves only 12-1/2 inches. Comparing every
pair of the other points, we find the same to hold good; the western
stake moves in each case faster than the corresponding eastern one.
Hence, _the entire western half of the Mer de Glace, at the place
crossed by our fourth line, moves more quickly than the eastern half of
the glacier_.
We next proceeded farther up, and tested the contrary curvature of the
glacier, opposite to Trelaporte. The station chosen for this purpose was
on a grassy platform of the promontory, whence, on the 28th of July, a
row of stakes was fixed at right angles to the axis of the glacier.
Their motions, measured on the 31st, gave the following results:--
Fifth Line.[B]--Daily Motion.
No. of stake. Inches.
West 1 moved 11-1/4
2 " 13-1/2
3 " 12-3/4
4 " 15
5 " 15-1/4
6 " 16
7 " 17-1/4
8 " 19-1/4
9 moved 19-3/4
10 " 19
11 " 19-1/2
12 " 17-1/2
13 " 16
14 " 14-3/4
15 " 10 East.
This line was set out and numbered from the Trelaporte side of the
valley, and was also measured by Mr. Hirst, over boulders, ice-ridges,
chasms, and moraines. The entire width of the glacier here was found to
be 893 yards, or somewhat wider than it is at the Ponts. It will also be
observed that its motion is somewhat slower.
An inspection of the notes of this line showed me that stakes 3 and 14,
4 and 12, 7 and 10, were "corresponding points;" the first of each pair
standing as far from the western side, as the second stood from the
eastern. In the following table these points and their velocities are
arranged exactly as in the case of the fourth line.
Numbers and Velocities of the Corresponding Points on the Fifth Line.
No. Vel. No. Vel. No. Vel.
West 3 12-3/4 4 15 7 17-1/4
East 14 14-3/4 12 17-1/2 10 19
[Sidenote: EASTERN HALF MOVES QUICKEST.]
In each case we find that the stake on the eastern side moves more
quickly than the corresponding one upon the western side: so that where
the fifth line crosses the glacier _the eastern half of the Mer de Glace
moves more quickly than the western half_. This is the reverse of the
result obtained at our fourth line, but it agrees with that obtained on
our first three lines, where the curvature of the valley is similar. The
analogy between a river and a glacier moving through a sinuous valley is
therefore complete.
Supposing the points of maximum motion to be determined for a great
number of lines across the glacier, the line uniting all these points is
what mathematicians would call the _locus_ of the point of maximum
motion. At Trelaporte this line would lie east of the centre; at the
Ponts it would lie west of the centre; hence, in passing from Trelaporte
to the Ponts, it must cross the axis of the glacier. Again, at the
Montanvert, it would lie east of the centre, and between the Ponts and
the Montanvert the axis of the glacier would be crossed a second time.
Supposing the dotted line in Fig. 21 to represent the middle line of the
glacier, then the defined line would represent the locus of the point of
maximum motion. _It is a curve more deeply sinuous than the valley
itself, and it crosses the axis of the glacier at each point of contrary
flexure._
[Sidenote: LOCUS OF POINT OF SWIFTEST MOTION.]
[Illustration: Fig. 21. Locus of the Point of Maximum Motion.]
To complete our knowledge of the motion of the Mer de Glace, we
afterwards determined the velocity of its two accessible
tributaries--the Glacier du Geant, and the Glacier de Lechaud. On the
29th of July, a line of stakes was set out across the former, a little
above the Tacul, and their motion was subsequently found to be as
follows:
Sixth Line.--Daily Motion.
No. of stake. Inches.
1 moved 11
2 " 10
3 " 12
4 " 13
5 " 12
6 moved 12-3/4
7 " 10-1/2
8 " 10
9 " 9
10 " 5
The width of the glacier at this place we found to be 1134 yards, and
its maximum velocity, as shown by the foregoing table, 13 inches a day.
On the 1st of August a line was set out across the Glacier de Lechaud,
above its junction with the Talefre: it commenced beneath the block of
stone known as the Pierre de Beranger. The displacements of the stakes,
measured on the 3rd of August, gave the following results:--
Seventh Line.--Daily Motion.
No. of stake. Inches.
1 moved 4-1/2
2 " 8-1/4
3 " 9-1/2
4 " 9
5 " 8-1/2
6 moved 7-1/2
7 " 6-1/4
8 " 8-1/2
9 " 7
10 " 5-1/2
The width of the Glacier de Lechaud at this place was found to be 825
yards; its maximum motion, as shown by the table, being 9-1/2 inches a
day. This is the slowest rate which we observed upon either the Mer de
Glace or its tributaries. The width of the Talefre-branch, as it
descends the cascade, or, in other words, before it is influenced by the
pressure of the Lechaud, was found approximately to be 638 yards.
[Sidenote: SQUEEZING AT TRELAPORTE.]
The widths of the tributaries were determined for the purpose of
ascertaining the amount of lateral compression endured by the ice in its
passage through the neck of the valley at Trelaporte. Adding all
together we have--
Geant 1134 yards.
Lechaud 825 "
Talefre 638 "
Total 2597 yards.
These three branches, as shown by the actual measurement of our 5th
line, are forced at Trelaporte through a channel 893 yards wide; the
width of the trunk stream is a little better than one-third of that of
its tributaries, and it passes through this gorge at a velocity of
nearly 20 inches a day.
[Sidenote: THE LECHAUD A DRIBLET.]
Limiting our view to one of the tributaries only, the result is still
more impressive. Previous to its junction with the Talefre, the Glacier
de Lechaud stretches before the observer as a broad river of ice,
measuring 825 yards across: at Trelaporte it is squeezed, in a frozen
vice, between the Talefre on one side and the Geant on the other, to a
driblet, measuring 85 yards in width, or about one-tenth of its former
transverse dimension. It will of course be understood that it is the
_form_ and not the _volume_ of the glacier that is affected to this
enormous extent by the pressure.
Supposing no waste took place, the Glacier de Lechaud would force
precisely the same amount of ice through the "narrows" at Trelaporte, in
one day, as it sends past the Pierre de Beranger. At the latter place
its velocity is about half of what it is at the former, but its width is
more than nine times as great. Hence, if no waste took place, its
_depth_, at Trelaporte, would be at _least_ 4-1/2 times its depth
opposite the Pierre de Beranger. Superficial and subglacial melting
greatly modify this result. Still I think it extremely probable that
observations directed to this end would prove the comparative
shallowness of the upper portions of the Glacier de Lechaud.
FOOTNOTES:
[A] Great care is necessary on the part of the man who measures the
displacements. The staff ought to be placed along the original line, and
the assistant ought to walk along it until the foot of a _perpendicular_
from the stake is attained. When several days' motion is to be measured,
this precaution is absolutely necessary; the eye being liable to be
grossly deceived in _guessing_ the direction of a perpendicular.
[B] The details of the measurement of the fourth and fifth lines are
published in the 'Philosophical Transactions,' vol. cxlix., p. 261.
ICE-WALL AT THE TACUL.
VELOCITIES OF TOP AND BOTTOM.
(11.)
As regards the motion of the _surface_ of a glacier, two laws are to be
borne in mind: 1st, that regarding the quicker movement of the centre;
2nd, that regarding the locus of the point of maximum motion. Our next
care must be to compare the motion of the surface of a glacier with the
motion of those parts which lie near its bed. Rendu first surmised that
the bottom of the glacier was retarded by friction, and both Professor
Forbes[A] and M. Martins[B] have confirmed the conjecture. Theirs are
the only observations which we possess upon the subject; and I was
particularly desirous to instruct myself upon this important head by
measurements of my own.
[Sidenote: FIRST ATTEMPT AT MEASUREMENT.]
During the summer of 1857 the eastern side of the Glacier du Geant, near
the Tacul, exposed a nearly vertical precipice of ice, measuring 140
feet from top to bottom. I requested Mr. Hirst to fix two stakes in the
same vertical plane, one at the top of the precipice and one near the
bottom. This he did upon the 3rd of August, and on the 5th I accompanied
him to measure the progress of the stakes. On the summit of the
precipice, and running along it, was the lateral moraine of the glacier.
The day was warm and the ice liquefying rapidly, so that the boulders
and debris, deprived incessantly of their support, came in frequent
leaps and rushes down the precipice. Into this peril my guide was about
to enter, to measure the displacement of the lower stake, while I was
to watch, and call out the direction in which he was to run when a stone
gave way. But I soon found that the initial motion was no sure index of
the final motion. By striking the precipice, the stones were often
deflected, and carried wide of their original direction. I therefore
stopped the man, and sent him to the summit of the precipice to remove
all the more dangerous blocks. This accomplished, he descended, and
while I stood beside him, executed the required measurement. From the
3rd to the 5th of August the upper stake had moved twelve inches, and
the lower one six.
Unfortunately some uncertainty attached itself to this result, due to
the difficulty of fixing the lower stake. The guide's attention had been
divided between his work and his safety, and he had to retreat more than
a dozen times from the falling boulders and debris. I, on the other
hand, was unwilling to accept an observation of such importance with a
shade of doubt attached to it. Hence arose the desire to measure the
motion myself. On the 11th of August I therefore reascended to the
Tacul, and fixed a stake at the top of the precipice, and another at the
bottom. While sitting on the old moraine looking at the two pickets, the
importance of determining the motion of a point midway between the top
and bottom forcibly occurred to me, but, on mentioning it to my guide,
he promptly pronounced any attempt of the kind absurd.
[Sidenote: STAKES FIXED AT TOP, BOTTOM, AND CENTRE.]
On scanning the place carefully, however, the value of the observation
appeared to me to outweigh the amount of danger. I therefore took my
axe, placed a stake and an auger against my breast, buttoned my coat
upon them, and cut an oblique staircase up the wall of ice, until I
reached a height of forty feet from the bottom. Here the position of the
stake being determined by Mr. Hirst, who was at the theodolite, I
pierced the ice with the auger, drove in the stake, and descended
without injury. During the whole operation however my guide growled
audibly.
On the following morning we commenced the ascent of Mont Blanc, a
narrative of which is given in Part I. We calculated on an absence of
three days, and estimated that the stakes which had just been fixed
would be ready for measurement on our return; but we did not reach
Chamouni until the afternoon of Friday, the 14th. Heavy clouds settled,
during our descent, upon the summits behind us, and a thunder-peal from
the Aiguilles soon heralded a fall of rain, which continued without
intermission till the afternoon of the 16th, when the atmosphere
cleared, and showed the mountains clothed to their girdles with snow.
The Montanvert was thickly covered, and on our way to it we met the
servants in charge of the cattle, which had been driven below the
snow-line to obtain food.
[Sidenote: THROUGH GLOOM TO THE TACUL.]
On Monday morning, the 17th, a dense fog filled the valley of the Mer de
Glace. I watched it anxiously. The stakes which we had set at the Tacul
had been often in my thoughts, and I wished to make some effort to save
the labour and peril incurred in setting them from being lost. I
therefore set out, in one of the clear intervals, accompanied by my
friend and Simond, determined to measure the motion of the stakes, if
possible, or to fix them more firmly, if they still stood. As we passed,
however, from l'Angle to the glacier, the fog became so dense and
blinding that we halted. At my request Mr. Hirst returned to the
Montanvert; and Simond, leaving the theodolite in the shelter of a rock,
accompanied me through the obscurity to the Tacul. We found the topmost
stake still stuck by its point in the ice; but the two others had
disappeared, and we afterwards discovered their fragments in a
snow-buttress, which reared itself against the base of the precipice.
