THE LIBRARY

OF

THE UNIVERSITY
OF CALIFORNIA

PRESENTED BY

PROF. CHARLES A. KOFOID AND
MRS. PRUDENCE W. KOFOID

OCCASIONAL PAPEES

ON

THE THEORY OF GLACIERS

OCCASIONAL PAPEES

THEORY OF GLACIERS

NOW FIRST COLLECTED AND CHRONOLOGICALLY ARRANGED.

A PREFATORY NOTE ON THE RECENT PROGRESS AND
PRESENT ASPECT OF THE THEORY.

BY

JAMES D. FORBES, D.C.L., F.E.S.,

SEC. R. S. ED., F.G.S., ETC.

Corresponding Member of the Imperial Institute of France, Associate or Honorary Member of

the Bavarian Academy of Sciences, of the Academy of Palenno, of the Dutch

Society of Sciences (Haarlem), of the Helvetic Society, of the Pontifical

Academy of " Nuovi Lincei " at Rome, and of the Natural

History Societies of Heidelberg, Geneva, and Vaud ;

Honorary Member of the Royal Medical Society of Edinburgh,

of the Cambridge, Yorkshire, St. Andrews, and Isle of Wight Philosophical Societies,

and of the Plymouth and Bristol Institutions, and

PROFESSOR OF NATURAL PHILOSOPHY IN THE UNIVERSITY
OF EDINBURGH.

EDINBURGH:
ADAM AND CHARLES BLACK, NORTH BRIDGE.

MDCCCL1X.

•

FEINTED BY R. AND K. CLARK, EDINBURGH.

EARTH
SCIENCES

UBRARY

DEDICATED
TO

JOHN ADDINGTON SYMONDS, M.D.

F.R.S.E. M.RC.PH.,

OF CLIFTON HILL HOUSE, BRISTOL,

TO WHOSE SKILL AS A PHYSICIAN,

AND
TO WHOSE UNWEAEIED KINDNESS AS A FEIEND,

THE WRITER

GRATEFULLY ACKNOWLEDGES

HIS DEEP AND LASTING

OBLIGATION.

CONTENTS.

Page

PREFATORY NOTE ON THE RECENT PROGRESS AND PRESENT

ASPECT OF THE THEORY OF GLACIERS . xiii

I. ON A REMARKABLE STRUCTURE OBSERVED BY THE AUTHOR

IN THE ICE OF GLACIERS [1841] .... 1

Obscurity of the Theory of Glacier Formation. . .Ribboned Struc-
ture described... Its Course traced on the Glaciers of the
Aar and Rhone . . . Probably perpendicular to Lines of ,^
greatest Pressure... Also perpendicular to Fissures or Cre-
vasses... Analogy to Slaty Cleavage in Rocks ... Physical
Cause of both perhaps similar,

II. FIRST LETTER ON GLACIERS. Addressed to PROFESSOR

JAMESON [1842] ' ^ . 9

Account of the First Experiments, undertaken in June 1842,
to determine the Laws of Motion of the Mer de Glace of
Chamouni.

III. SECOND LETTER ON GLACIERS. Addressed to PROFESSOR

JAMESON [1842] 13

The Laws of Motion of the Mer de Glace farther stated.

IV. THIRD LETTER ON GLACIERS, Addressed to PROFESSOR

JAMESON [1842] 17

The Laws of Structure of Glaciers generally detailed... The
" Dirt- Bands" described... Mechanical Theory of the Struc-
ture... Dirt- Bands correspond to Annual Intervals... Glaciers
probably move in Winter.

Viii CONTENTS.

Page

V. FOURTH LETTER ON GLACIERS. Addressed to PROFESSOR

JAMESON [1842] . -, .26

Change of Level and General Appearance of the Glacier in the
month of September... Collapse of the Walls of the Cre-
vasses.... Moulins.... Veined Structure.... Direction of the
Crevasses... Both Veined Structure and Crevasses renewed
from time to tune... The Motion of a Glacier resembles that
of a Viscid Fluid—Objections to the Dilatation Theory...
What the Cold of Winter may be supposed to accomplish.

VI. FIFTH LETTER ON GLACIERS. Addressed to the Bight

Honourable EARL CATHCART [1844] . . . 35

Motion of the Mer de Glace in the year 1842-3. . .Winter Move-
ment...The Veined Structure reproduced at the foot of an
Ice Cascade.., Cuts the Medial Moraines... Wrinkles in the
Ice of the Glacier du Geant and that of Grindelwald,
probably corresponding to the position of the Dirt-Bands...
An Ancient Moraine at Chamouni.

VII. SIXTH LETTER ON GLACIERS. Addressed to the Bight

Honourable EARL CATHCART [1844] ... 43

Analogies of Glaciers to Lava Streams,,.. Observations on
Mount Vesuvius... Moraines of Lava Streams. - Some Objec-
tions to the Plastic or Viscous Theory of Glaciers considered
,..Verticality of Crevasses accounted for.

VIII. SEVENTH LETTER ON GLACIERS. — ON THE VEINED STRUC-

TURE OF THE ICE, Addressed to the Bev. Dr. WHEWELL,
Master of Trinity College, Cambridge [1844] . . 56

Mechanical Considerations tending to explain the Forms of the
Veined Structure under different Conditions.

IX. EIGHTH LETTER ON GLACIERS. Addressed to PROFESSOR

JAMESON [1844] 61

Observations on the Motion of the Glacier of Aletsch, . .Experi-
ments on the Plasticity of the Ice of the Mer de Glace, by
Observing the Distortion of a Limited Space of Ice devoid
of Crevasses... Movement of a Glacier of the Second Order
8000 feet above the Sea.

CONTENTS. IX

Page

X. NINTH LETTER ON GLACIERS. Addressed to PROFESSOR

JAMESON [1845] 68

Remarks on the Recent Observations made on the Glacier
of the Aar (in 1844) by direction of M. Agassiz...New Con- „
firmation of the Plastic Theory.... Condensation of the
Glacier in its downward course in consequence of Frontal
Resistance... Continuity of Motion... Motion accelerated in
Fine Weather — Excess of Central Velocity of the Glacier
...Motion of Glaciers of Second Order, and of Snow Beds.

XL ILLUSTRATIONS OP THE Viscous THEORY OF GLACIER
MOTION.* — PART I. CONTAINING EXPERIMENTS ON THE
FLOW OF PLASTIC BODIES, AND OBSERVATIONS ON THE
PHENOMENA OF LAVA STREAMS [1845] . . .77

§ 1. Plastic Models. § 2. Analogy of Glaciers to Lava
Streams... Note on the Velocity of Lava,

XII. ILLUSTRATIONS OF THE Viscous THEORY OF GLACIER
MOTION.* — PART II. AN ATTEMPT TO ESTABLISH BY

OBSERVATION THE PLASTICITY OF GLACIER ICE [1845] 95

§ 3. De Saussure's Theory. § 4. Modifications of De Saus-
sure's Theory. § 5. Experiments at Chamouni on the
Plasticity of Ice.

XIII. ILLUSTRATIONS OF THE Viscous THEORY OF GLACIER

MOTION.*— PART III. [1846] . . . .119

§ 6. On the Motion of Glaciers of the Second Order. § 7. On
the Annual Motion of Glaciers, and on the Influence of
Seasons. § 8. Summary of the Evidence adduced in favour ^
of the Theory... Glacier not a Mass of Fragments...
Argument from the Equable Progression of Glaciers...
Formation of Crevasses... Law of Velocities... Ablation of
the Surface... Plasticity ; Veined Structure.

Addition on the Plasticity of Ice on the Small Scale 167
* From the Philosophical Transactions.

x CONTENTS.

Page

'XIV. ELEVENTH LETTER ON GLACIERS. Addressed to PRO-
FESSOR JAMESON [1846] . . .169

Observations on the Depression of the Glacier Surface...
Ablation and Subsidence distinguished and ascertained...
On the Relative Velocity of the Surface and Bottom of a
Glacier.

XV. TWELFTH LETTER ON GLACIERS. Addressed to PRO-

FESSOR JAMESON [1846] . . • • .176

On the Extraordinary Increase of the Glacier of La Brenva
from 1842 to 1846... Its Motion in Winter, as observed
by M. Guicharda... Observations on the Veined Structure
in 1846, and Experimentum Crucis respecting its Origin,
with measurements of the motion of the Ice... Analogy
from the motion of the Rhone.

XVI. THIRTEENTH LETTER ON GLACIERS. Addressed to PRO-

FESSOR JAMESON [1846] ..... 187

Acceleration of the Surface Motion of Glaciers confirmed...
Velocity of the Mer de Glace in the Summer of 1846 at
Stations C, D, P, and R... Velocity of the Glacier of
Talefre, near its point of Discharge . . .Discovery of a Knap-
sack ten years buried in the Ice, and deduction of the
motion. ..The Glacier of Nant Blancvisited, and its Motion
determined... The Glacier of Miage^re visited, and motion
measured... On the Conversion of the NeVe" into Ice...
Congelation of Infiltrated Water not necessary to produce
the Veined Structure... An attempt to explain the appa-
rent rejection of Stones from the Glacier... Ridges of Ice
in certain parts of Glaciers due to the bruising effect of
intense Pressure... Three Orders of discontinuity, Ridges,
Crevasses, Veined Structure.

XVII. FOURTEENTH LETTER ON GLACIERS. Addressed to PRO-
FESSOR JAMESON [1847] . . . . 205

On the Variation of the Motion at different Seasons... Obser-
vations on the Glacier of the Aar, considered and com-
pared with the Author's results.

CONTENTS. XI

Page

XVIII. FIFTEENTH LETTER ON GLACIERS. Addressed to the

Kev. Dr. WHEWELL [1848] . ' . . . .210

Observations on the Analogies derived from Mud- Slides
on a Large Scale, and [from some Processes in the Arts
in favour of the Viscous Theory of Glaciers.

Land-slips as observed by M. Collin...Mr. Milward on the
Phenomena of a large Mud-slide at Malta, and on the
Cause of the Dirt- Bands and Wrinkles of Glaciers... Con-
nection with Frontal Dip... Surfaces of Detrusion in Iron
Turnings, attended with Vertical Accumulation of
Material.

XIX. SIXTEENTH LETTER ON GLACIERS. Addressed to PRO-
FESSOR JAMESON [1850] 220

Observations on the Movement of the Mer de Glace down to
1850... Observations by Balmat, at different seasons, in
continuation of those formerly detailed... On the gradual
passage of Ice into the Fluid State ; observations of M.
Person... Notice of an undescribed Pass of the Alps, from
Chamouni to OrsiSres by the Glaciers of Tour and Salena.

XX. ON SOME PROPERTIES OF ICE NEAR ITS MELTING POINT

[1858] ~- . 228

XXI. ON GLACIERS IN GENERAL* . 233

APPENDIX.

No. I. AN ATTEMPT TO ILLUSTRATE THE ORIGIN OF u DIRT-
BANDS" IN GLACIERS. By A. MILWARD, Esq. . 261

No. II. MR. FARADAY ON THE PROPERTIES OF ICE . . 265

* From the Encyclopaedia Britannica, eighth edition.

XI 1 CONTENTS.

Page

No. III. EXTRACTS from LETTERS to PROFESSOR FORBES, on
the Analogy of the Structure of some Volcanic Rocks
with that of Glaciers. By C. DARWIN, Esq., F.R.S.* 266

No. IV. ACCOUNT OF AN EXPERIMENT ON STOCKHOLM PITCH,

CONFIRMING THE VlSCOUS THEORY OF GLACIERS. In

a Letter from PROFESSOR GORDON of Glasgow to PRO-
FESSOR J. D. FORBES of Edinburgh* . . . 269

No. V. EXTRACTS from a LETTER from E. BLACKWELL, Esq.,
containing Observations on the Movement of Gla-
ciers of Chamouni in Winter, with remarks by PRO-
FESSOR FORBES 271

* These articles ought chronologically to have preceded the two first of the
Appendix ; they ought also to have been referred to at pages 92, 201, and 215.

ERRATA.

Page 80, line 6 from bottom, for Plate II. read Plate I.
119, line 1, for XIV. read XIII.

PREFATORY NOTE

RECENT PROGRESS AND PRESENT ASPECT OF
THE THEORY OF GLACIERS.

IN 1850, Mr. Faraday delivered a lecture at the Royal In-
stitution on certain properties of water, and more especially of
water in the act of freezing. This lecture was never (I be-
lieve) published by authority. But an abstract of it appeared
in the Athenaeum Journal for June 15, 1850,* and also in the
Literary Gazette. In this brief and imperfect summary of what
must evidently have been an interesting and suggestive dis-
course, it is stated, that if a film of water be enclosed between
two plates of ice. even at a thawing temperature, the film of
water is frozen, and the plates of ice cohere ; and also that
damp snow becomes, by the same process, compacted into a
snow-ball, which will not occur if the snow is dry and hard
frozen.

These facts appear to have excited little notice, until atten-
tion was called to them by Dr. Tyndall in a lecture, also delivered
at the Royal Institution, on the 23d January 1857. He gave to
the phenomenon the name of regelation. He applied it to ex-
plain the observation, that portions of ice crushed in a mould
under Bramah's press may assume new and compact forms with-

* Keprinted in the Second Appendix to the present volume.
b

XIV RECENT PROGRESS OF THE GLACIER THEORY.

out showing any trace of flaws ; this he attributed to the
" regelation " of the water in the crevices. Mr. James Thom-
son and his brother, Professor William Thomson of Glasgow,*
however, ascribed this consolidation to the effect of intense
pressure, causing simultaneous liquefaction, which commences at
every point of the interior of the ice to which the pressure ex-
tends (according to a previous discovery made by them to that
effect), and to its subsequent solidification when the pressure is
removed.

Dr. Tyndall soon applied his experiments on the consolida-
tion or moulding of ice, and his adaptation to them of Mr.
Faraday's fact of " regelation," to the explanation of the veined
structure and movement of glaciers, which certain previous
speculations of Mr. Sorby and himself about " planes of cleav-
age " had brought under his notice.

Thus it will be seen how the theory of glaciers became
anew, in 1857, a matter of attention to men of science ; and,
considering the activity and ingenuity of those engaged in its
study, the received doctrines were not likely to be adopted
without being first thoroughly canvassed. My theory, among
others, was discussed, and I congratulated myself upon the
examination which it was likely to receive upon its intrinsic
merits. The fact that ice can be moulded under pressure, even
in hand specimens, so as completely to recover its continuity
under a changed form, was an argument in favour of my inter-
pretation of the similar fact occurring in glaciers on a great
scale, which appeared to me likely to remove some natural pre-
possessions, as well as to throw light on the precise relations of
water and ice near the freezing-point of the former or thawing-
point of the latter, to which in my writings I had repeatedly

* Proceedings of the Royal Society of London, 7th May 1857, 23d February
and 22d April 1858.

BEHAVIOUR OF ICE UNDER BRAMAH S PRESS. XV

referred. This practical argument was the more acceptable,
because the absence of such a power of being moulded under
intense, rapid pressure had been urged as an objection to my
theory by MM. Sehlagintweit, in their work on this subject.*
The fact is, that the confining of the ice by lateral compression,
whether in the great experiment of nature (in glaciers), or on
the small scale, is, generally speaking, requisite to its success.
I had, however, somewhat underrated the difficulties which my
opinions had to contend with. The new generation of thinkers,
whose powers of investigation were now first to be exercised
on the theory of glaciers, had to review and discuss all the pre-
liminary objections which fifteen years before had furnished the
weapons of the opponents of " plasticity " as a property of ice
on the great scale. Having said all that I could urge on that
subject, I had left my case with a calm and reasonable confi-
dence that Time would be the ablest advocate of my cause. I
never replied to MM. Schlagintweit's appeal to the evidence
derived from the pulverization of ice under Bramah's press,t
the reply being the very same as I had already made several
times to the popular argument derived from the fragility of
ice. It appeared to me that the difficulties felt by Dr. Tyndall
and Mr. Huxley, in admitting my theory, even after the inge-
nious experiments of the former had demonstrated on the small
scale the moulding power of ice, which I had long before
asserted to be unquestionably true on the large scale, were also
such as a longer familiarity with the subject, and perhaps a more

* Untersuchungen iiber die Physikalische Geographic der Alpen, pp. 24, 122.

f In this case the most extravagant distortion was sought to be produced in a
few moments of time. Whilst in the glacier, an almost inappreciable distortion
(for small areas or hand specimens) is produced in periods of many days or weeks.
Very probably also, MM. Sehlagintweit operated at temperatures considerably
below the freezing-point, otherwise they could hardly fail to have obtained the same
results as Dr. Tyndall.

Xvi RECENT PROGRESS OF THE GLACIER THEORY.

deliberate consideration of the whole of my views respecting it.
would materially modify. I hope I may be allowed to say that
the event has proved, partially at least, that I judged rightly.
It was natural that the author of so interesting an experiment
as the moulding of ice at a fusing temperature under Bramah's
press, should see in it the germ of a new theory. It is not less
natural that I, who rather hoped for than expected such a pal-
pable illustration of my opinion, should see in it, not a new
explanation of the phenomena of glaciers, but a new proof that
the explanation which I had advanced was correct.

These are points which naturally fall to be decided by those of
the scientific public who contemplate the question from an impar-
tial point of view. If the aspect in which I regard it be the
more correct — if the conclusions of Dr. Tyndall are rather con-
firmatory than subversive of my own — the result of the discus-
sion will be one more affecting personal credit than scientific
truth. If it be found that the limited plasticity of ice,
which, when ice is exposed in the glacier to a peculiarly violent
strain, necessitates the formation of an infinity of minute
rents, is really a part of my theory : — that it also embraces the
substitution of the finite sliding of the internally bruised sur-
faces over one another under the same circumstances, still pro-
ducing a quasi-fluid character in the motion of the whole : — if
it be granted, moreover, that the reconsolidation of the bruised
glacial substance into a coherent whole may be effected by pres-
sure alone acting upon granular snow or upon ice softened by
imminent thaw into a condition more plastic than ice of low
temperature, and that the terms " bruising and re-attachment,"
" incipient fissures reunited by time and cohesion," were equiva-
lent in 1846° to the phrase "fracture and regelation " applied

* The following are specimens of the phraseology used by me in that year, or
previously, with reference to the pages where they will he found in the present

" BRUISING AND RE-ATTACHMENT." Xvii

in 1857 : — If it shall be further found that I argued from nature's
own experiment on the modelling of ice on the great scale in the
irregular cavities of a mountain valley, to the same purpose as
Dr. Tyndall does from his beautiful laboratory experiment,
whence he retraces the steps of the process to apply it to the
actual glacier, and that there is no reason for accepting the ex-
periment in the laboratory as more certain or more conclusive
than the observation in the mountain valley, but that each obser-
vation confirms and illustrates the other : — Should all this be
admitted on due examination, I shall, I trust, still be held to
have laid just and solid foundations for a Plastic or Viscous
Theory of Glaciers, without the desire or pretension to have
credit for exhausting the subject in such a manner tl&t future
discoveries in physics can throw no more light upon it. . I
utterly disclaim so unworthy a pretension, and I appeal to every
passage of my writings in which I have referred to the more
obscure questions of physics and mechanics, as bearing on the
Glacial Theory, in corroboration of this statement.

I am aware that when the Theory of Glaciers was again
in 1857 brought under discussion in the way I have already
mentioned, it was not easy for those who desired to know what
could be said for the Plastic Theory to obtain an exact acquaint-
ance with all that had been urged in its favour, or with the
modifications, if any, of the author's original views derived from
farther experience or consideration. Some might hold that the
" Travels in the Alps," as published in 1843, contained the

work : — » The body of the glacier itself . . . yields, owing to its slightly ductile
nature, in the direction of least resistance, retaining its continuity, or recovering it
by re-attachment after its parts have suffered a bruise, according to the violence of
the action to which it has been exposed," p. 166. " In this condition [on the ' very
border of thawing'] molecular attachment amongst the granules must be com-
paratively easy, and the opacity disappears in proportion as optical contact is
attained," p. 201. "Multitudinous incipient fissures occasioned by the intense
strain are reunited by the simple effects of time and cohesion." Hid.

RECENT PROGRESS OF THE GLACIER THEORY.

authentic exposition of the theory ; and this is quite true. But
the criticism which the theoretical part of that work called
forth at the time and for several years subsequently, naturally
compelled the writer to attempt the removal of objections, to
enter into more detail on obscure points, and to bring together,
from various quarters, analogical arguments in favour of the
principles adopted and their applications. Indeed it would
even now perhaps be difficult for any one approaching the sub-
ject for the first time, to frame an objection which has not
either been anticipated by myself or suggested by others. So
numerous were these elucidations and replies to objections, that
they amounted to at least twenty detached essays, the greater
part of which are now for the first time collected in the present
volume.

The revival of attention to the Theory of Glaciers, by the
writings of Dr. Tyndall and other recent explorers of the Alps,
and by some philosophers at home, suggested the present pub-
lication. It contains a literal reprint of those Minor Writings
in which I from time to time endeavoured first to expound the
Plastic Theory and afterwards to defend it. The course thus taken
is at least impartial — in some respects it is disadvantageous to me.*
These pages exhibit, without reservation, the progress of the
writer's opinions from the very time of their formation within
view of the glaciers to their final homologation, after infinite
discussion and after years of reflection. But I accept the ordeal.
Without asserting that an absolute identity of opinion is to be
found in each page of writings composed at so many different

* An acute observer and essayist not unjustly says, " If you want to be admired
for attainments do not exhibit the several steps of your progress. It makes no real
difference, but fools have little respect for what they can measure. This is often
the reason of the prophet being without ' honour in his own country.' It is only
with the best judges that the highest works of art would lose none of their honour
by being seen in their rudiments." — Life and Books, by J. F. Boyes.

CONGELATION OF INFILTRATED WATER DISCUSSED. xix

times, and many of them during journeys, I believe that a sub-
stantial unity pervades the whole.

On only one point of any importance was the original theory,
as delivered in the " Travels in the Alps," subsequently modi-
fied. In the earlier of my writings, I attributed the conversion
of the powdery neve into the perfect glacier (as I believe nearly
all preceding and contemporary authors had done), as well the
alternations of perfect and porous ice in the " veined structure"
of the latter (which at that time no one else had tried to ac-
count for), to the congelation by the winter's frost of water
percolating through their interstices. In the former case, the
interstitial spaces were those of the snowy granules ; in the
latter, the flaws due to the differential motion of the central
and lateral parts of the glacier.

The congelation of infiltrated water was not a doctrine
which originated with me, but rather one which I had assisted
in banishing as much as possible from glacial speculations.
Having endeavoured to demonstrate that no efficient congela-
tion of this description can take place in summer when the
motion of the glacier is most rapid, I would gladly have dis-
pensed with it altogether. Still it was impossible to deny that
the winter's cold must penetrate to some depth in the glacier,
and at first I was willing to admit not only that it was the
efficient cause of the conversion of the neve into ice, and of the
glassy bands of the veined structure, but that it might by its
expansion restore the level of the glacier, during winter, to the
point from which it had fallen by the waste of the previous
summer. This was the idea I had in 1842.* But by the time
of the publication of the first edition of my Travels in the Alps
in 1843, I had already got rid of the necessity for allowing this
vertical dilatation, by including the ascent of the glacial surface

* Fourth Letter on Glaciers (1842), p. 34 of this volume.

XX RECENT PROGRESS OF THE GLACIER THEORY.

among the direct effects of plasticity.* I considered that my
theory was strengthened by being rendered so far independent
of an effect which I admitted with reluctance.f

There remained, however, three classes of facts requiring
explanation, and which were evidently connected with one
another — (1) there was the reconsolidation of a glacier more or
less extensively fissured by open cracks, which is yet seen ulti-
mately to recover its continuity ; (2) the reunion by pellucid
ice of the surfaces rent by the bruising action producing the
veined structure ; (3) the transformation of the neve into per-
fect ice. On the first point I had early come to the conclusion
that the greater fissures of ice were sealed up merely by the
collapse and reunion of the particles influenced by time and
pressure, and aided by the softening effects of the plentiful affu-
sion of water. Next I was disposed to account, at least in part,
for the reunion of the surfaces of internal sliding, or the forma-
tion of the veined structure when it arises or is modified in the
glacier proper, by a similar result of pressure and cohesion. I
do not think that any recognition of infiltration in connection
with this structure will be found in my writings subsequent to
1844. A third fact was, however, to be explained before the
congelation of water in the depths of the glacier could be dis-
pensed with ; — and until that was effected, it was, to say the least,
superfluous to affirm that congelation had no effect in producing
the " blue veins." The difficulty now centered on the conversion
of the n£ve into glassy ice, a process (in my opinion) very inti-
mately connected with the original formation of the veined struc-
ture in the higher glacier, as the following passage, written in
Italy early in 1844 (therefore founded on my observations of at
least the previous year) , clearly shows : — After explaining the

* Travels, 1st Edit., p. 384 ; 2d Edit., p. 386.
f See the limitations mentioned in Travels, pp. 232, 360, 372 of both editions.

CONSOLIDATION BY FRICTION AND PRESSURE. XX

effects of differential motion in developing the veined structure,
it is added, " I believe that it is during the progress of the glacier
thus subjected to a new and peculiar set of forces depending on
gravity, and which remodel its internal constitution by substi-
tuting hard blue ice in the form of veins for its previous snowy
texture, that the horizontal stratification observed in the higher
part of the glacier or ntoe is gradually obliterated."* It will
be seen in the pages of this volume that the identity of the
process which (at least in the higher glacier) produces the blue
veins of the ribboned structure, with the conversion of granular
snow into glassy ice, remained for nearly three years a strongly
fixed idea in my mind, but it only received a satisfactory
development when I returned to Chamouni in 1846, thoroughly
unsatisfied with the explanation of the conversion of the ntve
into ice by thaw and congelation, and determined, if possible,
to find a better solution for it. In the meantime I avoided in
my writings any farther allusion to the mode of this conversion,
as to which I had merely sanctioned the traditional opinions
adopted even by De Saussure, notwithstanding his well-founded
objections to the dilatation theory. f

But in my autumn journey of 1846 this difficulty was
removed, and I hastened, on my return, to record in my
Thirteenth Letter on Glaciers! my now clear conviction that
all the phases of consolidation of a glacier are due to the effects
of time and cohesion alone acting on a substance softened by
the imminent approach of the thawing state, in opposition to
the belief which I formerly, in common with most other persons,
entertained, that snow could not pass into pellucid ice without

* Fifth Letter, p. 53 of this volume.

•f " Ces memes neiges . . . abreuvees des eaux des pluies et des neiges fondus,
se gelent pendant I'hiver, et forment ces glaces poreuses dont les glaciers sont com-
poses."— Voyages, torn. i. § 527.

| Reprinted in the present volume, page 199.

xxii RECENT PROGRESS OF THE GLACIER THEORY.

being first melted and then frozen. Friction and pressure alone
I affirmed to effect the change, especially in the glacier,
which, during a great part of the year, is kept on the very
border of thawing by the ice-cold water which infiltrates it. In
this condition, molecular attachment I stated to be comparatively
easy, the opacity disappearing as optical contact is attained.
The " glacification" of the neve takes place by the kneading or
working of the parts under intense pressure,^and the multitu-
dinous incipient fissures are reunited by the simple effects of
time and cohesion. Thus the conversion into ice is simultaneous,
and in this case identical with the formation of the blue bands,
which are formed where the pressure is most intense, and
where the differential motion is a maximum, that is, near the
walls of the glacier. *

Thus the whole phenomena of the transformation of the
glacier from fresh fallen snow into laminated cohering ice,
which constitutes the character of the true glacial substance,
even at great depths, were accounted for without the embar-
rassing admission of the penetration of the winter frosts to an
unlimited extent, f

It was important that I should specifically point out this
change in my opinion, of which the record is to be found in these
pages, because it is the only one (I hope) which occasions a
discrepancy of any consequence between my earlier and later

* These expressions are verbally taken from the Thirteenth Letter, pp. 200, 201.

f Since a certain amount of congelation of infiltrated water must necessarily
take place during winter, at least near the surface of a glacier, it is easy to see
that it is a matter of nice discrimination to limit precisely its agency in a theory of
glaciers. Certain phenomena cannot be produced without it. Such are the lenti-
cular frozen cavities which were described by myself on the Glacier of Bossons, and
which have been since particularly noticed by Messrs. Tyndall and Huxley, and by
Mr. Ball. I have referred to them more than once in the papers printed in this
volume, where I have described them as the limit of the veined structure when the
dislocating forces are very great and the lateral compression small, and I have also

M. PERSON'S AND MB. FARADAY'S OBSERVATIONS. xxiii

writings reprinted in this volume. It will also, I think, be
admitted on examination, that the new doctrine preserved all
that was sound and important in the old one, and added a new
feature not only to the explanations given by myself, but to what
was true in the writings of De Saussure and other older
authors.

In 1850 I noticed M. Person's interesting observation de-
duced from his own experiments and those of M. Regnault, that
the dissolution of ice is a gradual, not a sudden process, and so
far resembles the more tardy liquefaction of fatty bodies, or of the
metals which absorb their latent heat by degrees, and pass
through intermediate stages of softness or viscosity. I hastened
to avail myself of this new confirmation of views which my
observations on glaciers had suggested. In like manner, had
Mr. Faraday's fact of the congelation of water placed between
two plates of ice, even though that ice be exteriorly melting,
been noticed by me when it was first published, I would have
unquestionably quoted it as a valuable auxiliary in explaining
the possible congelation of water in the minuter fissures of the
glacier, although since the change of views respecting the
consolidation of the glacier mentioned above as adopted by
me in 1846, the congelation of water in the crevices of the
glacier ceased to be essential to the mechanical explanation of
the movement and structure of the ice. The function of the
infiltrated water seems to be that of preserving the whole ice in
that state of softness which immediately precedes its dissolution,
as well as of conveying hydrostatic pressure ; and the cohesion of

(pp. 90, 162) pointed out a similar accident in lavas. The width of the longitudinal
fissures in question shows that no lateral pressure was exerted sufficient to bring
their bounding surfaces into contact, and that they could not have been filled with
pellucid ice by any process short of the prolonged action of external cold upon infil-
trated water. I believe, however, that these appearances are confined to the
neighbourhood of the surface and sides of glaciers.

XXIV RECENT PROGRESS OF THE GLACIER THEORY.

the softened ice under pressure still appears to me to be a
sufficient explanation of all the appearances. Mr. Faraday's
most interesting observation shows, however, the extreme limit
of the law of graduated softening, and is, as I have recently
endeavoured to prove (in a short paper printed at page 228 of
the present volume), a consequence of M. Person's law.

In like manner, Professor W. Thomson's interesting experi-
ment on the lowering of the freezing point of water under
pressure, and the consequent thaw of ice below 32°, may
reasonably be included amongst the causes which, in peculiar
circumstances, may impart to glacier ice a portion of its plasti-
city. I am not disposed to ascribe to it, by any means, the
whole of the plastic qualities of the glacier, as Mr. James
Thomson seems inclined to do. In the rapid alternations of
pressure which take place in the moulding of ice under Bramah's
press, it cannot, I think, be doubted that the opinions of the
Messrs. Thomson are verified.*

All these results of the discriminating study of the familiar
substance of ice near 32°, — the deduction of M. Person, the fact
of Mr. Faraday, the experiment of Dr. Tyndall, the prediction
of Mr. James Thomson and its verification by his brother, —
instead of militating against the correctness of my Theory of
Glaciers of 1842, seem to me to afford so many independent
confirmations of it. The larger and more correct views which
may now be taken of the entire subject, and for which we are*
indebted to so many eminent men, cannot, I should hope, render
valueless the generalization which was made without the full
advantage of them.

With these prefatory observations, I leave to the judgment
of the reader the historical records of my own successive endea-

* See the very interesting experiments of M. Mousson in the Bibliotheque
Universelle, September 1858.

HISTORY OF THE PAPERS IN THIS VOLUME. XXV

vours to place the Theory of Glaciers in a clear point of view,
with the aid of such lights as the science of the time enabled
me to use. I trust that the circumstances of their detached
composition and publication will be taken into account in judg-
ing of them. There will be found amongst them many things
besides those already referred to, which, owing to the same
cause, have been inevitably overlooked.*

I will now say a few words as to the contents of this
volume. They are, with the exception of the last paper,
arranged accurately in chronological order, beginning with 1841,
and ending with 1858.

The First Article on the Veined Structure of Glaciers pre-
ceded any attempt made by myself to account for the pheno-
mena of glaciers.

The first four of the Letters on Glaciers, which follow next,
were written from Switzerland in 1842, and contain the origi-
nal draft of the Plastic or Viscous Theory, which was expounded
in 1843 in a more methodical and detailed manner in my
Travels in the Alps, which appeared in that year.

In 1843 and 1844 I again visited Switzerland, as well as
Italy and Sicily, though not in favourable circumstances for
making extended observations. The Fifth to the Ninth Letters
inclusive contain the results of these journeys, with the reflections
which farther consideration of the Theory had suggested to my
own mind, or which the objections made to it had called forth.

The discussion of these objections continued in 1845, and
it then appeared to me desirable to bring down the subject of
the Glacier Theory to that period in a methodical form, which I
did in three papers on the Viscous Theory, which were commu-

* Besides the Thirteenth Letter already quoted, I would mention the " Sum-
mary of Evidence," at p. 168 of this volume, and the ohservations on plastic bodies
in the Fifteenth Letter, as not perhaps generally known.

XXVI RECENT PROGRESS OP THE GLACIER THEORY.

nicated to the Royal Society of London. These papers are
reprinted here ; and as they form a supplement to the Theory, as
delivered in my Travels, they include not only many additional
facts, but answers to objections or difficulties given in a form
which has none of the unpleasantness of controversy. I ven-
ture to think that some parts of these papers contain valuable
matter, which makes it desirable that they should be republished
in this more convenient form.

Subsequently to the composition of these papers, in 1846,
I once more returned to the Alps, and made the observations
referred to in a previous part of this preface, which altered some-
what my opinions on the consolidation of the glacier, and
afforded several other new observations, particularly as to the
more rapid surface motion of the ice stream. These gave rise to
the Eleventh, Twelfth, and Thirteenth Letters which follow. The
remaining Letters contain additional illustrations of the Theory
derived from various quarters, and the results of my latest visit
to Chamouni — that of 1850.

All these documents have been reprinted with scrupulous
attention to accuracy ; nor has the alteration even of a word
been made without an indication by a foot-note or brackets [ ] ;
the latter being exclusively reserved for additions made in the
course of the present impression. In the papers from the Philo-
sophical Transactions, only, I have consulted the reader's con-
venience by a few abridgements of formal and geometrical de-
tails (all noticed where they occur), but absolutely none which
affect the sense.

The concluding paper is a popular sketch of glaciers in
general, and of my theory, being the article GLACIER from the
Eighth Edition of the Encyclopaedia Britannica, published in
1855. As it contains no novelty, and professes no originality,
I have here and there altered a few words, chiefly in consequence

AIM OF THE PRESENT PUBLICATION.

of the impersonal character of the authorship. It is, I am aware,
very imperfect and indeed unfinished, for the limits to which
I was confined obliged me to bring it to an abrupt close. As,
however, it contains general information which may be accep-
table to some readers (and in this respect might be read first
by those to whom the subject is not already familiar), I have
thought it best to retain it here.

The Appendix contains a few documents by different writers,
referred to in the volume ; and a good Index has been added, by
the aid of which I trust that it will be easy to ascertain the pro-
gress of my observations or opinions on any part of the subject.

The whole of my minor writings on glaciers are not included
in this volume. I shall close this introduction with what is, I
believe, a correct list of those which are not reprinted. The
reason of their omission has been, for the most part, my reluc-
tance to recal the memory of controversies long since concluded.
My desire is, that this volume may contain as few traces as
possible of personal discussions, nor one sentence which might
occasion pain to any reader.

The main intention, indeed, of the present publication is to
appeal directly to what I have written as evidence of my share
in establishing a just theory of a great natural phenomenon, — that
of glaciers ; and to do this by rendering accessible to all readers
the documents by means of which my claims to originality must
be decided. It is not for me to add to or take from the text
of what I have written. It is also more becoming that others,
rather than myself, should be its interpreters, and the judges
of what, after a fair and impartial study of the whole, my writings
shall be held to establish. This is the only favour that I ask.

COLLEGE OF EDINBURGH,
February 1859.

XXV111 RECENT PROGRESS OF THE GLACIER THEORY.

PAPERS ON GLACIERS NOT INCLUDED IN THIS VOLUME.

1842. The Glacier Theory. Edinburgh Review for April 1842.

[I at first intended to reprint this article in the present
volume. But I found that it would ill accord with the character
of the other contents. Being based, so far as the physics of
glaciers are concerned, on the rude and imperfect data which
alone existed in 1841 when it was written, its speculations and
its questionings are already in a great measure obsolete, and
would have served no other purpose but to show the great advance
which the subject has since made. The longest part of the
article was upon the geological aspect of glaciers, one hardly
touched upon from the succeeding papers of the volume. Alto-
gether it seemed to me that, though historically not without
interest, yet its insertion would distract the attention of the
reader from the main purposes of this publication. Some of
the more popular and descriptive parts of the article were re-
printed as an introduction to the abridged edition of my Travels.*
The paper in the Edinburgh Review was translated into French,
and printed entire in the Annales de Chimie, under the direction
of M. Arago.]

1843. Historical Remarks on the Discovery of the true Structure
of Glacier Ice. Edin. New Phil. Journal, Jan. 1843.

1843. Three papers on Glaciers. Proceedings of the Royal
Society of Edinburgh. Read 6th and 27th February, and
20th March 1843.

[These papers contain the substance of some of the chapters
of the Travels in the Alps, then in preparation. The third con-
tains the first description of the plastic models, with illustrative

* Tour of Mont Blanc and of Monte Rosa. A. and C. Black. 12mo. 1855.

PAPERS NOT INCLUDED IN THIS VOLUME. XXIX

figures. The Keith Prize was awarded to the author for these
communications.]

1845. Reply to Mr. Hopkins on the Motion of Glaciers, with
reasons for avoiding further controversy. London and Edin.
Phil. Magazine, May 1845.

1845. Tenth Letter on Glaciers. Edin. New Philos. Journal,
January 1846. [Chiefly controversial. See page 169 of
this volume.]

1846. The Article on Glaciers in Johnstons Physical Atlas.

[Principally geographical and topographical.]

1855. Remarks on the Rev. H. Moseleys Theory of the Descent
of Glaciers. Proceedings of the Royal Society of London.
Uth June 1855.

1859. Remarks on a paper " On Ice and Glaciers" in the last
number of the Philosophical Magazine, in a letter to Professor
Tyndall. [Will probably appear in the Phil. Magazine for
March 1859.]

The first edition of Travels in the Alps of Savoy, &c., was
published in July 1843 ; the second edition in June 1845.

The work entitled Norway and its Glaciers, etc., was pub-
lished towards the end of 1853.