They had been hit by the falling stones, and crushed to pieces. Having
thus learned the worst, we descended to the Montanvert amid drenching
rain.
[Sidenote: DESCENT OF BOULDERS.]
On the morning of the 18th there was no cloud to be seen anywhere, and
the sunlight glistened brightly on the surface of the ice. We ascended
to the Tacul. The spontaneous falling of the stones appeared more
frequent this morning than I had ever seen it. The sun shone with
unmitigated power upon the ice, producing copious liquefaction. The
rustle of falling debris was incessant, and at frequent intervals the
boulders leaped down the precipice, and rattled with startling energy
amid the rocks at its base. I sent Simond to the top to remove the
looser stones; he soon appeared, and urged the moraine-shingle in
showers down the precipice, upon a bevelled slope of which some blocks
long continued to rest. They were out of the reach of the guide's baton,
and he sought to dislodge them by sending other stones down upon them.
Some of them soon gave way, drawing a train of smaller shingle after
them; others required to be hit many times before they yielded, and
others refused to be dislodged at all. I then cut my way up the
precipice in the manner already described, fixed the stake, and
descended as speedily as possible. We afterwards fixed the bottom stake,
and on the 20th the displacements of all three were measured.[C] The
spaces passed over by the respective stakes in 24 hours were found to be
as follows:--
Inches.
Top stake 6.00
Middle stake 4.50
Bottom stake 2.56
[Sidenote: MOTION OF STAKES.]
The height of the precipice was 140.8 feet, but it sloped off at its
upper portion. The height of the middle stake above the ground was 35
feet, and of the bottom one 4 feet. It is therefore proved by these
measurements that the bottom of the ice-wall at the Tacul moves with
less than half the velocity of the top; while the displacement of the
intermediate stake shows how the velocity gradually increases from the
bottom upwards.
FOOTNOTES:
[A] 'Edinb. Phil. Journ.,' Oct. 1846, p. 417.
[B] Agassiz, 'Systeme Glaciaire,' p. 522.
[C] On this latter occasion my guide volunteered to cut the steps for me
up to the pickets; and I permitted him to do so. In fact, he was at
least as anxious as myself to see the measurement carried out.
WINTER MOTION OF THE MER DE GLACE.
(12.)
The winter measurements were executed in the manner already described,
on the 28th and 29th of December, 1859. The theodolite was placed on the
mountain's side flanking the glacier, and a well-defined object was
chosen at the opposite side of the valley, so that a straight line
between this object and the theodolite was approximately perpendicular
to the axis of the glacier. Fixing the telescope in the first instance
with its cross hairs upon the object, its end was lowered until it
struck the point upon the glacier at which a stake was to be fixed.
Thanks to the intelligence of my assistants, after the fixing of the
first stake they speedily took up the line at all other points,
requiring very little correction to make their positions perfectly
accurate. On the day following that on which the stakes were driven in,
the theodolite was placed in the same position, and the distances to
which the stakes had moved from their original positions were accurately
determined. As already stated, the first line crossed the glacier about
80 yards above the Montanvert Hotel.
[Sidenote: HALF OF SUMMER MOTION.]
Line No. I.--Winter Motion in Twenty-four Hours.
No. of stake. Inches.
West 1 7-1/4
2 11
3 13-1/2
4 13
5 13-3/4
6 14-1/4
7 15-3/4
8 15-3/4
9 12-1/4
10 12
11 6-1/2 East.
[Sidenote: THE SAME LAW IN SUMMER AND WINTER.]
The maximum here is fifteen and three-quarters inches; the maximum
summer motion of the same portion of the glacier is about thirty
inches. These measurements also show that in winter, as well as in
summer, the side of the glacier opposite to the Montanvert moves quicker
than that adjacent to it. The stake which moved with the maximum
velocity was beyond the moraine of La Noire. The second line crossed the
glacier about 130 yards below the Montanvert.
Line No. II.--Winter Motion in Twenty-four Hours.
No. of stake. Inches.
1 7-3/4
2 9-1/2
3 13-3/4
4 16
5 16
6 15-3/4
7 17-1/2
8 16-1/2
9 14-1/2
10 14
The maximum here is an inch and three-quarters greater than that of line
No. 1. The summer maximum at this portion of the glacier also exceeds
that of the part intersected by line No. 1. The surface of the glacier
between the two lines is in a state of tension which relieves itself by
a system of transverse fissures, and thus permits of the quicker advance
of the forward portion.
My desire, in making these measurements, was, in the first place, to
raise the winter observations of the motion to the same degree of
accuracy as that already possessed by the summer ones. Auguste Balmat
had already made a series of winter observations on the Mer de Glace;
but they were made in the way employed before the introduction of the
theodolite by Agassiz and Forbes, and shared the unavoidable roughness
of such a mode of measurement. They moreover gave us no information as
to the motion of the different parts of the glacier along the same
transverse line, and this, for reasons which will appear subsequently,
was the point of chief interest to me.
CAUSE OF GLACIER-MOTION.
DE SAUSSURE'S THEORY.
(13.)
Perhaps the first attempt at forming a glacier-theory is that of
Scheuchzer in 1705. He supposed the motion to be caused by the
conversion of water into ice within the glacier; the known and almost
irresistible expansion which takes place on freezing, furnishing the
force which pushed the glacier downward. This idea was illustrated and
developed with so much skill by M. de Charpentier, that his name has
been associated with it; and it is commonly known as the Theory of
Charpentier, or the Dilatation-Theory. M. Agassiz supported this theory
for a time, but his own thermometric experiments show us that the body
of the glacier is at a temperature of 32 deg. Fahr.; that consequently there
is no interior magazine of cold to freeze the water with which the
glacier is supposed to be incessantly saturated. So that these
experiments alone, if no other grounds existed, would prove the
insufficiency of the theory of dilatation. I may however add, that the
arguments most frequently urged against this theory deal with an
assumption, which I do not think its author ever intended to make.
[Sidenote: THE GLACIER SLIDES.]
Another early surmise was that of Altmann and Gruener (1760), both of
whom conjectured that the glacier slid along its bed. This theory
received distinct expression from De Saussure in 1799; and has since
been associated with the name of that great alpine traveller, being
usually called the 'Theory of Saussure,' and sometimes the 'Sliding
Theory.' It is briefly stated in these words:--
"Almost every glacier reposes upon an inclined bed, and those of any
considerable size have beneath them, even in winter, currents of water
which flow between the ice and the bed which supports it. It may
therefore be understood that these frozen masses, drawn down the slope
on which they repose, disengaged by the water from all adhesion to the
bottom, sometimes even raised by this water, must glide by little and
little, and descend, following the inclinations of the valleys, or of
the slopes which they cover. It is this slow but continual sliding of
the ice on its inclined base which carries it into the lower
valleys."[A]
[Sidenote: STRAINED INTERPRETATION.]
De Saussure devoted but little time to the subject of glacier-motion;
and the absence of completeness in the statement of his views, arising
no doubt from this cause, has given subsequent writers occasion to affix
what I cannot help thinking a strained interpretation to the sliding
theory. It is alleged that he regarded a glacier as a perfectly rigid
body; that he considered it to be "a mass of ice of small depth, and
considerable but uniform breadth, sliding down a uniform valley, or
pouring from a narrow valley into a wider one."[B] The introduction "of
the smallest flexibility or plasticity" is moreover emphatically denied
to him.[C]
It is by no means probable that the great author of the 'Voyages' would
have subscribed to this "rigid" annotation. His theory, be it
remembered, is to some extent _true_: the glacier moves over its bed in
the manner supposed, and the rocks of Britain bear to this day the
traces of these mighty sliders. De Saussure probably contented himself
with a general statement of what he believed to be the substantial
cause of the motion. He visited the Jardin, and saw the tributaries of
the Mer de Glace turning round corners, welding themselves together, and
afterwards moving through a sinuous trunk-valley; and it is scarcely
credible that in the presence of such facts he would have denied all
flexibility to the glacier.
The statement that he regarded a glacier to be a mass of ice of uniform
width, is moreover plainly inconsistent with the following description
of the glacier of Mont Dolent: "Its most elevated plateau is a great
circus, surrounded by high cliffs of granite, of pyramidal forms; thence
the glacier descends through a gorge, in which _it is narrowed_; but
after having passed the gorge, it _enlarges again_, spreading out like a
fan. Thus it has on the whole the form of a sheaf tied in the middle and
dilated at its two extremities."[D]
[Sidenote: GLACIER OF MONT DOLENT.]
Curiously enough this very glacier, and these very words, are selected
by M. Rendu as illustrative of the plasticity of glaciers. "Nothing," he
says, "shows better the extent to which a glacier moulds itself to its
locality than the form of the glacier of Mont Dolent in the Valley of
Ferret;" and he adds, in connexion with the same passage, these
remarkable words:--"There is a multitude of facts which would seem to
necessitate the belief that the substance of glaciers enjoys a kind of
ductility which permits it to mould itself to the locality which it
occupies, to grow thin, to swell, and to narrow itself like a soft
paste."[E]
FOOTNOTES:
[A] 'Voyages,' Sec. 535.
[B] James D. Forbes, 'Occasional Papers on the Theory of Glaciers,'
1859, p. 100.
[C] "I adhere to the definition as excluding the introduction of the
smallest flexibility or plasticity." 'Occ. Pap.,' p. 96.
[D] 'Voyages,' tome ii. p. 290.
[E] In connexion with this brief sketch of the 'Sliding Theory,' it
ought to be stated, that Mr. Hopkins has proved experimentally, that ice
may descend an incline at a sensibly uniform rate, and that the velocity
is augmented by increasing the weight. In this remarkable experiment the
motion was due to the slow disintegration of the lower surface of the
ice. See 'Phil. Mag.,' 1845, vol. 26.
RENDU'S THEORY.
(14.)
[Sidenote: RENDU'S CHARACTER.]
M. Rendu, Bishop of Annecy, to whose writings I have just referred, died
last autumn.[A] He was a man of great repute in his diocese, and we owe
to him one of the most remarkable essays upon glaciers that have ever
appeared. His knowledge was extensive, his reasoning close and accurate,
and his faculty of observation extraordinary. With these were associated
that intuitive power, that presentiment concerning things as yet
untouched by experiment, which belong only to the higher class of minds.
Throughout his essay a constant effort after quantitative accuracy
reveals itself. He collects observations, makes experiments, and tries
to obtain numerical results; always taking care, however, so to state
his premises and qualify his conclusions that nobody shall be led to
ascribe to his numbers a greater accuracy than they merit. It is
impossible to read his work, and not feel that he was a man of
essentially truthful mind, and that science missed an ornament when he
was appropriated by the Church.
The essay above referred to is printed in the tenth volume of the
Memoirs of the Royal Academy of Sciences of Savoy, published in 1841,
and is entitled, '_Theorie des Glaciers de la Savoie, par M. le Chanoine
Rendu, Chevalier du Merite Civil et Secretaire perpetuel_.' The paper
had been written for nearly two years, and might have remained
unprinted, had not another publication on the same subject called it
forth.
I will place a few of the leading points of this remarkable production
before the reader; commencing with a generalization which is highly
suggestive of the character of the author's mind.
[Sidenote: "THEORIE DES GLACIERS DE LA SAVOIE."]
He reflects on the accumulation of the mountain-snows, each year adding
fifty-eight inches of ice to a glacier. This would make Mont Blanc four
hundred feet higher in a century, and four thousand feet higher in a
thousand years. "It is evident," he says, "that nothing like this occurs
in nature." The escape of the ice then leads him to make some general
remarks on what he calls the "law of circulation." "The conserving will
of the Creator has employed for the permanence of His work the great law
of _circulation_, which, strictly examined, is found to reproduce itself
in all parts of nature. The waters circulate from the ocean to the air,
from the air to the earth, and from the earth to the ocean.... The
elements of organic substances circulate, passing from the solid to the
liquid or aeriform condition, and thence again to the state of solidity
or of organisation. That universal agent which we designate by the names
of fire, light, electricity, and magnetism, has probably also a
_circulation_ as wide as the universe." The italics here are Rendu's
own. This was published in 1841, but written, we are informed, nearly
two years before. In 1842 Mr. Grove wrote thus:--"Light, heat,
magnetism, motion, and chemical affinity, are all convertible material
affections." More recently Helmholtz, speaking of the "circuit" formed
by "heat, light, electricity, magnetism, and chemical affinity," writes
thus:--"Starting from each of these different manifestations of natural
forces, we can set every other in action." I quote these passages
because they refer to the same agents as those named by M. Rendu, and to
which he ascribes "_circulation_." Can it be doubted that this Savoyard
priest had a premonition of the Conservation of Force? I do not want to
lay more stress than it deserves upon a conjecture of this kind; but
its harmony with an essay remarkable for its originality gives it a
significance which, if isolated, it might not possess.
[Sidenote: GLACIERS RIGHTLY DIVIDED.]
With regard to the glaciers, Rendu commences by dividing them into two
kinds, or rather the selfsame glacier into two parts, one of which he
calls the "_glacier reservoir_," the other the "_glacier
d'ecoulement_,"--two terms highly suggestive of the physical
relationship of the _neve_ and the glacier proper. He feeds the
reservoirs from three sources, the principal one of which is the snow,
to which he adds the rain, and the vapours which are condensed upon the
heights without passing into the state of either rain or snow. The
conversion of the snow into ice he supposes to be effected by four
different causes, the most efficacious of which is _pressure_.[B] It is
needless to remark that this quite agrees with the views now generally
entertained.
In page 60 of the volume referred to there is a passage which shows that
the "veined structure" of the glacier had not escaped him, though it
would seem that he ascribed it to stratification. "When," he writes, "we
perceive the profile of a glacier on the walls of a crevasse, we see
different layers distinct in colour, but more particularly in density;
some seem to have the hardness, as they have the greenish colour, of
glass; others preserve the whiteness and porosity of the snow." There is
also a very close resemblance between his views of the influence of
"time and cohesion" and those of Prof. Forbes. "We may conclude," he
writes, "that _time_, favouring the action of _affinity_, and the
pressure of the layers one upon the other, causes the little crystals of
which snow is composed to approach each other, bring them into contact,
and convert them into ice."[C] Regelation also appears to have attracted
his notice.[D] "When we fill an ice-house," he writes, "we break the ice
into very small fragments; afterwards we wet it with water 8 or 10
degrees above zero (Cent.) in temperature; but, notwithstanding this,
the whole is converted into a compact mass of ice." He moreover
maintains, in almost the same language as Prof. Forbes,[E] the opinion,
that ice has always an inner temperature lower than zero (Cent.). He
believed this to be a property "inherent to ice." "Never," he says, "can
a calorific ray pass the first surface of ice to raise the temperature
of the interior."[F]
[Sidenote: OBSERVATIONS AND HYPOTHESES.]
He notices the direction of the glacier as influencing the wasting of
its ridges by the sun's heat; ascribing to it the effect to which I have
referred in explaining the wave-like forms upon the surface of the Mer
de Glace. His explanation of the Moulins, too, though insufficient,
assigns a true cause, and is an excellent specimen of physical
reasoning.
With regard to the diminution of the _glaciers reservoirs_, or, in other
words, to the manner in which the ice disappears, notwithstanding the
continual additions made to it, we have the following remarkable
passage:--"In seeking the cause of the diminution of glaciers, it has
occurred to my mind that the ice, notwithstanding its hardness and its
rigidity, can only support a given pressure without breaking or being
squeezed out. According to this supposition, whenever the pressure
exceeds that force, there will be rupture of the ice, and a flow in
consequence. Let us take, at the summit of Mont Blanc, a column of ice
reposing on a horizontal base. The ice which forms the first layer of
that column is compressed by the weight of all the layers above it; but
if the solidity of the said first layer can only support a weight equal
to 100, when the weight exceeds this amount there will be rupture and
spreading out of the ice of the base. Now, something very similar occurs
in the immense crust of ice which covers the summits of Mont Blanc. This
crust appears to augment at the upper surface and to diminish by the
sides. To assure oneself that the movement is due to the force of
pressure, it would be necessary to make a series of experiments upon the
solidity of ice, such as have not yet been attempted."[G] I may remark
that such experiments substantially verify M. Rendu's notion.
But it is his observations and reasoning upon the _glaciers
d'ecoulement_ that chiefly interest us. The passages in his writings
where he insists upon the power of the glaciers to mould themselves to
their localities, and compares them to a soft paste, to lava at once
ductile and liquid, are well known from the frequent and flattering
references of Professor Forbes; but there are others of much greater
importance, which have hitherto remained unknown in this country.
Regarding the motion of the Mer de Glace, Rendu writes as follows:--
[Sidenote: MEASUREMENT OF MOTION.]
[Sidenote: THE SIDES OF THE GLACIER RETARDED.]
"I sought to appreciate the quantity of its motion; but I could only
collect rather vague data. I questioned my guides regarding the position
of an enormous rock at the edge of the glacier, but still upon the ice,
and consequently partaking of its motion. The guides showed me the place
where it stood the preceding year, and where it had stood two, three,
four, and five years previously; they showed me the place where it would
be found in a year, in two years, &c.; _so certain are they of the
regularity of the motion_. Their reports, however, did not always agree
precisely with each other, and their indications of time and distance
lack the precision without which we proceed obscurely in the physical
sciences. In reducing these different indications to a mean, I found the
total advance of the glacier to be about 40 feet a year. During my last
journey I obtained more certain data, which I have stated in the
preceding chapter. _The enormous difference between the two results
arises from the fact that the latter observations were made at the
centre of the glacier_, WHICH MOVES MORE RAPIDLY, _while the former were
made at the side, where the ice_ IS RETAINED BY THE FRICTION AGAINST ITS
ROCKY WALLS."[H]
An opinion, founded on a grave misapprehension which Rendu enables us to
correct, is now prevalent in this country, not only among the general
public, but also among those of the first rank in science. The nature of
the mistake will be immediately apparent. At page 128 of the 'Travels in
the Alps' its distinguished author gives a sketch of the state of our
knowledge of glacier-motion previous to the commencement of his
inquiries. He cites Ebel, Hugi, Agassiz, Bakewell, De la Beche,
Shirwell, Rendu, and places them in open contradiction to each other.
Rendu, he says, gives the motion of the Mer de Glace to be "242 feet per
annum; 442 feet per annum; a foot a day; 400 feet per annum, and 40 feet
per annum, or _one-tenth_ of the last!" ... and he adds, "I was not
therefore wrong in supposing that the actual progress of a glacier was
yet a new problem when I commenced my observations on the Mer de Glace
in 1842."[I]
In the 'North British Review' for August, 1859, a writer equally
celebrated for the brilliancy of his discoveries and the vigour of his
pen, collected the data furnished by the above paragraph into a table,
which he introduced to his readers in the following words:--"It is to
Professor Forbes alone that we owe the first and most correct researches
respecting the motion of glaciers; and in proof of this, we have only to
give the following list of observations which had been previously made.
Observers. Name of glacier. Annual rate of motion.
Ebel Chamouni 14 feet
Ebel Grindelwald 25 "
Hugi Aar 240 "
Agassiz Aar 200 "
Bakewell Mer de Glace 540 "
De la Beche Mer de Glace 600 "
Shirwell Mer de Glace 300 "
M. Rendu Mer de Glace 365 "
Saussure's Ladder Mer de Glace 375 "
... Such was the state of our knowledge when Professor Forbes undertook
the investigation of the subject."
I am persuaded that the writer of this article will be the first to
applaud any attempt to remove an error which, advanced on his great
authority, must necessarily be widely disseminated. The numbers in the
above table certainly differ widely, and it is perhaps natural to
conclude that such discordant results can be of no value; but the fact
really is that _every one of them may be perfectly correct_. This fact,
though overlooked by Professor Forbes, was clearly seen by Rendu, who
pointed out with perfect distinctness the sources from which the
discrepancies were derived.
[Sidenote: DISCREPANCIES EXPLAINED.]
"It is easy," he says, "to comprehend that it is impossible to obtain a
general measure,--that there ought to be one for each particular
glacier. The nature of the slope, the number of changes to which it is
subjected, the depth of the ice, the width of the couloir, the form of
its sides, and a thousand other circumstances, must produce variations
in the velocity of the glacier, and these circumstances cannot be
everywhere absolutely the same. Much more, it is not easy to obtain this
velocity for a single glacier, and for this reason. In those portions
where the inclination is steep, the layer of ice is thin, and its
velocity is great; in those where the slope is almost nothing, the
glacier swells and accumulates; the mass in motion being double, triple,
&c., the motion is only the half, the third, &c.
[Sidenote: LIQUID MOTION ASCRIBED TO GLACIER.]
"But this is not all," adds M. Rendu: "_Between the Mer de Glace and a
river, there is a resemblance so complete that it is impossible to find
in the latter a circumstance which does not exist in the former._ In
currents of water the motion is not uniform, neither throughout their
width nor throughout their depth; _the friction of the bottom, that of
the sides_, the action of obstacles, cause the motion to vary, _and only
towards the middle of the surface is this entire...._"[J]
In 1845 Professor Forbes appears to have come to the same conclusion as
M. Rendu; for after it had been proved that the centre of the Aar
glacier moved quicker than the side in the ratio of fourteen to one, he
accepted the result in these words:--"The movement of the centre of the
glacier is to that of a point five metres from the edge as FOURTEEN to
ONE: such is the effect of plasticity!"[K] Indeed, if the differences
exhibited in the table were a proof of error, the observations of
Professor Forbes himself would fare very ill. The measurements of
glacier-motion made with his own hands vary from less than 42 feet a
year to 848 feet a year, the minimum being less than _one-twentieth_ of
the maximum; and if we include the observations made by Balmat, the
fidelity of which has been certified by Professor Forbes, the minimum is
only _one-thirty-seventh_ of the maximum.