PAPERS

ON

THE THEORY OF GLACIERS,

I. ON A REMARKABLE STRUCTURE OBSERVED BY
THE AUTHOR IN THE ICE OF GLACIERS.—
(Read to the Royal Society, Edinburgh, Dec. 6, 1841).*

Obscurity of the Theory of Glacier Formation — Ribboned Structure described — Its
Course traced on the Glaciers of the Aar and Rhone — Probably perpendicular
to Lines of greatest Pressure — Also perpendicular to Fissures or Crevasses —
Analogy to Slaty Cleavage in Rocks — Physical Cause of both perhaps similar, f

THE object of the present short communication is little more
than to announce and describe a peculiarity which the Ice of
Glaciers frequently exhibits, interesting in itself as connected
with the theory of their formation and propagation, and per-
haps having a bearing upon the explanation of some facts long
felt by geologists to be perplexing.

Had I yielded to my own first impulse, this communication
would have formed but a part of a much more extensive one,
intended to give such an account, as I best might, of the pre-
sent views entertained respecting the mechanism and conser-
vation of glaciers, and the curious and interesting question of
their ancient extension, and perhaps vast geological influence
in producing some of the latest evidences of revolution on the
surface of the globe. When I considered, however, the great

* Edinburgh New Philosophical Journal for January 1842.
•j- [The contents have been added in this reprint.]

2 STRUCTURE OBSERVED IN THE ICE OF GLACIERS. [1841.

extent which such a communication, to be generally intelligible,
must necessarily have, — and farther, that a large share of the
material must be drawn from the works and the observations
of others, — when I recollected, besides, that, however earnest
and sustained had been my investigation of these curious points,
there was still much left obscure or unproved to my own mind ;
in short, that the communication I should lay before the Society
could not have that completeness, determination, and originality,
which could properly entitle it to a permanent place in the
Transactions of our Body, it seemed to me that the wish which
had been expressed by very many of those to whose judgment
I am most willing to defer, that I should make such a detailed
communication, was one with which, in my official position as
Secretary, and having in some degree the control of the order
and distribution of business, I could not properly comply.

I do not, however, relinquish the idea of laying before the
Society, and even at considerable length, the conclusions which
I may ultimately form respecting the great physical and geo-
logical questions now at issue, and the facts and reasonings
upon which these conclusions are founded. The Glacier
Theory, whether it regards the present or past history of those
mighty and resistless vehicles of transport and instruments of
degradation, yields to no other physical speculation of the pre-
sent day in grandeur, importance, interest, and, I had almost
said, novelty. I look forward to the prospect, which I hope
may be realized, of extending much farther, during another
summer, my direct observations and experiments, and in the
meantime I desire to prepare myself for the task, by a thought-
ful review of the experience I have already had, and a close
anatysis of what has been already argued and written upon the
subject.* Should the result be successful, the Society may, a
year hence, expect the communication of it. For the present
I mean to confine myself to the description of a single fact,

* [Such an analysis and review was published in the Edinburgh Beview for
April 1842, and was afterwards translated into French and published in the Annales
de Chimie.]

1841.] RIBBONED APPEARANCE OF THE GLACIER OF THE AAR. 3

which appears generally, if not universally, to have escaped the
notice of former travellers amongst the Glaciers.

On the 9th of August last (1841) I paid my first visit to
the Lower Glacier of the Aar, upon or near which I spent the
greater part of three weeks in company with Professor Agassiz
of Neufcliatel, and Mr. J. M. Heath of Cambridge. It is
surprising how little we see until we are taught to observe. I
had crossed and recrossed many glaciers before, and attended
to their phenomena in a general way ; but it was with a new
sense of the importance and difficulty of the investigation of
their nature and functions that I found something to remark at
every step which had not struck me before ; and even in the
course of the walk along our own glacier (as we considered that
of the Aar, when we had taken up our habitation upon it), we
found on its vast and varied surface something each day which
had totally escaped us before. It was fully three hours' good
walking on the ice or moraine from the lower extremity of the
glacier to the huge block of stone, under whose friendly shelter
we were to encamp ; and in the course of this walk (a distance
of eight or nine miles, on a moderate computation, allowing for
the roughness of the way) on the first day, I noticed in some
parts of the ice, an appearance which I cannot more accurately
describe, than by calling it a ribboned structure, formed by thin
and delicate blue and bluish-white bands or strata, which
appeared to traverse the ice in a vertical direction, or rather
which, by their apposition, formed the entire mass of the ice.
The direction of these bands was parallel to the length of the
glacier, and, of course, being vertical, they cropped out at the
surface, and wherever that surface was intersected and smoothed
by superficial water-courses, their structure appeared with the
beauty and sharpness of a delicately-veined chalcedony. I was
surprised, on remarking it to Mr. Agassiz as a thing which
must be familiar to him, to find that he had not distinctly
noticed it before, at least if he had, that he had considered it
as a superficial phenomenon, wholly unconnected with the
general structure of the ice. But we had not completed our

4 STRUCTURE OBSERVED IN THE ICE OF GLACIERS. [1841.

walk before my suspicion that it was a permanent and deeply-
seated structure was fully confirmed. Not only did we trace it
down the walls of the crevasses by which the glacier is inter-
sected, as far as we could distinctly see, but, coming to a great
excavation in the ice, at least 20 feet deep, formed by running
water, we found the vertical strata or bands perfectly well
defined throughout the whole mass of ice to that depth. An
attempt has been made to convey some idea of their appear-
ance in Plate L* Where the plane of vertical section was
eroded by the action of water, the harder seams of blue ice
stood protuberant ; whilst the intermediate ones, partaking of a
whitish-green colour and granular structure, were washed out.
We did not sleep that night until we had traced the structure
in all directions, even far above the position of our cabin, and
quite from side to side across the spacious glacier of the Finster
Aar.

During the whole of our subsequent residence amongst the
glaciers, the phenomena and causes of this structure occupied
our thoughts very frequently. We had much difficulty in
arriving at a correct description of the manner of its occurrence,
and still more in forming a theory in the least plausible re-
specting its origin.

Its importance, however, as an indication of an unknown
cause, is very great ; not only because all that can illustrate
what is so obscure as the manner of glacier formation and
movement, is so, but because it is precisely on this very point
of "What is the internal structure of the ice of a glacier?"
that the question now pending respecting internal dilatation
as a force producing progression, mainly hangs. Some con-
sider ice as compact, others as granular ; some as crystallized,
others as fractured into angular fragments ; some as horizon-
tally stratified, others as homogeneous ; some as rigid, others
as plastic ; some as wasting, others as growing ; some as ab-
sorbing water, others as only parting with it ; — and yet no one
seems to have observed, or at least observed as an object of

* [It has not been thought necessary to reproduce the figure.]

1841.] COURSE OF THE VEINED STRUCTURE. 5

study, this pervading slaty or ribboned structure, to be found
probably in one part or other of every true glacier.

With regard to extent, this structure was observable on the
Lower Glacier of the Aar, from its lower extremity up to the
region of the firn or neve, where, the icy structure ceasing to
exist, it could not be looked for ; yet even there, where fre-
quent thaws, induced by the neighbourhood of rocks or stones,
produced a compacter structure, the veins became apparent.
In some parts of the glacier, it appears more developed than
in others ; in the neighbourhood of the moraines, and the wall's
of the glacier, it was most apparent. This would seem to infer
a relation to the frequency of thaws and recongelation.

It penetrates the thickness of the glacier to great depths.
It is an integral part of its inmost structure. That it could
not be the production of a single season I was speedily con-
vinced, by observing that where old crevasses fissured the gla-
cier transversely, the veined structure not only was reproduced
on either side, but frequently with a shift or dislocation, or
series of parallel fissures, presenting sometimes a series of dis-
locations advancing in one direction.

The course of the veined structure was, generally speaking,
on the Glacier of the Aar, strictly parallel with its length, and
that with a degree of accuracy which seems extraordinary, if
we attribute its production to the remote influence of the
retaining walls of the glacier, distant at least half a mile. Near
the inferior extremity, where the declivity becomes rapid, the
structure varies its position in a manner very difficult to trace
satisfactorily. There can be little doubt, however, that the
nearly horizontal bands which appear on the steep declivity of
the glacier at its lower termination, are nothing else than the
outcropping of these bands, which have there totally changed
their direction, being transverse instead of longitudinal, and
leaning forwards in the direction in which the glacier moves
at a very considerable angle. The ice in this part of the glacier
is distinctly granular, being composed of large fissured morsels,
nicely wedged together ; and the ribboned structure is greatly

6 STRUCTURE OBSERVED IN THE ICE OF GLACIERS. [1841.

obscured. There seems no doubt, however, that the horizontal
stratification in the lower part of glaciers, insisted on by several
writers, is merely a deception arising from this cause, so familiar
to the geologist who gets a section perpendicular to the dip of
strata, which, therefore, appear horizontal. Towards the sides
or walls of the glacier, at its lower extremity, the veins have
their plane twisted round a vertical axis, having now their dip
towards the centre of the glacier, and rising against the walls ;
and this inclination sometimes extends nearly to the axis of the
glacier, or the medial moraine, where I have observed the veins
deviating from the vertical by an angle of about twenty degrees,
the bands inclining from the centre (or rising towards the
walls), as if the pressure arising from the superior elevation ol
the glacier under the moraine had squeezed them out. The
whole phenomenon has a good deal the air of being a structure
induced perpendicularly to the lines of greatest pressure, though
I do not assert that the statement is general. Whilst the
glacier is confined between precipitous barriers with a feeble
inclination, the structure is longitudinal. As the glacier, by
its weight, falls over the lower part of its bed, and moulds itself
into the form which the continued action of gravity on its
somewhat plastic structure impresses, the longitudinal structure
is first annihilated (for throughout a certain space we could
detect no indications of one kind or other), and the bands then
reappear in a transverse direction, as if generated by the down-
ward and forward pressure, which, at the lowest part of the
glacier, replaces the tight wedging which higher up it received
laterally. It is not easy to convey without a model a clear
idea of the forms of surface here intended, and which yet re-
quire considerable correction.

I may mention, however, that the glacier of the Rhone,
which I have carefully examined, presents a structure in con-
formity with the view thus developed. It will be recollected
by all who have seen that magnificent mass, that it pours in
colossal fragments over the rocky barrier which separates the
Gallenstock from the valley of the Rhone, and having reached

1841.] VEINED STRUCTURE ON THE GLACIER OF THE RHONE. 7

the last-named valley, it spreads itself across and along it
pretty freely — much as a pailful of thickish mortar would do in
like circumstances. The form into which it spreads is rudely
represented in the annexed figure. In this particular case,
even the strongest partisans of the dilatation theory will
hardly deny, that the accumulated ice descending from the
glacier cataract A would form a centre of pressure at C, and
that the lines of equal pressure
would be found in the direction of
the dotted lines, following nearly
the periphery of the glacier. Now
these dotted lines precisely trace
out the course of the veined struc-
ture alluded to ; and, moreover,
they bend more and more forwards
as we proceed from the centre of
pressure C, especially in the direc-
tion of D, the line of greatest in-
clination of the bed, and down
which gravity urges the icy mass.
The front of the glacier, about E D
F, presents the fallacious apearance
of horizontal strata, as in the Aar glacier ; but these are found
to dip inwards at an angle of 10° or 15°, which angle continually
increases as we approach the heart of the glacier, rising to 40°,
50°, 60°, and even 70°, as we approach C. It cannot be
doubted, that these facts are so far favourable to the view which
we have taken, although the establishment of it would require
far more extensive observation ; and in several glaciers which I
have visited, the observation of the convolutions of the veined
structure is very difficult and obscure. Before quitting the sub-
ject, I must add an observation which I made on the Glacier
of the Rhone, arid which I am pretty confident is well founded.
The lines of fissure, or crevasses, are always perpendicular to
the conical surfaces of the veined structure. These fissures are
denoted in the figure by the% full lines a a a. Perhaps the

Moraines

cut by the
River.

The Rhone.

8 STRUCTURE OBSERVED IN THE ICE OF GLACIERS. [1841.

primary cause of these fissures is, that the pressure of the ice
at C forces the glacier to distend itself into continually widen-
ing rings, which its rigidity resists, and therefore it becomes
traversed by radial crevasses.

The veined structure itself, I have already said, arises from
the alternation of more or less compact bands of ice. The
breadth of these varies from a small fraction of an inch to
several inches. The more porous of these bands are the likeliest
vehicles for the transmission of water from the higher to the
lower part of a glacier ; and that opinion receives some con-
firmation from the fact, that, at a certain depth, in crevasses,
we may see the veined structure marked out and exaggerated
"by the frozen stalagmite, which is protruded from the section of
jhe more porous layers.

In conclusion (for the present), this structure deserves the
attention of geologists generally, as showing how the appearance
of the most delicate stratification, and of sedimentary deposition,
may be produced in homogeneous masses, where nothing of the
kind has occurred. For a short time, indeed, I was of opinion
that this structure resulted from true stratification ; but a
closer examination of the mass convinced me that, inexplicable
as the fact remains, it must be accounted for in some other
way. We have endeavoured to show an empirical connection
which appears to exist between the structural planes and the
sustaining walls of the glacier, and likewise that the recurrence
of congelation and thaw appears to strengthen the formation of
the bands. But this cannot be considered as in any degree
amounting to an explanation. The analogous difficulty of slaty
cleavage in rocks presents itself as not improbably connected
with a similar unknown cause, whose action pervaded the mass
of the crystallizing rock undergoing metamorphic change, as
this pervades the mass of the crystallizing glacier. In the
former case, we have cleavage planes perfectly parallel, almost
indefinitely extending with unaltered features over vast surfaces
of the most rugged country, changing neither direction, dip, nor
interval, with hill or valley, cliff or scar, and passing alike

1841.] ANALOGOUS STRUCTURE IN SLATY ROCKS. 9

through strata whose planes of stratification, horizontal, ele-
vated, undulating, or contorted, offer no obstacle or modifica-
tion to the omnipotent energy which has rearranged every
particle in the mass subsequent to deposition. The supposition
of Professor Sedgwick, who has minutely described and con-
sidered this geological puzzle, that " crystalline or polar forces
acted on the whole mass simultaneously in given directions,
and with adequate power,"* can hardly be considered as a
solution of the difficulty, until it is shown that the forces in
question have so acted, and can so act. The experiment is
one which the boldest philosopher would be puzzled to repeat
in his laboratory ; it probably requires acres for its scope, and
years for its accomplishment. May it not be that Nature is
performing in her icy domain a repetition of the same mys-
terious process, and that in another view from the one which
has recently been taken, the Theory of Glaciers may lead to
the true solution of geological problems ?

II. FIRST LETTER on GLACIERS, addressed to
PROFESSOR

Account of the First Experiments, undertaken in June 1842, to determine the Laws
of Motion of the Mer de Glace of Chamouni.

COURMAYEUR, PlEDMONT, 4th July 1842.

My Dear Sir — Knowing that you will be glad to hear of
my safe arrival amongst the Alps, and of my farther proceed-
ings, I hasten to give you an account, in a few words, of what
I have as yet done. Finding the season more than usually
advanced, I hastened to reach Chamouni, in order to ascertain
whether the Mer de Glace was as yet accessible in all its ex-
tent ; and I arrived at the Mon tan vert on the 24th June, and

* Geological Transactions, Second Series, iii. 477.
f Edinburgh New Philosophical Journal, October 1842.

10 FIRST LETTER ON GLACIERS. [1842.

remained there for a week. I was fortunate enough to convey
all my instruments to their destination, without, I believe,
injury to any one of them. The Mer de Glace, so continually
visited by the curious, but so little studied, seemed to me to
offer great advantages for the prosecution of the objects which
I proposed to myself. At first sight it appeared .to me steeper
and more crevassed than I recollected it to be, and I doubted
for a moment whether it was adapted for my experiments ; but
that doubt vanished upon closer examination ; and in the course
of the single week which I have been able to spend there, being
favoured by most excellent weather, I have obtained results so
far definite and satisfactory, that, imperfect as they necessarily
are, and only the commencement of what I expect to accomplish
during the remainder of the season, I will state them shortly.

You will recollect that, in my lectures on glaciers delivered
last December and January, and afterwards in an article
written by me in the Edinburgh Review,* I insisted on the
importance of considering the mechanism of glaciers as a ques-
tion of pure physics, and of obtaining precise and quantitative
measures as the only basis of accurate induction. I pointed
out, also, the several experiments of a critical kind which might
be made ; such, for instance, as the determination of the motion
of the ice at different points of its length, in order to distinguish
between the theories of De Saussure and De Charpentier ; for,
if the glacier merely slides, the velocity of all its points ought
(in the simplest case) to be the same ; if the glacier swells in

* [For April 1842, as, for example, the following passages : —

" The mechanism of a glacier is a problem of natural philosophy, and one much
more difficult and embarrassing than it has commomy been supposed" [to be], —
Ed. Rev., p. 52.

" The solution of this important problem [the theory of glacier motion] would
be obtained by the correct measurement, at successive periods, of the spaces between
points marked on insulated boulders on the glacier ; or between the heads of pegs
of considerable length, stuck into the matter of the ice, and by the determination of
their annual progress." — Ed. Itev., p. 77.

The lectures referred to in the text were delivered by the author in the Uni-
versity of Edinburgh to a numerous audience, including many men of science.
They traced out mainly the same views as are embraced in the article in the Edin-
burgh Review, but with larger details of the author's experience of 184] .]

1842.] PRECISION ATTAINABLE BY OBSERVATION. 11

all its mass, the velocity of the inferior part ought to be greatest.
Of course, I do not now advert to the many causes which might
accidentally invert this law, and which would require to be fully
taken into account ; still less do I mean to say that anything I
have now to state can be considered as critically decisive between
rival theories ; but my experiments certainly do show that the
kind of precision which I desired to see introduced into reason-
ings about this subject, is practically attainable, even in a far
higher degree than I expected.

For example : — The motion of glaciers by the measurement
of the distance of blocks upon its surface from a fixed point,
from one year to another, has marked indubitably the annual
progress of the ice. I do not know that any one has attempted
to perform the measurement in a manner which could lead to
any certain conclusion respecting the motion of the ice at one
season compared with another, or from month to month ; still
less has any one been able to state, with precision) whether the
glacier moves by starts and irregularly (as we should certainly
expect on the sliding theory), or uniformly and evenly ; and
if so, whether it moves only at one part of the twenty-four
hours, and stands still during the remainder (as we should expect
on the dilatation theory, as commonly expounded). Now, I have
already been able —

1st. To show and measure the glacier motion, not only from
day to day, but from hour to hour ; so that I can tell nearly
what o'clock it is by the glacier index. That you may have
an idea of the coincidence which these experiments present, I
give you the longitudinal motion of a point on the Mer de
Glace during four consecutive days.

15.2 inches. 16.3 inches. 17.5 inches. 17.4 inches.

2d. This motion, evidently incompatible with sudden starts,
takes place in the glacier as a whole, undisturbed by the most
enormous dislocations of its surface, for these measures were
taken where the glacier was excessively crevassed.

3d. This motion goes on day and night, and if not with

12 FIRST LETTER ON GLACIERS. [1842.

absolute uniformity, at least without any considerable anomaly.
On the 28-29th June the motion

from 6 P.M. to 6 A.M. was 8.0 inches,
.. 6 A.M. to 6 P.M. ... 9.5 ...
29th-30th, ... 6 P.M. to 6 A.M. ... 8.5 ...
... 6 A.M. to 6 P.M. ... 8.9 .».

seeming to show a greater motion during the day.

4th. Tn the particular case of the Mer de Glace, the higher
part (the Glacier de Lechaud) moves slower than the lower
part near the Montanvert in the proportion of 3 to 5.

5th. The central part of the glacier moves faster than the
edges in a very considerable proportion ; quite contrary to the
opinion generally entertained.

There cannot be a doubt of the accuracy of these results
within the limits in which the experiment has been made.
They prove how completely problems of a purely physical cha-
racter admit of accurate investigation ; and when a larger
induction shall have freed the results from local errors, it is
evident that we shall have the solid foundations of a theory.
My wish to see the total eclipse of the sun on the 8th, has
brought me to the south side of the Alps sooner than I could
have wished ; but I have now fixed so many points on the Mer
de Glace, that, on my return thither, I shall be able to obtain
more comprehensive results. But what is most important in
the whole matter is this — that an observer furnished with the
proper instruments and methods may, by spending a few days
on a glacier, determine at any particular season the amount of
its motion at all the essential points, within the limits which
any glacier theory can require.

1842.] LAWS OF MOTION OF THE MER DE GLACE. 13

III. SECOND LETTER on GLACIERS, addressed to
PKOFESSOR JAMESON.*

The Laws of Motion of the Mer de Glace farther stated.

CHAMOUNI, 10th August 1842.

My Dear Sir — Since I last wrote to you on the 4th July
from Courmayeur, I have examined, in detail, the two principal
glaciers of the Alice Blanche ; and having re-crossed the Alps
from Courmayeur by the Col du Geant, where I had the satis-
faction of still finding the remains of Saussure's Cabane of 1788,
I have pursued for a fortnight my experiments on the motion
of the Mer de Glace. Being composed, as you know, of several
tributaries which are in some degree independent, and presenting
also a considerable variety of surface, this glacier seems as
proper as any for detailed experiments, such as those which I
am attempting. Being about to quit this place on a tour to
Monte-Rosa and the glaciers east of the Great St. Bernard, I
wish to explain to you now in what respect my observations
differ from those formerly undertaken on the glaciers, and to
mention a few results, which, of course, being as yet only
partial, ought not to be considered as altogether decisive of the
truth or falsehood of any theory ; still I believe it will be
admitted that the facts established in my last (and which farther
experience has confirmed), militate strongly against some of the
received opinions as to the cause of glacier motion.

You are aware that, in my lectures on glaciers in December
and January last, and in an article in the Edinburgh Review
for April, I insisted, and so far as I know it was for the first
time, on the importance of considering the glacier theory as a
branch of mechanical physics, by which I mean that the cause

* Edinburgh New Philosophical Journal, October 1852. [The coincidence of
a few expressions with those in the First Letter will be pardoned when it is recol-
lected that these letters were printed precisely as written from the scene of the
observations described, and that any comparison of one letter with another was
impossible.]

14 SECOND LETTER ON GLACIERS. [1842.

of movement should be ascertained inductively from the observed
motion, carefully and numerically ascertained at different points.
It is because authors have considered the problem as too simple
a one to require a systematic analysis, that we find little or
nothing done in this respect ; and it may be affirmed, without
any disrespect to the ingenious persons who have assigned pro-
bable causes for the movement of these masses of ice, that their
solutions have been, like the astronomical theories of the earlier
cosmogonists, based upon somewhat vague analogies with better
understood phenomena, as when the analogy of magnetic attrac-
tions seemed to offer a parallel to the mechanism of the heavens
in the theory of Gilbert, and that of fluid currents gave rise to
the Cartesian vortices. The Newtonian theory was based upon
its coincidence with the empirical laws of planetary motion.
We have as yet no empirical laws of glacier motion, conse-
quently no proper mechanical theory can as yet be adequately
tested. I endeavoured to point out in my lectures how a
mechanical theory might be deduced from observation, and how
these observations might be practically made. I believe that I
have also obtained, for the first time, the numbers on whose
importance I insisted. I am not aware that any one had
hitherto proposed to determine the diurnal velocity of a given
point of a glacier with reference to three co-ordinates. The
analogy with the empirical laws of astronomy is both striking
and just ; an exact acquaintance with the path described by any
molecule of a glacier, will almost as certainly lead to a know-
ledge of the cause of its motion, as the theory of gravitation
sprung from the three laws of Kepler. We have to deal,
indeed, with an effect more complex and varied ; but the results
contained in my last letter already show how much of numeri-
cal precision may be attained. I have already determined the
diurnal motion of 10 points of the Mer de Glace with a probable
error, not exceeding, I think, a quarter of an inch in any case ;
and when these observations shall have been pursued, as I
expect to do, until the end of September, there will be a toler-
able basis for sound speculation.

1842.] LAWS OF MOTION OF THE HER DE GLACE. 15

In particular, you will recollect that I pointed out last winter
two experiments for distinguishing between the prevailing
theories of De Saussure and De Charpentier, — those of gravita-
tion and of dilatation. One was the exact measurement of a
space along the ice to be measured after a certain time, in order
to ascertain whether any expansion had occurred. The other
was the determination of the linear velocity of the glacier at
any point, which, on the theory of Saussure, ought (if the
glacier be of nearly uniform section) to be uniform throughout ;
on the theory of Charpentier it ought to increase from nothing
at the upper extremity of the glacier, to a maximum at its
lower end. The former experiment had, I have since learned,
been suggested by Professor Studer to M. Escher last year, and
attempted to be put in practice (though unsuccessfully) by the
latter, on the glacier of Aletsch. Admitting Charpentier 's
theory, however, this dilatation would be too small to be suc-
cessfully observed in a moderate time, and with the geometrical
methods which the uneven and varying surface of the glacier
enables us to employ ; I have therefore not attempted it. The
other method, in fact, embraces both ends ; for if the movement
of the glacier in its upper and lower part be determined (by
upper I mean near its origin), the difference of the motions
determines the dilatation or contraction of the intermediate pail
of the ice, and is liable to none of the great errors arising from
the measurement of long distances. The observation, in the
simplest and best form which I employ, resembles perfectly that
of determining with the transit instrument the progress of a
planet.

I have already said that my later observations confirm those
which I previously communicated ; any variations, indeed, arise
solely from a change of circumstances or season, and not from
errors of observation. (1.) The continuous imperceptible
motion of the glacier is entirely confirmed ; its bearing upon the
sliding theory is very obvious. (2.) This motion is not by
any means the same, however, from day to day and from week
to week, as indeed already appeared from my first results. (3.)

16 SECOND LETTER ON GLACIERS. [1842.

This variation of motion appears to be common to every part
of the glacier, as well where compact and completely even, as
where most fissured ; nor perhaps is the variation of velocity
greater in one case than in the other. (4.) From numerous
observations, made in all parts of the glacier, it invariably
results as before, that the centre moves faster than the sides of
the ice-stream. In the lower and faster-moving part of the
glacier, this disproportion is greatest, varying from one-third to
one-half of the smaller velocity. Near the origin of the glacier
it appears to be one-fourth or one-fifth of the smaller velocity.
(5.) The variations of glacier motion affect the central parts most
sensibly. (6.) The greatest daily motion which I have observed,
nearly opposite the Montanvert, amounts to 27.1 inches. (7.)
I have ascertained the velocity of motion much nearer the
origin of the glacier than when I last wrote. This, which
would appear to be nearly, if not quite, an experimentum crucis
between the sliding and dilatation theories, does not yield a
result so favourable to the latter as I had at first supposed ; for
though it is undoubtedly true, as stated in my last, that the
head of the glacier moves slower than the foot, the middle part
moves rather slower than either, owing probably to the greater
width and thickening of the ice there. This source of error,
from the varying section of the glacier, I had fully anticipated ;
but still, when we push the experiment to a limit, and take the
velocity very near the origin, the velocity ought to diminish,
on the theory of Charpentier, with a rapidity not to be mis-
taken. Yet very near the head of the Glacier de Lechaud,
the diurnal velocity is considerably more than a foot per day.
I am far, however, from thinking that I am yet in a position to
judge finally of the merits of any theory ; my belief is, that both
of those cited will as yet require great modification.

By insisting upon the treatment of the problem as one of
pure mechanics, I am far from denying that the kind of inves-
tigations to which the glacier theorist have hitherto almost
exclusively referred, are also of great value, such as those on
the temperature and structure of the ice. The latter, in par-

1842.] THEORY TO INCLUDE BOTH STRUCTURE AND MOTION. 17

ticular, is a sort of standing evidence of its mechanism, and,
rightly understood, must lead to the most important confirmation
of any mechanical theory. This, you may believe, I have made
an object of very particular attention. I have now examined
so many glaciers as to have a very clear idea of the empirical
laws which that structure follows. Lately, I begin to perceive
a connection between that structure and the facts of motion
already cited. If these two classes of facts can be well brought
into harmony with one another, we should have a very good
chance of consolidating them into something like a theory. In
my next letter, I will give you some account, at all events, of
my observations on the subject, which are sufficiently definite,
and probably also (without considering it as proved), of what
seems likely enough to be its true explanation. I go to-morrow
to the Great St. Bernard, to meet M. Studer. — Believe me, very
sincerely yours.

IV. THIRD LETTER on GLACIERS, addressed to
PROFESSOR JAMESON.*

The Laws of Structure of Glaciers generally detailed — The " Dirt-Bands " described.
Mechanical Theory of the Structure — Dirt-Bands correspond to Annual Inter-
vals— Glaciers probably move in Winter.

ZERMATT, North Side of Monte Eosa,
22d August 1842.

My Dear Sir — I arrived here two days ago by a very
interesting and unfrequented route. I mentioned in my last,
that M. Studer and I had agreed to visit together the valleys
eastward of the great St. Bernard. The Convent was our
place of rendezvous, and we afterwards descended to Orsieres,
and turned into the valley of Bagnes. Crossing the Alpine
chain at the head of the valley, by the Col de Fenetres, we
went down to Valpelline on the Italian side, and ascended that

* Edinburgh New Philosophical Journal, October 1842.
C

18 THIRD LETTER ON GLACIERS. [1842.

valley quite to its origin. We then crossed to the western
branch of the valley of Erin, by the Col de Collon or Arolla,
a very striking glacier pass. Thence M. Studer went to the
Val d'Anniviers, and rejoins me here by the way of Yisp, whilst
I ascended the other branch of the Eringer Thai from Evolena
by way of the Ferpecle glacier, and crossed over the mountains
to this place, by a pass higher and much longer than the Col
du Geant, which presents, certainly, the grandest views I have
hitherto met with in the Alps. I must not, however, stop to
describe, as my present object is to fulfil the promise in my
last respecting the structure of glacier ice.

The internal veined or ribboned structure presented by all
glaciers in a greater or less degree, appears to be the only true
essential structure which they possess, and which, you will
recollect, I described in a paper printed in your Journal for
January last.* The existence of granules divided by capillary
fissures, as well as of large crevasses, are equally unessential to
glacier structure, and subordinate to the other. Whatever
other result may flow from the examination of glaciers this
summer, by the many persons who are probably at this moment
directing their attention to them, this, I am sure, will be ad-
mitted, that the veined structure is not peculiar to some glaciers,
as some would maintain, nor to some years, as has been alleged
by others ; but that it is perfectly general and systematic, having
one general type or form, which is varied according to external
mechanical circumstances. Being then the most essential and
intimate part of the glacier formation, as well as one of its most
obvious and universal features (especially on those glaciers which
are most commonly visited), it is equally singular that it should
not have been sooner noticed, or if noticed, never once alluded
to by the eminent and ingenious authors who have treated of
existing glaciers and their effects.

With respect to the general type or form of this structure,
I am happy to say that I have found not the slightest reason
to modify the description which I have given in the paper above

* [See page 1 of this volume.]

1842.] FORMS OF SURFACES OF STRUCTURE. 19

alluded to, of the conformation of the glacier of the Rhone.
The description is characteristic, not of that glacier only, but of
every other, with certain modifications similar to the variation
of the parameter of a curve ; variations, therefore, not in kind
but in degree. The most beautiful structure I have ever met
with is in the glacier of La Brenva, in the Allee Blanche, which
was one of the earliest I examined this season, and in which I
found all that I had seen, though imperfectly, on the glacier of
the Rhone (which it resembles in the circumstances of being
derived from an icy cascade, and in having a considerable
breadth in proportion to its length), developed in a manner so
clear and so geometrically precise, as gave me the most lively
satisfaction. I refer to my former paper for the figure and
description of that structure ; I have found the same conoidal
surfaces, and the same false appearance of horizontal stratifica-
tion on the terminal face of the glacier, arising from the veins
dipping inwards at first at an angle of only 5°, rising to 10°,
20°, up to 60° and 70°, if we follow the medial line of the
glacier, or axis parallel to its length. The sides of the glacier,
in like manner, have their cleavage planes or veins dipping
inwards towards the centre at an angle determined by the
declivity of the rock or moraine which supports them, gradually
becoming more vertical as the centre of the glacier is ap-
proached, where they twist round by degrees, so as to become
transverse to its length, and to form part of the system of planes
dipping inwards first described. Fig. 1 exhibits a section parallel
to the length. Fig. 2, a transverse section.

Fig. 1. Fig. 2.

You are already aware that this structure consists in the
alternation of more or less perfectly crystallized ice in parallel
layers, often thinning out altogether like veins in marble, not

20 THIRD LETTER ON GLACIERS. [1842.

unfrequently parallel and uniform like a ribboned calcedony or
jasper.

I will, for brevity, merely state the modifications which this
fundamental type undergoes, bringing together glaciers of all
classes, but reserving the detail of examples and proofs, of which
my experience has already furnished me with a great number,
to another occasion. If a glacier be long and narrow, as the
Lower Aar, or the Mer de Glace of Chamouni, the frontal dip
is the least conspicuous part of the phenomenon ; and if it ter-
minate in an icy cascade, as in the second case, it might escape
observation altogether. The vertical planes parallel to the
length, or nearly so, usurp nearly all the breadth of the glacier,
and^ only in the centre is a narrow space, where not unfrequently
the structure appears quite undefined. I have satisfactorily
made out, however, in every glacier which I have had the
means of examining^with that view, that the conoidal structure,
however obscured, exists in all parts of the true glacier, modified,
according to its length and breadth, in the manner which figs.
3 and 4 indicate. I need not add, that these rude sketches are

Fig. 3. Fig. 4.

ife

not intended to be considered as rigorously exact, but only to
explain generally my meaning.

There is yet another modification, but only a modification,
of the above, namely, in the case of extremely steep glaciers, but
which are coherent, and not crevassed into pyramids. There
are numberless examples of these in all the higher valleys of
the Alps, which do not descend into the hollows, but festoon

1842.] GLACIERS OF SECOND ORDER — DIRT-BANDS. 21

the steep sides of snowy mountains. They are, I believe, what
Saussure called glaciers of the second order, and have no relation
to neves, so far as I can attach a meaning to that term. They
are of hard ice, and almost invariably present an appearance of
stratification parallel to the soil on which they rest. This
stratification is only apparent ; the cleavage planes dip forwards
and outwards, instead of dipping inwards, as in the terminal
portion of glaciers of less inclination. The surfaces of crystal-
lization have, in this case, absolutely the form of a scallop-
shell, the lip or front being always inclined below the horizon.
I attach importance to the community of feature in glaciers of
every form and inclination, because it indicates that the origin
of the structure cannot be unimportant, considering its gene-
rality ; and in this particular case of small steep glaciers, it
appears, I think, that M. de Charpentier, who has justly denied
the stratification of glaciers in general, has wrongly admitted
the existence of strata in the case in question, which he regards
as formed by the intercalation of mud from the soil in some
manner, which, if I recollect rightly, he does not very clearly
describe. Now, these seeming strata of mud I have examined
in a multitude of cases, and found invariably to result merely
from the percolation of dirt from the moraine, sometimes even
accompanied by small fragments of rock, into the more spongy
arid less crystalline veins of the glacier mass which already
existed ; and the proof is that, by cutting with a hatchet, we
gradually gain the pure ice, equally veined with the exterior,
but not discoloured. I may observe, in passing, that the fissures
which, in the lower part and near the sides of glaciers, form
the granules, about which so much has been written, are stopped
by the independent formation of the veins in the ice, which thus
demonstrate their prior origin.

One afternoon I happened to ascend higher than usual
above the level of the Mer de Glace, and was struck by the
appearance of discoloured bands traversing its surface nearly
in the form indicated in fig. 4. These shades, too indistinct to
be noticed when near or upon the surface, except upon very

22 THIRD LETTER ON GLACIERS. [1842.

careful inspection, are very striking and beautiful when seen at
a distance by a light not too strong, as in the afternoon, or by
moonlight. They are evidently bands of dirt on the surface
of the ice, having nearly the form of very elongated parabolas
merging in the moraines on either side, widest apart from one
another in the centre, and confounded towards the edge. For
some time I was at a loss to conceive how these sort of false
moraines could spread from side to side of the glacier, but I at
length assured myself that it was entirely owing to the struc-
ture of the ice, which retains the dirt diffused by avalanches
and the weather on those parts which are most porous, whilst
the compacter portion is washed clean by the rain, so that these
bands are nothing more than visible traces of the direction of
the internal icy structure, and of course correspond
with what has been already stated as to the forms
in which the conoidal surfaces intersect the plane of
the glacier. I counted distinctly sixteen of these
bands on the surface of the ice then in view. I
afterwards traced them to the higher part of the,
ice-field ; and the only distinction which I there
observed was, that the loops of the curves were, less
acute, or more nearly circular, fig. 5. All glaciers
do not shew this external evidence of their structure
equally, as there are some glaciers which possess the structure
itself more developed than others. The cause of the dazzling
whiteness of the Glacier des Bossons at Chamouni is the com-
parative absence of these layers of granular and compact ice ;
the whole is nearly of uniform consistence, the particles of rock
scarcely find a lodgment, the whole is washed clean by every
shower. The superficial bands are well seen on the Mer de
Glace of Chamouni, and, to quote another example, one of the
last I have seen, very admirably on the glacier of Ferpecle in
the valley of Erin, where I counted above thirty in view at
once.

I am quite persuaded that these bands, and of course the
structure which they represent, have their origin in the move-

1842.] STRUCTURE THE RESULT OF UNEQUAL PROGRESSION. 23

inent of the glacier ; and if the laws of movement, ascertained
independently, shall coincide with, or confirm, the phenomena
of structure, we shall be better able, from the comparison of the
two classes of facts, to decide upon the cause of movement.

What I have hitherto stated is matter of fact. I will state
very briefly what I am disposed to deduce by way of hypothesis.