[Sidenote: NORTH BRITISH REVIEW.]
There is another point connected with Rendu's theory which needs
clearing up:--"The idea," writes the eminent reviewer, "that a glacier
is a semifluid body is no doubt startling, especially to those who have
seen the apparently rigid ice of which it is composed. M. Rendu himself
shrank from the idea, and did not scruple to say that 'the rigidity of a
mass of ice was in direct opposition to it;' and we think that Professor
Forbes himself must have stood aghast when his fancy first associated
the notion of imperfect fluidity with the solid or even the fissured ice
of the glacier, and when he saw in his mind's eye the glaciers of the
Alps flowing like a river along their rugged bed. A truth like this was
above the comprehension and beyond the sympathy of the age; and it
required a moral power of no common intensity to submit it to the ordeal
of a shallow philosophy, and the sneers of a presumptuous criticism."
These are strong words; but the fact is that, so far from "shrinking"
from the idea, Rendu affirmed, with a clearness and an emphasis which
have not been exceeded since, that all the phenomena of a river were
reproduced upon the Mer de Glace; its deeps, its shallows, its
widenings, its narrowings, its rapids, its places of slow motion, and
the quicker flow of its centre than of its sides. He did not shrink from
accepting a difference between the central and lateral motion amounting
to a ratio of ten to one--a ratio so large that Professor Forbes at one
time regarded the acceptance of it as a simple absurdity. In this he was
perhaps justified; for his own first observations, which, however
valuable, were hasty and incomplete, gave him a maximum ratio of about
one and a half to one, while the ratio in some cases was nearly one of
_equality_. The observations of Agassiz however show that the ratio,
instead of being ten to one, may be _infinity_ to one; for the lateral
ice may be so held back by a local obstacle that in the course of a year
it shall make no sensible advance at all.
[Sidenote: THE ICE AND THE GLACIER.]
From one thing only did M. Rendu shrink; and it is _the_ thing regarding
which we are still disunited. He shrank from stating the physical
quality of the ice in virtue of which a glacier moved like a river. He
demands experiments upon snow and ice to elucidate this subject. The
very observations which Professor Forbes regards as proofs are those of
which we require the physical explanation. It is not the viscous flow,
if you please to call it such, of the glacier as a whole that here
concerns us; but it is the quality of the _ice_ in virtue of which this
kind of motion is accomplished. Professor Forbes sees this difference
clearly enough: he speaks of "fissured ice" being "flexible" in hand
specimens; he compares the glacier to a mixture of ice and sand; and
finally, in a more matured paper, falls back for an explanation upon the
observations of Agassiz regarding the capillaries of the glacier.[L]
FOOTNOTES:
[A] Expressions such as "last summer," "last autumn," "recently," will
be taken throughout in the sense which they had in the early half of
1860, when this book was first published.--L. C. T.
[B] 'Memoir,' p. 77.
[C] P. 75.
[D] P. 71.
[E] 'Philosophical Magazine,' 1859.
[F] 'Memoir,' p. 69.
[G] Page 80.
[H] Page 95.
[I] At page 38 of the 'Travels' the following passage also occurs:--"I
believe that I may safely affirm that not one observation of the rate of
motion of a glacier, either on the average or at any particular season
of the year, existed when I commenced my experiments in 1842."
[J] 'Theorie,' p. 96.
[K] 'Occ. Pap.,' p. 74.
[L] In all that has been written upon glaciers in this country the above
passages from the writings of Rendu are unquoted; and many who mingled
very warmly in the discussions of the subject were, until quite
recently, ignorant of their existence. I was long in this condition
myself, for I never supposed that passages which bear so directly upon a
point so much discussed, and of such cardinal import, could have been
overlooked; or that the task of calling attention to them should devolve
upon myself nearly twenty years after their publication. Now that they
are discovered, I conceive no difference of opinion can exist as to the
propriety of placing them in their true position.
(15.)
The measurements of Agassiz and Forbes completely verify the
anticipations of Rendu; but no writer with whom I am acquainted has
added anything essential to the Bishop's statements as to the identity
of glacier and liquid motion. He laid down the conditions of the problem
with perfect clearness, and, as regards the distribution of merit, the
point to be decided is the relative importance of his idea, and of the
measurements which were subsequently made.
[Sidenote: OBSERVATIONS OF FORBES.]
The observations on which Professor Forbes based the analogy between a
glacier and a river are the following:--In 1842 he fixed four marks upon
the Mer de Glace a little below the Montanvert, the first of which was
100 yards distant from the side of the glacier, while the last was at
the centre "or a little beyond it." The relative velocity of these four
points was found to be
1.000 1.332 1.356 1.367.
The first observations were made upon two of these points, two others
being subsequently added. Professor Forbes also determined the velocity
of two points on the Glacier du Geant, and found the ratio of motion, in
the first instance, to be as 14 to 32. Subsequent measurements, however,
showed the ratio to be as 14 to 18, the larger motion belonging to the
station nearest to the centre of the glacier. These are the only
measurements which I can find in his large work that establish the
swifter motion of the centre of the glacier; and in these cases the
velocity of the centre is compared with that of _one side_ only. In no
instance that I am aware of, either in 1842 or subsequent years, did
Professor Forbes extend his measurements quite across a glacier; and as
regards completeness in this respect, no observations hitherto made can
at all compare with those executed at the instance of Agassiz upon the
glacier of the Aar.
In 1844 Professor Forbes made a series of interesting experiments on a
portion of the Mer de Glace near l'Angle. He divided a length of 90 feet
into 45 equal spaces, and fixed pins at the end of each. His theodolite
was placed upon the ice, and in seventeen days he found that the ice 90
feet nearer the centre than the theodolite had moved 26 inches past the
latter. These measurements were undertaken for a special object, and
completely answered the end for which they were intended.
In 1846 Professor Forbes made another important observation. Fixing
three stakes at the heights of 8, 54, and 143 feet above the bed of the
glacier, he found that in five days they moved respectively 2.87, 4.18,
and 4.66 feet. The stake nearest the bed moved most slowly, thus
showing that the ice is retarded by friction. This result was
subsequently verified by the measurements of M. Martins, and by my own.
If we add to the above an observation made during a short visit to the
Aletsch glacier in 1844, which showed its lateral retardation, I believe
we have before us the whole of the measurements executed by Professor
Forbes, which show the analogy between the motion of a glacier and that
of a viscous body.
[Sidenote: MEASUREMENTS OF AGASSIZ.]
Illustrative of the same point, we have the elaborate and extensive
series of measurements executed by M. Wild under the direction of M.
Agassiz upon the glacier of the Aar in 1842, 1843, 1844, and 1845, which
exhibit on a grand scale, and in the most conclusive manner, the
character of the motion of this glacier; and also show, on close
examination, an analogy with fluid motion which neither M. Agassiz nor
Professor Forbes suspected. The former philosopher publishes a section
in his 'Systeme Glaciaire,' entitled 'Migrations of the Centre;' in
which he shows that the middle of the glacier is not always the point of
swiftest motion. The detection of this fact demonstrates the attention
devoted by M. Agassiz to the discussion of his observations, but he
gives no clue to the cause of the variation. On inspecting the shape of
the valley through which the Aar glacier moves, I find that these
"migrations" follow the law established in 1857 upon the Mer de Glace,
and enunciated at page 286.
To sum up this part of the question:--The _idea_ of semi-fluid motion
belongs entirely to Rendu; the _proof_ of the quicker central flow
belongs in part to Rendu, but almost wholly to Agassiz and Forbes; the
proof of the retardation of the bed belongs to Forbes alone; while the
discovery of the locus of the point of maximum motion belongs, I
suppose, to me.
FORBES'S THEORY.
(16.)
The formal statement of this theory is given in the following words:--"A
glacier is an imperfect fluid, or viscous body, which is urged down
slopes of a certain inclination by the mutual pressure of its parts."
The consistency of the glacier is illustrated by reference to treacle,
honey, and tar, and the theory thus enunciated and exemplified is called
the 'Viscous Theory.'
It has been the subject of much discussion, and great differences of
opinion are still entertained regarding it. Able and sincere men take
opposite sides; and the extraordinary number of Reviews which have
appeared upon the subject during the last two years show the interest
which the intellectual public of England take in the question. The chief
differences of opinion turn upon the inquiry as to what Professor Forbes
really meant when he propounded the viscous theory; some affirm one
thing, some another, and, singularly enough, these differences continue,
though the author of the theory has at various times published
expositions of his views.
[Sidenote: "FACTS AND PRINCIPLES."]
The differences referred to arise from the circumstances that a
sufficient distinction has not been observed between _facts_ and
_principles_, and that the viscous theory has assumed various forms
since its first promulgation. It has been stated to me that the theory
of Professor Forbes is "the congeries of facts" which he has discovered.
But it is quite evident that no recognition, however ample, of these
facts would be altogether satisfactory to Professor Forbes himself. He
claims recognition of his _theory_,[A] and no writer with whom I am
acquainted makes such frequent use of the term. What then can the
viscous theory mean apart from the facts? I interpret it as furnishing
the principle from which the facts follow as physical consequences--that
the glacier moves as a river because the ice is viscous. In this sense
only can Professor Forbes's views be called a theory; in any other, his
experiments are mere illustrations of the facts of glacier motion, which
do not carry us a hair's breadth towards their physical cause.
[Sidenote: VISCOUS THEORY;--WHAT IS IT?]
What then is the meaning of viscosity or viscidity? I have heard it
defined by men of high culture as "gluey tenacity;" and such tenacity
they once supposed a glacier to possess. If we dip a spoon into treacle,
honey, or tar, we can draw the substance out into filaments, and the
same may be done with melted caoutchouc or lava. All these substances
are viscous, and all of them have been chosen to illustrate the physical
property in virtue of which a glacier moves. Viscosity then consists in
the power of being drawn out when subjected to a force of tension, the
substance, after stretching, being in a state of molecular equilibrium,
or, in other words, devoid of that elasticity which would restore it to
its original form. This certainly was the idea attached to Professor
Forbes's words by some of his most strenuous supporters, and also by
eminent men who have never taken part in any controversy on the subject.
Mr. Darwin, for example, speaks of felspathic rocks being "stretched"
while flowing slowly onwards in a pasty condition, in precisely the same
manner as Professor Forbes believes that the ice of moving glaciers is
stretched and fissured; and Professor Forbes himself quotes these words
of Mr. Darwin as illustrative of his theory.[B]
The question now before us is,--Does a glacier exhibit that power of
yielding to a force of tension which would entitle its ice to be
regarded as a viscous substance?
[Sidenote: THEORY TESTED.]
With a view to the solution of this question Mr. Hirst took for me the
inclinations of the Mer de Glace and all its tributaries in 1857; the
effect of a change of inclination being always noted. I will select from
those measurements a few which bear more specially upon the subject now
under consideration, commencing with the Glacier des Bois, down which
the ice moves in that state of wild dislocation already described. The
inclination of the glacier above this cascade is 5 deg. 10', and that of the
cascade itself is 22 deg. 20', the change of inclination being therefore 17 deg.