It is impossible to consider these structural bands on the
surface of the glacier, in combination with the fact established
in my former letters, that the centre of the glacier moves con-
siderably faster than its edges, without believing that the bands
are an indication of the motion, and that the motion gives rise
to the veined structure. These dirt-bands perfectly resemble
those of froth and scum which every one has seen upon the
surface of slowly-moving foul water ; and their figure at once
gives the idea of fluid motion, freest in the middle, obstructed
by friction towards the sides and bottom. It will be found
that the analogies are entirely favourable ; the glacier struggles
between a condition of fluidity and rigidit}^ It cannot obey
the law of semifluid progression (maximum velocity at the
centre, which is no hypothesis in the case of glaciers, but a
fact), without a solution of continuity perpendicular to its sides.
If two persons hold a sheet of paper, so as to be tense, by the
four corners, and one moves two adjacent corners, whilst the
other two remain at rest, or move less fast, the tendency will
be to tear the paper into shreds parallel to the motion ; in the
glacier, the fissures thus formed are filled with percolated water,
which is then frozen. It accords with this view — 1. That the
glacier moves fastest in the centre, and that the loop of the
curves described coincides (by observation) with the line of
swiftest motion. 2. That the bands are least distinct near the
centre, for there the difference of velocity of two adjacent stripes
parallel to the length of the glacier is nearly nothing ; but near
the sides, where the retardation is greatest, it is a maximum.
3. It accords with direct observation (see my last Letter), that
the difference of velocity of the centre and sides is greatest near
the lower extremity of the glacier, and that the velocity is more

24 THIRD LETTER ON GLACIERS. [1842.

nearly uniform in the higher part ; this corresponds to the less
elongated form of the loops in the upper part of fig. 5. 4. In
the highest parts of such glaciers, as the curves become less
bent, the structure also vanishes. 5. In the wide saucer-shaped
glaciers already spoken of, which descend from mountain-slopes,
the velocity being, as in shallow rivers, nearly uniform across
their breadth, no vertical structure is developed. On the other
hand, the friction of the base determines an apparent stratifica-
tion, parallel to the slope down which they fall. 6. It also
follows immediately (assuming it as a fact very probable but
still to be proved, that the deepest part of the glacier moves
slower than the surface), that the frontal dip of the structural
planes of all glaciers diminishes towards their inferior extremity,
where it approaches 0, or even inclines outwards, since there
the whole pressure of the semifluid mass is unsustained by any
barrier, and the velocity varies (probably in a rapid progression)
with the distance from the soil ; whilst, nearer the origin of the
glacier, the frontal dip is great, because the mass of the glacier
forms a virtual barrier in advance ; and the structure is com-
paratively indistinct for the same reason that the transverse
structure is indistinct, viz., that the neighbouring horizontal
prisms of ice move with nearly a common velocity. 7. Where
two glaciers unite, it is a fact that the structure immediately
becomes more developed. This arises from the increased velo-
city, as well as friction of each, due to lateral compression.
8. The veined structure invariably tends to disappear when a
glacier becomes so crevassed as to lose horizontal cohesion, as
when it is divided into pyramidal masses. Now, this immediately
follows from our theory ; for so soon as lateral cohesion is de-
stroyed, any determinate inequality of motion ceases, each mass
moves singly, and the structure disappears very gradually.

I might add more illustrations ; but let these suffice for the
present. It is not difficult to foresee, that, if my view should
prove correct, a theory of glaciers may be formed, which, with-
out coinciding either with that of Saussure or Charpentier, shall
yet have some thing in common with both. Whether that of

1842.] GLACIERS NOT STATIONARY IN WINTER, 25

M. Rendu may not avail something, I am unable to say, not yet
having been able to procure his work.

It yet remains to decide, what is the cause of the succession
of dirt-bands at considerable distances on the surface of the
glacier, indicating the succession of waves of more or less com-
pact ice. In all the glaciers where I have yet distinctly
observed them, they appear to follow a regulated order of dis-
tances, nearly the same for a considerable space, but closer the
farther we ascend the glacier. I cannot help thinking that
they are the true annual rings* of the glacier, which mark its
age, like those of a tree, only increasing instead of diminishing
in breadth as the ice grows older, coinciding again with the fact
which I formerly established, that the higher part of a glacier
moves, generally speaking, more slowly than its lower extremity.
The different states of the glacier at different seasons, the
presence or absence of snow, or even the simple difference of
velocity at different seasons, would be sufficient to account for
this alternation of structure. There is no cause so likely to
produce it as some annual change. I may add, that ^some
observations which I have already made on the distances of
these bands, as well as information which I have endeavoured
to collect, lead me at least to have some doubt as to the correct-
ness of the opinion generally entertained, that the glaciers are
stationary in winter, perhaps even, that there is any very great
inequality in their march at different times of the year. — I am,
my dear Sir, yours very truly.

* [Originally printed annular rings, by a typographical error. See Editor's
Note, Edin. New Phil. Journal, April 1843, p. 382.]

26 FOURTH LETTER ON GLACIERS. [1842.

V. FOURTH LETTER on GLACIERS, addressed to
PROFESSOR JAMESON.*

Change of Level and General Appearance of the Glacier in the month of September —
Collapse of the Walls of the Crevasses— Moulins — Veined Structure — Direction
of the Crevasses — Both Veined Structure and Crevasses renewed from time to
time — The Motion of a Glacier resembles that of a Viscid Fluid— Objections to
the Dilatation Theory — What the Cold of Winter may be supposed to accom-
plish.

GENEVA, 5th October 1842.

My Dear Sir — Since my last letter from Zermatt, I have
had an opportunity of examining the glaciers on different sides
of Monte Rosa, particularly those of Lys and Macugnaga, and
those near the Valley of Saas ; and on my return to Chamouni
early in September, I devoted a day to each of the glaciers of
Trient and Argentiere, before resuming my station at the
Montanvert, where I remained until almost the last days of the
month.

What I think it most interesting now to add, as supplemen-
tary to my former statements, is not a description of these
various glaciers, but with particular reference to the Mer de
Glace, to mention what the extended period of examination
which I have been able to give to it, has enabled me to con-
clude beyond what is contained in my previous letters, respecting
the Theory of glacier-movement generally. Having accurately
observed the condition and motions of this glacier throughout
by far the greater part of the season at which it, or indeed any
glacier is easily accessible, or sufficiently free from snow for
accurate observations, — having also, especially during the month
of September, observed it under every circumstance of weather
and a great range of atmospheric temperature, I believe that I
have obtained the chief data necessary for basing a theory of
its motion, upon sound mechanical principles. The changes
which I have witnessed upon its surface, during the period of

* Edinburgh New Philosophical Journal. Jan. 1843.

1842.] THE HER DE GLACE AT THE END OF SUMMER. 27

above three months during which I have studied it, are so great
and remarkable, and in some respects so unexpected, as to be
of capital importance in any theory which may be proposed.

I was very greatly struck with the change in the general

appearance of the glacier during my absence, from the 10th

August to the 10th September. I left it comparatively high

and tumid in the centre, at no great depth below the arrete of

its natural boundary, the moraine by its side ; and fissured by

crevasses, deep and rather narrow, with well defined vertical

walls. — On my return, the icy mass had most visibly sunk in

its bed ; it seemed to me to have a wasted, cadaverous look ;

the moraines protruded far higher than before from its sides,

and the ice itself clinging to the moraine at a considerable

height above its general level, was covered by the fallen masses

of stone and gravel which had rolled down the inclined plane

formed by this central subsidence. The whole resembled

somewhat the Wye, or some of those narrow tidal rivers whose

muddy banks are left exposed by the retreat of the ocean.

That this subsidence was in a good measure occasioned by the

melting of the ice in contact with the bottom of the valley in

which it lies, and by the falling together of the parts in a soft

and yielding state, owing to a complete infiltration of the

whole mass with water during the warm season of the year,

was proved by a variety of circumstances which I shall not stop

to detail. I may mention however, that the crevasses were

wider but less deep and regular — excessively degraded on the

side to which the mid-day sun had free access, and in many

places where several crevasses nearly joined, the icy partitions

had sunk gradually towards a level, and thus rendered the

fissured parts of the glacier more easily traversed than at an

earlier part of the season. It is plain, too, that the fact of the

more rapid advancement of the centre of the glacier mentioned

in my earliest letter, implies a subsidence of that part, and a

consequent drain from the lateral ice, to supply the vacuity

which it leaves.

It will at once be understood that the change of which I

28 FOURTH LETTER ON GLACIERS. [1842.

speak in the external figure of the ice, its crevasses and ine-
qualities, is an effect due to the season, and must be repeated
every year. Were the summer considerably prolonged, the
annihilation of the glacier would take place from a simple con-
tinuation of the process, namely, the increased velocity of the
central part, the exaggeration of the crevasses in width, and
the falling of their walls, or rather the gradual subsidence of
the elevations, softened by the warmth, into the hollows which
separate them, whilst the moraine would be left in all its
continuity as a witness of the original boundary of the glacier.
The ice must possess within itself some reproductive power (if
the phrase may be permitted), to restore it in spring to the
level from whence it had descended ; and since crevasses thus
form, extend, and again vanish — perhaps in a single season,
but certainly in a very few years — we must consider the
glacier as a much more plastic body than it has commonly been
imagined.

I state it, then, as a result of observation the most direct,
that, in the early part of summer, the glacier level is highest,
and the fissures least numerous. The latter form and widen
especially during the months of June and July ; and, in the
beginning of August, the glacier is most difficult to traverse
(generally speaking), owing to the multitude and sharpness of
these cracks ; but later, the prolonged sunshine and autumnal
rains not only reduce the ice to water, and thus carry off a
part of its surface, but leave the remainder in a softened and
plastic state, in which the tendency is to a general subsidence
of all the elevations, whilst the prolonged excess of velocity of
the central above the lateral parts, causes an increased hollow-
ness and subsidence there, and produces a great fissuring, the
lateral ice still clinging to the moraines, which it is compelled
gradually to uncover. Before spring, by some process which
it remains to explain, the level of the ice is restored (supposing
the glacier not to be permanently wasting).

Another mode of considering the successive conditions of a
certain portion of the glacier, will lead also to the admission of

1842.] PERMANENCE OF THE " MOULINS." 29

the ever- varying state of its aggregation and subdivision. In
a glacier, like the Mer de Glace of Chamouni, which presents a
great many and well-marked "accidents" of surface in its
different parts, it is yet perfectly well known, that, though
continually moving and changing, the distribution of these
" accidents" is sensibly invariable. Every year, and year after
year, the water-courses follow the same lines of direction, — their
streams are precipitated into the heart of the glacier by vertical
funnels called "moulins" at the very same points; the fissures,
though forming very different angles with the axis or sides of
the glacier at different points of its length, opposite the same
point are always similarly disposed, — the same parts of the
glacier, relatively to fixed rocks, are every year passable, and
the same parts are traversed by innumerable fissures. Yet the
solid ice of one year is the fissured ice of the next, and the very
ice which this year forms the walls of a " moulin," will next year
be some hundred feet farther forward and without perforation,
whilst the cascade remains immovable, or sensibly so, with
reference to fixed objects around. All these facts, attested by
long and invariable experience, prove that the ice of the glaciers
is insensibly and continually moulding itself under the influence
of external circumstances, of which the principal, be it remarked,
is its own weight affecting its figure, in connection with the
surfaces over which it passes, and between which it struggles
onwards. It is, in this respect, absolutely comparable to the
water of a river, which has here its deep pools, here its constant
eddy, continually changing in substance, yet ever the same in
form.

With reference to the yet more essential modifications of
structure, I mean the veined structure which I formerly de-
scribed ; I shewed in my last letter, that it is equally mutable
and subjected to the momentary conditions of external restraint ;
and that, far from being an original structure in the higher part
of the glacier, variously modified in its subsequent course, but
never annihilated, it owes its existence at any moment to the
conditions of varying velocity in different parts of the transverse

FOURTH UCTTWl ON ULAOIKHN, [1842.

of tho glador, and that it in not unfroquontly ontiroly
doMtroyod in one, part of tho glaoior, to bo renewed in n totally
d.iieront direction iu another, A molecule of ice is a« passive
and NtruoturoloMN a unit an a molecule of water, to far ad it IUIH
not that Ntructuro improved by Momothing external at the time,
hike tho walor in die river, wyriatlN Ninverd ,.,,,- another, and
MM hi 1 10 iniMluhon for (ho MIUH

Kow WOIH!M will MM (Hoe to ihew how intimately what 1 havn
dUted ii connected with tho lirni nidiiuonU of a thoory of glacier
motion, wluoli 1 ondoavottrod to Nkctolt in my IUH(. lottor, and
ilio truth of whidi all that 1 havo ninro MOOH has Imdrd •.••r«t;iil\
to ..'ni.in. Th<< oontro of tho glacior ntroant in urgod onwafdl
by proHMuro from abovo (how oausod wo nhall immodiatoly ron-
id'1!1), which in tlioro n»HiMt.od IOMH llian .n ilt> id< . and liolloiu,
nwiny; h» |.ho rumpamtivo ubmMu'o of iVirlion. Thr lai.-nil purl.';
ilt dl'IVggOll oii\vaid'. liy lli«- UK. lion t>!' (hr tMMilrr. and inovr

H!MO, but it in quito oompatiblo with tin !•!•• i ->i ..•nnilmd mntion,
tliut tho bottom of the glaoior nhould romain Iroxon to iu* bod,
a« Nomo writorn havo Ntipponod to bo ihr rase, i hough I am fur
from aborting this to bo tho faot, or ovon NuppoHtng it probablo.
Why, thon, aro tho tinNuroN gt»norally wtfmt, and also \\IUMV .
glaoior i» mont rogular, nimply tmnsn-rsr. and not convex
towardn tho lower oxtromity ? Tho tlrnt of thom^ quoNt.ionn
had alwn\ nil latol^ appoaral to mo a MoriouM dillioulty, 'I'lu-
/.'/.-/ i.i.ji.-d iu ill.- ...-i-iMid, 1-ond'Mn-d \\itli iln> poNJiivc owtainly

tluit- tho OOntl'O of a j-danrr nin\r. I'a'.i.M ill. in H-. ides, in (lie
ralio IVripu-nllv ol' .> l.» ."», .lu-\\ . dial an answt-r inusf !•»• lonnd,

and, thoroforo, that it olVorn no innurmountablo objootlon. Tho
explanation tN to bo nought in tho oontinuftlly varying condition
«>l tho glaoior, tho porpotual ronowal of tho rrovaMHo?.. d.. action

of wator in tond'mg to preserve verlicalilV, .ind llie r.-ally small
variation of Volooity of dillnvnl pail-. i>l' Ihc iro lowardw ill.-
,-rnlro o!' a ",laci«M •>!' itniiMMiNo depth. I'Yoin llir?;e circnm •

ntanooH, it followN (hai i > \ fiso JH oither renewod or altogothor
.-\i n paird l.eforo iU vortioality IH nonnibly allootod. For tho
•MOM Ninon, a Mick novorai foot long, innortod vortioully in tho

IS 12. 1 I'.MAUINO OF TIIK < H'-SKl! V ATluNS 1'1'oN KXISTlNd TIIICOIUKH. 31

ice, remains sensibly vortical so long as it stands at all ; for
the velocity of the surface is sensibly the same as that at 10 or
20, or probably even 100 feet deep in most glaciers. It is only
near the bottom or bed that the velocity is materially affected,
as I have found also, that, in respect to breadth, it is in the imme-
diate neighbourhood of the sides that the velocity diminished
rapidly, and that, for half its breadth in the centre, the
velocity does not vary by more than from -j^ to -fa of its
amount. It is farther worthy of notice, that whenever a glacier
is of no great thickness, and, at the same time, highly inclined,
is, in circumstances calculated to produce a great difference
the motions of points of the glacier in a vertical line,
there the fissures are not transverse but radiated, as in almost
all glaciers of the second order, and, therefore, the fissures are
not liable to distortion.

I might put it rather as a direct result of observation than as
;i hypothesis, that the motion of a glacier resembles that of a
viscid fluid, not being uniform in all parts of the transverse
section, but the motion of the parts in contact with the walls
being determined mainly by the motion of the centre; but it
yet remains to be shewn what is the cause of the pressure which
conveys the motion, whether il, i,s the mere weight of the He.mi-
II 1 1 id mass, or the dilatation of the head of the glacier pushing
onwards. The answer to this question involves the fate of the
rival theories of Do Saussuro and De Charpcntier. 1 still
entertain the same difficulties with respect to both, which I have
stated in an article in the Edinburgh Review ; but these diffi-
culties amount, I think, to a proof of insufficiency, if taken in
connection with the observations which I have made this sum-
mer. On the one hand, if it were possible that the glaeier
could slide by the mere action of gravity in a trough inclined
only 3, or 4, or 5 degrees, it is probable that one of two things
would happen ; either it would slide altogether with an accele-
rated velocity into the valley beneath, or else it would move
by fits and starts, being stayed by obstacles until these were
overcome by the melting of the ice beneath, or by the aecumu-

32 FOURTH LETTER ON GLACIERS. [1842.

lated weight of snow above and behind. Now, neither of these
things happen ; the glacier moves on day and night, or from
day to day, with a continuous regulated motion, which, I feel
certain, could not take place were the sliding theory true.

But if possible, still stronger, as well as more multiplied,
objections are to be found to the theory of dilatation, and I
trust I shall not be accused of levity in thus, as it were, in a
few lines, dismissing a theory which has so much primd facie
plausibility to recommend it, and which has been maintained
with so much ingenuity by men such as Scheuchzer, De Char-
pentier, and Agassiz. It is essential to the aim of this letter,
that I state briefly the grounds of the conclusions at which I
have arrived, whilst it is equally essential that my observations
should be confined within small compass. In another place I
shall give them all the development that may be requisite.

Summarily, then (1.) The motion of the glacier, in its
several parts, does not appear to follow the law which the
dilatation theory would require. It has been shewn (Ed. Rev.,
April 1842, p. 77) that the motion ought to vanish near the origin
of the glacier, and increase* continually towards its lower
extremity. I have found the motion of the higher part of the
Mer de Glace to differ sometimes very little from that several
leagues farther down ; whilst in the middle, owing to the
expansion of the glacier in breadth, its march was slower than
in either of the other parts. (2.) Whilst I admit that the
glacier is, during summer, infiltrated with water in all or most
of its thickness (a point on which I had last year great doubts),
I feel quite confident that, during some months of the year
during which the glacier is in most rapid motion, no congelation
take* place in the mass of the ice beyond a depth of a very
few inches, much less during the cold of each night, and least
of all, at all times, as appears to be now the opinion held upon
the subject. Whilst I say that I am confident of this, I will
state one proof. Less than ten days since I traversed the Mer
de Glace up to the higher part of the Glacier de Lechaud,
whilst it was covered with snow to a depth of six inches at

1842.] DILATATION THEORY REFUTED. 33

Montanvert, and three times as much in the higher part. It
was snowing at the time, and for a week the glacier had been
in the same state nearly, the thermometer having fallen in the
meanwhile to 20° Fahr. Yet I had abundant evidence that
the effect of the frost had not penetrated farther into the ice
than it might be expected to have done into the earth under
the same circumstances. All the superficial rills were indeed
frozen over; there were no cascades in the "moulins;" all
was as still as it could be in mid-winter; yet even on the
Glacier de Lechaud, my wooden poles, sunk to a depth of less
than a foot in the ice, were quite wet, literally standing in
water, and consequently unfrozen to the walls; and in the
hollows beneath the stones of the moraines, by breaking the
crust of ice, pools of unfrozen water might be found almost on
the surface. Is it possible, then, that the mere passing chill of a
summer night, or the mere cold of the ice itself at all times, can
produce the congelation which has been so much insisted on ?

But (3.) What was the effect of the congelation, trifling as
it was, upon the motion of the glacier ? So sharp and sudden
a cold succeeding summer weather, must inevitably, it seems to
me, were this theory true, have produced an instantaneous
acceleration of the mean motion of the glacier. But the con-
trary was the fact ; the diurnal motion fell rather short of its
previous value, and [no sooner was] the severe weather past,
and the little congelation which had taken place thawed, and
the snow reduced to water, than the glacier, saturated in all
its pores, resumed its march nearly as in the height of summer.

(4.) It has been inferred from the dilatation theory, that
whilst the surface of the glacier continually wastes, it [is] at the
same time heaved bodily upwards from beneath, so that its
absolute level is unchanged. My experiments, as well as the
most ordinary observation (as has been already remarked),
disprove this hypothesis. I find that, between the 26th June
and the 16th September, the surface of the ice near the side of
the Mer de Glace had lowered absolutely TWENTY-FIVE feet 1/5
inches, and the centre had undoubtedly fallen more. The

D

34 FOURTH LETTER ON GLACIERS. [1842.

observation of the waste of the surface by the protrusion of a
stick sunk to a determinate depth in a hole, is very inaccurate,
and gives results Mow the truth.

I ain perfectly ready to admit, with M. de Charpentier, that
the congelation of the infiltrated water of glaciers is an import-
ant part of their functions ; only, I conceive that it occurs but
once a year to any effective extent, instead of daily or
continually, as he supposes. Every thing which I have seen
on the glacier, during cold weather and when covered with
snow, confirms the idea I have always entertained, that the
progress of congelation in the mass of the glacier is very similar
to that of a mass of moist earth, and that, therefore, the daily
variations of temperature can make no sensible impression, with
respect to the mass of the infiltrated ice. The prolonged cold
of winter must, however, produce a very sensible effect ; and
considering that the temperature of the mass is never above
32°, it may be expected that the congelation of the water in
capillary fissures in ice will, in the course of months of tran-
quillity, reach a great depth. I apprehend that there is only
an annual congelation, and that its effect is not to move the
glacier onwards by sliding down its bed — for that the friction
of so enormous a body seems evidently to render impossible —
but (what Mr. Hopkins has very well shewn is the only alter-
native, and which he has used as an argument against Char-
pentier's theory) to dilate the ice in the direction of least resist-
ance, that is, vertically, and consequently to increase its
thickness. The tendency of such a force would, therefore, be
to restore during the winter the thickness of ice lost during the
summer; and in those winters which are less severe, a less
depth of ice being frozen, a less expansion would occur, and a
permanent diminution of the glacier would result. Nothing can
be more certain than the fact, so well stated by Charpentier in
his 10th section, that the glacier does not owe its increase to
the snow of avalanches, nor indeed to any snow which falls on
the greater part of its surface.

In conclusion, the admission of semifluid motion produced

1844.] MOTION OF A SEMIFLUID. 35

by the weight of the ice itself, appears to explain the chief
facts of glacier-movement, viz. (1.) That it is more rapid at
the centre than at the sides. (2.) For the most part, most
rapid near the lower extremity of glaciers, but varying rather
with the transverse section than the length. (3.) That it is
more rapid in summer than in winter, in hot than in cold
weather, and especially more rapid after rain, and less rapid
in sudden frosts. (4.) It is farther in conformity with what we
know of the plasticity of semisolids generally, especially near
their point of fusion. Many examples will occur to every one
of what they have observed of the plasticity of hard bodies —
such as sealing-wax, for example — exposed for a long time to
a temperature far below their melting heat, and which have
moulded themselves to the form of the surfaces on which they
rest. (5.) When the ice is very highly fissured, it yields
sensibly to the pressure of the hand, having a slight determi-
nate play, like some kinds of limestone, well-known for this
quality of flexibility. (6.) I have formerly endeavoured to
shew how such a condition of semirigidity, combined with the
determined movements of the glacier, accounts for the remark-
a,ble veined structure which pervades it.*

VI. FIFTH LETTER on GLACIERS, addressed to the
Right Honourable EARL CATHCART.t

Motion of the Mer de Glace in the year 1842-3 — Winter Movement — The Veined
Structure reproduced at the foot of an Ice Cascade — Cuts the Medial Moraines —
Wrinkles in the Ice of the Glacier du Geant and that of Grindelwald, probably
corresponding to the position of the Dirt-Bands — An Ancient Moraine at

Chamouni.

ROME, 29th January 1844.

My Lord — In reply to your kind letter of the 14th Decem-
ber last, requesting me to communicate to the Royal Society
any observations upon glaciers which I was enabled to make

* [See page 23.]

f Edinburgh New Philosophical Journal, July 1844. [Read to the Royal
Society of Edinburgh.]

36 FIFTH LETTER ON GLACIERS. [1844.

during last summer, I may mention, that the state of my health
was so indifferent during the finer months of the year, and the
caution which it required so great, that I was quite unable to
prosecute, as I had hoped, the subject of my previous inquiries
in Switzerland. As, however, the journey was not quite unpro-
ductive, I will very shortly state the additional facts which I
was enabled to observe, claiming from you and from the Society
the indulgence which their scantiness requires.

At Chamouni, the most obvious consideration was to deter-
mine the actual annual motion of the ice, the partial motions
of which during the summer months had been carefully ascer-
tained by me, as stated in my former communications. For
this purpose, I had two marks of a permanently distinguishable
kind, namely, blocks of stone lying on the surface of the ice ;
the one, formerly marked D 7, and referred to in my Travels
by that name, situated a little lower than the position of the
Montanvert ; the other, marked C, or " Pierre platte," on the
Glacier de Lechaud, near its junction with the Glacier du Geant.
It was the former of these masses which had been approximately
observed in position by my guide, Auguste Balmat, during the
winter of 1842-3, with great labour and fidelity — observations
which first conclusively proved the fact which I had previously
suspected,* although opposed to the received opinions — that the
glacier moves with considerable velocity even in winter. By
going to the spot with Balmat, and verifying the marks which
he had from time to time made, I ascertained that his measure-
ments, if not absolutely correct, did not admit of being
materially improved, owing to the great size and repeated turn-
ing over of the block in question. His measurements between
October 1842 and June 1843 have been published in the
volume already cited.f I had the mortification, however, to
find, on the llth September 1843, when I visited the block,
that though still upon the ice, it had got shoved so near the
moraine of the glacier near an angle of its course, as to be well
nigh stranded ; and that, in fact, since Balmat's last mark in

* [See page 25.] f [Travels in the Alps, 1st edition, p. 151.]

1844.] STATE OF THE HER DE GLACE IN 1843. 37

June, its motion had been scarcely perceptible. It farther
appeared, that the part of the glacier with which it had recently
been moving was so crevassed and steep, that the vast block
must have rolled and tossed about, or even been precipitated
occasionally forwards by the failure of the ice beneath it on the
steep, in a way which amply accounts for any want of regularity
in its winter progress, as indicated by Balmat's measurements.
It therefore became the more interesting and important to de-
termine with care the motion of a point of the glacier removed
from the accidental local influence of the sides and irregularities
of the surface, in order to compare the mean annual motion with
the summer motion of the ice. The " Pierre platte " was, in
every way, an unexceptionable landmark ; and I resolved to
cross the Mer de Glace for the purpose of accomplishing it —
an exertion which I should hardly have ventured for a less inte-
resting result. In the course of this walk, which was fraught
with interest to me, as enabling me to compare the existing
condition of a glacier with the appearances which had been so
familiar to me just twelve months before, I found the state of
the ice just such as might be expected after a very severe and
snowy winter, and a very cold and late summer. The glacier
opposite the " angle " (station A), had now a much higher level
than it had at the same time in 1842 ; evidently, therefore, it
had, during the winter, regained its usual volume ; and then,
during the ensuing summer, it had wasted less than it had done
during the summer before. The glacier also bore other testi-
mony to the same circumstances ; for the crevasses were far
sharper and better defined, and the whole appearance of the
ice less collapsed, than at the same season in 1842. The sur-
face also at the " angle " was extensively covered with the
unmelted snow of the winter, which, as I have often observed,
never admits for a moment of being confounded with the matter
of the glacier. The general direction and appearance of the
crevasses, and of the position of the " moulins," was the same
as in 1842 ; as if the glacier had remained at rest, though it
had really moved some hundred feet forwards. - The moraines

38 FIFTH LETTER ON GLACIERS. [1844.

were unaltered in appearance, only perhaps less prominent (at
least, this was the remark of the guides), which would naturally
arise from the less superficial waste of the ice.

There was no difficulty in recognising the " Pierre platte,"
which, indeed, had recently slid off an ice pedestal similar to
that of the preceding year (as figured in the frontispiece to
my volume of Travels), but far less stupendous. As all the
marks in the rock which I had at different places cut with a
pick, and painted red, were as visible and fresh as on the day
they were fixed, there was no difficulty in recovering, to a
nicety, the exact position of the block on any day on which it
was observed in 1842, and comparing it with its new position.
Accordingly, referring to the starting point on the 27th June
1842, I found that it had moved, down to the 12th Sep-
tember 1843, or in 442 days, 320 feet, that is, 8.7 inches
daily. I have not now my own work to refer to ; but 1
believe it will be there found, that the motion of the " Pierre
platte," during the hottest summer months, was only between
9 and 10 inches at a mean.* It is plain, therefore, that during
the remainder of the year (throughout the greater part, or nearly
the whole of which, this part of the glacier is covered with
snow), the motion, though somewhat diminished, was very far
indeed from ceasing, — thus entirely confirming the observations
of Balmat, near the lower end of the glacier.

Finding that my strength and the time permitted, I pur-
sued my excursion up to the level of the Jardin, opposite the
glacier of Talefre, near the Aiguille du Moine. I had a lively
satisfaction in comparing my engraved map with the natural
features of the country, and finding it a tolerably faithful repre-
sentation ; and I checked everywhere, with minute care, the
definition I had given of the direction of the ribboned structure
of the ice at different parts of the glacier. The observations
formerly made I found to be rigorously exact ; and especially
these two facts, which at once put an end to any idea of the
ribboned structure being a prolongation or deformation of the

* [See Travels in the Alps, p. 140.]

1844.] NEW DISCOVERY RESPECTING THE DIRT BANDS. 39

strata of the Neve ; viz. (1.) the structure assumed by the ice
of the Talefre is extirpated entirely by its precipitous descent
to the level of the Glacier de Lechaud. where it reappears, or
rather is reconstructed out of the bruised fragments, according
to a wholly different scheme ; (2.) the veined structure often
cuts the medial moraines, i.e., a glacier composed of two,
having originally distinct looped structures, assumes finally, after
being for some time united, a single looped structure.

From the heights above the Egralets, which command a
most extensive bird's-eye view of nearly the whole Mer de
Glace, about 2000 feet below, I was led to make a very inte-
resting observation, — on the whole, the newest of the season.
I need not remind your Lordship, that I first observed, in 1842,
the existence of certain wave-like marks on the surface of the
Mer de Glace, figured in my map of that year, and represented
in the models submitted to the Royal Society last winter. These
waves, or " dirt-bands," as I termed them, were parallel in their
course to the veined or ribboned structure of the ice, and
recurred at pretty regular intervals upon the surface of the
glacier, — the loops pointing in the direction of its motion, — at
an average distance, as I think, of between 600 and 700 feet.
(The exact value is stated in my book.)* I was prevented, by
a premature fall of snow, from tracing these bands (which I also
termed " annual rings") higher up on the glacier than the point
called Trelaporte. Standing, on the 12th September last, above
the precipices of the Couvercle, at the foot of the Aiguille du
Moine, as above mentioned, I not only saw with admirable dis-
tinctness the " dirt-bands" between Montanvert and Trelaporte
delineated, as it were, upon a plan ; but I was enabled to count
six new ones higher up in the direction of the Glacier du
Geant. Then followed a space corresponding to three intervals
of dirt-bands, which were, however, not perceptible. Higher
up, on the Glacier du Geant, was a most striking and beautiful
appearance, quite new to me. The heavy snow of the previous
winter had not been entirely melted during the whole summer,

* [711 feet,— Travels, 1st Edition, p. 165.]

40

FIFTH LETTER ON GLACIERS.

[1844.

and still lay in all the hollows where it could accumulate. A
series of snowy bands having this origin appeared at regular
intervals upon the upper part of the Glacier du Geant. corre-
sponding in distance and form to the arrangement of the " dirt
bands" in the lower part of the glacier, as I have endeavoured
to represent below : — thus ascertaining a most curious and un-

Rocks.

Fig. 6.

suspected fact, namely, the existence of a series of curvilinear
hollows on the nearly plane surface of the ice, which the eye
would probably have in vain striven to detect, but for the pal-
pable evidence of the accumulations of snow lodged in the inter-
vals of these vast waves. In fig. 6 the ground-plan of a part
of the Glacier du Geant is shown, where it is divided longitu-
dinally by the medial moraine descending from the Aiguille
Noire. The portion of the icy stream descending from the
ridge, called " les Periades," bears that name. The masses of
dots indicate the position of the snow wreaths which mark the
indentations of the ice ; these appeared to be confined to the ice
of the proper Glacier du Geant ; the lines indicate the direction
of the most distinct veined structure in the ice, which are visible
in a mass from a distance, as the finely veined structure of
Cipollino marble is,' even when the laminae composing it cannot
be individually seen. The Conclusion from this is, that the
surface of a glacier is not an inclined plane, nor an inclined

1844.] WRINKLES OF THE ICE — GLACIER OF GRINDELWALD. 41

convex surface, but that it is indented by furrows or wrinkles,
which would give a section like fig. 7, in a direction parallel
to the length of the glacier A B, fig. 6, the dots here indicating
the snow wreaths as before, and the broken lines approximating
to the direction of the frontal dip of the veined structure in
this part of the glacier. These periodical undulations (to use a
technical scientific term) are, beyond question, very important
in the theory of glaciers generally, and of the dirt bands in par-
ticular. Their existence will, probably, be thought to yield
some confirmation of the theory of semifluid motion of the ice,

Fig. 7.

even although their precise origin be still obscure. A homely
comparison, but a striking one, may be found in the wrinkles of
the horns of many animals ; and, though it may appear fanciful,
there is perhaps more than a vague analogy between the facts.
I hope, at some future time, to offer a better elucidation.

After some stay at Chamouni, I proceeded, by easy journeys,
to Grindelwald, whose beautiful and easily accessible glaciers
I had not visited for many years. I there found an exact
confirmation of the views which I have published respecting
the origin and structure of glaciers. I found the forms, simple
and compound, of the ribboned structure, to be such as I have
described, including the gradual rise of what I have called the
" frontal dip," from a very low angle near the lower end of the
glacier up to 75° towards its origin, especially on the Glacier
of the Strahleck, above the Zasenberg, where it forms the Mer
de Glace of Grindelwald. The other principal affluent of that
icy sea, viz. the glacier descending from the Viescher horner,
exhibits the same wrinkles exactly as those which I have just
described upon the Glacier du Geant, perhaps even better
marked. As the lower glacier of Grindelwald furnished an
excellent example of all the modifications which I have elsewhere

42 FIFTH LETTER ON GLACIERS. [1844.

shown to belong to the canal-shaped glacier, with branches ; so
the upper glacier is an exact representative, in its lower part,
of the oval glacier, for which I have taken that of the Rhone
as a type ; whilst many of the tributary glaciers of Grindelwald
and the Jungfrau bear ample testimony to the general fact, that
the structure of glaciers is developed during their progression,
and after their primitive stratification has been annihilated,
by their being projected in avalanches over appalling preci-
pices.

To these brief notes, I have only to add one interesting
discovery, though of a somewhat local importance, which I
made at Chamouni. The ancient lateral moraine of the Glacier
des Bois is acknowledged by De Saussure, and all subsequent
writers, to be found in the barrier of debris which crosses the
valley of Chamouni, at Les Tines ; but very feeble traces have
(I believe) been observed of the corresponding lateral moraine
of the left bank of the glacier, excepting those between the
Chalet of Montanvert, and the descent of La Filia. I have
ascertained, however, that a good part of the ascent to the
Montanvert, and especially near the chalets of Planaz, passes
over a vast accumulation of debris, whose nature corresponds
to that of the granites of the central chain, and which lies to
an immense thickness against the rocky slopes of the valley,
at the foot of the Aiguille de Blaitiere. The resistance offered
by this mass of debris to the progress of the torrents, which
descend from the glaciers of Grepon and Blaitiere towards
the Arve, has diverted their course in a direction parallel to
that of the valley of Chamouni, and it was the observation of
this singularity which led me to the detection of the moraine
first mentioned, which I could hardly believe had escaped me
so long.

1844.] ANALOGY OF LAVA STREAMS TO GLACIERS. 43

VII. SIXTH LETTER on GLACIERS, addressed to the
Right Honourable EARL CATHCART.*

Analogies of Glaciers to Lava Streams — Observations on Mount Vesuvius —
Moraines of Lava Streams — Some Objections to the Plastic or Viscous Theory
of Glaciers considered — Verticality of Crevasses accounted for.

EOMB, February 5, 1844.

My Lord — In a letter which I addressed to you on the
29th ult., I gave some account of the few new observations
which untoward circumstances permitted me to make, last
autumn, upon the glaciers of Switzerland and Savoy. I have,
howrever, had leisure to reflect maturely upon the theory of
glaciers, which I have been occupied for two years in endeavour-
ing to mature ; and, without pretending to find in it a complete
solution of every problem which might be proposed respecting
these wonderful bodies, I am perfectly satisfied that it is funda-
mentally conformable to the laws by which they are governed.
Some new analogies, to which your Lordship has referred in
your last letter, such as that between glaciers and lava streams,
may serve to render the subject more popularly intelligible ; and
in explaining them, I may have an opportunity of removing, in
some degree, the difficulties which have arisen in the minds of
candid and intelligent persons, who have studied this theory for
the first time — difficulties which would probably disappear of
themselves by a more prolonged attention.

I have not had the advantage of seeing the eruption of
Etna, to which your Lordship alludes, which was indeed over
before I arrived at Naples, and of which I did not even hear
for a considerable time after ; so small is the sensation which
such events excite in the country. I have, however, had an
opportunity — probably not less favourable, though far less im-
posing— of studying the mechanism of plastic lava, in the small
currents which, during the months of November and December,

* Edinburgh New Philosophical Journal, October 1844.

44 SIXTH LETTER ON GLACIERS. [1844.

were very frequently flowing from mouths within the crater of
Vesuvius. On the 30th November, in particular, I descended
to the bottom of the crater, in order to examine a current of
very liquid lava, fifteen or twenty feet wide, which issued from
a cavity near the foot of the small cone which .occupied the
centre of the crater, and from whose top (in the shape of an
inverted funnel, or of a blast furnace) there issued smoke and
flames,* occasionally accompanied by a discharge of volcanic
projectiles. The lava issued in a very steady rapid stream, and
spread itself over a gentle declivity with a velocity of not less,
I think, than a foot per second.

Admitting the plastic or viscous theory of glaciers, the
resemblance to lava fails (1.) In respect of the great liquidity
of the lava near its source ; (2.) From its very unequal rate of
consolidation ; a crust being very soon formed upon the surface,
which becoming more and more massive, the principle of fluidity
is not uniformly distributed throughout the mass, as in the
glacier, but a tolerably perfect fluid struggles with the increasing
load of its ponderous crust, which it tears and rends by the
mighty energy of hydrostatic pressure ; and here and there
finding a freer exit far removed from its source, tosses high
those mighty fragments of the stony arch which confined it into
the wild shapes which strike the eye in crossing the wastes of
a lava stream, and which seem at first incompatible with the
fluid or semifluid principle of motion. This second circum-
stance, then, — the very unequal and rapid superficial consoli-
dation of the lava near its source, — has no analogy in a glacier,
nor even in a river, unless when breaking up a ponderous crust
of ice after a sudden thaw. The regulated progression of the
glacier, swiftest in its centre, and with a graduated retardation
towards the sides, has a much more precise analogy to that of
a river than the lava stream has, which is subdivided (when it

* I am able to add my distinct testimony to that of M. Pilla, as to the emission
of flames by the crater of Vesuvius. I spent part of the evening of the 1st January
on the top, and had not the least doubt that what I saw were actual flames, which
issued from time to time from the orifices of the small cone, and which were of a
pale colour, often inclining to blue.