10'.
[Illustration: Fig. 22. Inclinations of ice cascasde of the Glacier des
Bois.]
In Fig. 22 I have protracted the inclination of the cascade and of the
glacier above it; the line A B representing the former and B C the
latter. Now a stream of molten lava, of treacle, or tar, would, in
virtue of its viscosity, be able to flow over the brow at B without
breaking across; but this is not the case with the glacier; it is so
smashed and riven in crossing this brow, that, to use the words of
Professor Forbes himself, "it pours into the valley beneath in a cascade
of icy fragments."
[Sidenote: INCLINATIONS OF THE MER DE GLACE.]
But this reasoning will appear much stronger when we revert to other
slopes upon the Mer de Glace. For example, its inclination above l'Angle
is 4 deg., and it afterwards descends a slope of 9 deg. 25', the change of
inclination being 5 deg. 25'. If we protract these inclinations to scale, we
have the line A B, Fig. 23, representing the steeper slope, and B C
that of the glacier above it. One would surely think that a viscous body
could cross the brow B without transverse fracture, but this the glacier
cannot do, and Professor Forbes himself pronounces this portion of the
Mer de Glace impassable. Indeed it was the profound crevasses here
formed which placed me in a difficulty already referred to. Higher up
again, the glacier is broken on passing from a slope of 3 deg. 10' to one of
5 deg. Such observations show how differently constituted a glacier is from
a stream of lava in a "pasty condition," or of treacle, honey, tar, or
melted caoutchouc, to all which it has been compared. In the next
section I shall endeavour to explain the origin of the crevasses, and
shall afterwards make a few additional remarks on the alleged viscosity
of ice.
[Illustration: Fig. 23. Inclinations of Mer de Glace above l'Angle.]
FOOTNOTES:
[A] "Mr. Hopkins," writes Professor Forbes, "has done me the honour, in
the memoirs before alluded to, to mention with approbation my
observations and experiments on the subject of glaciers. He has been
more sparing either in praise or criticism of the theory which I have
founded upon them. Had Mr. Hopkins," &c.--_Eighth Letter_; 'Occ.
Papers,' p. 66.
[B] 'Occ. Papers,' p. 92.
THE CREVASSES.
(17.)
[Sidenote: CREVASSES CAUSED BY THE MOTION.]
Having made ourselves acquainted with the motion of the glacier, we are
prepared to examine those rents, fissures, chasms, or, as they are most
usually called, _Crevasses_, by which all glaciers are more or less
intersected. They result from the motion of the glacier, and the laws of
their formation are deduced immediately from those of the motion. The
crevasses are sometimes very deep and numerous, and apparently without
law or order in their distribution. They cut the ice into long ridges,
and break these ridges transversely into prisms; these prisms gradually
waste away, assuming, according to the accidents of their melting, the
most fantastic forms. I have seen them like the mutilated statuary of an
ancient temple, like the crescent moon, like huge birds with
outstretched wings, like the claws of lobsters, and like antlered deer.
Such fantastic sculpture is often to be found on the ice cascades, where
the riven glacier has piled vast blocks on vaster pedestals, and
presented them to the wasting action of sun and air. In Fig. 24 I have
given a sketch of a mass of ice of this character, which stood in 1859
on the dislocated slope of the Glacier des Bois.
[Sidenote: FANTASTIC ICE-MASSES.]
[Illustration: Fig. 24. Fantastic Mass of ice.]
It is usual for visitors to the Montanvert to descend to the glacier,
and to be led by their guides to the edges of the crevasses, where,
being firmly held, they look down into them; but those who have only
made their acquaintance in this way know but little of their magnitude
and beauty in the more disturbed portions of glaciers. As might be
expected, they have been the graves of many a mountaineer; and the
skeletons found upon the glacier prove that even the chamois itself,
with its elastic muscles and admirable sureness of foot, is not always
safe among the crevasses. They are grandest in the higher ice-regions,
where the snow hangs like a coping over their edges, and the water
trickling from these into the gloom forms splendid icicles. The Goerner
Glacier, as we ascend it towards the old Weissthor, presents many fine
examples of such crevasses; the ice being often torn in a most curious
and irregular manner. You enter a porch, pillared by icicles, and look
into a cavern in the very body of the glacier, encumbered with vast
frozen bosses which are fringed all round by dependent icicles. At the
peril of your life from slipping, or from the yielding of the
stalactites, you may enter these caverns, and find yourself steeped in
the blue illumination of the place. Their beauty is beyond description;
but you cannot deliver yourself up, heart and soul, to its enjoyment.
There is a strangeness about the place which repels you, and not without
anxiety do you look from your ledge into the darkness below, through
which the sound of subglacial water sometimes rises like the tolling of
distant bells. You feel that, however the cold splendours of the place
might suit a purely spiritual essence, they are not congenial to flesh
and blood, and you gladly escape from its magnificence to the sunshine
of the world above.
[Sidenote: BIRTH OF A CREVASSE.]
From their numbers it might be inferred that the formation of crevasses
is a thing of frequent occurrence and easy to observe; but in reality it
is very rarely observed. Simond was a man of considerable experience
upon the ice, but the first crevasse he ever saw formed was during the
setting out of one of our lines, when a narrow rent opened beneath his
feet, and propagated itself through the ice with loud cracking for a
distance of 50 or 60 yards. Crevasses always commence in this way as
mere narrow cracks, which open very slowly afterwards. I will here
describe the only case of crevasse-forming which has come under my
direct observation.
On the 31st of July, 1857, Mr. Hirst and myself, having completed our
day's work, were standing together upon the Glacier du Geant, when a
loud dull sound, like that produced by a heavy blow, seemed to issue
from the body of the ice underneath the spot on which we stood. This was
succeeded by a series of sharp reports, which were heard sometimes above
us, sometimes below us, sometimes apparently close under our feet, the
intervals between the louder reports being filled by a low singing
noise. We turned hither and thither as the direction of the sounds
varied; for the glacier was evidently breaking beneath our feet, though
we could discern no trace of rupture. For an hour the sounds continued
without our being able to discover their source; this at length revealed
itself by a rush of air-bubbles from one of the little pools upon the
surface of the glacier, which was intersected by the newly-formed
crevasse. We then traced it for some distance up and down, but hardly at
any place was it sufficiently wide to permit the blade of my penknife to
enter it. M. Agassiz has given an animated description of the terror of
his guides upon a similar occasion, and there was an element of awe in
our own feelings as we heard the evening stillness of the glacier thus
disturbed.
[Sidenote: MECHANICAL ORIGIN.]
With regard to the mechanical origin of the crevasses the most vague and
untenable notions had been entertained until Mr. Hopkins published his
extremely valuable papers. To him, indeed, we are almost wholly indebted
for our present knowledge of the subject, my own experiments upon this
portion of the glacier-question being for the most part illustrations of
the truth of his reasoning. To understand the fissures in their more
complex aspects it is necessary that we should commence with their
elements. I shall deal with the question in my own way, adhering,
however, to the mechanical principles upon which Mr. Hopkins has based
his exposition.
[Illustration: Fig. 25. Diagram explanatory of the mechanical origin of
Crevasses.]
Let A B, C D, be the bounding sides of a glacier moving in the direction
of the arrow; let _m_, _n_ be two points upon the ice, one, _m_, close
to the retarding side of the valley, and the other, _n_, at some
distance from it. After a certain time, the point _m_ will have moved
downwards to _m'_, but in consequence of the swifter movement of the
parts at a distance from the sides, _n_ will have moved in the same
time to _n'_. Thus the line _m n_, instead of being at right angles to
the glacier, takes up the oblique position _m' n'_; but to reach from
_m'_ to _n'_ the line _m n_ would have to stretch itself considerably;
every other line that we can draw upon the ice parallel to _m' n'_ is in
a similar state of tension; or, in other words, the sides of the glacier
are acted upon by an oblique pull towards the centre. Now, Mr. Hopkins
has shown that the direction in which this oblique pull is strongest
encloses an angle of 45 deg. with the side of the glacier.
[Sidenote: LINE OF GREATEST STRAIN.]
[Illustration: Fig. 26. Diagram showing the line of Greatest Strain.]
What is the consequence of this? Let A B, C D, Fig. 26, represent, as
before, the sides of the glacier, moving in the direction of the arrow;
let the shading lines enclose an angle of 45 deg. with the sides. _Along_
these lines the marginal ice suffers the greatest strain, and,
consequently _across_ these lines and at right angles to them, the ice
tends to break and to form _marginal crevasses_. The lines, _o p_, _o
p_, mark the direction of these crevasses; they are at right angles to
the line of greatest strain, and hence also enclose an angle of 45 deg. with
the side of the valley, _being obliquely pointed upwards_.
[Sidenote: MARGINAL AND TRANSVERSE CREVASSES.]
This latter result is noteworthy; it follows from the mechanical data
that the swifter motion of the centre tends to produce marginal
crevasses which are inclined from the side of the glacier towards its
source, and not towards its lower extremity. But when we look down upon
a glacier thus crevassed, the first impression is that the sides have
been dragged down, and have left the central portions behind them;
indeed, it was this very appearance that led M. de Charpentier and M.
Agassiz into the error of supposing that the sides of a glacier moved
more quickly than its middle portions; and it was also the delusive
aspect of the crevasses which led Professor Forbes to infer the slower
motion of the eastern side of the Mer de Glace.
The retardation of the ice is most evident near the sides; in most
cases, the ice for a considerable distance right and left of the central
line moves with a sensibly uniform velocity; there is no dragging of the
particles asunder by a difference of motion, and, consequently, a
compact centre is perfectly compatible with fissured sides. Nothing is
more common than to see a glacier with its sides deeply cut, and its
central portions compact; this, indeed, is always the case where the
glacier moves down a bed of uniform inclination.
But supposing that the bed is not uniform--that the valley through which
the glacier moves changes its inclination abruptly, so as to compel the
ice to pass over a brow; the glacier is then circumstanced like a stick
which we try to break by holding its two ends and pressing it against
the knee. The brow, where the bed changes its inclination, represents
the knee in the case of the stick, while the weight of the glacier
itself is the force that tends to break it. It breaks; and fissures are
formed across the glacier, which are hence called _transverse
crevasses_.
[Sidenote: GRINDELWALD GLACIER.]
No glacier with which I am acquainted illustrates the mechanical laws
just developed more clearly and fully than the Lower glacier of
Grindelwald. Proceeding along the ordinary track beside the glacier, at
about an hour's distance from the village the traveller reaches a point
whence a view of the glacier is obtained from the heights above it. The
marginal fissures are very cleanly cut, and point nearly in the
direction already indicated; the glacier also changes its inclination
several times along the distance within the observer's view. On crossing
each brow the glacier is broken across, and a series of transverse
crevasses is formed, which follow each other down the slope. At the
bottom of the slope tension gives place to pressure, the walls of the
crevasses are squeezed together, and the chasms closed up. They remain
closed along the comparatively level space which stretches between the
base of one slope and the brow of the next; but here the glacier is
again transversely broken, and continues so until the base of the second
slope is reached, where longitudinal pressure instead of longitudinal
strain begins to act, and the fissures are closed as before. In Fig. 27A
I have given a sketchy section of a portion of the glacier, illustrating
the formation of the crevasses at the top of a slope, and their
subsequent obliteration at its base.