1844.] ANALOGY OF LAVA STREAMS TO GLACIERS. 45

has any considerable breadth) into many little currents, each
rolling past, and being retarded by its more sluggish or already
consolidated neighbour ; so that its surface resembles that of the
bed of many torrents in the Alps, where the more solid matters,
the rocks, stones, gravel, sand, and clay, trace out the form of
a sluggish mass propelled downwards by gravity, whilst its
surface is seamed by the trickling of innumerable rills of water,
charged with the more portable materials which have been
washed down, or squeezed from the general mass.

There are other circumstances, however, in which the
analogy of the glacier with the lava stream is more complete ;
and of these I will observe —

I. That the cracks of the dark-coloured slag on the surface
of the liquid lava, as it spreads itself abroad, on issuing from
the fiery mouth, are radiated exactly as those of a glacier under
similar circumstances, and which I have represented in the
margin as I saw them on Vesuvius, the lines of fissure being

Fig. 8.
Fissures in the Crust of Lava during Crystallization.

marked by the liquid fire shining through. A perfect analogy
here exists with the phenomena of radiating fissures in ice, which
I first described in the glacier of the Rhone, and afterwards in
the ice of the Glacier du Talefre, where it joins the Glacier de
Lechaud, in the Glacier of Arolla, and very many other in-
stances.

II. That the slags, where solidified, presented strice or
ripple-marks along their surface, parallel to the direction of the

46 SIXTH LETTER ON GLACIEKS. [1844.

" ribboned structure " of glacier ice, i. e.9 inclining slightly from
the sides towards the centre of the current, in the direction in
which the current is moving. These striae, or ripple-marks,
which have a striking analogy in certain cases of the retarded
movement of rivers, are carefully to be distinguished, on the
one hand, from the cracks or flaws, and, on the other, from the
direction of motion of the fluid particles.*

III. When, at some distance from the source, the lava
became viscid and tenaceous, and forced itself, in streamlets of
a pasty consistence; through the interstices of its slag, thence it
became streaky and drawn out, in the direction last mentioned,
as molten glass does in the hands of the workman.

IV. But there is a more striking analogy to the ribboned
structure of glacier ice, to be found in lava currents at a distance
from their origin, and where by any circumstance their surface
has been broken up, and their internal structure exposed. In
the Fossa della Vetrana, for instance, and other places, I have
found the lava divided into thin layers parallel to the interior
of the surface of the channel through which it flowed, evidently
produced by the adhesion or retardation which the soil exerted
upon its adjoining film of lava, and the successive portions of
lava upon one another, in proportion as the semifluid mass,
rolling upon its own particles (or rather sliding imperfectly over
them), produced a solution of continuity and a series of shells,
parallel in direction to the bed upon which the whole rests.
The thickness of these shells varies from one-third of an inch
upwards. I have never, however, observed a structure in the
interior of the lava except that parallel to the sides and bottom
of the canal in which it moves ; nothing, in short, corresponding
to the frontal dip in glaciers. But this is quite natural and
conformable to the very different constitution of a glacier ; and,
in particular, it corresponds to the fact so often urged as a diffi-
culty to the semifluid theory of glaciers, namely, the want of

* A long accidental delay in the printing of this letter enables me to add, that
I have found in the lavas of Etna a yet far more perfect analogy to the veined
structure of glaciers than that described in the text. It is, indeed, so completely
developed as to leave no doubt as to the identity of origin.

1844.] VEINED STRUCTURE IN LAVAS. 47

ductility or tenacity of their parts. It is that 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
and leaves a bruise, within which the infiltrated water finally
freezes and forms a blue vein. In the lava, on the other hand,
where the tenacity is great, the discontinuity, if produced at all,
is soldered up by the plasticity of the parts, whose small crystal-
line structure farther tends to obliterate the separation. The
layers just mentioned, parallel to the bed, are perhaps produced
by the successive adhesion of warmer streams of lava to the
colder parts already deposited, and, consequently, their analogy
to the glacier structure must not be pushed farther than as
showing the directions of the tendency to separation of a very
viscid stream, powerfully retarded by its bed. It is the con-
gealing of the lava which makes its adhesion to the sides great
enough, and its own fluidity small enough, to bear a comparison
with the far less ductile body of a glacier. In the heart of the
mass where the same intestinal motions take place (as I have
shown conclusively by using coloured layers of plastic matter in
the models formerly exhibited to the Royal Society), the dis-
placed particles reunite and consolidate into a homogeneous
mass without any trace of dislocation.*

V. The convexity or concavity of a semifluid stream like a
current of lava or of a glacier, depends entirely upon the rela-
tions or conditions in which it is placed. Upon the same slope,
a fluid of one degree of consistence will run off in a concave
stream, whilst a more viscid one, which must accumulate in
thickness, in order to overcome the resistance in front (just as
water which meets a sudden obstacle), rises into a convex curve.
This is perfectly seen in the case of a substance like plaster of

* The following passage from M. Dufrenoy's Account of Vesuvius, is interesting,
if it were only as recording his remark, that the variation of velocity in different
parts of a stream must produce longitudinal striae. " La plupart des coulees pre-
sentent des bandes longitudinales assez paralleles entre elles ; ces larges stries sail-
lantes sur la surface sont les traces du mouvement de la lave qui ne s'avance pas
d'une seule piece, mais par bandes paralleles." — Sur les Environs de Naples,
p. 324.

48 SIXTH LETTER ON GLACIEIiS. [1844.

Paris, mingled with water, whose consistence may be varied at
pleasure, and a stream of which may be made either concave or
convex, or concave at its origin and convex at its termination,
as is the case with a glacier. The evidence on this subject,
afforded by the models formerly laid before the Royal Society,
is so complete and conclusive, that, however interesting it might
be to put into a mathematical form the relations of the constants
of the effect of gravity, the viscosity of the body, and the retar-
dation of the sides, as affecting the form of the surface, it is
sufficient for my present purpose to appeal to facts so familiar,
and experiments so easy, that their evidence may well be pre-
ferred to the more casual and embarrassed case of lava streams,
which, as I have already observed, are seldom or never to be
regarded, on a great scale, as simple moving masses. I may,
however, add, that when the inclination is small, the surface is
convex, at a certain distance from the origin.

VI. There is a circumstance attendant on the motion of
lava streams, which has struck several geologists, before the
viscous theory of glaciers had been proposed — I mean the ex-
istence of moraines. The moraines of lava are best seen in
the more defined and united lava streams on rather a small
scale, — those, in short, which have the unity and character of
a proper stream, moving at once in its various parts. The
moraine is composed of stranded masses of lava crust, thrown
aside by the liquid fiery stream, and partly, perhaps, of the
yielding matter of the bed of the stream pressed outwards and
upwards by the hydrostatic pressure of the centre. The former
is chiefly, perhaps, the case when streams of tolerably fluid lava
flow down a steep inclination, as on the exterior of the cone of
Vesuvius ; the latter, when the inclination is small and the
weight of accumulated lava great. The igneous moraines,
though noticed by various geologists, are most emphatically
described by M. Elie de Beaumont, in his masterly memoir on
Etna, in the following words : — " Une des circonstances que les
coulees de lave presentent le plus invariablement toutes les fois
qu'elles ont parcouru des talus ou elles pouvaient acquerir une

1844.] LAVA STREAMS — HOW THEY TERMINATE. 49

certaine vitesse, caracteres que j'ai observes sur toutes sortes
de pentes depuis 33° jusqu'a 2° et que je n'ai cesse d'observer
que la ou les coulees se sont ^arretees faute de pente, consiste en
ce que chaque coulee est flanquee de part et d'autre par une
digue de scories accumulees qui rappelle par sa forme la moraine
d' un glacier ; digue que s' eleve constamment a une hauteur
superieure a celle a laquelle la coulee est reduite a la fin du
mouvement, et qui marque le maximum de hauteur qu'elle a
atteint dans le moment de son plus grand gonflement. Souvent
aussi les coulees presentent de pareilles digues vers leur milieu,
lorsqu'elles sont partagees en plusieurs en courants distincts
coulant Tun a cote de Fautre."*

VII. The termination of a lava stream on a level or slightly
inclined surface due to its increasing viscidity, presents appear-
ances almost identical with those of a glacier. The same pro-
tuberant convexity of surface, the same steeply-inclined sides
and front, and nearly the same ground-plan, all bespeak a simi-
larity in the circumstances of motion. I may add, that in some
experiments which I made some years ago upon the flowing of
melted iron in narrow channels, and upon small slopes, with a
view to illustrate some phenomena of lava streams, before I had
commenced a particular study of glaciers, I arrived at similar
results, and obtained the same convexity of surface which is
produced in the plaster models before cited.

It is very interesting to observe how many intelligent per-
sons have been struck with the similarity between glaciers and
lava streams, without, however, pushing the parallel beyond a
general resemblance. M. Elie de Beaumont, we have seen,
speaks of the moraines of volcanoes ; but in various parts of his
writings, as well as those of his colleague, M. Dufrenoy, we find
the mention of glaciers as continually suggested to his mind
when surveying the wastes of Etna and Vesuvius. One of these
passages is the following : — " L'ecorce superieure d'une coulee
separee de 1'ecorce inferieure et du sol sousjacent par une cer-

* E. de Beaumont, Recherches sur le Mont Etna, p. 184.
E

50 SIXTH LETTER ON GLACIERS. [1844.

taine epaisseur de lave liquide ou du moins visqueuse, se trouve
dans im etat comparable a celui d'un glacier, qui, ne pouvant
adherer au sol sousjacent a cause de la fusion continuelle de sa
couche inferieure, se trouve contraint a glisser;"* shewing that
the author then adopted the theory of Saussure (since ably
defended by Mr. Hopkins), in which the fusion of the ice by
the heat of the earth, might be said, in some sense, to float
down the superincumbent solid ; an opinion best controverted
by the fact which M. E. de Beaumont has since clearly brought
into notice, that under existing circumstances such fusion is
perfectly insigriificant.t

The writer of a popular Italian guide-book, Mrs. Starke,
is perhaps one of the first who indicated the striking general
resemblance of a stream of lava to a glacier. She describes the
former (which she saw during a small eruption of Vesuvius) as
" rolling, wave after wave, slowly down the mountain with the
same noise (?) and in the same manner, as the melting glaciers
roll into the valley of Chamouni ; indeed, this awful and extra-
ordinary scene would have brought to mind the base of the
Montanvert, had it not been for the crimson glare and excessive
heat of the surrounding scoriae. "J

Mr. Auldjo, the author of a Narrative of an Ascent of Mount
Blanc, and therefore acquainted with the appearance of glaciers,
has renewed Mrs. Starke's comparison in very similar expres-
sions, in a work more recently published upon Mount Vesuvius.
Captain Basil Hall has, if I mistake not, in more than one part
of his writings suggested the picturesque analogy of volcanoes
and icy mountains, the cradle of glaciers.

We have seen how far there is a real analogy between the
mechanism of these two terrible scourges of Almighty power —
the ice-flood and the fire-flood, both of which invade the homes
and the labours of man, with a force alike irresistible. But
to render the analogy more than apparent or poetical, it was

* Recherches sur 1'Etna, p. 177.

-J- Annales des Sciences Geologiques par Riviere.

| Starke's Travels. French edition, p. 311.

1844.] PLASTICITY OF GLACIERS — OBJECTIONS REMOVED. 51

required that several difficulties, very obvious, and seemingly
insuperable, should be removed ; and the chief of these was the
texture of ice compared to the texture of lava — the former
passing from a brittle solid into limpid fluid by heat, the latter
passing like sealing-wax through every intermediate degree of
viscidity. This difficulty could only be met by an exact deter-
mination of the question — how far a glacier is to be regarded
as a plastic; mass ? Were a glacier composed of a solid crystal-
line 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 pres-
sure horizontally, and whose parts might change their mutual
position, 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. Tt 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 as it is 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 communi-
cates 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, crackling mass of seemingly
rigid ice, in which it is, as it were, bound up.

But farther than this, the experiments first announced in
the earliest of these letters, shewed, that whatever be the con-
stitution of a glacier, and whatever be the cause of its motion,
THE FACT is, that it does not move like a solid body sliding
down a bed or channel, but that the velocity of each part of
its breadth is different. It was demonstrated by the most
clear and plain geometrical measurements, that whilst the
centre of a glacier moves 500 feet, the side of the glacier moves
only 300 ; consequently, the portions of ice which started
together soon part company, and the central molecule has com-

52 SIXTH LETTER ON GLACIERS. [1844.

pleted its course, or arrived in the lower valley, whilst the
other, which was its companion, has advanced only three-fifths

A

B C

D E F a

a

f

a

&

J g

c

d e

. it

g

i

J

c'

, e

f

c e

d

Fig. 9.

of the distance, or remains perhaps several miles behind.
Thus it has been shewn from multiplied measurements of the
most precise and accordant kind, that a series of stones or
marks being supposed to be laid across a glacier in the line
ABCDEFG; they will be found, after a certain time, in the
position abcdefg, after other equal intervals at a'b'cd'e'fg', and
at a%"c'd"e'f"g"t by which time it will be seen that the neigh-
bour particles have entirely changed their relative positions,
and that the mass can have no pretension to be called rigid,
but moulds itself after the manner that a fluid or semifluid body
does in like circumstances — the centre advancing fastest, and,
for some space in the centre, nearly uniformly, whilst the retar-
dation produced by the friction of the banks is most intense in
their neighbourhood, which is conformable to what we know of
the movement of viscous fluids. It is, therefore, no hypothesis,
but a simple statement of a demonstrated fact, that the, manner
of movement of the surface of a glacier is not such as is consistent
with the continuity of a rigid body, but that it coincides with the
manner of motion of a viscous or semifluid body. Whatever may
be the difficulty of conceiving the glacier to be a body thus
constituted, the fact admits of no doubt ; — the effects of forces

1844.] ICE MAY BE RENT AND REUNITED. 53

applied on a great scale to bodies, are the best and only con-
clusive proofs of their real constitution, and worth all molecular
theories and minute experiments put together.

If a body be really of a pasty consistence, ductile and plas-
tic like lava or tar, such transpositions taking place in the
interior of the mass are effected without any injury to the tex-
ture or continuity of the substance. With a degree less of
plasticity, a violent separation of the parts may take place, but
they will, by juxtaposition, soon reunite and take a new set.
With a degree more of rigidity, there must be a permanent
'bruising and rending of the parts, in order that a semirigid
body may assimilate in all its movements to a fluid. It must,
therefore, be considered as entirely confirmatory and explanatory
of the preceding statements of the seeming plasticity of a body
so fragile in its elements as pure ice, that the ice of glaciers is
found rent in many parts by the forces tending to dislocation,
and that, besides, it contains within itself a testimony to the
internal partial movements by which its total motion is effected,
in the veined structure already alluded to, occasioned by the
varying velocity of the adjacent icy strips A. a a a", B b b' l>",
etc. This structure is not exactly parallel to the direction of
motion of the ice, for reasons which I have elsewhere stated,
but which need not now be adverted to. My present object is
to shew, that the rigidity of ice, as a physical fact, cannot con-
tradict the mathematical evidence of the manner in which
glaciers do move, and that the seeming contradiction is recon-
ciled by showing, that the ice bears permanent traces of the
violent strain to which it is subjected, and of the actual bruising
and disseverment of its parts, producing a phenomenon otherwise
impossible to be explained.

I believe that it is during the progress of the glacier thus
subjected to a new and peculiar set of forces depending upon
gravity, and which remodel its internal constitution, by sub-
stituting hard blue ice, in the form of veins, for its previous
snowy texture, that the horizontal stratification observed in the
higher part of the glacier or nM, is gradually obliterated.

54 SIXTH LETTER ON GLACIERS. [1844.

If, as we cannot doubt, the slower motion of the glacier
near its sides be owing to the retardation which their excessive
friction occasions, there must necessarily be a retardation at the
bottom in a similar manner, and the surface of the glacier will
move faster than the strata in contact with the ground ; to
which it is even supposable, that, in some cases, they may be
entirely frozen. This retardation may, perhaps, be less than
the lateral retardation, because the slope of the valley in which
the glacier lies is probably more even, generally speaking, than
its breadth is regular. In fact, so great is the irregularity of
the ground-plan of any compound valley — so frequent the inter-
fering ridges or promontories, the bays formed by adjoining
tributary valleys — and so numerous the gorges or contractions
— that we cannot properly call the lateral resistance to the
onward motion of a glacier, friction, but rather a direct opposi-
tion to the exit of a solid body, which renders its plasticity
absolutely essential to its progression. Nevertheless, the infe-
rior slope of the glacier bed being also irregular, and its friction
great, must cause a retardation in the lower strata of ice, which
must be continually overtaken by the superior ones : and this
appears to me to be so plain and necessary a consequence of
the combination of facts which we have to consider, that per-
haps the direct proof of it would not repay the labour which it
would involve, which would be of the most serious kind ; — for
we must not expect to find the difference of velocity apparent
in the superficial strata, even to a considerable depth, since we
know that the retardation is a maximum near the sides and
bottom, and that, for the same reason, the motion of all the
central part of a glacier is nearly uniform, so will the motion
of all the part of the ice near the surface be nearly uniform.

These considerations suggest the explanation of a difficulty,
kindly suggested to me by a most competent judge, who ex-
pressed himself at the same time persuaded of the truth of the
viscous theory of glaciers. " How comes it, that, if the motion
of the different parts of a glacier diminishes from the surface to
the bottom, the * trou de sonde ' or bore, 140 feet deep, made

1844.] UPPER STRATA OF ICE MOVE ALMOST UNIFORMLY. 55

by M. Agassiz in the glacier of the Aar, is stated to have re-
mained vertical for a period of many weeks?" In the first
place, the fact of the verticality requires confirmation ; for it is
difficult to understand how, by means of a plummet, a hole 140
feet deep, and only 3 or 4 inches in diameter, could have its
verticality tested. Such bores, so far as I have seen them, are
more or less twisted, owing to the softness of the material, and
the method of working ; and it seems beyond all probability,
that a hole of such a depth constructed in the ordinary way,
should be either mathematically straight or vertical. I appre-
hend that the verticality alluded to by M. Agassiz, or his
coadjutors, is merely that of popular language, indicated by the
boring rods standing vertically outwards when plunged into the
hole, which, on account of their flexibility, would not be an
indication of the verticality of more than the upper twenty or
thirty feet of the bore at the most.*

Bat, even setting aside this important consideration, the
principle of the variation of velocity being chiefly confined to
the neighbourhood of the sides and bottom, and the compara-
tively quiescent and passive state of the central and superficial
part, seem sufficient to explain the facts within the reasonable
limits of error. The depth of 140 feet appears, from M.
Agassiz' s own observations, not to exceed ONE SIXTH, at most,
of the depth of the glacier of the Aar in that part. Now, let
ABC, etc., represent points in the vertical section of the
glacier ; then, from all that we know of the superficial motion
of glaciers, or of the parallel case of rivers whose velocity has
been ascertained at different depths, the velocities will vary
in some such manner as A a, B b, C c, etc., — the variation
being scarcely sensible at first, and very rapid at the bottom,
where the velocity may even be zero, if the curve be prolonged

* Since this passage was written, I have had an opportunity of referring to the
description of the experiments of Agassiz in the JBibliotheque Universelle ; and I find
that there is no evidence whatever of the continued verticality of the bore of 140
feet, which existed (to that depth), I believe, but a few days ; the observations of
continued verticality, such as they are, applied to small bores only, not exceeding
25 or 30 feet, which, of course, greatly increases the force of the reasoning in the
text. — Aug. 1844. [See also page 30 of the present volume.]

56 SEVENTH LETTER ON GLACIERS. [1844.

to the point h. But, supposing G to be the bottom vof the
glacier, it will be seen how insignificant may be expected to be

7

Fig. 10.

the variation of velocity between A, the surface, and B, one-
sixth of the depth, during the short period of a few weeks, or
even months.

VIII. SEVENTH LETTER on GLACIERS.* — ON THE

VEINED STRUCTURE OF THE ICE. Addressed to the Rev.
Dr. WHEWELL, Master of Trinity College, Cambridge.

Mechanical Considerations tending to explain the Forms of the Veined Structure
under different Conditions.

SALERNO, May 18, 1844.
****** *

You object that the shells produced by the rupture of the parts
of the ice caused by excessive friction should be all parallel to
the sides and bottom of the trough of the glacier, instead of being
inclined from the sides inwards and forwards towards the centre,
as in fig. 11, and from the bottom upwards and forwards, as in

Fig. 12.

Fig. 11.

fig. 12. You will find that I have endeavoured to explain this

* Edinburgh New Philosophical Journal, October 1844.

1844.] EXPLANATION OF THE VEINED STRUCTURE. 57

in the last chapter of my book of Travels ; but not having it
by me, I cannot refer you to the particular passages. The
point in question is undoubtedly the least obvious and most
difficult part of the theory, but as I have no doubt of its exact-
ness, it will have a proportionate weight in deciding in its
favour the opinion of persons accustomed to mechanical theories.
It would be difficult to bring it home to the apprehension of
ordinary readers ; and, for this reason, I have dwelt upon it,
perhaps, too shortly in the chapter alluded to.

You will readily admit, that if I shall demonstrate separate
reasons for the existence of each of the structures figured
above (the first a plan, the second a section), the result will
be the spoon-shaped structure which I have shewn to exist in
glaciers.

(1.) The tearing asunder of the particles of the glacier
owing to the friction of the sides is nearly but not quite, parallel
to the sides ; for this reason, that the lines of greatest strain are
determined, not merely by the force of gravitation which urges
the particles forwards, but there is a drag towards the centre of
the stream, in consequence of the greater velocity there.

^ Let A B be the side of the

*~~"~---.^ glacier, whilst the particle a

A^MMMI^M^MB II • . ..^ L _ ^— MM^MMM^B^^BMMM*.}}

a ^:V_7:::_-:=^C a moves to a , the central par-

~"B' tide Z> moves to I', which,

ft H

"~P owing to the cohesive bond
between a and £, must pro-

c- ' D duce a strain oblique to the

Rg' 13> axis of the glacier.

Or view the matter thus — the movement of the ice stream
(considered just now solely as respects its surface), is effected
against a varying resistance. The line of particles in the direc-
tion a a present a greater force of opposition to the movement
of the particle a, than the line of particles b 13 present to the
movement of I. This is owing to the lateral friction acting
more powerfully in retarding the first than the second ; conse-
quently the virtual wall of the glacier, or plane of complete

58

SEVENTH LETTER ON GLACIERS.

[1844.

resistance, will be no longer A B, but inclined (for the particle
a) in the direction A' B'.

If this reasoning require support from experiment, it is
easily had. I have described, in a foot-note to my last chapter,
the experiment of dusting powder upon a moving viscous
stream; and our friend Heath has now a specimen of the
result, shewing the lines of separation in the direction I have
stated. The same is remarkably shown in the case of a stream
of water, for instance, a mill-race. Although the movement of
the water, as shown by floating bodies, is exceedingly nearly
(for small velocities, sensibly) parallel to the sides, yet the
variation of speed from the side to the centre of the stream
occasions a ripple, or molecular discontinuity, which inclines
forward from the sides to the centre of the stream at an angle
with the axis, depending on the ratio of the central and lateral
velocities. The veined structure of the ice corresponds to the
ripple of the water, a molecular discontinuity whose measure is
not comparable to the actual velocity of the ice ; and, therefore,
the general movement of the glacier, as indicated by the
moraines, remains sensibly parallel to the sides.*

(2.) If I have explained myself distinctly as respects the

Fig. 14.

fissures produced by lateral friction, there will be little difficulty

* I have lately identified completely the planes of separation in the lava
streams of Etna, which correspond perfectly to those of the glacier, heing nearly
vertical at the sides, and directed slightly towards the centre of the stream.

1844.] SURFACES OF DISCONTINUITY IN A VERTICAL PLANE. 59

in applying the same reasoning to the resistance of the frontal
dip, exhibited in the second figure of this letter. When a
fluid, or semi-fluid, is very viscous, there is a great resistance
to its onward motion in the direction which gravity and the
fall of the bed prescribe. Let L M be the surface, N 0 the
bed of a glacier ; then the resolved force is usually considered
as acting on the particles m n, in the directions m m ', n ri ,
parallel to the bed. But if we reflect that, owing to the length
of the glacier, and the toughness or consistency in its mass, the
resistance of the line of particles n v is enormous, the plane of
complete resistance N 0 will virtually be twisted in the direc-
tion W 0', and the particle tends to be thrust forwards and
upwards, which will evidently produce the frontal dip.

(3.) Bat there is a peculiarity in the vertical plane which
did not exist in the horizontal one. In the case we first con-
sidered, the veined structure exists almost entirely in the neigh-
bourhood of the sides of the glacier, and is lost towards its
centre, being due to the influence of friction, which varies with
the distance from the side; the central part, efgh (fig. 11),
moving nearly uniformly, would cease to exhibit a linear ar-
rangement. The completion of the curve is due to the influence
of the curvilinear bottom, combined with the opposing mass of
the glacier in front ; and this
influence will extend to the very — ^^^ ,

surface, as a little consideration
will show. For, resuming the
construction of fig. 14, since a
vertical series of particles, m 1 . .
-m6 (fig. 15) are supposed to be
acted on by a force partaking of
the nature of hydrostatic pres-
sure, derived from a great eleva-
tion, each particle is ready to Kg. 15.
move onward in the direction in which the effective pressure
is greatest ; and it is plain, that, owing to the diminishing rela-
tion between the weight of the superincumbent particles and

60 SEVENTH LETTER ON GLACIERS. [1844.

the frontal resistance, the direction in which the particles will
tend to slide over one another, or to produce rents, will approach
verticality at the surface, and on the whole will, therefore, tend
to produce lines of discontinuity, such as N M.

(4.) Considering the glacier at different points of its length,

Fig. 16.

it is evident, by similar reasoning, that near the region of the
neve a the frontal dip will be all but vertical, because there the
horizontal resistance is enormous ; whilst at the lower end &>
where it tends to vanish, the shells will tend to parallelism with
the bed. It is needless to add, that the relative movements of
the particles over one another, producing discontinuity, are not
to be confounded with their absolute motions in the glacier,
exactly as under head (1.) I must, however, observe, that as
the tendency of any particle due to the hydrostatic pressure will
be to describe ultimately the whole curve N w4 M within the
glacier, this may account for some of the facts, or supposed facts,
which indicate a tendency in the ice to expel bodies engaged
in it, as well as the convexity of the glacier at all times, and its
remarkable rise of surface during winter.

Lastly r, The ablation of the surface of the glacier during its
descent from a to b (fig. 16) will tend continually to give the
observed elongated forms of the superficial bands, by cutting the
shells of structure obliquely.

1844.] GREAT GLACIER OF ALETSCH. 61

IX. EIGHTH LETTER on GLACIERS * addressed to PRO-
FESSOR JAMESON by PROFESSOR FORBES.

Observations on the Motion of the Glacier of Aletsch — Experiments on the Plas-
ticity of the Ice of the Mer de Glace, by Observing the Distortion of a Limited
Space of Ice devoid of Crevasses — Movement of a Glacier of the Second Order
8000 Feet above the Sea.

GENEVA, 30th August 1844.

My Dear Sir — The theory of glaciers has now reached that
point when it can only receive some material addition by the
multiplication of accurate measurements; and these measure-
ments must be conducted in the manner which will best discri-
minate between rival hypotheses, and, if possible, yield direct
instead of indirect proofs of each fundamental fact assumed. In
my former letters I have insisted sufficiently upon the import-
ance of the results which a system of nice measurement has
introduced into this branch of science, and their value to the
theorist who afterwards wishes to put numerical for unknown
quantities in his investigations ; I also showed that there is a
continuity and approximate constancy in the motions of glaciers,
which permits us to obtain, with certain precautions, in a few
days, better results than any one had previously acquired during
the lapse of months or years. I have now to announce to you
that I have pushed these measurements to a still greater degree
of minuteness, and with results which show that the methods I
have employed are trustworthy, and are able to afford the direct
solution of questions which at first appeared to admit of only
indirect or inductive proof.

Of this class, by far the most important appeared to be the
manner in which the glacier alters its form in such a way, and
to such a degree, as to suffer its central portion to descend
towards the valley with double or treble the velocity of its
lateral parts. Such, for instance, I have found to be the case
in the middle region of the great glacier of Aletsch, where its
inclination is small (about 4°), and where the continuity of the

* Edinburgh New Philosophical Journal, October 1844.

62 EIGHTH LETTER ON GLACIERS. [1844.

ice with the side wall is preserved without the interference of
large fissures. I there found that, whilst the velocity of the ice
at 1300 feet, or about a quarter of a mile, from the side, is 14
inches in 24 hours ; at 300 feet distant from the side it was but
3 inches in the same time ; and, close to the side, it had nearly,
if not entirely, vanished. Facts like this seem to show, with
evidence, what intelligent men, such as Bishop Rendu, had only
supposed, previously to the first exact measures in 1842, that
the ice of glaciers, rigid as it appears, has in fact a certain
" ductility " or " viscosity," which permits it to model itself to
the ground over which it is forced by gravity — and that, re-
taining its compact and apparently solid texture, unless the
inequalities be so abrupt as to force a separation of the mass
into dislocated fragments, such as it is well known that every
glacier presents, when the strain upon its parts reaches a certain
amount — as when it has to turn a sharp angle, or to descend
upon a rapid or convex slope.

The mutual action of the parts of the glacier, the drag
which the centre exerts upon the sides (and, by an exact parity
of reasoning, the top upon the bottom), seemed to me so ob-
vious, after measurement had proved their variable velocity, and
observation had shown that this was not necessarily accompanied
by a general dislocation of the mass — that I should scarcely
have thought of attempting a direct proof of the yielding and
ductile nature of glacier ice, had I not been favoured by Mr.
Hopkins with copies of his two ingenious papers on the subject
of glaciers, read to the Cambridge Philosophical Society on the
1st May and llth December 1843, which were put into my
hands here less than a month ago, by his friend Mr. Williamson.
1 there found it stated that there is " a necessity of proving, by
independent experimental evidence, that glacier ice does possess
this property of semi-fluidity or viscosity, if we would attribute
to that property the effectiveness of gravity, in setting a glacier
in motion." — First Memoir, p. 3.

Since Mr. Hopkins admits the fact of the swifter central
motion of the glacier, he must have recourse to some mechani-

1844.] MR. HOPKINS' DIFFICULTIES CONSIDERED. 63

cal explanation of the fact. This he does by assuming the
existence of vertical fissures, parallel to the sides of the glacier,
dividing it into a series of longitudinal stripes, whose adjacent
surfaces, according to him, slide over one another, arid, in the
case of a glacier forcing its way through a gorge, the lateral
portions are altogether arrested, whilst the central parts slip
down between them.*

These parallel stripes of ice are supposed by Mr. Hopkins
to be of considerable breadth, and to have no sort of analogy
with the ribboned structure, to which the readers of my earlier
letters will recollect that I have ascribed a similar origin, being
lines of discontinuity arising from the crushing of one portion
of the semirigid glacier past another. This Mr. Hopkins
regards as "no more possible than that a mass should per-
manently maintain a position of unstable equilibrium." The
veined structure of glaciers he considers to be unexplained, and,
in the present state of science, inexplicable.

Although the general absence of such a system of longitu-
dinal fissures as Mr. Hopkins has figured in page 14 of his First
Memoir, and the regularity and continuity of motion of the
glacier and of its parts, wholly inconsistent with the jostling of
huge masses of dislocated ice, might be considered as a sufficient
answer to this modification of the theory of De Saussure,
the consideration of this demand for a direct proof of the flexi-
bility of glacier ice led me to think of its practicability ; and I
shall now state what I have succeeded in doing, towards the
solution of this practical question in the only way in which it
admits of being treated, namely, by the assiduous observation of
the motion and change of form of a small compact space of ice
on a glacier. The Mer de Glace of Chamouni offers fewer fit
points for such an experiment than many other glaciers, since in
all its middle and lower portions the ice is excessively crevassed
near the sides. There is one spot, however, between the
" Angle " and Trelaporte, below the little glacier of Charmoz,

* [Mr. Hopkins' figures are reproduced in Plate II., figs. 1, 2, and are referred
to again, later in this volume.]

64 EIGHTH LETTER ON GLACIERS. [1844.

where the ice is extremely flat and compact for a space of about
seventy yards in width, and several hundred yards in length,
which is wholly devoid of open crevasses, and where I expected
to find the variation of velocity from the side towards the centre
very sensible, because the veined structure is there more per-
fectly developed than in any other part of the glacier. In this
anticipation I was not disappointed. The ice in question is
separated from the western moraine of the glacier by a space
deeply crevassed 50 or 60 yards wide. The entire breadth of
the glacier is here at least 800 yards. The central part has
great transversal crevasses due to the rapid descent of the
glacier where it sweeps round the promontory of Trelaporte
immediately above. There is no trace of longitudinal fissures
of any kind, except the true blue veined or ribboned structure,
which, as already mentioned, is here exceedingly developed ;
giving to the even part of the glacier already specified the
appearance of exquisite veined chalcedony of an aqua-marine
colour ; and the vertical plates of ice thus subdivided are so
distinct as to produce a true cleavage when the ice is broken by
a hammer or cut with an axe. When the glacier is wet, the
blade of a knife may be introduced to a depth of some inches
between the laminae, which are commonly not more than a
quarter of an inch apart.

I fixed in a line transverse to the axis of the glacier six
stations. Over the first of these the theodolite was regularly
centred, in order to observe the relative motions of the others,
which were respectively 30, 60, 90, 120, and 180 feet distant.
Finding that, even in the course of a single day, the accelera-
tion of the more central parts was evident, and the six points
in question formed a portion of a continuous curve, I subdi-
vided the first 90 feet from the theodolite into 45 spaces of 2
feet, each of which was marked by a perforation in the ice
into which short pins could be accurately fitted, and the de-
formation of this straight line of 90 feet in length was carefully
observed at short intervals. The errors of the original places
of the marks were determined by a simple but nice process, and

1844.] PLASTICITY OF ICE ASCERTAINED BY OBSERVATION. 65

their daily progress was similarly noted. I have now before
me the registers and also the graphical projections of the actual
places of this portion of the curve of flexure of the ice, cleared
of the errors arising from the movement of the theodolite, which
was itself placed upon the ice, which error was independently
determined. You will probably be surprised when I state, that
in seventeen days, the part of the glacier 90 feet nearer the
centre than the theodolite, had moved past the theodolite by a
space of 26 inches, and the intermediate spaces in proportion.
When I was reluctantly compelled to cease my observations on
the 45 marks, they had, in the course of six days, formed a
beautiful curve slightly convex towards the valley ; and as the
vertical wire of the theodolite ranged over them, their devia-
tions from a perfect curve were slight and irregular, nor was
there any great dislocation to be observed in their whole extent ;
proving the general continuity of the yielding by which each
was pushed in advance of its neighbour. During these six
days the 45th mark had shifted 10 inches ; and besides this
obliquity of the line of pins (= 31' 46"), they had a convexity
whose versed sine was about an inch. All this, viewed in
prospective with the theodolite, left no remaining doubt as to
the plasticity of the glacier on the great scale.

Lest, however, the convexity should have been too small,
in so short a time, to admit of measurement, I had provided
another test, in order to show that the progressive advance-
ment of the line of marks was due to the actual deformation
of the ice, and not to the mass of the glacier in this part
revolving round some fixed or moveable centre. For this
purpose, I fixed a mark in the glacier, 20 feet from the
theodolite, and in a direction perpendicular to the before-
mentioned line of marks. It was, therefore, seen from the
theodolite in the direction of the length of the glacier, and,
consequently, was not liable to displacement by its motion. I
measured, from time to time, the angle between this mark
and the several marks transverse to the glacier, and I found

F

66 EIGHTH LETTER ON GLACIERS. [1844.

that this angle became continually, and without any exception,
more and more obtuse. During seventeen days, it revolved
through an angle of about a degree and a half.

I reserve to another opportunity the publication of the
details of the measurements and the graphical ' projections,
which offer, when minutely examined, some interesting pecu-
liarities too long to specify. The main conclusion is, that even
the most compact parts of the ice yield to pressure, and that
where no fissures exist, there is a sliding of the parts of the ice
over one another, or else a plasticity of the whole mass. With
the abundance of blue bands before us in the direction in which
the differential motion must take place (in this case sensibly
parallel to the sides of the glacier), it is impossible to doubt that
these infiltrated crevices (for such they undoubtedly are) have
this origin, and are the main mechanism of the forward motion ;
but it occurred to me, on one occasion (the 23d August), to
obtain all but ocular evidence of the fact. Standing at the theo-
dolite with an assistant, we heard a dull noise in the ice within
a very few feet of us, attended (I think) with a slight tremor,
and followed by a rushing and hissing sound. As we were
very near the great crevasses of the moraine, it was, no doubt,
a subsidence of a portion of the glacier, and the rushing was
occasioned by the more rapid flow of the superficial streamlets
in the direction of increased inclination of the ice. I instantly
searched in all directions, but in vain, for the slightest evidence
of the fracture of the ice. All that I could see was, that where
the veined structure was best developed, innumerable air bubbles
escaped through the superficial water, which was slowly imbibed
in those parts where the strain had expanded the ice, and thus
enlarged the capillary fissures between the blue bands.

Mr. Hopkins 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 1 have
founded upon them. Had Mr. Hopkins applied himself with
equal care to that as to other parts of my writings, he would

1844.] MOTION OF A GLACIER OF THE SECOND ORDER. 67

have observed coincidences in our views which he appears not
to have noticed ; and he would probably have hesitated before
laying down so broadly as he has done an objection to the
Viscous Theory, very easily refuted, and some peculiar views
which he considers distinctive of his manner of considering the
subject, from De Saussure's and my own. I shall probably, on
another occasion, endeavour to show that, by following out his
own principles, the results must inevitably merge in mine, when
what is inadmissible shall have been subtracted.

P. 8. — The influence of the Dimension, Slope, and absolute
Elevation (or surrounding temperature) of glaciers upon their
motion, is a matter of observation in detail which offers no
peculiar difficulty, and which deserves to be extended. Having
measured the rate of motion of perhaps the largest glacier in
Switzerland (the Aletsch), I have also measured one of the
smallest, a glacier of the second order, near the Hospice of the
Simplon, almost 8000 feet above the sea, and not many hundred
feet in length. The velocity was little more than an inch in
twenty-four hours, a result corresponding with the extreme
dryness of the neve at that elevation, indicated by the very
trifling issue of water from beneath, and to the insignificant
vertical pressure of so small a mass, notwithstanding its consi-
derable slope. A similar result, it must be owned, might be
expected in this case upon almost any theory.

68 NINTH LETTER ON GLACIERS. [1845.

X. NINTH LETTER on GLACIERS, addressed to
PROFESSOR JAMESON. *

Remarks on the Recent Observations made on the Glacier of the Aar (in 1844) by
direction of M. Agassiz. New Confirmation of the Plastic Theory. Conden-
sation of the Glacier in its downward course in consequence of Frontal Resis-
tance— Continuity of Motion — Motion accelerated in Fine Weather— Excess of
Central Velocity of the Glacier — Motion of Glaciers of Second Order, and of
Snow Beds.*

EDINBURGH, 7th March 1845.

My dear Sir — However satisfied one may be with the con-
clusiveness of their own experiments, it is always pleasing when
they are confirmed by others even in their minuter particulars,
especially if the observations have been made in circumstances
at all different. In this respect, I find with pleasure, from a
communication read at the Institute on the 9th December last,
that M. Agassiz's coadjutors on the glacier of the Aar have
obtained results so perfectly accordant with those which you
have done me the favour of publishing on former occasions, that
they would have satisfactorily established, had earlier observa-
tions been awanting, the viscous theory of glacier motion with
which alone they are reconcileable ; the single seeming antago-
nism to my own measurements being one which tells still more
in favour of that view.

I propose to give a brief summary of these results, and to
show their correspondence with my own. This correspondence —
amounting almost to coincidence — is, of course, to me, a satis-
factory guarantee for their accuracy, as far as they go. By
others, the goodness of the instruments, and the expertness of
the observers, must, in the mean time, be taken for granted.

It is hardly necessary to premise that M. Agassiz and his
friends now admit that all glaciers move fastest at the centre,
and slowest at the sides.

* Edinburgh New Philosophical Journal, April 1845.

1845.] PECULIARITIES IN THE MOTION OF THE AAK GLACIER. 69

But a new fact still less reconcileable with the dilatation
theory resulted from the first attempt to apply geometrical
measurement to the motions of this glacier; viz.,

I. The glacier of the Aar moves fastest in its middle region,
and slower in its upper and lower regions (i. e. towards the
origin and termination). The slowness in the upper region does
not so plainly follow from the facts at present before us, but the
retardation towards the termination of the glacier is undoubted.
The following are the motions originally ascertained, in f ths of a
year, or, more exactly, 289 days. We prefer retaining the
original measures in Swiss feet ; the stations are in descending
order, and a quarter. of league (4000 feet) apart* (the second
in order indicates the rock called Hotel des Neufchatelois.)

169.2 Swiss feet.

177.1

141.3

150.1

133.1
83.7 %
58.3

This result is very different numerically from that which I
obtained on the Mer de Glace of Chamouni, but the difference
is of the kind which might have been expected from their great
diversity of situation and circumstances. I never expected, or
pretended to find in the Mer de Glace the same peculiarities of
velocity as in other glaciers ; on the contrary, I endeavoured
to showf what were the local peculiarities as to slope and
breadth, which probably produced the law of variation of the
motion which I observed, slowest in the middle, and quickest
towards either end, precisely the reverse of that observed by
M. Agassiz ; but I neither depreciate the accuracy of his sur-
veyor, nor contend that one cause of motion sways the glacier
of the Aar, and another that of the Montanvert. On the con-
trary, the difference appears to me entirely conformable to the

* Bulletin de la Societe de Neufchatel, 8th November 1843.
f Travels in the Alps, p. 145, 371.

70

NINTH LETTER ON GLACIERS.

[1845.

viscous theory ; and the glacier of the Aar, in this respect, a
more instructive example than the Her de Glace of Chamouni.
I have shown in one of the passages of my work just cited,
that the velocities of the different portions of the glacier depend,
among other things, on their inclination or slope ; and hence, I
should have inferred, that in a glacier which did not slope faster
and faster towards its lower end, till it becomes almost precipi-
tous, there would be accumulating resistance due to the friction
of the ice on the bed of a long, nearly uniform, gently sloping
valley, such as that which contains the glacier of the lower Aar,
which must magnify the tendency which the ice has to be
squeezed forwards and upwards against the mass immediately
in advance of it, which produces the frontal dip of the ribboned
structure or slaty cleavage of the ice, in the way that I have
explained in my Seventh Letter.* In a glacier then, whose
slope is nearly constant and small, I should expect a condensa-
tion of the ice longitudinally, and a swelling of the surface de-
pending upon the motion of the plastic ice in the direction of
least resistance. Now this is exactly what we have in the
results of measurement. If the annexed figure represent the

/

Kg. 17.

plan of the glacier, and the ice be divided into imaginary com-
partments by vertical sections ; since, whilst AB moves 177
feet, CD moves but 141 feet, there is a condensation of the
mass of ice ABC D, from back to front, of no less than 36 feet
in that time, and so for the successive slices EF, etc. How
then, is this shrinking to be accounted for ? Not by the mere

* [See page 59.]

1845.]

MOTION OF THE AAR GLACIER ACCOUNTED FOR.

71

internal melting, for that would produce merely a lowering of
the surface, and a subsidence of the level of the ice ; such as I
have shown* actually takes places in other glaciers whose
sections move with increasing velocity on the whole. There is
only a vis a tergo which can approximate the sections together,
and, as we read in the Comptes Rendus, squeeze the moraine
longitudinally, giving it a greater breadth,t and condense the
entire body of the ice so as to make it more compact in texture. \
If we take a vertical section instead of a plan (see next page),
the slice abed must be condensed into the higher and shorter
solid cdef, and so of the rest, and the surface will be a swelling
one, as acen, which might even rise towards valley, but

Fig. 18.

generally need only be less sloped than the bed. The effect of
superficial thaw and internal subsidence diminishes this again,
and gives it the form of the dotted curve an'.

In these diagrams the varying velocity in different parts of
the transverse section is, for simplicity, kept out of view.

A retardation of the foremost portion of a viscid stream,
and consequent heaping of its surface, is exactly imitated in the
models formed of plaster of Paris, which I have elsewhere
described, and which, though of uniform fluidity from end to
end, and therefore not subject to the objection arising from the
cooling of lava, where a precisely similar fact is observed, repro-
duce faithfully the motions of the glacier of the Aar.

* Travels, p. 153.

f Les " Moraines medianes s'elargissent dans la meme proportion que le
mouvement se ralentit." Comptes Kendus, 9th Dec. 1844, p. 1301.
\ Ibid, p. 1306, line 29.

72 NINTH LETTER ON GLACIERS. [1845

The fact established on the glacier of the Aar satisfactorily
refutes the notion that a predominant state of compression in a
glacier is incompatible with the existence of transverse crevasses.

II. Continuity of motion. Until recently it was a question
entirely unresolved, whether the glaciers move by insensible
and nearly uniform degrees, or whether they start forward by
short jerks, as might be expected if the movement in their
irregular channels were effected by piecemeal fractures, local
subsidences, arid the justling of independent fragments. Accord-
ingly, when I determined, for the first time, in June 1842, the
absolute continuity of the motion down even to the interval of
an hour,* it seemed impossible to reconcile this to the only
modification of De Saussure's theory applicable to the case, and
the fact seemed to point, as a necessary consequence, to an
insensible yielding of parts throughout the whole mass, which
therefore moves as a whole, and not by jerks occasioned by
strains upon a nearly rigid mass when they attain the limit
consistent with the small play of flexibility of the particles, as
some authors would have us believe.f

It is satisfactory to have an entire confirmation of these
particulars, from the observations made on the glacier of the
Aar in 1844. The observations were made " to the accuracy
of a millimetre on the movement of the glacier from hour to
hour"^ and " at the lower extremity of the glacier, as in the
upper part of its course, the glacier does not advance abruptly,
by jerks (saccades) as formerly § supposed, but its march is
gradual and continuous "\\

III. The influence of warm and damp weather in accelerating
the continuous march of the glacier, and of cold weather in
checking it, 1 deduced in 1842 from a careful comparison of its

* See first Letter on Glaciers, [p. 1 1 of this volume.]

f Mr. Hopkins' experiment of a box full of ice descending an even plane, does
not apply to this case, because, though it moves as a whole, it does so without
change of figure, and without the resistance arising from the irregularity of the
channel of a glacier ; and hence the seeming analogy to a glacier entirely fails.

I Comptes Kendus, p. 1302, line 12.

§ Jadis. It is to be inferred that the writer meant previous to 1842-

|| Comptes Rendus, p. 1303, line 9.

1845.] FURTHER CONFIRMATIONS OF THE AUTHOR'S RESULTS. 73

motion for three months with the state of the thermometer,
and exhibited the result in numbers and diagrams.* Here,
again, the observers on the Aar glacier have confirmed this fact,
so important in a theoretical point of view. " The advancement
of the glacier," they say, " was far from uniform ; it varied
considerably, according to the condition of the atmosphere."
During nine days of cold snowy weather in August 1844, the
mean daily advance was 155 millimetres, but during the sixteen
fine days which followed, it moved through 230 millimetres
per day.f

It is worthy of remark, that the entire annual motion of
the part in question of the glacier of the Aar was ascertained
to be 60 metres, or 164 millimetres per day.;); Now the mean
motion during 35 days of August and September was 203
millimetres, or but one-fourth above the annual mean (and
during part of the time, we have seen, it fell to 155 millimetres,
or below the annual mean) ; proving sufficiently that the annual
motion is not entirely effected during the warm season, and that
even in winter it must bear a very sensible proportion to its
summer motion, as it has been directly proved in the case of
the Mer de Glace of Chamouni.§

IV. The extreme inequality of motion of the central and
lateral parts of glaciers is the best direct proof of the very
considerable plasticity of their mass ; and in the paper before
us this is shewn, in a still more striking manner, than in the
experiments which I have published. A glacier, like the Mer
de Glace of Chamouni, has so considerable a velocity (on an
average at least three times that of the glacier of the Aar), that
the ice is impetuously borne along, and torn from the sides at
the expense of innumerable lacerations and crevasses. So that
in the whole extent of the middle and lower regions of that
glacier, in no place do the ice and ground meet without the

* Travels in the Alps, p. 148. f Comptes Rendus, p. 1301, at the bottom.

| Ibid, p. 1301, line 29.

$ Travels in the Alps, p. 151, 420 ; and Fifth Letter on Glaciers, [p. 38 of this
volume].

74 NINTH LETTER ON GLACIERS. [1845.

former being more or less fissured by rents. But the contrary
is the case on the great glaciers which move on small slopes,
and with smaller velocities ; and the discovery of this fact
rewarded me for the labour of a short visit which I made the
Great Aletsch glacier, in July 1844, when I ascertained, not
merely the small daily progress of the mass of the glacier, but
the astonishing retardation produced by the sides, whilst the
surface remained compact and wholly undivided by longitudinal
crevasses. In that case, I found that, " whilst the velocity of
the ice at 1300 feet, or about a quarter of a mile from the side,
is 14 inches in 24 hours, at 300 feet distant from the side it
was but 3 inches in the same time ; and close to the side it had
nearly, if not entirely, vanished."* Now this observation, a
hasty one, and which, therefore, I am happy to have confirmed,
is more than borne out by the observations on the glacier of
the Aar, detailed in the Comptes Rendus, and which were
made shortly after. The movement of the centre of the glacier
is to that of a point 5 metres from the edge as FOURTEEN to
ONE ; such is the effect of plasticity ! Thirteen-fourteenths of the
motion of the glacier of the Aar are due to the sliding of the ice over
its own particles, and one-fourteenth only to its motion over the soil.
V. Motion of Glaciers of the Second Order. It is a question
of considerable interest to know how those small glaciers, called by
De Saussure glaciers of the second order, advance, compared to
the great ice masses which fill the bottoms of valleys. These
little glaciers, on the contrary, are usually isolated, extending
but a small way, occupying a nook or niche in a mountain side,
and though persisting in their occupancy, and shewing signs of
motion and activity, like other glaciers, yet stretch forward but
a small way, then cease abruptly, as if foiled in the struggle to
join their icy contribution to the magnificent glacier which often
fills the valley immediately below them.f Their isolated

* Eighth Letter on Glaciers, Edin. Phil. Journal, Oct. 1844 [p. 62 of this
volume].

f See a plate, giving a correct idea of a glacier of the second order, in my
Travels, Plate ix.

1845.] MOTION OF GLACIERS OF THE SECOND ORDER. 75

position, their great absolute height, and their usually very
steep declivity and small surface, give considerable interest to
the determination of their rate of motion, at least approximately.
Accordingly, I seized the occasion of spending some days in
July 1844, at the Hospice of the Simplon (already at a height
of 6600 feet above the sea), to examine and measure the pro-
gress of the small glacier which hangs from the slope of the
Schonhorn, immediately behind it, and 1400 feet higher. I
intend to give elsewhere a minute account of this glacier, and
my observations upon it ; but in the mean time I may state that
one of the marks observed, at a point having an inclination of
10°, moved at the rate of 1.4 inches in 24 hours ; and another,
at an inclination of 20°, moved 1.8 inches in the same time.
This small result is quite conformable with the dry and powdery
condition of such elevated glaciers, yielding little water, and
capable of exerting, on their under parts, a very trifling hydro-
static pressure.*

Exactly analogous results were obtained by M. Agassiz's
coadjutors at a somewhat later period of the same year. The
experiments are fully detailed in the Comptes Rendus ; f and
the conclusions which are deducible from them, are — (1.) That
the daily motions of these small glaciers, which rested on beds
so highly inclined as from 15° up to 33°, are included between
20 and 72 millimetres (0.79 to 2.84 English inches) per diem.
(2.) The observers think that their observations go to prove
that when these glaciers are prolonged far enough to meet the
main glacier below, and to unite their streams, then the lower
part of the tributary glacier, or that nearest the point of union,
moves SLOWER than the upper part, or that nearest the origin
of the little glacier; but, on the contrary, if the glacier be
pendant on the slope, and the lower end decays away without
joining the principal, then the inferior extremity moves FASTER
than the origin. Now the cause of this variation in the two

* This experiment is briefly mentioned at the close of my Eighth Letter on
Glaciers.

f P. 1303, and two following pages.

76 NINTH LETTER ON GLACIERS. [1845.

cases (should the fact really appear to be general, as is not un-
ikely, provided the lower station be always chosen low enough)
seems to be, that the main glacier resists the interference of its
tributary with its course, and consequently represses its stream,
causing a heaping up in front, such as mere friction on a low
inclination alone produces, and is thus in conformity with the
viscous theory. In the other case, that of the free glacier of
the second order, the difference of velocity at the upper and
lower station (one-seventh part only) is not more than the dif-
ference of slope (15° and 25°) will readily explain.

VI. Movements of Bas-Neves or Snow-beds. One observa-
tion remains which completes my analysis of the measures
of M. Agassiz's coadjutors. It is one of considerable interest,
and I believe is new. It is the establishment of the fact that
the highly inclined beds of old snow, formed by avalanches,
which lie unmelted in the ravines, without assuming any external
trace of glacial structure, have a proper motion of their own.
This, though to me not unexpected, is very interesting ; for the
most attached advocate of either the dilatation or the sliding
theory, will hardly maintain, on the one hand, that the congela-
tion of soft snow could act here as a propelling force, or on the
other, that the motion can take place without acceleration in
the totality of a mass, inclined (in this case) at an angle even
of 43°, over the bed on which it rests : especially since the
actual movement under this enormous inclination was only 7
millimetres, or three-tenths of an inch, per day;* or one-thirtieth
of that of the great glacier under an inclination of but a few
degrees. The velocity increased towards the lower extremity,
as in the free glacier of the second order. On the plastic theory,
this evidently presents the extreme case of a body, approaching
in its nature to a soft heavy powder slightly moistened, which
gives way by the yielding of its parts, and so far resembles a
fluid (as a bank of earth, slightly glutinous, rather than sand
does) ; and the slowness of movement is in conformity with the
imperfectness of the fluid pressure, and with the fact already

* Comptes Rendus, p. 1305, line 32 ; and p 1306.

Flg:lP.7S.

Fig: 6. p. 89.

' ' : ' Jj^LJ-tt f

\ TTTTiT-n

^^^feki-^-

AA.HB.Sides
da<. bb. Fredomirumt rippl&.TfiA dfftizd;
Jji/tfs shew the dtrtcfooTv of'flotttuiq
Ttodits tocantiy parallel, ~to die, sia

6'ecti-arv

Fig:7. Section; p. 89

(/. A

WXmffKfflllZmmi-^rrrrr*.^

Fr SchoockladL1: Edmburgk.

1845.] VISCOUS THEORY OF GLACIER MOTION ILLUSTRATED. 77

stated (under Head III. above) that the velocity of any glacier
is proportional to the completeness of its saturation with water
at the time. The bas-nev6s, or old avalanches, furnish very
little water at their lower extremities.

I have now gone through these observations, made by persons,
it may be assumed, not particularly desirous to find results con-
firming a theory which they have opposed, but which it may be
hoped they will oppose no longer, when their own results speak
in language so unequivocal. My analysis has been succinct,
but complete and impartial. The facts are stated as given,
without selection or suppression.

XL ILLUSTRATIONS OF THE VISCOUS THEORY OF
GLACIER MOTION.— PART I. CONTAINING EXPERI-
MENTS ON THE FLOW OF PLASTIC BODIES, AND OBSERVA-
TIONS ON THE PHENOMENA OF LAVA STREAMS.*

§ 1. Plastic Models. § 2. Analogy of Glaciers to Lava
Streams. — Note on the Velocity of Lava.

§ 1. PLASTIC MODELS.

In the concluding chapter of my " Travels in the Alps of
Savoy," I have shown how the obscure relations of the parts of
a semifluid or viscous mass in motion (such as I have attempted
to prove that the glaciers may be compared to) may be illus-
trated by experiment.

The larger models, there described and figured, showed
very clearly the precise effects of friction upon the motion of
such a mass. They were formed of plaster of Paris, mixed
with glue, and run in irregular channels, and the relative velo-

* From the Philosophical Transactions for 1846, p. 143. Received by the Royal
Society of London March 15— Read April 10, 1845.

78 VISCOUS THEORY OF GLACIER MOTION. [1845.

cities of the top and bottom, the sides and centre of such a
pasty mass were displayed by the alternating layers of two
coloured pastes, which were successively poured in at the head
of the model valleys. The boundaries of the coloured pastes
were squeezed by the mutual pressures into greatly elongated
curves, whose convexity was in the direction of motion ; and in
a vertical medial section, the retardation of the bottom and the
mutual action of the posterior and anterior parts shaped the
bounding surface of two colours into a spoon-like form.

Now these models convey a very palpable commentary
upon the effects of friction on a plastic mass, and likewise on
the influence of the mutual pressures of its parts ; but in further
illustration of the same thing I constructed another model, only
executed as the printing of my volume approached its close, and
which is cursorily described in a long note (page 377),* whence
its real importance may perhaps have been pretty generally
overlooked.

The models in question, of which I have since made many,
are formed by accumulating in one end of a long narrow box
AB, Plate I. fig. 1, a deep pool of the viscid material already
mentioned, which is retained there by a sluice or partition C,
which may be withdrawn at pleasure.

The surface of the pool abed is then pretty thickly dusted
over with a coloured powder, and the sluice is withdrawn.

The pasty mass subsides slowly under its own weight into
the lengthened form efgh. The film of colour on the surface is
therefore broken up so as to cover three or four times the sur-
face it did at first ; and its new distribution marks the lines of
greatest separation of the superficial particles of the mass. The
appearance of such a model when run is shown in fig. 2 of the
same plate, and it manifests in the plainest manner the twofold
tendency to separation in such a case where the channel is
narrow and confined, and there is a certain mass of matter in

* In this paper reference is of course made to the first edition of my " Travels,"
the second not having been then published. [The substance of the note referred to
was re-written and introduced into the text at page 381 of the Second Edition.]

1845.] MODELS ILLUSTRATING THE VEINED STRUCTURE. 79

front. Plate V.* shows a more accurate drawing taken from
such a model.

The lines of sliding separation occur most distinctly marked
near the sides, where the friction is greatest, and the central
parts are forced past the lateral parts, on account of the less
embarrassed and consequently swifter motion of the centre ;
and they incline to the centre although the breadth of the
channel be perfectly uniform. But the forces which tear asunder
the parts (when such exist) act perpendicularly to the former,

Fig. 19.

and produce dislocations and fissures, which perfectly correspond
to the direction and appearance of the crevasses of a glacier,
that is, they are convex upwards or towards the origin of the
glacier. It is the former of these lines of separation, or diffe-
rential motion, which constitute and trace out with an exact
parallelism the veined structure which I have described as form-
ing the normal structure of all true glaciers. Plate VI.* is a
representation of a very beautiful plaster model of more con-
sistence than the other, in which the swelling of the surface and
the direction of the open cracks produced by direct thrusts are

* [Plates V. and VI. of the Philosophical Transactions for 1846, which exhibit
more elaborately the markings shown in the diagram of Plate I., fig. 2, of this
volume, are not reproduced on account of their elaborate nature, but fig. 19 of this
page gives a somewhat rude idea of one of them. J

80 VISCOUS THEORY OF GLACIER MOTION. [1845.

most beautifully shown ; and are even more so in the model
than in the engraving. The fissures are transverse and slightly
convex to the origin in the higher part of the glacier, then
gradually turning round they radiate from a centre in the lower
part, exactly as in the glacier of Arolla (Travels in the Alps,
Plate VI.) , and in all similar cases.

The experiment above detailed was suggested to me by
studying the ripple of streams of water, which appears to have
the same origin : and in very weak currents moving through
very smooth and uniform channels (as the chiselled sides of
water conduits) the same may be made manifest by throwing a
handful of light powder on the surface, which then becomes
divided into threads of particles inclined in the manner I have
described at a certain angle from the side towards the centre,
depending on the velocity of the stream.

The slightest prominence of any kind in the wall of such a
conduit, a bit of wood or tuft of grass, is sufficient to produce a
well-marked ripple-streak, from the side towards the centre,
depending upon the sudden and violent retardation of the lateral
streamlets and the freer central ones being momentarily edged
away from them. The general course of the motion of the par-
ticles is, however, scarcely affected by such a circumstance, for
the differential velocities which cause the ripple and the sepa-
ration are always small compared to the absolute velocity of
the stream ; and thus a floating body on the water (just as the
moraine on the glacier) perseveres in its course parallel to the
side with scarcely any perceptible disturbance. When, however,
the descent is violent and the friction great, floating bodies are
gradually drawn towards the centre, and this happens also in
exactly the same circumstances to the moraine of the glacier.
Plate II. figs. 3 and 4, show the relation of the ripple-marks
to the channel of a very flat smooth gutter in one of the side
streets of Pisa, sketched after heavy rain.

These ripple-marks in water are well seen near the piers of
a bridge, or when a post is inserted in a stream and makes a
fan-shaped mark in the water cleft by it: such marks have

1845.] ANALOGY OF GLACIERS TO LAVA STREAMS. 81

been much neglected by writers on hydraulics ; but in one of
the most ancient hydraulic treatises, that of Leonardo da Vinci,
lately printed from the MS. in the Italian collection of writers
on hydraulics, they are very well described and figured. A
case parallel to the last mentioned, where a fixed obstacle
cleaves a descending stream and leaves its trace in the fan-
shaped tail, is well seen in several glaciers, as in that at Fer-
pecle, and the Glacier de Lys on the south side of Monte Rosa,
particularly the last, where the veined structure follows the law
just mentioned.* And I desire here to record that the views
just presented as to the origin of the veined structure of ice
were confirmed, but were not suggested, by the experiments on
viscous fluids just mentioned. The necessity of the tearing
up of a solid mass, if it moved at all in a bed presenting insur-
mountable resistances on all sides, in directions such as the
veined structure presents, was foreseen by me whilst dwelling
amongst the glaciers themselves, at a distance from books or
the means of experiment. . * . * My Third Letter to Pro-
fessor Jameson, written in 1842 from the remote village of
Zermatt, contains the substance of all that I have since deve-
loped and illustrated at greater length and in different ways,
rather to meet the difficulties of others, than to confirm what
was plainly fixed in my own mind.f

In explaining the theory of the veined structure at a
meeting of the Royal Society of Edinburgh on the 20th
of March 1843, I stated that I had arrived at the conclusion
that crevasses resulting from tension in certain parts of a
glacier must be formed at right angles to the surfaces of
discontinuity or structural veins where they intersect the
surface : a law conformable to the empirical one discovered
by me on the glacier of the Rhone in 1841 4 since general-
ized in other cases, and which even the adversaries of my

* [See Travels in the Alps, p. 328. J

f [See page 23 of this volume. The quotation from it is therefore omitted.]

^ Edinburgh PhilosophicalJournal, January 1842, [and p. 7 of this volume.]

82 VISCOUS THEORY OF GLACIER MOTION. [1845

theoretical views have admitted to be a correct statement of
the facts.*

My attention was at that time (March 1843) turned by my
learned and acute friend, Mr. W. A. Cadell, to the veined struc-
ture of the slag of iron furnaces as due to the difference of
velocity of the parts producing surfaces of separation and peculiar
molecular condition. The transition was easy to the case of
volcanic rocks and lava streams ; and this case was pressed on
my attention by an unexpected journey which I soon after
undertook to Italy and Sicily.

§ 2. ANALOGY OF GLACIERS TO LAVA STREAMS.

There is something pleasing to the imagination in the
unexpected analogies presented by a torrent of fiery lava and
the icy stream of a glacier. But when we look upon the com-
parison historically and critically, and find how generally this
analogy has been perceived and adverted to by persons of very
different views and talents of observation, we are strongly
tempted to suspect that some latent cause confers the marked
resemblance.

This cause I of course consider to be the laws and condition
of their motion, the struggle of a semi-fluid mass of enormous
weight creeping down a mountain side, in which fluidity and
solidity are so curiously combined, that we should be at a loss
in either case how to name it ; a straining, crackling, splinter-
ing solid, heaved on by the internal energy of the latent
fluidity which pervades it, and which at last succeeds in
giving to the general character of the motion and the moving
mass, those of fluid bodies subject to the law of gravity ;
whilst the parts, themselves almost rigid, have that rigidity
most fantastically subjected to the action of the dominant
principle.

In illustration of what has now been said, I shall quote
passages from some authors which, without particular research,

* Bibliotheque Universelle, tome xliv. p. 153. " C'est en effet un fait assez
general que les bandes bleues coupent a angle droit les crevasses," etc.

1845.] MR. AULDJO AND CAPTAIN HALL QUOTED. 83

have come under my notice expressive of the analogy just men-
tioned.*

Mr. Auldjo, an intrepid alpine traveller, writing about
Vesuvius in 1832, says, " The field of lava in the interior of
the crater, inclosed within a lofty and irregular bank, might be
likened to a lake whose agitated waves had been suddenly
petrified ; and in many respects resembles the Mers de Glace, or
level glaciers of Switzerland, although in its origin and materials
so very different." f And the view in the same work of "streams
of lava on the south-east of the cone" presents a perfect ana-
logy to a glacier, bearing on its surface three medial and two
lateral moraines.

Captain Basil Hall, writing of Vesuvius at a later period,
uses these remarkable expressions whilst describing an eruption
of lava : — " The colour of this stream was a brilliant pink, much
brighter at the sides than in the middle, where, either from the
cooling of the surface, or the accumulation of cinders and broken
pieces of stone, a sort of dark ridge or backbone was visible
from end to end, not unlike the moraine on the top of a glacier.
This reminds me of a curious analogy which often struck me,
between two objects so dissimilar as a glacier and a lava stream.
They are both, more or less, frozen rivers ; they both obey the
law of gravitation with great reluctance, being essentially so
sluggish, that although they both move along the bottoms of
valleys with a force well nigh irresistible, their motion is some-
times scarcely perceptible." J This remarkable passage, worded
with the usual scrupulous care of the author, combined with his
account of the mechanism of a glacier in the description of the
glacier of Miage in the same work, show that he had arrived at
more correct notions on the subject than any of his contempo-
raries ; notions which chiefly required careful observation to
give them the force of demonstration. The allusion to moraines

* [A quotation from Starke's Italy, previously given in the Sixth Letter, p. 50.
of this volume, is here omitted, but the words of Mr. Auldjo and Captain Hall, not
before quoted, are retained.]

f Auldjo's Sketches of Vesuvius, p. 10, published 1833.

J Patchwork, by Captain Hall, vol. iii. p. 118, published. 1841.

84 VISCOUS THEORY OF GLACIER MOTION. [1845.

as characteristic of lava streams as well as glaciers, in the pre-
ceding extract, is perfectly borne out by the view of the lava of
1831 given by Mr. Auldjo, and already cited ; the same appear-
ance is mentioned by M. Elie de Beaumont in his account of
Etna in the following terms : " Une des circonstances que les
coulees de lava presentent le plus invariablement .... con-
siste en ce que chaque coulee est flanquee de part et d'autre par
une digue de scories accumulees qui rapellent par sa forme la
moraine d'un glacier, . . . souvent aussi les coulees presentent
de pareilles digues vers leur milieu, lorsqu'elles sont partagees
en plusieurs courants distincts coulant 1'un a cote de 1'autre."*

In another place the same author compares the movement
of the upper crust of the lava to that of glaciers according to
the then prevalent theory : — " L'ecorce superieure d'une coulee
separee de 1'ecorce inferieure et du sol sousjacent par une cer-
taine epaisseur de lave liquide, on du moins visqueuse, se trouve
dans un etat comparable a celui d'un glacier, qui, ne pouvant
adherer au sol sousjacent a cause de la fusion continuelle de sa
couche inferieure, se trouve contraint de glisser."f

Finally, M. Rendu, Bishop of Annecy, in his excellent
Essay on Glaciers, refers in one passage (and I believe in one
only) to the possible analogy with a lava stream, " [le glacier]
s'affaisse-t-il sur lui-meme pour couler le long des pentes comme
le ferait une lave a la fois ductile et liquide ?"J

The following considerations seem to show more than a
general external analogy between lava streams and glaciers.

Their velocities are sometimes equally slow. Although
common lava is nearly as liquid as melted iron, when it issues

* R'echerches sur le Mont Etna, p. 184. Published in the Memoires pour
servir a une Description Geologique de la France, tome iv. 1838.

f Recherches sur le Mont Etna, p. 177.

J Theorie des Glaciers, Mem. de 1' Academic de Savoie, tome x. p. 93, published
1841. [I may perhaps be allowed to add my own impressions to the same effect in
1841, founded, for the most part (at that time), on the same general and external
analogies which had guided my predecessors : — " None who has ever seen, or even
clearly conceived a lava-stream, can fail to find in it the nearest analogue of a
glacier. Stiff and rigid as it appears, it either flows or has once flowed." Ed. Rev.,
April 1842, p. 53.]

1845.] IMPERFECT FLUIDITY OF LAVAS EXEMPLIFIED. 85

from the orifice of the crater, its fluidity rapidly diminishes, and
as it becomes more and more burdened by the consolidated slag
through which it has to force its way, its velocity of motion
diminishes in an almost inconceivable degree, and at length,
when it ceases to present the slightest external trace of fluidity,
its movement can only be ascertained by careful and repeated
observations, just as in the case of a glacier. In November
1843, I watched lava issuing rapidly from a small mouth in the
crater of Vesuvius at the rate of about one foot in a second. The
eruption of Etna in 1832 advanced at the rate of five miles in
two days, which is at the rate of one foot in about six seconds.*
We may contrast with this the eruption of Etna in 1614, which
yielded a lava which advanced but two miles in ten years,
according to Dolomieu,t during the whole of which time its
motion was sensible. This gives a mean rate of rather more
than three feet per day ; but at the conclusion it was no doubt
much slower.

Mr. Scrope J saw the lava of 1819 in the Val del Bove
moving down a considerable slope at the rate of a yard a day,
nine months after its eruption. It had, he adds, the appear-
ance of a huge heap of rough cinders ; its progression was
marked by a crackling noise due to friction and straining, and,
" on the whole, was fitted to produce any other idea than that of
fluidity. In fact," he continues, " we must represent to our-
selves the mode in which the crystalline particles of lava move
amongst one another, rather as a sliding or slipping of their
plane surfaces over each other, facilitated by the intervention
of the elastic (?) fluid, than as the rotatory movement which
actuates the molecules of most other liquids." It is generally
conformable to this view that we find in Hamilton's Campi
Phlegrcei (fol. i. 38, Note] the curious remark that some lava is

* E. de Beaumont, Recherches sur le Mont Etna.

f Quoted by E. de Beaumont, p. 85. The original is in the Journal de Phy-
sique, vol. i. of the New Series, where it is mentioned that the same slowness of
motion has been observed in lavas of Vesuvius. Ferrara (Descrizione del Etna,
Palermo, 1818) denies this statement, but not I think on sufficient grounds.

\ On Volcanoes, p. 102.

86 VISCOUS THEORY OF GLACIER MOTION. [1845.

so incoherent, or whilst fluid has so little viscosity, that in issuing
from the volcano (Vesuvius) it has appeared "farinaceous, the
particles separating as they forced their way out, just like meal
coming from under the grindstones." ».-

From all this it is quite clear that the seeming rapidity of
the parts of a glacier, or the slowness of its motion, cannot be
taken as the slightest evidence of its moving otherwise than as
a fluid, contending with the rigor of the parts which include
and resist the moving force, which is truly hydrostatic, though
limited in its exercise.

It is manifestly futile and unphilosophical to seek one cause
of motion in a lava which, like that of Vesuvius in 1805, must
have described as many hundred feet in a minute as that of
1614 from Etna probably did in a year* for the mean daily
motion of the latter during ten years was three feet ; but toward
the end of that time it must evidently have had, for a long
period, an average motion of one-half or one-quarter of this,
and therefore below the observed mean movements of certain
glaciers. Fluidity, in the first instance, as in the second, was
the propelling vehicle or manner in which gravity acted, and
this is a sufficient answer to any attempt to maintain that the
plasticity of a glacier is a collateral but not a primary cause of
motion — a distinction surely without a difference.

As in the case of all imperfect fluids, the central and super-
ficial particles move faster than the lateral and inferior ones ;
and when the fluidity is exceedingly imperfect, as in those long-
flowing lavas, there must be a rupture of continuity between
the parts to permit them to slide and jostle past one another.
This is evidently the cause of the noise referred to by Mr.
Scrope and other writers. This tearing up of the stream into
longitudinal stripes, occasioned by the varying velocity of the
parts, is thus described by M. Dufrenoy in his account of Vesu-
vius : — " La plupart des coulees presentent des bandes longitu-
dinales assez paralleles entre elles : ces larges stries saillantes

* See Note on the Velocity of Lava Streams at the end of this paper.

1845.] INSTANCES FROM VESUVIUS AND ETNA. 87

sur la surface sont les traces du mouvement de la lave qui ne
s'avance pas d'une seule piece, mais par bandes paralleles."*

And M. Elie de Beaumont describes a lava stream at Etna
in these terms: — " La surface offrait de profondes cannelures
paralleles entre elles, dirigees dans le sens du mouvement qui
1'avoit deversee a 1'exterieur et qui etaient croisees par de nom-
breuses gerqures transver sales," f Here then is evidently the
twofold system of rents and perpendicular fissures described in
the commencement of this paper as being found in the models,
and as being conformable to the phenomena of glaciers.

During the winter 1843-44 which I spent in Italy, I had
an opportunity of testing these resemblances, and tracing others
to glaciers in the lavas of Vesuvius and Etna. I entered on the
inquiry with a very jealous care of being drawn into the admis-
sion of fanciful or imperfect analogies ; and I shall confine
myself to the statement of one or two most plain and undeniable
confirmations, selected from the results of many fatiguing
rambles.

The plastic nature of the viscous lavas of Vesuvius and
Etna is such as well might obliterate any internal traces of
rents due to differential velocity, which, in the mass, are speedily
closed and reunited as in a stream of treacle, or in the plaster
models before explained, where the interior is homogeneous and
the superficial coating above is permanently dislocated.

In lavas the indescribable ruggedness of the surface very
generally prevents any record of the gentler play of forces.
The following facts appear to me quite conclusive as to the
manner in which a mass partially solidified, yet moving as a
fluid, is torn up by the interior forces which act upon it.

1. At Vesuvius, the Fossa della Vetrana, between the Her-
mitage and Monte Somma, is a valley lined with the lava of
1751. I here observed that the lava was in some places de-
tached from the wall of the valley, leaving a cavity on the shel-
tered side of a projecting elbow of rock, just as a glacier does

* Dufrenoy sur les Environs des Naples, p. 324.
fE.de Beaumont, p. 38.

88 VISCOUS THEORY OF GLACIER MOTION. [1845.

in similar circumstances, showing the considerable consistence
which the lava possessed.

In the upper part of this Fossa the lava has a distinct linear
structure where broken, in shells parallel to the sides, whose thick-
ness varies from one-third of an inch upwards. The position of
these surfac.es of dislocation is indicated (for illustration) in
fig. 5 of Plate I.

2. In the vast lava wastes of Etna we encounter not only
a greater extent of surface, but a greater variety of condition
as to cohesion of the lava streams, and the slope down which
it has descended, and thus we have a better chance of meeting
with specimens of the manner in which the semi-solid crust of
a lava stream is torn up and crevassed by the effect of gravity
compelling it into the circumstances of fluid motion. From this
tendency of all lavas to form slags, and of these slags to be
splintered, tossed, and remoulded by the action of the still
liquid portion of the stream below or around, not one-thousandth
of the surface bears marks of the simple condition of fluidity
under which it was originally moulded ; and though, when viewed
from a distance, and in connexion with the form of the ground
over which it has passed, we see plainly enough that it has
flowed like a stream, the absence of any trace of easy undulating
forms which characterise fluids or plastic masses, gives to the
sciarre of Etna (the cheires of Auvergne) an appearance far
more removed from pristine fluidity than the glacier masses of
Switzerland.

In traversing many miles of lava wastes between Nicolosi
and Zafarana, on the eastern slope of Etna, I met with one
singularly favourable specimen of a branch of a stream consoli-
dated exactly as it had moved, and undisturbed afterwards. It
is the part of the current of 1763, called Lava delle Cerve. The
branch stream in question may be ten yards wide, and presents
a thin crust, which has floated on the viscid lava below, and
which, while yet imperfectly solidified, has been urged to move
with the rest of the stream, and has undergone a process of
division and rending accordingly. The stream has flowed in the

1845.] LAVAS OF ETNA — FRONTAL DIP. 89

direction from left to right in Plate L, fig. 6. The lateral parts,
PP, QQ,* have been literally torn to pieces longitudinally (as I
wrote on my note-book on the spot) by the multiplied rents
which showed the dislocation of the quicker moving central
from the lateral parts, and these rents inclined towards the centre
of the stream in the direction in which it moved. The length of
the stream was divided by transverse rents strikingly convex
towards the origin of the stream, as shown in the same figure.
These cracks were marked by another peculiarity ; the cake of
floating scoria had not only been cracked across, but pushed
upwards, generally forwards and upwards, before it was finally
included in the cooling mass of the stream ; the result was the
arrangement shown in the longitudinal section, fig. 7, which it
will be seen resembles the tiling of a house, only that the frac-
tured parts do not always overlap, but the anterior edge is
tilted upwards. It will thus be seen that this tendency to
separation acts also in the vertical plane, and the dotted lines
aa', bb', etc., indicate the direction of its action, coinciding with
the surfaces of differential motion, which produce what I have
called the frontal dip of the veined structure of the ice of
glaciers.