[Sidenote: COMPRESSION AND TENSION.]
[Illustration: Fig. 27A, B. Section and Plan of a portion of the Lower
Grindelwald Glacier.]
Another effect is here beautifully shown, namely, the union of the
transverse and marginal crevasses to form continuous fissures which
stretch quite across the glacier. Fig. 27B will illustrate my meaning,
though very imperfectly; it represents a plan of a portion of the Lower
Grindelwald glacier, with both marginal and transverse fissures drawn
upon it. I have placed it under the section so that each part of it may
show in plan the portion of the glacier which is shown in section
immediately above it. It shows how the marginal crevasses remain after
the compression of the centre has obliterated the transverse ones; and
how the latter join on to the former, so as to form continuous fissures,
which sweep across the glacier in vast curves, with their convexities
turned upwards. The illusion before referred to is here strengthened;
the crevasses turn, so to say, _against_ the direction of motion,
instead of forming loops, with their convexities pointing downwards, and
thus would impress a person unacquainted with the mechanical data with
the idea that the glacier margins moved more quickly than the centre.
The figures are intended to convey the idea merely; on the actual slopes
of the glacier between twenty and thirty chasms may be counted: also the
word "compression" ought to have been limited to the level portions of
the sketch.
[Sidenote: LONGITUDINAL CREVASSES.]
Besides the two classes of fissures mentioned we often find others,
which are neither marginal nor transverse. The terminal portions of many
glaciers, for example, are in a state of compression; the snout of the
glacier abuts against the ground, and having to bear the thrust of the
mass behind it, if it have room to expand laterally, the ice will
yield, and _longitudinal crevasses_ will be formed. They are of very
common occurrence, but the finest example of the kind is perhaps
exhibited by the glacier of the Rhone. After escaping from the steep
gorge which holds the cascade, this glacier encounters the bottom of a
comparatively wide and level valley; the resistance to its forward
motion is augmented, while its ability to expand laterally is increased;
it has to bear a longitudinal thrust, and it splits at right angles to
the pressure [strain?]. A series of fissures is thus formed, the central
ones of which are truly longitudinal; but on each side of the central
line the crevasses diverge, and exhibit a fan-like arrangement. This
disposition of the fissures is beautifully seen from the summit of the
Mayenwand on the Grimsel Pass.
[Illustration: Fig. 28. Diagram illustrating the crevassing of Convex
Sides of glacier.]
Here then we have the elements, so to speak, of glacier-crevassing, and
through their separate or combined action the most fantastic cutting up
of a glacier may be effected. And see how beautifully these simple
principles enable us to account for the remarkable crevassing of the
eastern side of the Mer de Glace. Let A B, C D, be the opposite sides of
a portion of the glacier, near the Montanvert; C D being east, and A B
west, the glacier moving in the direction of the arrow; let the points
_m n_ represent the extremities of our line of stakes, and let us
suppose an elastic string stretched across the glacier from one to the
other. We have proved that the point of maximum motion here lies much
nearer to the side C D than to A B. Let _o_ be this point, and, seizing
the string at _o_, let it be drawn in the direction of motion until it
assumes the position, _m_, _o'_, _n_. It is quite evident that _o' n_ is
in a state greater tension than _o' m_, and the ice at the eastern side
of the Mer de Glace is in a precisely similar mechanical condition. It
suffers a greater strain than the ice at the opposite side of the
valley, and hence is more fissured and broken. Thus we see that the
crevassing of the eastern side of the glacier is a simple consequence of
the quicker motion of that side, and does not, as hitherto supposed,
demonstrate its slower motion. The reason why the eastern side of the
glacier, as a whole, is much more fissured than the western side is,
that there are two long segments which turn their convex curvature
eastward, and only one segment of the glacier which turns its convexity
westward.
[Sidenote: CREVASSING OF CONVEX SIDE.]
The lower portion of the Rhone glacier sweeps round the side of the
valley next the Furca, and turns throughout a convex curve to this side:
the crevasses here are wide and frequent, while they are almost totally
absent at the opposite side of the glacier. The lower Grindelwald
glacier turns at one place a convex curve towards the Eiger, and is much
more fissured at that side than at the opposite one; indeed, the
fantastic ice-splinters, columns, and minarets, which are so finely
exhibited upon this glacier, are mainly due to the deep crevassing of
the convex side. Numerous other illustrations of the law might, I doubt
not, be discovered, and it would be a pleasant and useful occupation to
one who takes an interest in the subject, to determine, by strict
measurements upon other glaciers, the locus of the point of maximum
motion, and to observe the associated mechanical effects.
[Sidenote: BERGSCHRUNDS.]
The appearance of crevasses is often determined by circumstances more
local and limited than those above indicated; a boss of rock, a
protuberance on the side of the flanking mountain, anything, in short,
which checks the motion of one part of the ice and permits an adjacent
portion to be pushed away from it, produces crevasses. Some valleys are
terminated by a kind of mountain-circus with steep sides, against which
the snow rises to a considerable height. As the mass is urged downwards,
the lower portion of the snow-slope is often torn away from its higher
portion, and a chasm is formed, which usually extends round the head of
the valley. To such a crevasse the specific name _Bergschrund_ is
applied in the Bernese Alps; I have referred to one of them in the
account of the "Passage of the Strahleck."
(18.)
The phenomena described and accounted for in the last chapter have a
direct bearing upon the question of viscosity. In virtue of the quicker
central flow the lateral ice is subject to an oblique strain; but,
instead of stretching, it breaks, and marginal crevasses are formed. We
also see that a slight curvature in the valley, by throwing an
additional strain upon one half of the glacier, produces an augmented
crevassing of that side.
But it is known that a substance confessedly viscous may be broken by a
sudden shock or strain. Professor Forbes justly observes that
sealing-wax at moderate temperatures will mould itself (with time) to
the most delicate inequalities of the surface on which it rests, but may
at the same time be shivered to atoms by the blow of a hammer. Hence, in
order to estimate the weight of the objection that a glacier breaks when
subjected to strain, we must know the conditions under which the force
is applied.
The Mer de Glace has been shown (p. 287) to move through the neck of the
valley at Trelaporte at the rate of twenty inches a day. Let the sides
of this page represent the boundaries of the glacier at Trelaporte, and
any one of its lines of print a transverse slice of ice. Supposing the
line to move down the page as the slice of ice moves down the valley,
then the bending of the ice in twenty-four hours, shown on such a scale,
would only be sufficient to push forward the centre in advance of the
sides by a very small fraction of the width of the line of print. To
such an extremely gradual strain the ice is unable to accommodate itself
without fracture.
[Sidenote: NUMERICAL TEST OF VISCOSITY.]
Or, referring to actual numbers:--the stake No. 15 on our 5th line, page
284, stood on the lateral moraine of the Mer de Glace; and between it
and No. 14 a distance of 190 feet intervened. Let A B, Fig. 29, be the
side of the glacier, moving in the direction of the arrow, and let _a b
c d_ be a square upon the glacier with a side of 190 feet. The whole
square moves with the ice, but the side _b d_ moves quickest; the point
_a_ moving 10 inches, while _b_ moves 14.75 inches in 24 hours; the
differential motion therefore amounts to an inch in five hours. Let _a
b' d' c_ be the shape of the figure after five hours' motion; then the
line _a b_ would be extended to _a b'_ and _c d_ to _c d'_.
[Illustration: Fig. 29. Diagram illustrating test of viscosity.]
The extension of _these_ lines does not however express the _maximum_
strain to which the ice is subjected. Mr. Hopkins has shown that this
takes place along the line _a d_; in five hours then this line, if
capable of stretching, would be stretched to _a d'_. From the data given
every boy who has mastered the 47th Proposition of the First Book of
Euclid can find the length both of _a d_ and _a d'_; the former is
3224.4 inches, and the latter is 3225.1, the difference between them
being seven-tenths of an inch.
This is the amount of yielding required from the ice in five hours, but
it cannot grant this; the glacier breaks, and numerous marginal
crevasses are formed. It must not be forgotten that the evidence here
adduced merely shows what ice cannot do; what it _can_ do in the way of
viscous yielding we do not know: there exists as yet no single
experiment on great masses or small to show that ice possesses in any
sensible degree that power of being drawn out which seems to be the very
essence of viscosity.
I have already stated that the crevasses, on their first formation, are
exceedingly narrow rents, which widen very slowly. The new crevasse
observed by our guide required several days to attain a width of three
inches; while that observed by Mr. Hirst and myself did not widen a
single inch in three days. This, I believe, is the general character of
the crevasses; they form suddenly and open slowly. Both facts are at
variance with the idea that ice is viscous; for were this substance
capable of stretching at the slow rate at which the fissures widen,
there would be no necessity for their formation.
[Sidenote: STRETCHING OF ICE NOT PROVED.]
It cannot be too clearly and emphatically stated that the _proved_ fact
of a glacier conforming to the law of semi-fluid motion is a thing
totally different from the _alleged_ fact of its being viscous. Nobody
since its first enunciation disputed the former. I had no doubt of it
when I repaired to the glaciers in 1856; and none of the eminent men who
have discussed this question with Professor Forbes have thrown any doubt
upon his measurements. It is the assertion that small pieces of ice are
proved to be viscous[A] by the experiments made upon glaciers, and the
consequent impression left upon the public mind--that ice possesses the
"gluey tenacity" which the term viscous suggests--to which these
observations are meant to apply.
FOOTNOTES:
[A] "The viscosity, though it cannot be traced in the parts _if very
minute_ nevertheless _exists_ there, as unequivocally proved by
experiments on the large scale."--Forbes in 'Phil. Mag.,' vol. x., p.
301.
HEAT AND WORK.
(19.)
[Sidenote: CONNEXION OF NATURAL FORCES.]
Great scientific principles, though usually announced by individuals,
are often merely the distinct expression of thoughts and convictions
which had long been entertained by all advanced investigators. Thus the
more profound philosophic thinkers had long suspected a certain
equivalence and connexion between the various forces of nature;
experiment had shown the direct connexion and mutual convertibility of
many of them, and the spiritual insight, which, in the case of the true
experimenter, always surrounds and often precedes the work of his hands,
revealed more or less plainly that natural forces either had a common
root, or that they formed a circle, whose links were so connected that
by starting from any one of them we could go through the circuit, and
arrive at the point from which we set out. For the last eighteen years
this subject has occupied the attention of some of the ablest natural
philosophers, both in this country and on the Continent. The connexion,
however, which has most occupied their minds is that between _heat_ and
_work_; the absolute numerical equivalence of the two having, I believe,
been first announced by a German physician named Mayer, and
experimentally proved in this country by Mr. Joule.
[Sidenote: MECHANICAL EQUIVALENT OF HEAT.]