3. At no great distance from this lava, and near the foot
of the hillock called the Serra Pizzuta, between the last-named
point and the valley of Tripodo, I observed a transverse section
of a lava stream exhibiting an arrangement in bands or plates,
nearly parallel to the side of the current, but inclining towards
the centre.

4. Between Zafarana and the Porta Calanna (Etna), a
remarkably pretty illustration occurs in the surface of an old
lava stream, worn and polished by the action of a brook.
Where the lava has had to turn an abrupt corner of a rock, A, fig.
8 (which represents a ground plan) , the progress of the lava being

* [The lines PP, QQ do not (I believe) represent the converging banks of the
lava stream, which was probably of uniform breadth, but the converging lines of
structural dislocation merely. In the sketch done on the spot which is now before
me, the central and unstriated portion is marked as being 6 paces wide. Nov.
1858.J

90 VISCOUS THEORY OF GLACIEK MOTION. [1845.

violently checked by the resistance of the projecting mass, has
been torn up into longitudinal shreds, which from imperfect
fluidity have not reunited, but have left open cavities of the
form represented in the figure, which exhibit with, remarkable
fidelity the forms of the fissures with which glaciers are some-
times traversed, when they are subjected to sudden transitions
in their states of motion (as in the Glacier des Bossons at Cha-
mouni, and which coincide in direction with the veined struc-
ture, and pass into it by imperceptible gradations.

5. What I have called the frontal dip of the veined struc-
ture in glaciers,* I have explained by the accumulation of a
sluggish mass of considerable extent upon a floor or bed offering
the resistance of intense friction ; in consequence of which the
mass of ice, urged downwards and forwards by its intense
weight, being resisted by the friction of that which immediately
precedes it, must yield in the direction of least resistance, or
squeeze itself in a slanting direction forwards and upwards, and
thus sliding over the resisting mass immediately in front, will
produce surfaces of discontinuity or differential velocity in that
direction. Such a result I inferred from general principles,
without reference to any particular example, and the explana-
tion of the superficial convexity of the lower part of many
glaciers was evidently satisfactorily explained by it.

The convex swelling form of a viscous stream will depend
principally upon the relative measure of two quantities, the
stiffness or viscosity of the fluid, and the inclination of the
surface ; although it will also depend on the part of the
stream, whether near the origin or the termination, which we
consider.

I have found this variation from concave to convex, depend-
ing upon circumstances, alike in glaciers and lava streams.
Some very highly inclined small glaciers existing at considerable
heights, and therefore very hard and consistent, are, nevertheless,
deeply concave from end to end, the slope compensating for the

* See my Travels in the Alps, 1st edition, pp. 167, 376, and letter to Dr. Whe-
well in Jameson's Journal. Oct. 1844, [page 59 of this volume.]

1845.] TERMINATION OF A LAVA STREAM — INTUMESCENCE. 91

stiffness of the matter ; such is a beautiful glacier, named, as
far as I can learn, La Gria, or Glacier de Bourget, which de-
scends from the Aiguille de Goute towards the valley of Cha-
mourii. See Plate IV., fig. 9.*

Many, perhaps most, lava streams, where they have well-
determined hanks, are concave during the longer part of their
course, but towards their termination they become convex as
their viscosity increases. Nevertheless, I have seen portions of
well-bounded streams decidedly convex.

The appearance of the termination of a lava stream ap-
proaches strikingly that of a glacier. But this is much more
than a vague analogy, and the accounts of faithful eye-witnesses
prove the resisted motion of the doughty stream to be such as
I anticipated. We find it explicitly stated over and over again
in the writings of Dolomieu f and Delia Torre f (and more par-
ticularly by the latter), that when a lava stream meets with any
obstacle in front which checks its course, or when its course is
checked by its own sluggishness, the stream swells, and gains
gradually in thickness by the fluid pressure from behind urging
its particles forwards and upwards. So striking was this natu-
ral effect of semi-fluid pressure, that these old observers attri-
buted it to a peculiar force developed in the lava, of the nature
of " fermentation," producing intumescence, the only way by
which they could account for the vertical rise of the fluid,
although it was very evident that the result was only what
might be expected from the nature of the lava. It was also
observed that when the lava stream had thus attained a certain
height, it began to move on again, the necessary result of the
increased hydrostatic pressure, although attributed by the authors
named to the heat developed by chemical action. The tenacity
with which the idea was long adhered to, that the residual
fluidity of a nearly cooled lava stream was insufficient to account
for its progress, without attributing to it the qualities of a

* [Of the Philosophical Transactions for 1846. It has not been thought essential
to reproduce this figure.]

f Papers in the Journal de Physique.

\ Histoire du Vesuve. Naples, 1771, 8vo, p. 207-9, and several other places.

92 VISCOUS THEORY OF GLACIER MOTION. [1845.

second volcanic focus, are curious proofs of how long a palpable
cause may be rejected as insufficient to explain a phenomenon,
and a totally imaginary one superadded.*

I may add, that lava streams sometimes push .their extre-
mities up hill ft glaciers do the same.|

In addition to the considerations already stated, which
illustrate the viscous theory of glaciers, I am glad to avail
myself of two which have reached me from independent and
impartial sources.

The first is by Mr. Darwin, who in a small book on
" Volcanic Islands," published about the time that I was en-
gaged in making the preceding observations on Etna and Vesu-
vius, pointed out in a very clear manner the explanation which
the veined structure of glaciers lends to that of volcanic rocks
belonging to the Trachytic arid Obsidian Series, where the
lamination, instead of being obscure and rare, as it generally is
in the Augitic lavas, owing perhaps to their greater fluidity,
and more viscid and homogeneous texture, is the general rule.
" The most probable explanation," says Mr. Darwin, " of the
laminated structure of these felspathic rocks appears to be that
they have been stretched whilst 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. In both cases the zones may be compared to the
finest agates ; in both they extend in the direction in which the
mass has flowed, and those exposed on the surface are generally
vertical. "§

The other illustration is contained in a communication with
which I have been favoured by Mr. Gordon, Professor of Civil

* See the view of the termination of a lava stream in Auldjo's Sketches of
Vesuvius, facing p. 92. The reader may also compare the view of a gi-otto in the
lava, in the same work, with that of the source of the Arveiron in my Travels, p. 387.

f Hamilton, Campi Phlegraei, folio, vol. i. p. 40, note.

£ [As in the Allalein glacier, described in my Travels, p. 352, showing the
remarkable development of the frontal dip under these circumstances. See also the
Section in a 6 in the Ninth Topographical Sketch in the same work.)

§ Darwin on Volcanic Islands, 1844. The whole passage, pp. 65-72, illustrates
this analogy.

1845.] EXAMPLES OF THE VELOCITY OF LAVA STREAMS. 93

Engineering in Glasgow, and which has been printed in the
Philosophical Magazine for March 1845, to which therefore I
may refer. I need only state at present that it demonstrates,
from observations on the flow of Stockholm pitch with a speed
wholly insensible, and which requires some months for its
accomplishment even in small masses, that a motion, of the
nature of fluid motion, takes place at temperatures at which
the pitch remains so hard as to be fragile throughout, and
presents angular fragments with a conchoidal fracture. Mr.
Gordon adds, that the resistance of the pitch to its own forward
motion produces bands of differential velocity, and having the
frontal dip.

EDINBURGH, FEBRUARY 26, 1845.

NOTE ON THE VELOCITY OF LAVA, REFERRED TO IN PAGE 85.

The following are a few facts which I have collected on the
velocity of lava. That of Vesuvius in 1805 appears to be the
most fluid on record. Von Buch, who was in company with
MM. de Humboldt and Gay-Lussac, describes it as shooting
suddenly before their eyes from top to bottom of the cone in
one single instant,* which must correspond to a velocity of
many hundred feet in a few seconds without interpreting it
literally. Melogrami, quoted by Breislak,f says it described
three miles in four minutes, or about seventy-five feet per
second at a mean. The same lava, when it reached the level
road at Torre del Greco, moved at the rate of only eighteen
inches per minute, or three-tenths of an inch per second. J The
lava of 1794 (Vesuvius) reached the sea, a distance of 12,961
feet, in six hours, or passed over one-third of a mile per hour,
or eight inches per second ;§ whilst the lava of Etna, in 1651,
described sixteen miles in twenty-four hours, or above a foot
per second the whole way. That of 1669 (Etna), which destroyed

* Bibliotheque Britaimique, vol. xxx. The vertical height of the cone proper
is 700 or 800 feet ; the length of the slope may therefore be 1300 feet,
f Institutions Geologiques, iii. 142.
| Nicholson's Journal, vol. xii. § Breislak, Campanie, i. 203.

94 VISCOUS THEORY OF GLACIER MOTION. [1845.

Catania, described the first thirteen miles of its course in twenty
days, or at the rate of 162 feet per hour, but required twenty-
three days for the last two miles, giving a velocity of twenty-two
feet per hour ;* and we learn from Dolomieu, that this same
stream moved, during part of its course, at the rate of 1500
feet an hour, and in others took several days to cover a few
yards.f

The lava of 1753 (Vesuvius), starting with a velocity of
2500 feet per hour, soon -diminished to sixty feet,J as did that
of 1754 to the same;§ and of 1766 to thirty feet per hour.||
The lava of 1831 (Vesuvius) moved over 3600 feet in twenty-
six hours, and finally advanced steadily at the rate of ten feet
an hour. If The lava of Etna of November 1843, is said to
have moved over three paces per second at the distance of a
mile from the crater.

The stream of 1761 (Vesuvius), before it stopped flowing,
advanced but three yards a-day ;** and that of 1766, which con-
tinued moving for about nine months, moved over but a small
space in that time. Had the attention of authors been equally
directed to the slow as to the rapid advancement of lava, there
is no doubt that we should find many instances besides these
recorded by Dolomieu and Scrope, of continuous movements of
three feet, and even one foot a-day, or less.

* Ferrara, Descr. del Etna, p. 105. This appears from the dates, though at
variance with one assertion of the author.

f Dolomieu, Isles Ponces, p. 286, Note. \ Delia Torre, Histoire, etc., p. 196.

§ Ihid, p. 130. j| Hamilton, Campi Phlegrsei, i. 19.

^f Auldjo, Sketches of Vesuvius, p. 79, with a sketch of the front of the stream
whilst advancing at this rate.

** Delia Torre, p. 182.

1845.] DE SAUSSURE'S THEORY OF GLACIER MOTION. 95

XII. ILLUSTRATIONS OF THE VISCOUS THEORY
OF GLACIER MOTION. — PART II. AN ATTEMPT TO
ESTABLISH BY OBSERVATION THE PLASTICITY OF GLACIER
ICE.*

§ 3. De Saussures Theory. § 4. Modifications of De
Saussures Theory. § 5. Experiments at Chamouni on the
Plasticity of j.ce.

§ 3. DE SAUSSURE'S THEORY.

When Gruner proposed the explanation of glacier motion
by the sliding of the ice over its bed, and De Saussure illus-
trated and confirmed it by considerations drawn from the lubri-
cating action of the earth's heat melting the ice in contact with
the soil,f there is no reason to suppose that either of them
thought it necessary to take into account the varying form of
the channel through which the glacier had to pass, and the
consequently invincible barrier presented to the passage of a
rigid cake of ice through a strait or narrow aperture when it
occurred. This is the more remarkable, because he [De Saus-
sure] conceives that the inequalities of the bed or bottom may be
overcome by the hydrostatic pressure of the water, which he
supposes may be imprisoned between the rock and the ice, so
as absolutely to heave the latter over the resisting obstacles.

I believe that in no part of De Saussure's writings will there
be found any, the slightest reference to the possibility of the
glacier, when fairly formed, moulding itself to the inequalities of
the surfaces over which gravity urges it ; nor is there any trace

* Philosophical Transactions, 1846, p. 137. Received July 28, 1845.— Read
January 15, 1846.

f To do Gruner justice, he appears to have heen aware of the effects of the
earth's heat and the lubricating action of the water thawed from the glacier :
" Lorsque les cotes de 1'amas [de glace] qui touchent la montagne, fondent en entier,
toute la masse entrainee par son poids glisse sur son fond et s'avance dans la vallee,"
French translation, p. 333 . . . " il est vraisemblahle que leur surface, inferieure
[i. e. des glaciers] se liquefie autant, et peut-etre plus que la superieure," ib. p. 289.

96 VISCOUS THEORY OF GLACIER MOTION. [1845.

of the correlative fact of an unequal motion of the sides and
centre of the ice, which may in some sense be considered as the
geometrical statement of the preceding physical fact. The fact
of plasticity was suspected by Basil Hall, and more distinctly
announced by Rendu, as shown in the first part of this paper ;
but it could not be proved until the geometrical fact of the
swifter motion of the centre of the glacier relatively to the
sides was established in 1842.* The contrary opinion at that
time generally entertained would have been conclusive against
the hypothesis of plasticity called forth by the gravity of the
mass.

So far then as appears from his writings, De Saussure con-
sidered the ice of glaciers to constitute a mass possessing rigidity
in the highest degree, such rigidity in short as common expe-
rience assigns to ice tranquilly frozen in small masses, which is
sensibly inflexible. It is in this sense in which I have spoken
of De Saussure's sliding theory, as one which " supposes the
mass of the glacier to be a rigid one sliding over its trough or
bed in the manner of solid bodies,"t and I adhere to the
definition as excluding the introduction of the smallest flexibility
or plasticity, to which the term rigidity is correctly opposed.
I consider too, that De Saussure's theory supposes the mass of
the glacier to slide over its trough or bed in the manner of solid
bodies, that is, not as a heap of rubbish or absolute fragments,
such as a glacier sometimes precipitates over a rock, but which
evidently did not enter into De Saussure's explanation, nor, in
fact, required any theory.

As to the crevasses which form so prominent a feature of
many glaciers (although many are in parts almost devoid of
them), I do not recollect that De Saussure alludes to them as
facilitating in any way the movement of the glacier, but simply
as results of its motion and of the rigid character of ice. And
I believe that this view (whether it was held by De Saussure

* Edinburgh Philosophical Journal, October 1842, and Travels in the Alps of
Savoy, p. 134.

f Travels, p. 362.

1845.] DE SAUSSURE'S THEORY DEFINED. 97

or not) is substantially correct. The regular system of
crevasses of a glacier is approximately transverse, rather arched
upwards towards the origin of the glacier ; and as De Saussure
supposes the glacier to be pressed downward by the mass of
snow accumulating at its head, it is hard to believe that he
could have regarded these fissures as in any way essential to
its movement, even were they very numerous ; the tendency
of such a pressure from above would rather seem to be to pack
the ice like an arch, opposing its convex side to the direction of
the pressure.

The view now given of De Saussure's theory of glacier
motion is not only conformable to what may be gathered from
his writings, but expresses the unanimous understanding of his
numerous commentators, followers, and opponents. As some
doubt has lately been hinted as to the definiteness of De Saus-
sure's conception of a glacier as a mass devoid of flexibility and
plasticity, and urged down a slope as a whole, by the lubricating
action of fusion in contact with the soil to an extent which, in
extreme cases, might even give it the character of buoyancy, I
will take the liberty of quoting some indisputable authorities
amongst writers of name in different countries.

And first from De Saussure himself: —

" La chaleur de la terre fait fondre les neiges et les glaces,
meme pendant les froids les plus rigoureux lorsque leur epaisseur
est assez grande pour preserver du froid exterieur les fonds

sur lesquels elles reposent." — Voyages, § 532 " C'est elle

qui entretient les torrents, qui, meme pendant les plus grands
froids, ne discontinuent jamais de sortir de tous les grands
glaciers."— § 533

" Presque tous les glaciers reposent sur des fonds inclines ;
et tous ceux d'une grandeur un pen considerable ont au-dessous
d'eux, meme en hiver, des courans d'eau qui coulent entre la
glace et le fond qui la porte. On comprend done que ces
masses glacees, entrainee par la pente du fond sur lequel elles
reposent, degagees par les eaux de la liaison qu'elles pourraient
contracter avec ce meme fond, soulevees meme quelquefois par

H

98 VISCOUS THEORY OF GLACIER MOTION. [1845.

ces eaux, doivent peu a peu glisser et descendre en suivant la
pente des vallees, ou des croupes qu'elles couvrent." § 535.

" Quand on considere que ces glaces reposent sur des plans
inclines, qu'il coule sous elles des torrens d'eaux qui les fondent
par en bas, les detachent et les soulevent, ne seiit-on pas que
leur permanence dans la meme place est une chose physiquement
impossible?" § 2284.

Kamond's account :

" La cause [de la marche des glaciers] est facile a concevoir ;
une masse qui pese sur un plan incline tend necessairement a
descendre, et cette tendance est favorisee dans les glaciers par
le choc des torrents qui roulent sous leur voutes, par 1'humidite
que leur masse communique au terrain qui les porte, enfin par
cette multitude innombrable de cavities qui creusent leur partie
inferieure, et dont 1'eifet est de diminuer le frottement en
diminuant 1'etendue des surfaces*."

Elie de Beaumont's account of De Saussure's theory ;
speaking of the lava of Etna, he says, " L'ecorce superieure d'une
coulee separee de 1'ecorce inferieure et du sol sousjacent par une
certain epaisseur de lave liquide, ou de moins visqueuse, se trouve
dans un etat comparable a celui d'un glacier, qui, ne pouvant
adherer au sol sousjacent a cause de la fusion contmuelle de la
couche inferieure se trouve contraint a glisser f."

BischofFs account :

" Das jahrliche Yorriicken der Gletscher welches Saussure
ganz einfach aus einem allmaligen Herunterrutschen der unteren
durch das Aufthauen des Eises schliipfrig gewordenen Seite des
Gletschers auf der schiefen Flache des Bodens erklart, ist eine
bekannte Thatsache J."

Agassiz's account :

" Autrefois on admettait tout simplement qu'ils glissaient
sur leur fond, en vertu de leur propre pesanteur, et que ce

* Voyages en Suisse par Coxe, traduit par Eamond, ii. 119, 1790.
f Memoires pour servir a la Description Geologique de la France, iv. 177,
published 1838.

t Warmelehre, p. 180, published 1837.

1845.] DE SAUSSURE'S THEORY DEFINED. 99

glissement etait favorise par les eaux au fond de leur lit. C'etait
1'opinion de Saussure.*"

Martins' account :

" De Saussure, Escher de la Linth, Andre de Luc, attribuent
cette progression au poids des glaces et a 1'affaissement produit
par la fonte de la face inferieure qui repose sur le sol.f "

Studer's account :

" Die bisher fast allgernein herrschende Theorie erklarte die
Bewegung der Gletscher aus der Schwere allein. Es soil die
Gletschermasse als starrer Korper auf ihrer Felsgrundlage, wie auf
einer schiefen Ebene, theils durch ihr eigenes Gewicht theils durch
dem Druck der hoheren Eis-und Firnmasse herunter gleiten
(Grimer, Ramond, Kuhn, De Saussure, Escher) 4"

This last testimony of the most exact and most learned of
the living Swiss geologists as to the sense in which De Saus-
sure's theory has always been understood, is so important that
I shall add a translation : — " The hitherto generally prevailing
theory explains the movement of the glaciers by gravity alone.
The glacier masses are considered as rigid bodies, which slide
down over their rocky beds partly by their own weight, partly
under the pressure of the higher ice and neve." My interpre-
tation of the views of De Saussure as regards the rigidity of the
glacier ice is thus borne out by an independent authority, for
M. Studer's work and my own appeared simultaneously. It is
further confirmed by private communication with another
eminent Swiss naturalist nearly connected by relationship with
De Saussure himself, who is more intimately acquainted with
the opinions and writings of his illustrious kinsman than any
other person now alive, and who considers that De Saussure's
views were confined to the general analogy of the glaciers to
solid masses sliding down inclined planes, and that the effects
of the inequalities of the channels and forms of the ice-basins
were not comprehended in his theory. §

* Etudes sur les Glaciers, p. 152, published 1840.

f Sur les Glaciers de Spitzberg. Bibliotheque Universelle, 1840, torn, xxviii.
p. 166. \ Lehrbuch der pbysikalischen Geographic und Geologic, 1844.

f [M. Necker-de-Saussure is here referred to.]

100 VISCOUS THEORY OF GLACIER MOTION. [1845

If we feel surprise that a naturalist and observer so eminent
had not adverted to the difficulty of imagining a solid cake of
ice, even though perfectly detached from its bed, to disengage
itself from the obstacles and sinuosities of its rocky channel, we
should remember, — first, that the explanation is given in the
most general terms, and there is no appearance that its author
looked more closely at its consequences and details than to
satisfy himself that a sliding motion in the abstract was ren-
dered possible by the action of the earth's proper heat, an
ingenious and philosophical element of the theory (however
inadequate), and that which being due principally to De Saussure,
renders the theory properly his, and connected it with his
ingenious inquiry into this curious part of physics as a distinct
and wholly independent investigation. Secondly. Every one
knows how an application of a principle so true and so ingenious
leads men of even the most exact habits of thought to overlook
difficulties in a subject almost unstudied. De Saussure did
much for our knowledge of glaciers, and he saw much which no
one had observed before him : we must not blame him if,
yielding to a true and natural analogy of sliding bodies, he
overlooked real and great difficulties inherent in the conception
of a glacier as a solid continuous mass and highly rigid. Thirdly.
In De Saussure's time no plan or map, worthy of the name, of
any glacier existed, and this was a blank which even De
Saussure did not attempt to supply. The popular notion of a
glacier, which it is certain he had in his mind when he penned
the passages which relate to their motion, is 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, as is the case with a vast majority of glaciers tolerably
accessible, and which alone were visited at the time of publica-
tion of the first edition of the Voyages dans les Alpes. In all
these cases the lateral resistance might easily be overlooked, and
the popular comparison to one solid body sliding on another and
lubricated by its own liquefaction, might be accepted as a com-
plete explanation ; as has even been done at a later period by

1845.] MODIFICATIONS OF DE SAUSSURE's THEORY. 101

those who have attempted to illustrate De Saussure's theory by
experiment, but who, like him, neglected the form and undula-
tions of the bed in which it rests.

§ 4 MODIFICATIONS OF DE SAUSSURE'S THEORY.

De Saussure and his immediate followers appear to have
considered the crevasses which occur transversely in most glaciers
as the result of the inequalities of the beds down which they
are constrained to move ; but other writers have imagined that
the part which these crevasses perform in the phenomena of
glacier motion is fundamental, and essential to the existence of
the movement at all. Some writers have remarked that the
fall of ice blocks over the precipice wnich often occurs near
the lower end of glaciers, leaving the superior portions unsup-
ported, allows them to advance to fill the position formerly
occupied by the portion of the now fallen ice. But in this
case it would appear that cause and effect are in some degree
confounded. The ice about to be projected over the cliff must
either advance towards its fall by its own gravity, or by the
pressure of the parts behind. If its own gravity suffices, the
same cause will urge the ice behind it to move similarly, whether
the block in question fall or not ; and if it be the pressure from
behind which shoves it on, then still more is the pressure of
the entire glacier the cause of motion of the entire glacier,
irrespective of the precipitation of its more advanced part.

Thus, M. Martins' theory of the progression of glaciers is,
that the weight of the parts causes them to separate by fissures
into wedge-shaped masses, without their sliding along the bottom ;
that the fissures become filled with frozen snow, and that thus
the glacier is perpetuated and extended year by year. " Cette
progression," he says, " n'est done ni un glissement ni un
affaissement difficiles a comprendre, puisque la glace doit adherer
au sol, mais un demembrement successif."* Besides other
objections, it is now universally admitted that the glacier-proper
does not grow by the consolidation of snow in its fissures.

* Martins sur les Glaciers de Spitzberg et de la Suisse. Bibl. Univ. Juillet 1840.

102 VISCOUS THEORY OF GLACIER MOTION. [1845.

But setting aside the attempt to render the sliding motion
of the entire glacier considered as a plane slab more easy, by
considering the motions of the parts instead of the motion of
the whole, we are led to notice the attempt to reconcile the
sliding theory to recent observation, by ascribing to the crevasses
of the glacier the important office of enabling it to accommodate
itself to the inequalities of its channel.

Our object is, in this section, merely to state the view in
its most plausible form, which in the succeeding section we
shall controvert by experiments giving it a direct negative. In
the third portion of this essay we shall enter more at large into
the phenomena of crevasses, and mention other objections to
this hypothesis and every modification of it.

According to this view, the friction of the ice against the
sides of the valleys will produce a dislocation of the glacier into
longitudinal stripes (as shown in Plate II. fig. 1.*), where a
transverse line W becomes by the irregular motions of the ice
distorted into the zigzag form hcc'h. Or if we suppose the
plasticity of the ice to be sensible, but that its action is accom-
panied with fractures, the abruptness of the angles of the figure
will be softened, as in the broken line Imm'l in the lower part
of the same figure. This latter hypothesis evidently merges into
the true plastic theory, when the part of the progression due to
the flexure of the transverse lines bears a large proportion to the
effect of the longitudinal slide, or more generally, when the
surfaces of sliding or yielding become greatly multiplied, when
the notched line will merge into a curve.

The passage of the glacier through a gorge or contraction
is explained on the same view by fig. 2, where the resistance
of the sides having occasioned a series of parallel longitudinal
rents as before, the portion of the glacier beyond the limits of
breadth of the gorge BB' is supposed to be detained or embayed
whilst the intermediate columns slip through.

* These figures and their interpretation are taken from Mr. Hopkins' First Memoir
in the Cambridge Transactions, vol. viii. part 1. A figure similar to the first is to
be found in a more recent paper by the same author in the Philosophical Magazine
for June 1845.

PLATE H

Fig i p.102,147

SUMMER.

Fig: 4. F 3, AUTUMN.

1845.] EXPERIMENTS ON THE PLASTICITY OF ICE. 103

§ 5. EXPERIMENTS AT CHAMOUNI ON THE PLASTICITY OF ICE.

It has been shown that in order to reconcile De Saussure's
theory of sliding motion with the ascertained fact that the centre
of the glacier moves faster than the sides, it had been assumed
that solutions of continuity or longitudinal crevasses were formed
parallel to the length of the glacier, by means of which the
central portion slides past that adjacent to it, and so on for
successive strips as we approach the sides, the more rapid
retardation near the sides being rendered mechanically possible
by the increased number of these longitudinal dislocations.

The result was therefore predicted to be, that the glacier
would be found to move by echelons, or that strips of ice of a
certain number of feet, or yards, or fathoms, would move either
suddenly or by gradual sliding, but at all events so as to mark
by an abrupt separation at the longitudinal fissure, that the one
portion of ice has slipped past the other by a distinct measurable
quantity.

When I first learnt at Geneva, in August 1844, from Mr.
Hopkins's published papers,* that this was really the author's
meaning, it occurred to me that the proof between the rival
theories was easy, and that it was only necessary to place a
series of marks in a right line transversely to the glacier, and
observe whether they were displaced by an imperceptible flexure,
or whether they slid past one another by sudden dislocations.

Such a proof was independent of any assertion as to the
existence or not of such fissures as those contended for, about
which different opinions might be formed, especially as they
might be asserted to exist although invisible to the eye. Being
satisfied in my own mind of the non-existence of such fissures
wherever the ice is not violently dislocated and descends a steep
place in a tumultuous manner (which, as already mentioned, is
not the case which we consider), I had no hesitation in predicting
that the result of the experiment would be confirmatory of my
theory, and contradictory of the other ; that the transverse line

* Cambridge Transactions, vol. viii.

104 VISCOUS THEORY OF GLACIER MOTION. [1845.

would be found to become a continuous curve, and that no other
system of fissures could be found in the glacier satisfying the
mechanical postulate of the greater velocity of the central parts
of the glacier, than the ribboned structure of the ice, which I
had already pointed out as resulting from a forced separation of
the semi-rigid ice, at a vast, though finite, number of points in
the breadth of the glacier, and which I showed to exist exactly
in the direction required for releasing the mass from the tension
induced by the gravity of its parts.

Having gone to Chamouni a few days later, I looked out
for a place where the ice should be as compact as possible,
wholly devoid of open fissures, and if possible continuous up to
the bank. This latter condition I found it impossible to fulfil
on the Mer de Glace, at least without ascending to the neve,
which might be objected to as less rigid than the glacier proper.
The former condition was well satisfied in a sort of bay on the
west side of the Mer de Glace between the Angle and Trelaporte,
exactly under the little glacier of Charmoz. The part adjoining
the western shore of the glacier is indeed highly crevassed, and
therefore unfit for this experiment ; but at the distance of fifty
or sixty yards from the moraine it becomes remarkably flat and
compact for a space of about seventy yards in width, and several
hundred yards in length, throughout which space there is not a
single open crevasse. Now this compact area of ice presents
the veined structure in a nearly longitudinal direction, with a
degree of delicacy and distinctness not to be found in any other
part of this glacier (as I had already remarked in my Travels,
p. 159), and it contains no other trace of a system of longitu-
dinal fissures or lines of separation of any kind, which could
render mechanically possible the distortion of this flat compact
surface of so great an extent. Now I have always observed
that the veined structure near the side of a glacier is best de-
veloped where the ice is least crevassed, or the continuity of
the mass most perfect; a fact stated and referred to its true
cause from the first date of my speculations on the origin of the
blue veins, in the following words : — " The veined structure

PLATE HI

400C' fccfs

110,114.

(Z) (1)

1845.] RIBBONED STRUCTURE PRODUCED IN SITU. 105

invariably tends to disappear when a glacier becomes so crevassed
as to lose horizontal cohesion, as when it is divided into pyra-
midal masses. Now this immediately follows from our theory ;
for as soon as lateral cohesion is destroyed, any determinate
inequality of motion ceases, each mass moves singly, and the
structure disappears very gradually."* Now the ice at the
point in question is the compacted ice which has just passed
round the great promontory of Trelaporte, having been rent
by numberless chasms, and which is consolidated by pressure
in the bay in question, whilst the centre of the glacier being
still on the steep is deeply crevassed. The structure of the
even ice is continuously striped .with a regularity comparable to
that of the finest chalcedony for a distance of some hundred
feet. This structure must have been produced on the spot, since
no such perfect structure exists higher up, and if it did, [it] must
have retained all the marks of dislocation due to the formation
and reconsolidation of the fissures, which are so numerous and
wide as to render the passage of the glacier quite impracticable
if we follow the same strip of ice up towards the promontory of
Trelaporte. Let it then be recollected that the structure is
produced here, under our eyes, on the very spot where the
experiments about to be detailed were made, and that the
structure in question produced a vertical slaty cleavage so
distinct, that the ice broken into hand specimens may be split
parallel to it like any slaty rock, and that the fine hard lamina
projecting vertically after the glacier has been washed by rain,
permitted the blade of a knife to be thrust between them to a
depth of several inches, although they are rarely more than a
quarter of an inch thick.

I shall now describe the actual measurements made upon
the glacier, in order that my method of proceeding in similar
cases (when I have only published results) may be understood.

The general position of the experimental surface will be
understood from the topographical sketch (Plate III. fig. 1).

* Third Letter on Glaciers, Edinburgh Philosophical Journal, October 1842,
[page 24 of the present volume].

106 VISCOUS THEORY OF GLACIER MOTION. [1845.

The theodolite was planted at a fixed point on the ice Q, just
within the crevassed portion, which intervened between it and
the western shore of the glacier. This point of fundamental
and constant reference was fixed by an exactly vertical hole
pierced with an iron jumper, or blasting iron, one inch in
diameter, and was frequently deepened in order to preserve the
centre as exactly as possible in the same vertical line in the ice.
The theodolite was centred over it at every observation by
means of a plummet, which nearly filled the cylindrical hole
and permitted an adjustment, which one day with another
might be accurate to about one-tenth of an inch. No stick was
placed in the hole, but when not in use it was covered by a
large flat stone, which effectually prevents congelation in ordi-
nary weather.* The adjustment of the theodolite on the ice is
always a matter for patience, but I succeeded in rendering it
perfectly stable when once erected, by inserting the three feet
in cavities in the ice, and filling them carefully with ice chips.

The theodolite, placed at Q, was pointed with its vertical
wire on the well-defined angle of an erratic block Ql on the
opposite eastern bank of the glacier, above Les Echellets (see
Plate III. fig. 2). By causing the telescope to traverse in a
vertical circle, a transverse line joining the points Q, Ql, was
determined, and several stations were fixed in the compact ice
eastwards of Q, at distances from it of 30, 60, and 90 English
feet, and subsequently at 120 and 180 feet. These were num-
bered in succession (1), (2), (3), (4), (5), and the permanence of
their positions in the ice was secured as before by carefully
driving vertical holes two feet deep, which were occasionally
deepened, and covered with flat stones when not in use. As these
points were in succession nearer to the centre of the glacier,
they were expected to move with gradually increasing velocity
in advance of the imaginary line Q, Ql, drawn across the ice.

* On one occasion this precaution having been neglected (in the case of a
different mark on the ice), the hole was found completely frozen up after exposure
to a day or two of severe weather in the month of August. It was, however,
recovered hy observing the beautiful stellar form of the ice- crystallization.

1845.] DETAILS OF THE EXPERIMENT ON THE HER DE GLACE. 107

But as the theodolite stationed on the glacier at Q must
partake of its motion whilst the mark Ql on the bank remained
at rest, the visual line QQ1 would appear to revolve towards
the origin of the glacier, and hence the relative advance of the
points (1), (2), etc., would seem too rapid. To estimate the
correction for this error the velocity of the glacier at Q must be
determined, and also the distance QQ1. For the former purpose
the following method was adopted. When an observation at
station Q had been completed, by pointing the telescope on Ql
and observing the apparent advance of the points (1), (2), etc.,
the telescope was reversed in the Y's, or turned 180° towards
the western moraine, upon which it indicated from day to day
a new position, owing to the angular revolution of the line
joining the fixed point Ql and the moveable point Q. The
point Q2 in the topographical sketch (Plate III. fig. 1) indi-
cates the point where the visual line touched the moraine at the
commencement of the observations on the 9th of August 1844.
By the application of a scale or a similiar method, the apparent
advance of Q referred to the moraine Q2 was regularly measured.
It is thus obvious that these apparent motions were too great
(by the property of diverging lines) in the ratio of the distance
Ql Q2, to Ql Q ; and hence it became necessary to ascer-
tain the position of Q2 as well as Ql. For this purpose a
base-line of 300 feet was measured on the ice parallel to the
length of the glacier,* [and the following results obtained] : —

Q Ql = 2569 feet. Q Q2 = 550 feet.

The apparent motion of Q measured on the moraine is greater
than the true motion in the ratio 2 5 26 1+95 5 ° or | nearly. The
actual motion of Q is readily deduced, as well as the apparent
rotation of the visual line QQ1. Thus during 16*75 days, the
duration of the experiment, the apparent advance of Q referred
to the moraine Q2 was twenty-three feet six inches, which at

the distance Ql Q2 (3119 feet) subtends an angle of 25' 54",

or almost exactly one and a half minutes daily.

* [The subordinate details are here abridged from the Philosophical Transactions.)

108

VISCOUS THEORY OF GLACIER MOTION.

[1845.

[Omitting the details of the corrections, the following table
presents the results of the observations for three periods,
Aug. 9-26, Aug. 14-26, and Aug. 17-26.]

a.)

(20

(3.)

(4-)1

(5.)

Actual

30 feet.

60 feet.

90 feet.

120 feet.

180 feet.

Interval
in days.

motion
of Q, or
zero
point.

Ratio to
actual

Mean
daily re-
la HVP

Ratio to
actual

Daily
relative

Ratio to
actual

Daily
relative

Ratio to
actual

Daily
relative

Ratio to
actual

Daily
relative

motion.

motion.

motion.

motion.

motion.

motion.

motion.

motion.

motion.

motion.

inch.

inch.

inch.

inch.

inch.

inch.

16-76

228-4*

0-59

0-043

1-17

0-086

1-60

0-118

11-96

159-8

0-56

0-042

1-12

0-084

1-59

0-119

2-03

0-152

9-00

115-8

0-56

0-014

1*16

0-090

1-62

0-126

2-16

0-167

2-94

0-229

This Table shows, first, in a striking point of view, the
regularity of action of the law by which the variable motion of
the different transversal points in the glacier is governed, since
the movement in the different intervals bears so near a propor-
tion, that when estimated in terms of the actual motion of the
glacier at the place, the relative motion of the parts scarcely
differs by unity in the second place of decimals, and is generally
much under it. Taking into account the inevitable errors of
observation and the extraordinarily unfavourable circumstances
of the weather, it is in the very highest degree improbable that
this law of continuity of the partial motions can be accounted
for by any casual justling or sliding of one finite portion of the
ice past another, which would inevitably have left some of the
points relatively at rest during some one of the many intervals
of observation, and given to others evidence of a starting motion
until friction had established a fresh position of repose amongst
the struggling masses.

Secondly, This Table enables us to establish not only the
continuity of motion of any one point, but the continuity of the
relation which connects the points (1), (2), (3), etc. For
instance, the relative motions of (1) being
•59 -56 -56.

* The true sum ought to be about four inches greater. The difference arises
from the impossibility of estimating the correct velocities for the fractional intervals.

1845.] DETAILS OF THE EXPERIMENT ON THE MER DE GLACE. 109

and those of (2) being

1-17 1-12 1-16,

the ratios are

1-98 2-00 2-07.

In like manner the ratios between (3) and (2) will be found to be
1-37 1-42 1-44.

From these data the simultaneous relative motions of these
six stations may be projected in a curve, or rather polygon, as
shown in Plate IX. fig. 1, [of the Phil. Trans, for 1846.*] This
is interesting, as showing very plainly, not only the regulated
increase of swiftness of the glacier towards the centre, but that
the variation of the variation is clearly brought out, indicated by
a convexity in the direction of the motion, and confirming the
general principle long ago announced by me, that the retarda-
tion is relatively greatest towards the side, and less towards
the centre. I appeal to any one conversant with the laws of
mechanics in their practical application, whether the manifest
continuity of such a law does not plainly include a continuity
in the mutual action of the parts of the mass under experiment,
and even independent of the manifest absence of great disloca-
tions, would not establish the doctrine of a molecular yielding,
or plasticity in the ice, as opposed to the irregular justling of
great blocks, admitting that such could exist unperceived.

The period through which this experiment extended (seven-
teen days) is conclusive against the idea that a small flexure
could take place until the accumulated strain on the solid pro-
duced a rupture, which relieved the strain, and so forth, per
saltum. The continuity of glacier motion in every case except
that of precipitous descents or ice-falls, first proved by my ex-
periments in 1842, is now universally admitted by those who
have had any personal experience in the measurement of glacier
motion, however opposed to my theoretical views, t The

* [To estimate rightly the result of the projection, it requires to be made on a
rather inconveniently large scale. It has not therefore heen reproduced in this
volume j.

f See proofs cited in my Ninth Letter on Glaciers, Edinburgh Philosophical
Journal, April 1845 [and page 68 of this volume].

110 VISCOUS THEORY OF GLACIEK MOTION. [1845.

changes for seventeen days were connected (as has been shown)
by a law of continuity established by numerous intervening ob-
servations ; and the flexure or distortion of the ice amounted in
this time to no less than four feet at the opposite ends of a line
180 feet in length. It is quite certain, from my own previous
observations and those since made by M. Agassiz's directions on
the glacier of the Aar, that the movement thus shown to have
continued seventeen days without a saltus would have continued
the whole season in the same manner. In fact, the deformation
or flexure thus observed being sufficient to account for the whole
excess of the central arbove the lateral motion, is in itself an
explanation, and a proof that the explanation is adequate, and
leaves nothing residual to be accounted for by saltus.*

I have more to add on this subject, but shall first give an
account of an extension of this experiment on the actual flexure
of the ice, upon so elaborate a scale as I scarcely ventured to
hope would prove successful, especially as the time I could
devote to watch its progress was small, and the circumstances
of weather excessively unfavourable.

Having succeeded so well with the thirty feet station in the
transverse line, I thought of multiplying the points of observation
still further, so as to obtain a polygon of flexure more nearly
approaching to a curve. This I did by making the first ninety
feet of the transverse line, *. e., the space between Q and (3),
Plate III. fig. 3, the subject of more immediate experiment,
fixing in it forty-five stations only two feet apart. After seve-
ral partial failures, which gave me, notwithstanding, encouraging
results, I selected this plan : — a space a foot wide and ninety
feet long, was cleared with hatchets and ice tools, so as to
arrive at a nearly even surface of the hard delicately-veined ice ;

* Mr. Williamson, Fellow of Clare Hall, Cambridge, to whom I proposed this
experimental test of the theory of movement by echelons, made a series of independ-
ent observations on the Mer de Glace, which coincided in result with what has been
stated above. After a patient examination of these facts, and of others which he
observed on different glaciers, I am glad to say that Mr. Williamson was led to
abandon the theory of sliding columns or fragments, and to accept that of plasticity
as connected with the mechanism of the veined structure which I have endeavoured
to illustrate above.

1845.] DISTORTION. OF THE ICE MEASURED. Ill

and gutters were made so as to drain as far as possible the sur-
face water from the part under experiment. The theodolite
being stationed as usual over Q, and the vertical wire of the tele-
scope describing a great circle passing through the line QQ1
transverse to the glacier, an assistant (Balmat), directed by my
signals, bored a series of holes from two feet to two feet, forty-
five in number, with a common carpenter's centre-bit, and as
nearly as possible in the visual line. The holes, which were
ffrth of an inch in diameter, and about five inches deep, were
immediately occupied by wooden pins prepared for the purpose.
These pins were placed as nearly as possible in the visual straight
line, but from the nature of the operation, some errors were in-
evitable. The amount of these errors of position or zero of the
marks was immediately determined by causing the vertical wire
again to traverse the series, the assistant placing over the centre
of the head of each pin in succession the zero point of a scale
of inches divided both ways, and held parallel to the length of
the glacier, so that (the divisions to tenths of an inch being very
plainly marked, and divisible by estimation by the telescope)
the fundamental position of each pin was determined, and con-
sidered as + if in advance of the traverse line (in the direction
of the glacier's motion), and — if behind it (or nearer the origin
of the glacier). The mere error of reading did not in any case
exceed ^Vtn °f an incn> though the uncertainty of centering of
the theodolite over Q might amount to j^h of an inch, or even
more. The two marks nearest Q had their positions deter-
mined by a thread stretched from the station-pointer of the
theodolite to the third mark, their distance being too small to
be distinctly seen by the telescope.

The very same process, as regards the placing the zero of
the scale on the head of the pin and reading off, was repeated
on subsequent days, and the new readings, minus the funda-
mental readings, gave the apparent relative motion in the inter-
val. This apparent motion had to be corrected, exactly as be-
fore explained, for the rotation of the visual line due to the
translation of the fundamental point Q. [Some details are here
omitted.] . .

112

VISCOUS THEORY OF GLACIER MOTION.

[1845.

Table shewing the True relative motions of forty-five points two feet apart, in a
line transverse to the axis of the Mer de Glace, 1844.*

Mark,
No.

Corrected Motion.

Remarks.

August
20—21.

August
20—23.

August
20—26.

1.

—0-25

—0-45

—0-15

Between 1 and 2 a slight

2.

—0-8

—0-6

—0-55

fissure.

3.

—0-05

+0-35

+0-4

4.

+0-05

+0-35

+0-3

5.

—0-15

+0-05

+0-25

6.

+0-2

+0-65

+1-1

7.

+0-3

+0-9

+1-6

Veined structure very strong.

8.

+0-2

+0-95

+2-0

9.

+0-4

+1-2.

+2-0

Between 9 and 10 a fissure.

JO.

+0-25

+1-0

+1-95

11.

+0-5

+1-4

+2-45

12.

+0-35

+1'25

+215

13.

+0-55

+1-6

+3-1

14.

+0-75

+1-65

+3-45

15.

+0-55

+1-75

+3-45

16.

+0-65

+1-9

+3-8

17.

+0-65

+1-95

+3-45

18.

+0-65

+2-0

+4-0

19.

+0-8

+2-05

+4-4

20.

+0-9

+2-55

+4-9

Two fissures between 19-20

and 20-21.

21.

+1-2

+2.7

+5-25

22.

+1-25

+2.75

+5-35

23.*

+1-1

+3.0

+5-95

22-23, slight fissure very ob-

24.

+1-15

+3.1

+6-35

lique.

25.

+1-15

+3.05

+6-9

26.

+1-05

+3-2

+6-1

26-27, a slight fissure ; it re-

27.

+1-2

+3-35

+6-65

crosses the line at 40-41 .

28.

4-1-45

+3'8

+7-25

29.

+1-45

+3-9

+7-3

30.

+1-4

+3-85

+7-45

31.

+ 1-65

+3'8

+7-25

32.

+1-7

+4-15

+7-7

33.

+ 1-5

+4-3

+7-95 '

34.

+1-6

+4-1

+8-15

35.

+1-55

+4'4

+8-25

34-35, a very slight fissure.

36.

+1-55

+4'35

+8-0

37.

+ 1-7

+4-3

+8-5

38.

+1-65

+4-75

+915

39.

+1-75

+5'0

+9'4

40.

+1-8

+4-7

+8-85

41.

+ 1-9

+5-1

+9-7

40-41. See 26-27.

42.

+1-8

+5-0

+9-4

43.

+1-8

+4-95

+9'3

44.

+1-85

+5-2

+9-85

45.

+2-05

+4-95

+9-9

[Abridged from the original table in Philosophical Transactions].

1845.] UNFAVOURABLE CONDITIONS OF WEATHER. 113

The results of these observations will be best understood
by consulting the projected results in Plate IX. fig. 2 [of the
Philosophical Transactions for 1846],* in which the advance
of the marks is magnified twofold relatively to the spaces
between them, which was necessary for the reduced scale of
the engraving, although it has the disadvantage of increasing
the deviations from symmetry, which might arise from errors
of observation. As those who have not attempted the actual
execution of such experiments cannot be aware of the difficulties
they entail, it may be just to mention, that during weather so
unfavourable as that which occurred during the continuance of
these experiments, nothing can be so irksome as the necessity
of persevering in the face of physical obstacles, the only alterna-
tive which the necessary limitation of my stay afforded being to
abandon them. I do not speak of the painful effort of conduct-
ing delicate observations for hours under a hot sun, whilst the
feet are immersed in the liquid sludge of decaying snow, for I
am not aware of having sacrificed the precision of a single
observation to such a cause (though in the course of my glacier
experience I have sometimes been compelled to abandon or dis-
continue observations), but it is easy to see that the success of
experiments like this depended upon the absolute fixity of the
marks inserted in the unstable and wasting surface of the gla-
cier, and that the most dry and uniform condition of the ice
seemed alone to promise a chance of finding the small pins in
the exact positions in which they had been planted a day or
two before. Instead of this, eleven out of nineteen days which
I spent at Chamouni were wet, and notwithstanding the sea-
son of the year, the glacier was repeatedly covered with snow,
which, in melting under a succeeding fierce sun, left the surface
honeycombed by infiltration, and streaming with wet, so that
the preservation of the holes was only effected by laboriously
covering every one with large flat stones during the intervals of
observation, and even this was not free from other disadvantages
which it would take too long to particularize. On the whole,

* [See note to page 109, above.]

114 VISCOUS THEORY OF GLACIER MOTION. [1845.

the uniformity of the triple curves exhibited in fig. 2 is surpris-
ing, considering the local errors to which the fixation of the
pins was liable, and the smallness of the quantities sought.
The first two curves, those for the 21st and 23d August, are
indeed as perfectly regular as it would be possible to expect from
this kind of observation, even much more so than I had ever
hoped to attain ; but on the 26th the holes containing the
pins were more degraded, and some manifest errors have arisen
from this cause, and evidently affect only single marks, such as
the twelfth and twenty-fifth mark, which singly have inclined
forward or backward, by the fusion of the ice. With these pre-
liminaries as to the reasons why the irregularities of these
curves should be judged [of] with indulgence, I will state briefly
the most apparent general results.

First. The flexure of the ice is proportional, almost exactly
to the time elapsed in the intervals of the observations ; and it
is also graduated from point to point, not staccato, as would
inevitably have been the case had the relative motion been due to
the sliding of finite portions past one another, as in Plate II.
fig. 3. ' We perceive nothing of the kind.

Secondly. A singular peculiarity strikes the eye, which at
first puzzled me, but when the cause was explained, confirms in
no slight degree the accuracy of the methods employed, and their
fitness to reveal the minutest motion of the glacier. The curves
of Plate IX. fig. 2 [of the Phil. Trans.], cut the axis, not all
exactly at the same point ; but on an average, this point may be
fixed with tolerable accuracy between the third and fourth mark,
or seven feet from the fundamental station Q. The first and
second marks moved evidently slower than the point Q, or the zero.
and the progress of the third and fourth is dubious or irregular.
The cause of this peculiarity was clearly ascertained on the spot
to be the existence of two crevasses belonging to the lateral
system of crevasses, between which, at their thinning out, the
station Q had been placed, under the idea that their distance
was sufficiently great not to affect the motion. The position of
these crevasses is shown in Plate III. fig. 3, by which it will

1845.] REMARKS ON THE CURVES OF DISTORSION. 115

be seen that they were fifty feet apart in the direction of the
length of the glacier, and that a line joining their extremities
passed eight feet nearer the centre of the glacier than the point
Q, thus almost coinciding with the point of contrary flexure,
which was evidently occasioned by a slightly superior advance
of the mass of ice on which Q was placed, thus insulated in
some degree between the two fissures. This enables us to
transfer the origin of our curve to a point of undoubted solidity,
namely, the fourth station, from which point it swells with the
regularity which has been described, and which establishes the
compactness of the ice experimented on in the most convincing
manner.

Thirdly. The curves reckoned from the origin, thus experi-
mentally indicated, show, in a beautiful manner, the convexity
in the direction of the glacier motion before alluded to, which
is singularly striking, considering the shortness of the space in
which it is developed with nearly mathematical precision, being
only about ^Vth Of the breadth of the glacier in this place (see
ground-plan, Plate III. fig. 1. Even an inspection of the curves
(Plate IX. fig. 2 of Phil. Trans.) can faintly convey the impression
made upon my own mind, when, upon the 26th of August, I placed
the theodolite for the last time over station Q, and caused the
vertical wire to pass in front of the line of pins bent into the
convex shape by the relative motion of six days' continuance.
Thus seen in foreshortened perspective, the eye would in an in-
stant have seized an abrupt motion or discontinuity of the line,
but " the appearance of the curve they formed was beautiful ;
the whole line of pins was deviated from the visual* line QQ1,
by an angle equal to 1245 inches, seen at a distance of ninety
feet, or about 40 ', and besides this, the pins lay in a beautiful
and nearly continuous curve, presenting its convexity towards
the valley, and decidedly without any great step or start.
This was beautifully seen when I directed the vertical wire of
the theodolite upon the forty-fifth pin, and caused it to describe

* [Misprinted usual.']

116 VISCOUS THEORY OF GLACIER MOTION. [1845.

a vertical plane.* I observed, however, a curious fact, plainly
indicated by the numerical results ; the curve crossed the axis
at the fourth pin, and attains its greatest convexity at the
twenty-fifth, "f

Fourthly. That no information might be wanting as to the
precise condition of the mass of ice under experiment, I made
a very minute examination of the state of the transverse line
with respect to the occurrence of flaws in the ice. The most
important of these was one which returned into itself, crossing
the line towards the origin of the glacier between the twenty-
sixth and twenty-seventh marks, and returning backwards
between the fortieth and forty-first without extending further
upwards. Such a flaw, even if devoid of cohesion, could only
act by allowing the piece of ice, contained between the twenty-
seventh and fortieth mark, to slip bodily forwards, leaving a
vacuity behind. No such slip is recorded in the observations,
and no such gap or vacuity was left during the continuance of
the observations for seventeen days. Nothing approaching to
an open fissure occurred in any part of the space of ice under
observation, and the few flaws noted (see the column of remarks
in the Table, p. 112, and the indications of them by dotted lines
and arrows in Plate IX. fig. 2 [of Phil. Trans.], showing their
directions) were perfectly close, and all more or less of zigzag forms,
preventing the possibility of sliding. The veined structure was
developed in every part of the section, in some parts more admi-
rably than others, as near the seventh mark, and between the for-
tieth and forty- fifth. These countless blue veins may be con-
sidered as so many flaws or partial solutions of continuity, existing
or having existed ; they are almost perfectly straight, and (as will
be shown immediately) exactly in the direction in which the rela-
tive motion of the parts of the ice demonstrated by these
experiments takes place. It would require strong evidence to
convince us that these veins are not occasioned by, and the
mechanism of, the plastic motion of the ice.

* An approximation to this effect will be obtained by stretching a fine thread
over the figure.

f From my notes made at the time.

1845.] ANALYSIS OF THE EXPERIMENT CONCLUDED. 117

The beautiful convexity of the curves in the direction of
motion which the eye at once seized, and could more accurately
distinguish by the theodolite, as having its largest sagitta at the
twenty-fifth mark, namely, almost exactly midway between the
fourth mark or convex origin, and the forty-fifth or extreme
mark, contains in itself an evidence with which no person of
correct habits of thought can fail to be struck, as proving a
regular plastic action of gravity, or other propelling force, acting
from point to point on the mass of the glacier. I made, how-
ever, a check experiment almost unnecessary, but which I will
here detail. Lest it should be alleged that the whole area under
experiment was a moving one, capable of being swung round
by the pressure upon the centre of the glacier, so that the dis-
placement of the transverse line was due to rotation of the mass
operated on round some distant centre, I took care, near the
commencement of my experiment, to fix a mark in the ice in
the same line with Q parallel to the length of the glacier, or
perpendicular to the transverse visual line at the commencement
of the experiment. This point is marked q in Plate III. fig.
3. Now, had the block of ice on which the marks Q, q, (1),
(2), (3), etc., were fixed, been revolving round some fixed or
moveable centre, the right angle (3) Q q would have remained
a right angle, and so for (2) Q q and (1) Q q. But these angles
constantly became more obtuse, and that in different degrees
depending on the convexity of the curve Q, (1), (2), (3) ; and
so of (4) and (5). It is impossible, therefore, to avoid the con-
clusion, that the solid ice was itself distorted to the amount of
the excess of progression of the more central above the lateral
stations.

The results are contained in the following Table, the five
stations being those formerly mentioned at 30, 60, 90, 120, and
180 feet from Q. The first three stations were fixed when the
visual line had an azimuth of 89° 55' [reckoned] from q [as an
origin] ; the fourth was fixed on the 14th of August, when the
visual line had revolved through 3' (its daily progression towards
q in consequence of the translation of the station Q being

118

VISCOUS THEORY OF GLACIER MOTION.

[1845.

1' 30" exactly), and the fifth on the 17th, when the visual line
was in azimuth 89° 47i'.

TABLE showing the variable azimuths (observed) of the transverse
stations with the longitudinal direction Q q.

Date.

(1.)

(2.)

(3.)

(4.)

(5.)

Angle Q1Q?.

0 i

o /

0 I

o /

o /

0 ,

August 12. 2 P.M.

89 55

89 55

89 55

89 55

14. 1 P.M.

90 6}

90 5

90 5

89 52

17. Noon.

90 26

90 24

90 23

90 8

89 47£

19.5 P.M.

90 47

90 46

90 44

90 10

89 52

21.6P.M.

90 56

90 56

90 53

90 21

90 3

23. 1 P.M.

91 9£

91 9£

91 4£

90 34

90 14

26. Noon.

91 24

91 23

91 18

90 45

90 24£

89 28J

EDINBURGH, July 1845.

[As the concluding paragraph of this paper contained a slight
numerical mistake, it has been omitted.]

[On reviewing these pages, after a lapse of thirteen years, I
feel afresh the interest of the experimental enquiry above
recorded, which, to the best of my knowledge, has not yet
been repeated. I ana impressed with the belief that it is well
worthy of repetition under more favourable circumstances. The
locality was by no means the best that might be found, although
it was probably the best amongst the glaciers of Chamouni.
The very level and almost uncrevassed portions of the glaciers
of the Aar and Aletch, and of others besides, present favourable
points for observing experimentally the plastic accommodation
of a definite portion of ice to the conditions of its motion. But
in the experiment of 1844 the drawback arising from the weather
was infinitely greater than that arising from the imperfect com-
pactness in the area of ice under observation. It is not too
much to say, that the state of the weather during my observa-
tions (to which I have referred at p. 113) could not possibly
have been more annoying and unfavourable. I hope that some
of the present race of active glacialists will resume the interest-
ing enquiry. — Nov. 1858.]

1846.] LAWS OF MOTION OF A SMALL GLACIER. 119

XIV. ILLUSTRATIONS OF THE VISCOUS THEORY
OF GLACIER MOTION.— PART III.*

§ 6. On the Motion of Glaciers of the Second Order. § 7.
On the Annual Motion of Glaciers, and on the Influence of Seasons.
§ 8. Summary of the Evidence adduced in favour of the Theory.

§ 6. ON THE MOTION OF GLACIERS OF THE SECOND ORDER.

Up to the year 1844 no attempt had been made, so far as
I am aware, to measure the rate of motion of those compara-
tively small isolated glacial masses reposing in the cavities of
high mountains, or on cols, called by De Saussure Glaciers of
the Second Order.

Some observations had indeed been made upon a glacier of
this description in 1841, and by MM. Martins and Bravais,
during a residence on the Faulhorn. But it was not at that
time known that the motion of glaciers was a continuous and
regular one, admitting of rigorous measurement even in short
intervals of time, and the importance of such observations was
overlooked. They accordingly believed that the glacier in
question had no sensible motion, and probably they did not
attempt to observe it until a subsequent year. It is impossible
now to doubt that the Blau Gletscher, near the Faulhorn, has a
movement like all other bodies of the kind.

In July 1844, I had an opportunity of passing some days
at the hospice of the Simplon, in the neighbourhood of which
exists a small glacier of the second order, easy of access, and
very fit for the experiment which I proposed to myself upon
such bodies. Its diminutive size made it all the more suitable ;
for should it be found to possess a regular motion, we are
certain that the mechanism of a glacier is contained! within the
small compass of a mass which may be conveniently examined

* [Philosophical Transactions, 1846, p. 177.] Received January 27, — Read
February 26, 1846.

f [Misprinted continued^]

120 VISCOUS THEORY OF GLACIER MOTION. [1846.

in detail in all its parts. It is lodged in a niche of the moun-
tain called the Schonhorn,* immediately behind the Simplon
hospice : we shall therefore call it the glacier of the Schonhorn.
From its inconsiderable extent, it might easily be overlooked
by a passing traveller amidst the multitude of vast and striking
objects by which he is surrounded, f It is perched, as has
been said, in a kind of niche on the northern face of the Schon-
horn, somewhere t about an hour's steep climb above the
hospice ; consequently about 1400 feet higher. The hospice
is itself 6580 English feet above the level of the sea ; the mean
height of the Schonhorn glacier may be taken at 8000 feet. I
had not an opportunity of ascertaining it more accurately.

Plate X. figs. 1 and 2 [of Phil. Trans. 1846],§ shows a sketch
of a front view of the Schonhorn taken from the opposite heights,
and a ground-plan of the glacier. The latter is sketched merely
by the eye, but the scale is furnished by some actual measures.
I first visited the glacier on the 20th July 1844. It was then
covered over, by far the greater part of its extent, with snow, as
shown in the plan. This snow is in great part manifestly perma-
nent, and the glacier is therefore in the state of neve: The general
slope is from top to bottom of the plan, and its inclination is
variable, depending upon the direction of the avalanches by
which it is fed, of which the principal descends the rapid couloir
marked C, [where] the inclination is about 35°. This avalanche
forms a sort of ridge down the glacier, as indicated by the
shading of the map, leaving a considerable space comparatively
flat to the eastward. On the west, the snow thins off from
the ridge until it exposes the ice near the part marked B, where
the slope is still considerable, being 20°, and here we have the
real mass of the glacier exposed, although the ice is not of an
exceedingly hard or crystalline character. The front or lower

* Also called Hiibsehorn, an equivalent epithet.

f The reader will not for a moment imagine that it is the Kaltwasser glacier of
which we speak, which lies also in the neighbourhood of the Schonhorn, descending
from the Monte Leone and Wasenhorn, and from which the Galerie du Glacier on
the Simplon road takes it name. \ [Misprinted somewhat]

§ [It has not been thought necessary to reproduce these figures.]

1846.] GLACIER OF THE SCHONHORN NEAR S1MPLON. 121

termination of the glacier all along presents a steep nearly pre-
cipitous surface of ice, sloping from 45° to 60°. This ice rests
on a bed of debris of rock, which appears to be inclined about
25°. Except near the precipitous termination of the glacier,
there are no apparent crevasses. The surface is uniform and
uninterrupted. Some water issues from beneath the steepest
part of the ice ; but even in the middle of the day, near the
end of July, there was exceedingly little. The length, if it may
be so termed, of the glacier, from back to front, is about 1000
feet, and its greatest breadth 1300 feet. Its surface may be
roughly estimated at twenty-six acres.

The rock of which the Schonhorn is composed, is an alter-
nation of the slaty rocks resembling gneiss with talc slate, which
are so common in this part of the Alps. To my great surprise,
on one of my visits, I heard the sound of hammers and blasting
in this elevated and remote spot ; and found two men employed
in quarrying Potstone (Lapis ollaris) for building ovens, from a
retired nook beyond the glacier ; the quarry is marked on the
plan at E.

On the 20th of July 1844, 1 ascended to the glacier, accom-
panied by M. Alt, one of the clerical members of the Simplon
establishment, and an assistant ; and I fixed upon a position,
marked St. on the rock on the east side of the glacier, for
planting the instrument, which was then directed, as nearly as I
could judge, in a line transverse to the prevailing slope of the
glacier, and the telescope was made to describe a vertical plane.
It was then sighted upon a well-marked quartz vein on the
rock on the distant side of the glacier, marked D, by which it
could at any time be brought into precisely the same position ;
the position of the instrument itself being referred to a mark
cut on the rock where it stood. Two marks were then fixed
on the glacier ; one was a pole stuck in at A, several feet into
the snow of the avalanche already described as traversing the
length of the glacier. The slope of the snow at the point A
was about 10° ; and the distance of A from the station St., by
an approximate measurement, 340 feet. 350 feet further in the

122 VISCOUS THEORY OF GLACIER MOTION. [1846.

same direction, a hole was made with a blasting iron into the
solid ice at B, where the inclination was 20°. The precise
position of these marks being determined relatively to the visual
line, the observation was finished at 4 o'clock P.M. .-

On the 23d of July we returned. The mark A in the snow
(which was so firmly driven in that it could not be withdrawn
without breaking the pole) had' advanced in the direction of the
slope exactly four inches at 1 P.M., or in sixty-nine hours ;
whilst the mark B in the ice had advanced 5J inches in the
same time ; whence we have

Velocity of A in twenty-four hours . . 1*4 inch.

Velocity of B in twenty-four hours . . 1*8 inch.
The result was what I had anticipated, although it must be con-
fessed it might be expected to be nearly the same upon any
theory of glacier motion yet proposed. The slope of a glacier,
per se, is not an index of what should be the velocity of motion
on the viscous theory. No doubt, other things being equal, the
velocity will be proportional to some function of the declivity,
and such we have seen to be fully borne out by experiments on
the Mer de Glace of Chamouni ; and in the present case, the
velocity under a slope of 20° was about one-third greater than
that under a slope of 10°. But the analogy of a river, as well
as theoretical considerations, show that the slope is but one of
numerous considerations ; such as (1.) the mass of the viscous
body ; the smaller the mass the smaller the velocity on a given
slope ;* (2.) the state of infiltration or wetness of the glacier
altering its resistance to change of form.f Without mentioning
other causes, these are quite sufficient to account for the small
velocity observed, when we recollect the very insignificant mass
of this glacier, and its dry state arising from its great elevation,
its northern exposure, and even the very inclination of its bed,
which keeps it in a state of perfect drainage, and leaves it
always in a state tending to the snowy, rather than that of
imbibition.

* Travels in the Alps of Savoy, 2d edit., p. 387. f Ibid, pp. 148, 371.

1846.] ANNUAL MOTION OF THE MER DE GLACE. 123

§ 7. ON THE ANNUAL MOTION OF GLACIERS, AND ON THE
INFLUENCE OF SEASONS.

The first estimate of the least authority on the advance of
any point of a glacier from year to year, was made by Hugi on
the glacier of the Aar, from 1827 to 1836. The method em-
ployed was to measure the distance of a well-marked block of
stone, resting on the ice from a transverse line determined by
the fixed objects on the shore. This is the only way, generally
speaking, practicable upon glaciers at a distance from habita-
tions, and where marks cannot be conveniently renewed in the
ice, from time to time, during the whole year. The velocity of
the part of the glacier immediately below the promontory, called
the Abschwung, was found to be about 240 feet per annum,
which, though neither confirmed nor invalidated by the discor-
dant measurements subsequently made by other observers on
the same glacier, has at length been substantially corroborated
by a professional surveyor, M. Wild, who has recently under-
taken the verification at M. Agassiz's request.

After having myself observed the motion of several points
of the Mer de Glace of Chamouni during the summer of 1842,
I fixed the positions of two conspicuous blocks, one near Mon-
tanvert, marked D 7 ; and another opposite the Tacul, marked
C, or the Pierre Platte (see my Map of the Mer de Glace), by
means of which I hoped to ascertain the mean annual motion
in succeeding years. With respect to the latter, or the Pierre
Platte, I was successful ; for in September 1843 I ascertained
geometrically its change of position, subject, however, to the
uncertainty of a few yards, owing to the sliding of the block
from the pedestal of ice upon which it was so picturesquely
poised,* a circumstance which happens once or twice in the
course of every summer.

From the 17th of September 1842, to the 12th of

September 1843, the advance was (in 360 days) 256'8 feet.
Or reduced to the exact year of 365 days, . 260-4 feet.
Mean daily motion, . - . . . . 8*56 inches,

* See Frontispiece to Travels through the Alps of Savoy.

124 VISCOUS THEORY OF GLACIER MOTION. [1846.

Again, being enabled to repeat the measures in 1844, I
found the advance —

From the 12th of September 1843, to the 19th of

August 1844 (342 days), . . . . 270 feet.

Proportional motion for 365 days, . . . 288*3 feet.

Mean daily motion, . . . ... . .." 9*47 inches.*

In the case of the block D 7, 1 was less fortunate. It was
very near the western side of the glacier, and though not thrown
up on the shore, yet the ice on which it rested got in some
manner so embayed or entangled, that though its motion had
been steadily watched during the winter of 1842-43 by my able
assistant, Auguste Balmat, it had scarcely moved since his
last observation on the 8th of June 1843, when I visited it in
September of the same year. It must be presumed that it had
been much retarded previously, and hence it is clearly inadmis-
sible to infer a proportional motion for the portion of the year
when it had not been observed, as I did in the Postscript at
the end of the first edition of my Travels, whilst in ignorance
of the then unsuspected retardation. The motion actually
observed was 432 feet in 322 days, being at the rate of 483
feet per annum, or 15*88 inches per day. This is, therefore,
undoubtedly below the true measure of the annual motion of the
side-part of the glacier somewhat in advance of the Chalet of Mon-
tanvert (see the position of D 7 in the Map). It may at least be
of some service as an inferior limit of the annual motion there.

In 1843 I fixed approximately the position of a block marked
P, higher up the glacier than the Montanvert, and near its left
bank, exactly opposite the spot called Les Ponts. The obser-
vation, being repeated the ensuing year, gave a motion of about
486 feet (the nature of the observation did not admit of the
same accuracy as at station C) from the 13th of September 1843
to the 9th of August 1844, or 331 days, being at the rate of

536 feet per annum,
or 17'62 inches per day.

* [The like annual motion for the interval 1844-46 was 323'8 feet, or 10'65
daily inches ; and for the interval 1846-50, 328'8 feet, or 10'81 inches daily. See
13th and 1 6th Letters below.]

1846.] BALMAT'S OBSERVATIONS THROUGHOUT THE YEAR. 125

In 1844 I made the casual discovery of one of my staves,
used to mark the position of the station A at the Angle, a little
higher up the glacier than the last, a point of which the motion
had been most carefully observed during the summer of 1842
(see Travels, p. 140). This stick still bore legibly written upon
it the date when it had been fixed in the ice at station A, and
as the painted marks on the rock of the Angle were still as fresh
as when they were made, I had no difficulty in finding the exact
position on the glacier which this mark had in any part of
the summer of 1842, and by measuring the distance to the place
where it was found (which was on a spot of the ice quite unfre-
quented by guides or any one else), I had good reason for
believing that this must be the space over which it had travelled
in the mean time ; although of course I do not ascribe to this
observation the weight of a direct measure, yet it proves an
interesting confirmation. Reckoning from the position it occu-
pied on the 1st of September 1842, it had advanced, down to the
26th of August 1844, or in' 720 days . ..952 feet,
or, per annum . . . 482*5 feet.
Mean daily motion . 15*87 inches.

It will be seen that this result is in close agreement with
that observed at station P above mentioned, which is a little
further down the glacier, but about the same distance from the
side ; for though the motion of P is somewhat greater for 1843-
44 than the mean motion of A for 1842-44, it will be seen by
the comparative observations at C already referred to, that the
glacier moved more rapidly in 1843-44 than in 1842-43.

But I am now enabled to present a view of the actual
progress of two glaciers during every part of the year from
direct observation. For these I am indebted to the intelligent
and persevering zeal of my excellent guide and assistant at
Chamouni, Auguste Balmat, of whose character I have had the
pleasure of forming a more and more favourable estimate the
longer I have been acquainted with him. To the long training
of the laborious summer of 1842, when he assisted me, he adds

126 VISCOUS THEORY OF GLACIER MOTION. [1846.

the further experience derived from my visits in 1843 and 1844,
in the latter of which especially he became familiar with the
nice precautions requisite in conducting the most accurate mea-
surements, and received instructions from me which rendered
him perfectly competent to continue by himself the simpler kind
of measurements which I have alone required of him. The
extraordinary exertions which he used to obtain the winter motion
of the block D 7, under the Montanvert, in 1842-3, have been
noticed in my former publications. On one or two occasions,
as 1 learned afterwards from himself, being unable to ascend the,
usual path to the Montanvert for fear of spring avalanches, he
actually clambered with a companion up the rugged ascent from
the source of the Arveiron, plunging continually up to the
middle in snow, for no other purpose than to make the obser-
vation which I had requested of him ; and it would be unjust
not to mention at the same time the admirable, because rare,
generosity, with which he positively refused for himself any
share of the remuneration which I pressed upon him the fol-
lowing summer, as some recompense for the fatigues and
dangers which he had braved to obtain for me this information.
With such a person, my confidence in the observations which
he has since made at points much more accessible, and with
the experience of some additional years, is complete. I do not
mean that mistakes may not occur, or even that the measures
may not be less exact than I might have taken myself ; but
from my knowledge of the man, I am nearly as confident in
their being faithfully reported, exactly as they were made, as if
I had done so myself.

With a view to lighten the labour as much as possible, I
selected two stations on the Glacier of Bossons, and desired
Balmat to select two on the Glacier des Bois (the outlet of the
Mer de Glace towards the valley of Chamouni) ; all these points
being tolerably accessible at every season of the year.

The general method of observation was the following : —
vertical holes were driven into the ice with a 4-foot blasting
iron, at the points whose motion was to be determined ; and

1846.] RESULTS OF BALMAl's MEASUREMENTS. 127

these holes were renewed from time to time as the surface of
the ice wasted. A staff of wood, 5j feet long, was stuck in
each, which projected sufficiently above the snow (which never
appears to have exceeded 2J feet deep on the glacier) to make
it visible at all seasons. During winter the staves were frozen
into the ice, and the waste being small, the holes did not re-
quire renewal. Two marks are then made of a permanent
kind ori the rocks of the moraine, or two staves driven in, or a
distant object on the farther side of the glacier was observed, so
as to mark out sufficiently a line transverse to the glacier, the
prolongation of which passes over the hole in the ice when first
made ; and the advance of the hole in the ice beyond this fixed
visual line marks the progress of the glacier. The want of a
theodolite is supplied by directing the eye past a plumb-line
suspended over the fixed mark on the moraine nearest to the
glacier, the eye of the observer being over the farthest mark.
As the spaces moved over were in most cases considerable, an
error of a few inches, or even a foot, is not important to the
result. The progress was in every case determined by means
of a line marked with English feet and inches, left by me at
Chamouni on purpose.

The results were communicated to me regularly by letter at
intervals of a few weeks during the whole year, and all questions
asked and explanations required by me were answered by return
of post.

Those who may look with suspicion upon observations made
in a remote place by a peasant of the better class, though they
may not partake of my security in the results from knowing the
character of the individual, will, I believe, have their doubts
removed by the internal evidence of this important series of
observations, which even a philosopher could not have in-
vented, and which, it will be seen, are confirmed by data of
quite another kind, over which the observer could have no
control, I mean the Meteorological Registers of Geneva and
St. Bernard.

The following Table contains, in a condensed form, the

128

VISCOUS THEORY OF GLACIER MOTION.

[1846.

results deduced from Balmat's observations at the four follow-
ing stations : — *

Bois I. On the Glacier des Bois a little way below the
Chapeau, at about one-third of the breadth of the glacier from
its eastern bank.

Bois II. On the Glacier des Bois near its lowest extremity,
behind the Cote du Piget, and near its centre.

BOSSONS I. Upper station on the Glacier des Bossons, some
way above the plateau where the glacier is usually crossed ; on
the west side, near the moraine.

BOSSONS II. Near the lowest extremity on the Glacier of
Bossons, where free from the moraine, on the western side.

INTERVALS OF OBSERVATION,

Mean daily Motion in English Inches

Bois,
No. I.

Bois,
No. II.

Bossons,
No. I.

Bossons,
No. II.

1844. Oct. 2 to Oct. 14

32-0

14-0

Oct. 14 to Nov. 2

27-8

17-0

Nov. 2 to Nov. 19

24-2

Nov. 20 to Dec. 4

11-8

...

17-3

131

Dec. 4 to Jan. 7 .

11-5

3-3

15-9

13-0

1845. Jan. 7 to Feb. 18

14-0

2-6

13-6

12-0

Feb. 18 to Mar. 18

17-0

3-0

15-4

12-8

Mar. 18 to April 17

16-9

4-6

12-9

10-2

April 17 to May 17

22-5

7-3

23-3

19-4

May 17 to May 31

37-0

8-8

429

30-2

May 31 to June 19

38-4

8-3

34-1

27-8

June 19 to July 4

42-3

ll'l

421

323

July 4 to July 18

52-1

14-6

30-6

26-4

July 18 to Aug. 6

49-0

11-9

28-8

21-4

Aug 6 to Oct. 6 .

35-7

9'9

20-6

16-5

Oct 6 to Nov. 8 .

36-4

9-8

19-4

5-9

Nov. 8 to Nov. 21

30-1

7-5

22-6

7-2

These four sets of observations are projected in Plate IV.,
where the four lower zigzag curves represent the gradation
of diurnal velocity by periods, according to the method adopted
in projecting my own observations in my Travels, p. 141.
The general accordance is sufficiently manifest, and the

* [This table is abridged from those in the Philosophical Transactions for 1846,
pp. 183 and 184, where the days and hours of each observation are more accurately
given, and the total motions. The days of observation are, in general, the same
for all the stations, but not precisely so in a few instances.]

1846.]

MOTION OF GLACIERS AT DIFFERENT SEASONS.

129

effect of the season of the year is beautifully shown, the follow-
ing being the minimum and maximum values :—

Glacier des Bois, No. I., minimum in December
Glacier des Bois, No. I., maximum in July .

Ratio of maximum to minimum

Glacier des Bois, No. II., minimum in January
Glacier des Bois, No. II., maximum in July

Ratio of maximum to minimum

Glacier des Bossons, No. I., minimum in March
Glacier des Bossons, No. I., maximum in May

Ratio of maximum to minimum

Glacier des Bossons, No. II., minimum in March
Glacier des Bossons, No. II., maximum in June

Ratio of maximum to minimum

Daily motion in inches.

3J:1

From these observations we may deduce the annual motion
from November 1844 to November 1845, with considerable ex-
actness. Allowing for the fractional parts of a year, we obtain
the following results, amongst which I have included a separate
computation of the mean daily motion for the summer period
(April — October), and the winter period (October — April) : —

Bois,
No. I.

Bois,
No. 11.

Bossons,
No. I.

Bossons,
No. II.

feet.

feet.

feet.

feet.

Motion for 365 days, November 1844 to

November 1845, . . .

847-5

220-8

657-8

489-1

inches.

inches.

inches.

inches.

Mean daily motion,

27-8

7-3

21-6

16-1

Mean daily motion, summer period,

April to October,

37-7

9-9

28-0

22-2

Mean daily motion, winter period, Octo-

ber to April, ....

191

4-7

15-8

10'7

Katio, summer: winter, motion,

2-0:1

2-1 : 1

1-8:1

2-1:1

I. From this Table we deduce, in the first place, a mean
annual motion far greater than has hitherto been observed, or
perhaps suspected in any glacier, that of near 300 yards, or

K

130 VISCOUS THEORY OF GLACIER MOTION. [1846.

almost one-sixth of a mile. This is on the Glacier des Bois be-
neath the Chapeau, where the inclination of the glacier is very
steep, adding a new illustration of the general principle,* that in
similar circumstances the velocity increases with the slope.
To this cause may be added the high temperature of the air of
the valley to which, in this part of its course, it is exposed ;
but this last cause is alone insufficient ; for

II. We find that the lowest part of the same glacier im-
mediately behind the Cote du Piget, a little way above the
source of the Arveiron, and therefore still deeper in the valley,
has a mean velocity nearly four times less, arising solely from
the diminished slope and volume of the glacier in that part.f
Hence there must be a condensation of the ice here, a pressure
a tergo, the quicker moving ice pressing against the slower, con-
solidating it, remoulding its plastic material, and sealing the
crevasses ; and a slight examination of the state of the glacier
at the points in question will show this to be the case.