A lead bullet may be made hot enough to burn the hand, by striking it
with a hammer, or by rubbing it against a board; a clever blacksmith can
make a nail red-hot by hammering it; Count Rumford boiled water by the
heat developed in the boring of cannon, and inferred from the experiment
that heat was not what it was generally supposed to be, an imponderable
fluid, but a kind of motion generated by the friction. Now Mr. Joule's
experiments enable us to state the exact amount of heat which a definite
expenditure of mechanical force can originate. I say _originate_, not
drag from any hiding-place in which it had concealed itself, but
actually bring into existence, so that the total amount of heat in the
universe is thereby augmented. If a mass of iron fall from a tower 770
feet in height, we can state the precise amount of heat developed by its
collision with the earth. Supposing all the heat thus generated to be
concentrated in the iron itself, its temperature would thereby be raised
nearly 10 deg. Fahr. Gravity in this case has expended a certain amount of
force in pulling the iron to the earth, and this force is the
_mechanical equivalent_ of the heat generated. Furthermore, if we had a
machine so perfect as to enable us to apply all the heat thus produced
to the raising of a weight, we should be able, by it, to lift the mass
of iron to the precise point from which it fell.
But the heat cannot lift the weight and still continue heat; this is the
peculiarity of the modern view of the matter. The heat is consumed, used
up, it is no longer heat; but instead of it we have a certain amount of
gravitating force stored up, which is ready to act again, and to
regenerate the heat when the weight is let loose. In fact, when the
falling weight is stopped by the earth, the motion of its mass is
converted into a motion of its molecules; when the weight is lifted by
heat, molecular motion is converted into ordinary mechanical motion, but
for every portion of either of them brought into existence an equivalent
portion of the other must be consumed.
What is true for masses is also true for atoms. As the earth and the
piece of iron mutually attract each other, and produce heat by their
collision, so the carbon of a burning candle and the oxygen of the
surrounding air mutually attract each other; they rush together, and on
collision the arrested motion becomes heat. In the former case we have
the conversion of gravity into heat, in the latter the conversion of
chemical affinity into heat; but in each case the process consists in
the generation of motion by attraction, and the subsequent change of
that motion into motion of another kind. Mechanically considered, the
attraction of the atoms and its results is precisely the same as the
attraction of the earth and weight and _its_ results.
[Sidenote: HEAT PRODUCED IF THE EARTH STRUCK THE SUN.]
But what is true for an atom is also true for a planet or a sun.
Supposing our earth to be brought to rest in her orbit by a sudden
shock, we are able to state the exact amount of heat which would be
thereby generated. The consequence of the earth's being thus brought to
rest would be that it would fall into the sun, and the amount of heat
which would be generated by this second collision is also calculable.
Helmholtz has calculated that in the former case the heat generated
would be equal to that produced by the combustion of fourteen earths of
solid coal, and in the latter case the amount would be 400 times
greater.
[Sidenote: SHIFTING OF ATOMS.]
Whenever a weight is lifted by a steam-engine in opposition to the force
of gravity an amount of heat is consumed equivalent to the work done;
and whenever the molecules of a body are shifted in opposition to their
mutual attractions work is also performed, and an equivalent amount of
heat is consumed. Indeed the amount of work done in the shifting of the
molecules of a body by heat, when expressed in ordinary mechanical work,
is perfectly enormous. The lifting of a heavy weight to the height of
1000 feet may be as nothing compared with the shifting of the atoms of a
body by an amount so small that our finest means of measurement hardly
enable us to determine it. Different bodies give heat different degrees
of trouble, if I may use the term, in shifting their atoms and putting
them in new places. Iron gives more trouble than lead; and water gives
far more trouble than either. The heat expended in this molecular work
is lost as heat; it does not show itself as temperature. Suppose the
heat produced by the combustion of an ounce of candle to be concentrated
in a pound of iron, a certain portion of that heat would go to perform
the molecular work to which I have referred, and the remainder would be
expended in raising the temperature of the body; and if the same amount
of heat were communicated to a pound of iron and to a pound of lead, the
balance in favour of temperature would be greater in the latter case
than in the former, because the heat would have less molecular work to
do; the lead would become more heated than the iron. To raise a pound of
iron a certain number of degrees in temperature would, in fact, require
more than three times the absolute quantity of heat which would be
required to raise a pound of lead the same number of degrees.
Conversely, if we place the pound of iron and the pound of lead, heated
to the same temperature, into ice, we shall find that the quantity of
ice melted by the iron will be more than three times that melted by the
lead. In fact, the greater amount of molecular work invested in the iron
now comes into play, the atoms again obey their own powerful forces, and
an amount of heat corresponding to the energy of these forces is
generated.
This molecular work is that which has usually been called _specific
heat_, or _capacity for heat_. According to the _materialistic_ view of
heat, bodies are figured as sponges, and heat as a kind of fluid
absorbed by them, different bodies possessing different powers of
absorption. According to the _dynamic_ view, as already explained, heat
is regarded as a motion, and capacity for heat indicates the quantity of
that motion consumed in internal changes.
The greatest of these changes occurs when a body passes from one state
of aggregation to another, from the solid to the liquid, or from the
liquid to the aeriform state; and the quantity of heat required for such
changes is often enormous. To convert a pound of ice at 32 deg. Fahr. into
water _at the same temperature_ would require an amount of heat
competent, if applied as mechanical force, to lift the same pound of ice
to a height of 110,000 feet; it would raise a ton of ice nearly 50 feet,
or it would lift between 49 and 50 tons to a height of one foot above
the earth's surface. To convert a pound of water at 212 deg. into a pound of
steam at the same temperature would require an amount of heat which
would perform nearly seven times the amount of mechanical work just
mentioned.
[Sidenote: HEAT CONSUMED IN MOLECULAR WORK.]
This heat is entirely expended in _interior work_,[A] and does nothing
towards augmenting the temperature; the water is at the temperature of
the ice which produced it, both are 32 deg.; and the steam is at the
temperature of the water which produced it, both are 212 deg. The whole of
the heat is consumed in producing the change of aggregation; I say
"_consumed_," not hidden or "latent" in either the water or the steam,
but absolutely non-existent as heat. The molecular forces, however,
which the heat has sacrificed itself to overcome are able to reproduce
it; the water in freezing and the steam in condensing give out the exact
amount of heat which they consumed when the change of aggregation was in
the opposite direction.
At a temperature of several degrees below its freezing point ice is much
harder than at 32 deg. I have more than once cooled a sphere of the
substance in a bath of solid carbonic acid and ether to a temperature of
100 deg. below the freezing point. During the time of cooling the ice
crackled audibly from its contraction, and afterwards it quite resisted
the edge of a knife; while at 32 deg. it may be cut or crushed with extreme
facility. The cold sphere was subjected to pressure; it broke with the
detonation of a vitreous body, and was taken from the press a white
opaque powder; which, on being subsequently raised to 32 deg. and again
compressed, was converted into a pellucid slab of ice.
[Sidenote: ICE NEAR THE MELTING POINT.]
But before the temperature of 32 deg. is quite attained, ice gives evidence
of a loosening of its crystalline texture. Indeed the unsoundness of ice
at and near its melting point has been long known. Sir John Leslie, for
example, states that ice at 32 deg. is _friable_; and every skater knows how
rotten ice becomes before it thaws. M. Person has further shown that the
latent heat of ice, that is to say, the quantity of heat necessary for
its liquefaction, is not quite expressed by the quantity consumed in
reducing ice at 32 deg. to the liquid state. The heat begins to be rendered
latent, or in other words the change of aggregation commences, a little
before the substance reaches 32 deg.,--a conclusion which is illustrated and
confirmed by the deportment of melting ice under pressure.
[Sidenote: ROTTEN ICE AND SOFTENED WAX.]
In reference to the above result Professor Forbes writes as follows:--"I
have now to refer to a fact ... established by a French experimenter, M.
Person, who appears not to have had even remotely in his mind the theory
of glaciers, when he announced the following facts, viz.--'That ice does
not pass abruptly from the solid to the fluid state; that it begins to
_soften_ at a temperature of 2 deg. Centigrade below its thawing point;
that, consequently, between 28 deg. 4' and 32 deg. of Fahr. ice is actually
passing through various degrees of plasticity within narrower limits,
but in the same manner that wax, for example, softens before it melts.'"
The "_softening_" here referred to is the "friability," of Sir J.
Leslie, and what I have called a "loosening of the texture." Let us
suppose the Serpentine covered by a sheet of pitch so smooth and hard as
to enable a skater to glide over it; and which is afterwards gradually
warmed until it begins to bend under his weight, and finally lets him
through. A comparison of this deportment with that of a sheet of ice
under the same circumstances enables us to decide whether ice "passes
through various degrees of plasticity in the same manner as wax softens
before it melts." M. Person concerned himself solely with the heat
absorbed, and no doubt in both wax and ice that heat is expended in
"interior work." In the one case, however, the body is so constituted
that the absorbed heat is expended in rendering the substance viscous;
and the question simply is, whether the heat absorbed by the ice gives
its molecules a freedom of play which would entitle it also to be called
viscous; whether, in short, "rotten ice" and softened wax present the
same physical qualities?
FOOTNOTES:
[A] I borrow this term from Professor Clausius's excellent papers on the
Dynamical Theory of Heat.
(20.)
There is one other point in connexion with the viscous theory which
claims our attention. The announcement of that theory startled
scientific men, and for two or three years after its first publication
it formed the subject of keen discussion. This finally subsided, and
afterwards Professor Forbes drew up an elaborate paper, which was
presented in three parts to the Royal Society in 1845 and 1846, and
subsequently published in the 'Philosophical Transactions.'
In the concluding portion of Part III. Professor Forbes states and
answers the question, "How far a glacier is to be regarded as a plastic
mass?" in these words:--"Were a glacier composed of a solid crystalline
cake of ice, fitted or moulded to the mountain bed which it occupies,
like a lake tranquilly frozen, it would seem impossible to admit such a
flexibility or yielding of parts as should permit any comparison to a
fluid or semifluid body, transmitting pressure horizontally, and whose
parts might change their mutual positions so that one part should be
pushed out whilst another remained behind. But we know, in point of
fact, that a glacier is a body very differently constituted. It is
clearly proved by the experiments of Agassiz and others that the glacier
is not a mass of ice, but of ice and water, the latter percolating
freely through the crevices of the former to all depths of the glacier;
and it is a matter of ocular demonstration that these crevices, though
very minute, communicate freely with one another to great distances; the
water with which they are filled communicates force also to great
distances, and exercises a tremendous hydrostatic pressure to move
onwards in the direction in which gravity urges it, the vast porous mass
of seemingly rigid ice in which it is as it were bound up."
[Sidenote: CAPILLARY HYPOTHESIS.]
"Now the water in the crevices," continues Professor Forbes, "does not
constitute the glacier, but only the principal vehicle of the force
which acts on it, and the slow irresistible energy with which the icy
mass moves onwards from hour to hour with a continuous march, bespeaks
of itself the presence of a fluid pressure. But if the ice were not in
some degree ductile or plastic, this pressure could never produce any
the least forward motion of the mass. The pressure in the capillaries of
the glacier can only tend to separate one particle from another, and
thus produce tensions and compressions _within the body of the glacier
itself_, which yields, owing to its slightly ductile nature, in the
direction of least resistance, retaining its continuity, or recovering
it by reattachment after its parts have suffered a bruise, according to
the violence of the action to which it has been exposed."
I will not pretend to say that I fully understand this passage, but,
taking it and the former one together, I think it is clear that the
water which is supposed to gorge the capillaries of the glacier is
assumed to be essential to its motion. Indeed, an extreme degree of
sensitiveness has been ascribed to the glacier as regards the changes of
temperature by which the capillaries are affected. In three succeeding
days, for example, Professor Forbes found the diurnal summer motion of a
point upon the Mer de Glace to increase from 15.2 to 17.5 inches a day;
a result which he says he is "persuaded" to be due to the increasing
heat of the weather at the time. If, then, the glacier capillaries can
be gorged so quickly as this experiment would indicate, it is fair to
assume that they are emptied with corresponding speed when the supply is
cut away.
[Sidenote: TEMPERATURE AT CHAMOUNI; WINTER 1859.]
The extraordinary coldness of the weather previous to the Christmas of
1859 is in the recollection of everybody: this lowness of temperature
also extended to the Mer de Glace and its environs. I had last summer
left with Auguste Balmat and the Abbe Vueillet thermometers with which
observations were made daily during the cold weather referred to. I take
the following from Balmat's register.
Minimum
Date. temperature
Centigrade.
December 16 -15 deg.
" 17 -20
" 18 -16-1/2
" 19 -9
" 20 -13
" 21 -20-1/2
" 22 -4-1/4
December 23 -4-1/2 deg.
" 24 -6-1/2
" 25 -2
" 26 +2
" 27 -3
" 28 -10-1/2
" 29 -6
The temperature at the Montanvert during the above period may be assumed
as generally some degrees lower, so that for a considerable period,
previous to my winter observations, the portion of the Mer de Glace near
the Montanvert had been exposed to a very low temperature. I reached
the place after the weather had become warm, but during my stay there
the maximum temperature did not exceed -4-1/2 deg. C. Considering therefore
the long drain to which the glacier had been subjected previous to the
29th of December, it is not unreasonable to infer that the capillary
supply assumed by Professor Forbes must by that time have been
exhausted. Notwithstanding this, the motion of the glacier at the
Montanvert amounted at the end of December to half its maximum summer
motion.
[Sidenote: BALMAT'S MEASUREMENTS.]
The observations of Balmat which have been published by Professor
Forbes[A] also militate, as far as they go, against the idea of
proportionality between the capillary supply and the motion. If the
temperatures recorded apply to the Mer de Glace during the periods of
observation, it would follow that from the 19th of December 1846 to the
12th of April 1847 the temperature of the air was constantly under zero
Centigrade, and hence, during this time, the gorging of the capillaries,
which is due to superficial melting, must have ceased. Still, throughout
this entire period of depletion the motion of the glacier steadily
increased from twenty-four inches to thirty-four and a half inches a
day. What has been here said of the Montanvert, and of the points lower
down where Balmat's measurements were made, of course applies with
greater force to the higher portions of the glacier, which are withdrawn
from the operation of superficial melting for a longer period, and
which, nevertheless, if I understand Professor Forbes aright, have their
motion _least affected_ in winter. He records, for example, an
observation of Mr. Bakewell's, by which the Glacier des Bossons is shown
to be stationary at its end, while its upper portions are moving at the
rate of a foot a day. This surely indicates that, at those places where
the glacier is longest cut off from superficial supply, the motion is
least reduced, which would be a most strange result if the motion
depended, as affirmed, upon the gorging of the capillaries.
[Sidenote: BAKEWELL'S OBSERVATIONS.]
The perusal of the conclusion of Professor Forbes's last volume shows me
that a thought similar to that expressed above occurred to Mr. Bakewell
also. Speaking of a shallow glacier which moved when the alleged
temperature was so enormously below the freezing point that Professor
Forbes regards the observation as open to question (in which I agree
with him), Mr. Bakewell asks, "Is it possible that infiltrated water can
have any action whatever under such circumstances?" The reply of
Professor Forbes contains these words:--"I have nowhere affirmed the
presence of liquid water to be a _sine qua non_ to the plastic motion of
glaciers." This statement, I confess, took me by surprise, which was not
diminished by further reading. Speaking of the influence of temperature
on the motion of the Mer de Glace, Professor Forbes says, the glacier
"took no real start until the frost had given way, and the tumultuous
course of the Arveiron showed that its veins were again filled with the
circulating medium to which the glacier, like the organic frame, owes
its moving energy."[B] And again:--"It is this fragility precisely
which, yielding to the hydrostatic pressure of the unfrozen water
contained in the countless capillaries of the glacier, produces the
crushing action which shoves the ice over its neighbour particles."[C]
[Sidenote: HUXLEY'S OBSERVATIONS.]
After the perusal of the foregoing paragraphs the reader will probably
be less interested in the question as to whether the assumed capillaries
exist at all in the glacier. According to Mr. Huxley's observations,
they do not.[D] During the summer of 1857 he carefully experimented with
coloured liquids on the Mer de Glace and its tributaries, and in no case
was he able to discover these fissures in the sound unweathered ice. I
have myself seen the red liquid resting in an auger-hole, where it had
lain for an hour without diffusing itself in any sensible degree. This
cavity intersected both the white ice and the blue veins of the glacier;
and Mr. Huxley, in my presence, cut away the ice until the walls of the
cavity became extremely thin, still no trace of liquid passed through
them. Experiments were also made upon the higher portions of the Mer de
Glace, and also on the Glacier du Geant, with the same result. Thus the
very existence of these capillaries is rendered so questionable, that no
theory of glacier-motion which invokes their aid could be considered
satisfactory.
FOOTNOTES:
[A] 'Occ. Pap.,' p. 224.
[B] 'Phil. Trans.,' 1846, p. 137, and 'Occ. Pap.,' p. 138.
[C] 'Occ. Pap.,' p. 47.
[D] 'Phil. Mag.,' 1857, vol. xiv., p. 241.
THOMSON'S THEORY.
(21.)
In the 'Transactions' of the Royal Society of Edinburgh for 1849 is
published a very interesting paper by Prof. James Thomson of Queen's
College, Belfast, wherein he deduces, as a consequence of a principle
announced by the French philosopher Carnot, that water, when subjected
to pressure, requires a greater cold to freeze it than when the pressure
is removed. He inferred that the lowering of the freezing point for
every atmosphere of pressure amounted to .0075 of a degree Centigrade.
This deduction was afterwards submitted to the test of experiment by his
distinguished brother Prof. Wm. Thomson, and proved correct. On the fact
thus established is founded Mr. James Thomson's theory of the
"Plasticity of Ice as manifested in Glaciers."
[Sidenote: STATEMENT OF THEORY.]
The theory is this:--Certain portions of the glacier are supposed first
to be subjected to pressure. This pressure liquefies the ice, the water
thus produced being squeezed through the glacier in the direction in
which it can most easily escape. But cold has been evolved by the act of
liquefaction, and, when the water has been relieved from the pressure,
it freezes in a new position. The pressure being thus abolished at the
place where it was first applied, new portions of the ice are subjected
to the force; these in their turn liquefy, the water is dispersed as
before, and re-frozen in some other place. To the succession of
processes here assumed Mr. Thomson ascribes the changes of form observed
in glaciers.
This theory was first communicated to the Royal Society through the
author's brother, Prof. William Thomson, and is printed in the
'Proceedings' of the Society for May, 1857. It was afterwards
communicated to the British Association in Dublin, in whose 'Reports'
it is further published; and again it was communicated to the Belfast
Literary and Philosophical Society, in whose 'Proceedings' it also finds
a place.
On the 24th of November, 1859, Mr. James Thomson communicated to the
Royal Society, through his brother, a second paper, in which he again
draws attention to his theory. He offers it in substitution for my views
as the best argument that he can adduce against them; he also
controverts the explanations of regelation propounded by Prof. James D.
Forbes and Prof. Faraday, believing that his own theory explains all the
facts so well as to leave room for no other.
[Sidenote: DIFFICULTIES OF THEORY.]
But the passage in this paper which demands my chief attention is the
following:--"Prof. Tyndall (writes Mr. Thomson), in papers and lectures
subsequent to the publication of this theory, appears to adopt it to
some extent, and to endeavour to make its principles co-operate with the
views he had previously founded on Mr. Faraday's fact of regelation." I
may say that Mr. Thomson's main thought was familiar to me long before
his first communication on the plasticity of ice appeared; but it had
little influence upon my convictions. Were the above passage correct, I
should deserve censure for neglecting to express my obligations far more
explicitly than I have hitherto done; but I confess that even now I do
not understand the essential point of Mr. Thomson's theory,--that is to
say, its application to the phenomena of glacier motion. Indeed, it was
the obscurity in my mind in connexion with this point, and the hope that
time might enable me to seize more clearly upon his meaning, which
prevented me from giving that prominence to the theory of Mr. Thomson
which, for aught I know, it may well deserve. I will here briefly state
one or two of my difficulties, and shall feel very grateful to have them
removed.
[Sidenote: IMPROBABLE DEDUCTION.]
Let us fix our attention on a vertical slice of ice transverse to the
glacier, and to which the pressure is applied perpendicular to its
surfaces. The ice liquefies, and, supposing the means of escape offered
to the compressed water to be equal all round, it is plain that there
will be as great a tendency to squeeze the water upwards as downwards;
for the mere tendency to flow down by its own gravity becomes, in
comparison to the forces here acting on the water, a vanishing quantity.
But the fact is, that the ice above the slice is more permeable than
that below it; for, as we descend a glacier, the ice becomes more
compact. Hence the greater part of the dispersed water will be refrozen
on that side of the slice which is turned towards the origin of the
glacier; and the consequence is, that, according to Mr. Thomson's
principle, the glacier ought to move up hill instead of down.
I would invite Mr. Thomson to imagine himself and me together upon the
ice, desirous of examining this question in a philosophic spirit; and
that we have taken our places beside a stake driven into the ice, and
descending with the glacier. We watch the ice surrounding the stake, and
find that every speck of dirt upon it retains its position; there is no
liquefaction of the ice that bears the dirt, and consequently it rests
on the glacier undisturbed. After twelve hours we find the stake fifteen
inches distant from its first position: I would ask Mr. Thomson how did
it get there? Or let us fix our attention on those six stakes which M.
Agassiz drove into the glacier of the Aar in 1841, and found erect in
1842 at some hundreds of feet from their first position:--how did they
get there? How, in fine, does the end of a glacier become its end? Has
it been liquefied and re-frozen? If not, it must have been _pushed_ down
by the very forces which Mr. Thomson invokes to produce his
liquefaction. Both the liquefaction, as far as it exists, and the
motion, are products of the same cause. In short, this theory, as it
presents itself to my mind, is so powerless to account for the simplest
fact of glacier-motion, that I feel disposed to continue to doubt my own
competence to understand it rather than ascribe to Mr. Thomson an
hypothesis apparently so irrelevant to the facts which it professes to
explain.