III. All that has now been said with respect to the two
stations on the Glacier des Bois, may be repeated with only
numerical differences with respect to the two stations on the
Glacier des Bossons ; the one set of observations confirming
the other.

IV. In both glaciers the summer motion exceeds the winter
motion in a greater proportion, as the station is lower, that is,
exposed to more violent alternations of heat and cold ; this we
shall find to be general.

Before continuing our deductions, we would call attention
to the close relation which may be established between the
mean temperature of any portion of the year, and the velocity
of the glacier corresponding to it. This is done in Plate
IV., exactly in the same way as I did when comparing my

* Travels, 2d edit,, p. 371.

f This explains a circumstance which has always hitherto heen a difficulty to
me ; the united testimony of the best-informed inhabitants, not only at Chamouni but
elsewhere (as at Zermatt and at the Simplon), to the effect that during winter the
lowest end of a glacier, which terminates in a valley, does not greatly protrude, nor
force the snow before it. This arises in fact from the comparative smallness of the
motion which the tongue of such a glacier appears to possess, especially in winter.

1846.] MOTION COMPARED WITH TEMPERATURE OF THE AIR. 131

observations in the summer of 1842 with the corresponding
changes of temperature.* That is to say, I have projected by
periods (corresponding to the intervals of observation on the
glaciers) the mean temperatures as observed at Geneva, and at
the Great St. Bernard, which are regularly published in the
Bibliotheque Universelle, the average of which (separately de-
duced from the mean of daily maxima and minima, and projected
in the upper part of the figure) may represent not inaptly
the average temperature to which the glaciers in question, and
especially the middle and lower regions of them, are exposed ;
and further, this average possesses the advantage of being de-
rived from data, wholly unconnected with the place or parties
where and by whom the observations on the motion of the
glaciers were made, and therefore are free from the remotest
suspicion of either in any degree influencing the other.

Mean Temperatures (by periods) on the Centigrade Scale,
observed at Geneva and the Great St. Bernard, f

Means of Max. and Min.

Geneva.

St. Bernard.

Cent.

Cent.

o

0

1844, Oct. 2 to Oct. 14

12-97

1-29

Oct. 1 4 to Nov. 2

8-45

— 1-66

Nov. 2 to Nov. 19

7-47

— 2-02

Nov. 19 to Dec. 4

2-27

— 8-18

Dec. 4 to Jan. 7

— 0-27

— 8-33

1845, Jan. 7 to Feb. 18

— 1-32

—10-39

Feb. 18 to March 18

1-01

— 8-24

March 18 to April 17

6-42

— 4-97

April 17 to May 17
May 17 to May 31

10-80
11-87

— 0-60
0-68

May 31 to June 19

18-20

6-29

June 19 to July 4

17-35

5-17

July 4 to July 18

18-42

6-14

July 18 to Aug. 6

18-24

5-46

Aug. 6 to Oct. 8

16-23

3-83

Oct. 8 to Nov. 8

7-81

— 0-60

Nov. 8 to Nov. 21

9-20

— 3-54

A general comparison of the curves of temperature and those

* Travels, p. 141. ^ f [Abridged from the original tables.]

132 VISCOUS THEORY OF GLACIER MOTION. [1846.

of glacier motion (more particularly on the Glacier des Bois)
affords a proof of the justness of the principle laid down by me
in 1842, that the motion of the ice " is more rapid in summer
than in winter, in hot than in cold weather, and especially more
rapid after rain,. and less rapid in sudden frosts ;"* the evidence
of the connection is plainer by mere inspection than any detail
could make it. But I request attention to the apparent anomalies
of the curves, as affording a stronger evidence of the fidelity with
which the measurements have been made, and to the truth of the
plastic theory, than perhaps even the general coincidence just
referred to.

If the velocity of the glacier depend upon the completeness
of its infiltration with water, rendering the whole an imbibed
porous mass like a sponge, it cannot depend solely on the mean
temperature of any period, but also upon the wetness of the sur-
face, whether derived from mild rain, from thawing snow, or
from any other meteorological accident which the register of
the thermometer cannot of itself indicate.f Further, a thick coat-
ing of snow on the glacier must defend it from the excessive
cold of winter just as it defends the earth and plants, and con-
sequently the minimum of motion will not necessarily coincide
with the minimum of temperature. Now, to estimate these more
irregular causes is not so easy ; but some light is thrown upon
them by a register of the weather and state of the snow, volun-
tarily kept for me at Chamouni by Auguste Balmat; which
forms a valuable supplement to the thermometrical register of
Geneva and St. Bernard. Although the daily details would
take up too much space, I will endeavour to give a faithful
abstract of them so far as to give a general idea of the climate
of Chamouni from October 1844 to November 1845. This
diary includes (at my request) occasional notes on the state of
the source of the Arveiron, which are of considerable interest.

* Fourth Letter on Glaciers, Edinburgh Philosophical Journal, Jan. 1843 ; and-
p. 35 of this volume.

f " The proportion of velocity does not follow the proportion of heat, because any
cause, such as the melting of a coating of snow by a sudden thaw, as in the end of
September 1842, produces the same effect as a great heat would do." — Travels,
2d edit. p. 372.

1846.] WEATHER AT CHAMOUNI DURING THE OBSERVATIONS. 133

WEATHER AT CHAMOUNI.

1844. October. — A good deal of rain during the month,
which, on the 10th and 16th, fell as snow on the hills (nine
inches at Montanvert), and subsequently to the latter day the
glacier at the Montanvert was not clear of snow during the
winter. 14th. Source of the Arveiron diminished to one-fourth
(of the summer volume.) Ice-vault more than half closed.

November. — Till 14th much rain and snow ; fine with frost
after. 20th. Source of the Arveiron very low ; has not shifted
its usual position.

December. — Weather generally fine throughout ; cold most
severe from 7th to 12th.

1845. January. — The weather continued splendid till the
20th ; greatest cold from — 2° to —5° Eeaumur. 19th. The
vault has disappeared at the source of the Arveiron. 20th,
The first snow fell which lay at Chamouni, and continued from
this day, attaining a depth of 1 J foot in February. Up to this
time all the secondary heights, even the Breven and Flegere,
were clear of snow, and the weather suitable for chamois hunt-
ing. Occasional snow till the end of the month.

February. — Snow at intervals all the month. 13th.
Greatest cold of the season ; thermometer — 15° Reaum. fol-
lowed by fine weather. 20th. Snow lies 2-^ feet deep at the
upper stations on the glaciers ; 1 J foot at Chamouni. The arch
of the source of the Arveiron has wholly disappeared, but the
water issues at the usual places as in summer. The water is
reduced to a small amount and may easily be stepped across.
It is still ichitish and dirty, though less so than in summer ;
except when a change -of weather is threatened (when it is as
dirty as in summer.)* Same date. The glacier of Bossons has
extended itself much. " On ne s'y reconnait presque plus." It
is advancing towards the moraine of 1818 ; and the lower end
is at least seventy feet high.

* This important remark proves that in the middle of winter a temporary ?
rise of temperature of the air over the higher glacier regions (which is the precursor
of bad weather) not only produces a thaw there, but finds the usual channels still
open for transmitting the accumulated snow water.

134 VISCOUS THEORY OF GLACIER MOTION. [1846.

March 1st — 3rd, mild, with rain; 3rd — 13th, cold; 15th,
heavy rain. Alternate rain and fine till end of the month.
27th. Not half a foot of snow lying at Chamouni. The source
of the Arveiron has not opened a vault. The quantity and
muddiness of the water the same as at the last report.

April. — First week fine ; second week cold with snow ;
•changeable to the end of the month. 16th. Source of Arveiron
has not much increased in water since the middle of March.
In the end of April the snow first disappeared from the lower
part of both glaciers.

May.— The first half of the month fine, with occasional
snow ; the second half changeable, with rain. 1 7th. The
source of the Arveiron has increased three-fourths (means pro-
bably in the ratio of four to one) since the middle of April, and
is dirty. The ice-vault is not yet formed. 26th. The Glacier
des Bossons advances rapidly, and is crumbling into pyramids.
The end of the glacier is at least eighty-five feet high, and
advances considerably, particularly during the month of May ;
and widens greatly.

June. — A changeable and wet month ; a very late season.*
The snow did not entirely disappear from the Mer de Glace
opposite the Montanvert till the beginning of July. 6th, 7th.
The vault opened at the source of the Arveiron. The quantity
of water since the end of May is the usual summer supply.

July. — Commenced with warm weather. 5th. Thermo-
meter 27° Reaum. The snow has disappeared from the ice
opposite Montanvert, but some patches remain on the way to
the Jardin. The Mer de Glace is much higher in level (about
forty feet) than in former years, and the marks made in [the
rock at the Angle (in 1842) are all covered. The crevasses
much the same as usual. The glacier of Bossons has also in-
creased greatly, and appears to be approaching its old moraines.
The register for the greater part of July has not come to hand.

August. — A very changeable rainy month. 8th or 9th.

* It will be seen from the temperature curves that the thermometer fell con-
siderably in the latter part of June, both at Geneva and St. Bernard.

1846.] DEDUCTIONS FROM OBSERVATIONS OF VELOCITY. 135

The arch at the source of the Arveiron fell in, and did not form
again during the season.

September. — Also a changeable month. Rain twelve days.

October. — A very fine month. No rain mentioned after
the 7th.

A careful examination of this interesting register will ex-
plain several of the apparently irregular inflections of the curves
of glacier motion. Thus (to continue our general remarks, p.
130) we find,

V. At the upper station on the Glacier des Bois the least
velocity occurred in December, whilst at the lower station (and
at both of those on the Bossons) a minimum, coinciding also
with that of the temperature of the air, took place in January.
This coincides with the important fact noted in the preceding
register, that the upper part of the Mer de Glace was covered
with snow from the 16th of October, which only lay in the valley
of Chamouni from the 20th of January ; the snow screening
the ice from the extremity of the cold.

VI. The comparative march of the two glaciers bears a
remarkable relation to their positions and form. In the Bossons
we detect at once the sudden transitions and seemingly capri-
cious changes of a torrent ; in the Mer de Glace we have the
stately and regulated flow of a river, in which the slighter
variations are absorbed by the predominant inertia of a com-
paratively stable mass. Now the glacier of Bossons is, as every
one who has seen it knows, a mere icy torrent, " a frozen cata-
ract," which descends in a continuous mass from the level of
the Grand Plateau of Mont Blanc to that of the Valley of
Chamouni with very little impediment, with no confining bul-
warks of rock, ho contracting straits ; and throughout this
great vertical height of at least 9000 feet, the angle of descent
is very steep indeed for so vast a mass. On the other hand,
though the part of the Mer de Glace, called the Glacier des
Bois under the Chapeau, is very steep, its " regime " is regulated
by the supply derived from the reservoir glacier above, and,

136 VISCOUS THEORY OF GLACIER MOTION. [1846.

precisely as in rivers of great magnitude and length of course,
and of moderate declivity, it yields sluggishly to impulsive or
retarding forces which are checked and opposed by the multi-
tude of sinuosities, the embaying of the ice in rock-bound
expansions of the channel, the struggle of its passage through
defiles, and the enormous friction of its lower surface. Yet,
lest we might attribute the irregularities of the torrential glacier
to causes quite local and uncertain, we find them reflected more
or less distinctly in the movements of the neighbouring one.
Thus the anomalous retardation in the end of March and be-
ginning of April appears in three stations out of four, as does
that in the first half of June, showing clearly that it is not an
error of observation. It appears that the thaw of the winter's
snow during the month of May, saturating the pores of the
glacier with water, produced (as we know that a thaw always
does) a sudden and violent march, especially of the more sus-
ceptible or torrential glacier. So completely had this sudden
move forced on the~glacier of Bossons, encumbered by the spring
avalanches, and loaded with all the fragments and snow masses
which had remained temporarily suspended during the winter
months, that the lower part of the glacier (as we read in the
memoranda to the register) advanced and widened greatly, to
an extent which it had not done for many years past, and
seemed to change its whole character ; and in February a
similar temporary increase of volume had taken place ; " on ne
s'y reconnait presque plus," writes Balm at ; thus accounting
for the particular accession of speed which appears in that
month. In both cases, after the rapid march in February and
in May, a reaction takes place ; the material is deficient, the
excessive pressure has been removed by the previous overflow,
and a lull occurs in March and in June.

VII. These irregularities, such as they are, even should we
fail in entirely explaining them, are at least not to be attributed
entirely to errors of observation, since different observations
(which, it is to be recollected, were sent to England in so rough
a state that they required to be reduced and computed before

1846.] DEDUCTIONS FROM OBSERVATIONS OF VELOCITY. 137

the variations of velocity could be deduced from them) agree
amongst one another, and agree with the phenomena casually
noted in the Meteorological Register. They are very trifling
in the movement of the Glacier des Bois, which presents a curve
of remarkable regularity, giving a minimum about the end of
December, and a maximum in July. The coincidence with the
curve of temperature is greater throughout than we could have
expected, considering the important difference of circumstances
which occur in autumn and in spring, when the thermometer
stands nearly alike ; the first chill of autumn depriving the
glacier of its fluid pressure more effectually than the severer
cold of winter which is tempered by its snowy covering, whilst
in spring the first relaxation of the bands of frost saturates the
icy mass with the impetuous streams of melted snow, as effectu-
ally as the intensest heat of summer. In fact, the velocity
would probably be greatest in spring, were it not that then the
ice has attained its greatest consolidation by the slow but con-
tinued effect of the winter's cold penetrating its upper layers,
though after all probably to no very great depth. But this is un-
doubtedly the reason why the minimum and maximum approach
so near to one another in point of time in the torrential glacier
of Bossons, and it receives an important illustration from the
independent fact of the observed condition of the source of the
Arveiron, which (see the Meteorological Register), though very
small in February, was still whitish and dirty before a change
of weather, showing that the bands of frost were not so strong
as to prevent a temporary relaxation of thaw throughout the
mass of the glacier even in winter ; and although the mean
temperature of the air had been rising ever since the middle of
January, and the greatest cold had occurred early in February,
we find that at the end of March the source of the Arveiron was
still as small as in February, and that owing to the coldness of
the spring, it had not even increased very much till the middle of
April, when it almost suddenly resumed its summer volume.
Now during all this time the velocities of the glaciers underwent
but little change, — some oscillations backwards and forwards, —

138 VISCOUS THEORY OF GLACIER MOTION. [1846.

but 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.

VIII. Being curious to see how far a relation might be
established between the temperature of the air and the motion
of the glacier, independent of the irregularly acting causes
above adverted to, I projected, in Plate V., the motions of the
several points of the glaciers in terms of the temperature of the
air for the periods already mentioned.* It is to be recollected,
however, that the observations of the thermometer were not
made on the spot, and, indeed, it would have been difficult to
have fixed upon a spot which should represent the mean cir-
cumstances of the whole glacier. Perhaps, therefore, the
average of the observations at Geneva and St. Bernard (the
mean of whose elevations is 4750 English feet above the sea,
and therefore between that of Montarivert and Chamouni) may
represent pretty fairly the climateric conditions of the inferior
parts of the Glaciers des Bois and Bossons. NoW, if we examine
the curves of Plate V., we are struck with their almost perfect
flatness until zero of the centigrade scale of temperature is reached ;
but, the thawing point of ice past, the velocity manifestly goes on
increasing with the temperature, in a ratio which would appear
to be tolerably uniform if we neglect the irregular inflections of
the curves.

IX. I am unwilling to multiply deductions which every
intelligent reader will draw for himself; but one more I must
add. It very clearly appears that the variations of velocity due
to season are greatest where the variations of temperature of
the air are greatest, as in the lower valleys ; but it also appears
from Remark VIII. , that variations of temperature below 0°
centigrade, or 32° Fahrenheit, produce almost inappreciable
changes in the rate of motion of the ice. Hence, from this
circumstance alone, we should deduce that in the higher parts

* [This diagram has been rendered a somewhat more complete and accurate
representation of the numbers given in pages 128 and 131, than the original figure
in the Philosophical Transactions]

>,

*.l

1846.] MUTUAL INFLUENCE OF THE PARTS OF A GLACIER. 139

of the glacier (where, for example, it freezes almost every night
in summer) the variations of velocity should be least, and, indeed,
comparatively small at different seasons. This is well illustrated
by comparing the summer motions of the stations [on the Mer
de Glace] D, A, and C, mentioned in the first part of this section
[page 123], with their annual motion, which exhibit a much slighter
excess in favour of the summer period than in the lower stations
which we are now discussing. The same thing was observed
by M. Agassiz's surveyors on the glacier of the Aar, who at
first saw, in this not very great inequality, an objection to my
theory. On a more searching investigation, however, the objec-
tion disappears, as in their later writings they have acknowledged.
Their position of observation far up on the glacier of the Aar,
in a spot having a mean temperature near the freezing point,
if not lower, had a summer daily motion of 7*99 inches, and a
mean daily motion during the whole year of 6*41 inches.* Now,
at station C, or the Pierre Platte, on the Mer de Glace, the
mean motion for July 1842 was 10 inches, and for the whole
year, 1842-43, it was 8*56 inches. It is quite evident that the
motion of any point in the midst of a glacier is controlled by
that of those which precede and follow it, and that it does not
necessarily result, either that all must at once suffer a similar
increase or diminution of speed, or that the times of maxima
and minima, or even the general form of the annual curve, shall
be the same.f This leads to an important practical result which
we shall follow out in the next section.

* Comptes Kendus, Dec. 9, 1844.

f [These considerations will go far to explain any seeming irregularities in the
results, not only of these tabular motions, but of those more recent observations
which will be cited in a later part of this volume [Sixteenth Letter on Glaciers].
Both the glaciers on which continuous measures have been taken in summer and
winter present an enormous variety in elevation and declivity at different parts of
their course. The points of observation to which the tables of page 128 refer, are
some thousand feet below the level of the greatest portion of either glacier to which
they belong. The warmth of spring, and the consequent gorging of the glacier with
water derived from thawing snow, occurs, in the localities observed, weeks, or even
months, before the same causes are in full action on the general course of the glacier.
Hence a local acceleration of the points of observation, followed later by an accelera-
tion due to the increased supply from the middle and higher regions. The result
may be either a double maximum of velocity (which occurs in some instances), or a

140 VISCOUS THEORY OF GLACIER MOTION. [1846.

§ 8. SUMMARY OF THE EVIDENCE ADDUCED IN FAVOUR OF THE
PLASTIC OR Viscous THEORY OF GLACIER MOTION.

It is often difficult to obtain a calm and full hearing for
any new theory or experimental investigation ; not because
there is any antipathy to novelty, or that experiment is under-
valued, but simply because, in an age of bustle and struggle for
pre-eminence, each man is so busy with his own reputation, or
the means of increasing it, that he has no leisure to attend to
the claims of others ; to which may, perhaps, be added, that in
the general diffusion of knowledge and acquirement, each reader,
finding something in every course of experiment or reasoning
which he knew, or thinks he knew before, is apt to run off with
the chain of ideas which that one familiar link suggests, and
losing patience to follow an argument of which he thinks he
can, by his own penetration, anticipate the close, he sits in
judgment on errors which are of his own invention, and confronts
the author with arguments and opinions already thrice refuted
and rejected by himself. In an age when all men would be
teachers, and all write for the press, the lot of an attentive
reader falls to few.

I am far from saying that I have been more than usually
unfortunate in this respect. But having, like others, seen my
opinions disfigured for want of sufficient attention to apprehend
them, or the arguments by which they are supported ; ignor-
ance of first principles hinted at, and even errors of observation
imputed, where it was convenient that such ignorance and such
errors should be presumed ; I claim the privilege of stating
afresh, though very briefly, the leading opinions which I do
hold, and some arguments for them, which, if not altogether
new, may be placed in a new light.

superposition of the velocities due to local and to general causes, which tends to
displace the seasons of maximum and minimum motion, and thus to render the con-
nection of velocity with temperature less direct and immediate than it would other,
wise be. The middle region of a glacier having a moderate and tolerably uniform
declivity is therefore most proper for studying the effects of temperature. Nov.
1858.J

1846.] ICE WHEN BRUISED FORMS NEW ATTACHMENTS. 141

My chief analogies for the illustration of glacier motion
have been drawn from the motion of a river, and by that
comparison it in a great measure stands or falls. Slight and
partial as is our knowledge of the mechanics of imperfect
fluids, the explanation which I have given is founded upon that
knowledge, and it appears to me to be sufficiently precise to
warrant the inference of an identity of the mechanism in the two
cases ; namely, that the movement is due to the internal pres-
sures, arising from the weight of the mass, communicated partly
or principally in the manner of hydrostatic pressure through-
out a body whose parts are capable of moving or being shoved
over one another (by that exertion of force which Dr. Thomas
Young calls Detrusive Force*, which overcomes what is com-
monly called the Friction of Fluids), so that the velocities vary
from point to point of the moving body, being most rapid near
the surface and centre, and least so near the banks and bottom.
So viscous fluids move, so bodies (even brittle solids, such
as hard-boiled pitch) possessing the ordinary properties of
solid bodies often do, if sufficient time and sufficient force be
allowed ; f the efficiency of time being chiefly this, that a
pressure insufficient to produce instant detrusion, will, sooner
or later, cause the particles to slide insensibly past one another,
and to form new attachments, so that the change of figure may
be produced without positive rupture, which would reduce the
solid to a heap of fragments. This change may either take
place without any loss of homogeneity, or by numerous partial
and minute rents not everywhere communicating, and therefore
not necessarily destructive of cohesion, which may be termed a
bruise.

A glacier is not a mass of fragments. — As the analogy of
the glacier to a river in which the fluid principle is greatly in
defect, and the cohering or viscous principle is greatly in
excess, is the theory which I maintain, it is evident that the
analogy of a stream of sand, or loose materials shot from a

* Lectures, I. 135.

f See Professor Gordon's Experiment, Philosophical Magazine, March 1845.

142 VISCOUS THEORY OF GLACIER MOTION. [1846.

cart, or any other comparison with an aggregate of incoherent
fragments or individual masses, must be wrong if mine be
right. And I feel confident, not only that such an incoherent
mass could not move after the manner of a glacier,, but also
that attentive inspection of a glacier at once contradicts such
an idea.

On the first point, I maintain that a rugged channel, like
that of a glacier, with a moderate slope, being packed with
angular solid fragments, would speedily be choked, and that
farther pressure from behind (for such a mass can only convey
thrusts, not strains) would tend to wedge the fragments more
tightly. Some grains of dry sand will slide easily down a
plate of glass; but try to thrust it forcibly through a narrowing
tube, or even a uniform one, the lower end of which rests on a
surface over which the sand has poured, and your effort is vain,
the tube will sooner burst ; and even rocks may be blasted
rather than the power of the wedge yield.* If the figure of
the bed or channel be in any degree irregular, that is, have
expansions and contractions, however smooth its surface, how-
ever small the sliding angle of ice upon that surface, the
choking of a strait or contraction by the piling of the fragments
will be as complete and effectual as if the lateral friction were
excessive. Now in point of fact we have such cases as this ; —
a glacier 2000 yards wide (the Mer de Glace at the Tacul)
issues by an orifice or strait 900 yards wide ; — the glacier of
Talefre, a nearly oval basin, pours out its annual overcharge by
an orifice the breadth of which is but oae-third of its lesser,
one-sixth of its greater diameter.f On the supposition of
jostling fragments, the facility of motion is increased, as the
comminution is greater. The impossibility of the discharge of
a fragmentary solid through a gorge by long stripes fractured
parallel to its length, and constituting parallelepiped ons of a
certain breadth, is evident.

* See Huber-Burnand's conclusive experiments on this subject, Ann. de Chimie
et de Physique, xli. 166, and Fechner's Repertorium, i. 65.

f See the Map of the Mer de Glace and its tributaries in my Travels in the
Alps of Savoy.

1846.] IMPORTANCE OF CREVASSES USUALLY OVERRATED. 143

Crevasses — In the second place, I maintain that actual in-
spection shows that a glacier is not the mass of fragments nor
of parallelopipedons which some persons have, naturally enough
at first sight, supposed it to be. In truth there is not an
approach to such a condition in those glaciers which move over
moderate slopes of considerable extent, which have very properly
been assumed by all writers as the criterial examples of any
theory; for it is not denied that portions of glaciers and glacier
tributaries do sometimes fall piecemeal over precipices, each
fragment descending by its separate and individual gravity, in
the manner of an avalanche, although I am disposed to believe,
indeed am sure, that the number of such instances is smaller
than is usually imagined ; and the angle requisite for such a
tumultuous mode of descent is far greater than it has, perhaps,
always hitherto been considered to be. To him who would
form a just estimate of the mechanical constitution of a glacier
— who would consider it as a whole — without always distracting
his attention from the length and breadth of the problem by a
minute attention to its lesser features, — I would earnestly
recommend the frequent and attentive survey of a glacier or
glaciers from a considerable elevation above their level and
under varying effects of light. Had I confined myself to
studying crevasses on the surface of the glacier, measuring
their depths, injecting the ice with fluids and taking its tem-
perature ; useful and important as these inquiries are (and I
might almost include the fundamental and most important
inquiry of all, that of ascertaining the velocity of its parts), I
should have been much longer in seizing the general truth of
the individual character of a glacier, the importance of the
fluid-like connection of its parts, the perfectly secondary
importance or unimportance of the fissures by which it is often
traversed. The traveller who winds his tortuous and sometimes
perilous path among these crevasses, forgets, in the fatigue of
his circumventions, in the wonder of his curiosity at their
beauty and seemingly unfathomable depth, in the appalling
steepness of their sides and the comparative insecurity of his

144 VISCOUS THEORY OF GLACIER MOT TON. [184C,

own footing — he forgets, I say, in the midst of all these claims
upon his attention, his curiosity, and his strength of mind, the
comparatively large surfaces of unbroken ice over which he
heedlessly walks, and the small, the very small depth, at which
most of the yawning crevasses which make such an impression
on his imagination, dwindle into mere slits ; — and when his
walk is finished, he imagines that a glacier is a mere network
of fissures interlacing in all directions. But let him gain a
bold height above its surface, 800 to 1000 feet at least,* so
that the whole may be spread somewhat like a map before
him, yet not too distant to prevent his seeing the number and
forms of the crevasses, and estimating their area compared to
that of the unbroken ice, his opinion is first shaken and then
changed. He sees in the glacier a whole, which, regarded as
such, is merely scarred, not dissected by these fissures ; — he
sees a mass as capable at least of conveying strains as thrusts ;
of which the cohesion is no more destroyed than (to use a
comparison which I long ago employed) a parchment sieve is
incapable of being stretched because it is covered with fine
slits.

I am confident that this will be plain to every unprejudiced
person who will make the observation which 1 have recom-
mended, and I have no hesitation in stating my belief that it
will be found to be fully confirmed by M. Wild's map of the
glacier of the Aar, should it ever be published; I say so without
having any recollection how the matter stands, although I once
had an opportunity of seeing that fine work for a few minutes ;
and the verification of this remark, by positive measurement,
will, so far as I see, be the chief result likely to flow from

* I may mention, as the very best stations which I am acquainted with, the
summit or higher slopes of the hill of Charmoz ahove Montanvert, Station G*,
above Trelaporte, and a point directly above the Couvercle at least 1200 feet
higher than the Mer de Glace, which may easily be reached from the glacier of
Talefre. Other glaciers offer of course similar points, but few so advantageous ;
the glacier of the Aar from the Schneebighorn, the lower glacier of Grindelwald
from the slopes of the Mettenberg, the glacier of the Khone from near the Mayen-
wand, and that of Zermatt from the Eiffelberg, are examples.

184G.] DISTRIBUTION OF CREVASSES. 145

the patient and disinterested labour of that competent sur-
veyor.

But if this be true in a merely superficial plan, how much
more true would it be if we could pare off the upper stratum
of the glacier, and view a horizontal section of it at a depth of
a hundred feet ! The depth of the crevasses has, I am per-
suaded, been as much exaggerated as the thickness of the ice
of the glacier has been underrated. In how few cases (where
a glacier does not descend tumultuously) can we let a plumb-
line down even fifty feet without grazing the sides ! and to
what an insignificant fissure has the gaping crevasse dwindled
even at that small fraction of the glacier's thickness! Supposing
the crevasse to become uniformly narrower, how soon would it
be extinct !

Again, the crevasses which traverse the surface of the
glacier, have almost always a determinate direction or direc-
tions, of which the simplest type seems to be that of perpen-
dicularity to the veined structure,* which, generally speaking,

* [Travels in the Alps, p. 171. As the differential velocities considered in
vertical plane, to which the frontal dip of the veined structure (see above, pp. 19, 59)
is principally due, are for the most part caused by intense resistance in the direction
of motion, the pressures accompanying or producing these differential velocities do
not tend directly to produce crevasses unless the glacier be free to spread itself
laterally. In this latter case the crevasses open parallel to the movement of the
ice or to the length of the glacier, and we have then the radiant system of cre-
vasses already several times referred to, as in the glacier of the Rhone, etc. (pp. 7,
20). The powerful lateral compression exerted by the confining walls of the Mer
de Glace, the glacier of the Aar, and other canal-shaped glaciers, prevent this mode
of fissure. Under these circumstances, as I have pointed out in a paper in the
Philosophical Magazine for May 1845 (which being chiefly controversial, is not re-
printed at length here), we must consider the perpendicularity of the crevasses
to the veined structure to have reference to the differential motions which produce
the veined structure estimated in a horizontal plane, or disembarrassed from the
effect of hydrostatic compression producing the frontal dip.f I have in the same
place illustrated the correctness of this view by a reference to the plastic models
described in § 1 of the present paper, where, practically, the frontal resistance
does not interfere with the result, and where, therefore, the lines of fissure or cre-
vasses due to tension, are throughout perpendicular to the lines of differential
motion or the veined structure due to horizontal forces only. The direction of
these last, as well as of the crevasses perpendicular to them, are sufficiently indi-
cated in figs. 2 and 6 of Plate I., and also in the woodcut on page 79 of the
present volume. Nov. 1858.]

f Phil. Mag., May 1845, p. 408.

146 VISCOUS THEORY OF GLACIER MOTION. [1846.

occasions a convexity of the lines of fissure towards the origin
of the glacier. Opposite Montanvert the crevasses form two
systems inclined 65° to one another, but this appears to be a
casual occurrence arising from a fresh strain being imposed on
the ice owing to its rigidity when the direction of the bed or
trough suddenly changes, and the two-fold systems probably
coexist but for a short space, one tending to close whilst the
other opens. Be this as it may, unless where a glacier is
falling headlong in the manner of a cascade, the crevasses do
not produce any actual dislocation of its mass into blocks or
fragments, since the crevasses rarely intersect even where most
numerous, but almost invariably thin out in the solid mass
whilst another crevasse takes its origin a little to one side or
other leaving a firm connection of ice between them ; and the
difficulty and danger of traversing a glacier where [it is] much
crevassed, does not arise from the necessity of leaping from square
to square of ice, but from having to traverse these bridges of
icy communication which even there link the glacier together,
and which are almost always sharp on their upper edge when
the season of the year is pretty far advanced, owing to the
continual dripping.

The occurrence of crevasses which cut up a glacier into
square or trapezoidal blocks, is sufficiently infrequent to deserve
notice. Such occur when a glacier of the second order de-
scends over a boss of granite, or a surface convex in all
directions. We have then radiating crevasses combined with
concentric ones, producing a tartan-like appearance. Such
may be seen in a glacier of the second order on the south side
of the Aiguilles of Charmoz and Grepon, above the Glacier du
Geant ; and it is a very convincing proof of the essential tenacity
of a glacier, that, with a surface so scarred and intersected, the
fragments do not fall away in avalanches. This only is to be
explained by the consideration that, thin as are the glaciers of
the second order, the apparent dislocation is only superficial.*

* [When they fall in avalanches, or are about to do so, on the verge of a pre-
cipice, the detached trapezoidal blocks received from De Saussure (or rather from

1846.] CREVASSES DO NOT MATERIALLY ASSIST PROGRESSION. 147

Were the inequality of the central and lateral movement of
the glacier mass to be attributed to longitudinal fissures or
discontinuities, by means of which broad stripes of ice slide
past each other, we should have to demonstrate the existence
of such fissures, which could not be always close unless either
(1) The surfaces were mathematically adapted to slide over one
another, or (2) The ice possessed sufficient plasticity to mould
the surfaces to one another's asperities, in which case the
plasticity would alone be sufficient without the discontinuity to
explain the motion of the ice. These longitudinal fissures,
cutting the common transverse fissures perpendicularly, would
divide the glacier even where most level into trapezia, and no
transverse crevasse could be straight-edged, but must be jagged
like a saw, or cut en echelon.* Such a phenomenon never occurs
unless where a glacier is moving torrentially, or with great dis-
turbance and down a steep. There such longitudinal fissures
may occasionally be seen, but they form the exception and not
the rule. It has been demonstrated by an elaborate proof in
§ 5, that the only trace of longitudinal discontinuity in the nor-
mal condition of the glacier is to be found in the veined struc-
ture, which, being caused by a partial discontinuity at a vast
number of points, admits of an insensible deformation of the
glacial mass without sudden or complete rents, or slips, or the
formation of zigzag crevasses.

The existence of the great transverse crevasses, which, even
in glaciers not moving torrentially, divide the surface of a glacier
by rents perhaps 2000 feet long,f have been thought by some
to be comparable to beams of an elastic material, supported at
the two ends, and bending under their own weight forward, in
the middle. Were this the case, it would scarcely modify the
plastic theory as I have propounded it ; because in order that
such a bar of ice should conform to the known movements of
the glacier, opposite the Montanvert for instance, the centre

his guides) the name of seracs, a term derived, if I recollect rightly, from a pro-
vincial word applied to masses of curd chopped into square pieces in the dairies of
the Alpine chalets. ]

* [See Plate II., fig. 1.] f Travels, p. 171, 2d ed.

148 VISCOUS THEORY OF GLACIER MOTION. [1846.

must continually gain upon the sides at the rate of 150 feet per
annum at least, consequently the limit of cohesion of an elastic
solid would soon be overpassed, and plasticity in the material
sufficient to explain the whole motion would inevitably be
admitted at last. Independently of this, it is evident, that were
such a flexure essential to the motion, the lines of crevasses
would be convex in the direction in which the glacier is moving
instead of towards its origin.

Argument from the Equable Progression of Glaciers. — The
equability of the motions of the various parts of a glacier, united
as I have shown them to be by intricate relations,* must, I
think, appear conclusive to every one capable of forming a just
opinion on the subject, that the relative movements of the
various parts of the glacier are due to the action of forces at
small distances and to the antagonism of molecular cohesions and
molecular strains, and not to the casual jumbling of a quantity
of rude fragments. To myself, I confess that this now appears
the strongest argument of all for considering the glacier as a
united mass like a river, in which there is a nice equilibrium
between the force of gravitation, acting by hydrostatic pressure,
and the molecular resistances of the semi-solid ; the degree of
regularity of the law which connects the partial movements is
wonderful, and I maintain that it is inexplicable except upon
the viscous theory. Thus (1.) The glacier moves continually,
summer and winter, day and night, and never by fits or starts ;
for if it does — if gravitation overcomes mere friction, it occasions
a shock or avalanche ; (2.) Its mean annual motion is nearly
alike from year to year ; (3.) The relative velocities of points
widely distributed over the glacier (but exposed to similar influ-
ences of climate), change simultaneously in the same directions,
often in the same proportions ; thus " the variation of velocity
in the breadth of a glacier is proportional to the absolute velo-
city at the time of the ice under experiment." f (4.) The pro-
gression of velocity from the side to the centre is marked by

* See § 5 of this paper, pages 108 and 109. f Travels, p. 149.

1846.] CONSTANCY OF THE RELATIVE VELOCITIES. 149

insensible gradations.* (5.) When we compare the motion of a
given point of a glacier any day of one year and the same day
of another, the probability is that the velocity will be exactly
the same, if the season be equally hot or cold ; hence, surely, a
most unexpected result, which I first announced in 1842, that a
few days observation of a glacier will enable any one to compare
its mean rate of motion over its various parts and with different
glaciers.^ Thus, the motion of a point marked D 2 on the Mer
de Glace, was in 1842, from August 1 to August 9, 16J inches
daily ; from August 9 to September 16, 18 inches ; now next
year, 1843, one observation at the same point in August gave
16 inches ; and in 1844, one observation in September gave 17J
inches. But still further (6.) The very law of flexure of the ice
is the same from year to year : a series of stations across the ice
at the Montanvert gave, in 1842, the following (simultaneous)
relative velocities : \ —

1-000 1-332 1-356 1-367

The same points being recovered in 1844, the relative motions
were (by a single observation of the space moved over in five
days) —

1-000 1-339 1-362 1-374,

ratios almost the same, but slightly increasing, which corresponds
with the fact mentioned above (3), that when the absolute velo-
cities are greater, the relative velocities are so too, which was
here the case, for the velocity denoted by 1*000 was a little
greater in the second case than in the first. §

Tensions and Thrusts. — The occurrence of open crevasses
plainly indicates the existence of strains in the ice of glaciers
producing disruption, at least partially. Hence some writers
have precipitately inferred that the whole glacier must be in a
state of tension ; an uncertain inference surely in a problem of

* See § 5 of this paper.

f [Regard must however be paid to the circumstance that the different parts even
of the same glacier are not exactly simultaneously affected by change of season.
See Note to page 139.J

\ Travels, 1st edit. p. 146.

§ [The continuation of these observations to 1846 is given in the Thirteenth
Letter below.]

150 VISCOUS THEORY OF GLACIER MOTION. [1846.

singular complexity, and one which is not warranted by a more
accurate analysis. Yet for a time rival theories seemed poised
on the inappropriate question, " Are glaciers in a state of
internal tension or compression '?" Even if the glacier moved
as a mass of fragments, therefore without tension, the cohesion
must first have been broken before it could be reduced into
fragments. I have been inconsiderately censured for quoting,
with approbation,* the observation of M. Elie de Beaumont, that
a glacier appears to be rather in a state of distension than com-
pression, whilst I adopted a hydrostatic pressure, acting from
the origin as the source of motion. A careful examination of
the passages in question will show that my assent to the view
of M. Elie de Beaumont was limited to portions of the glacier,
and especially to those portions most crevassed, the parts, namely,
which connect the sides and centre, and which serve to drag the
more sluggish, because retarded, lateral portions after the freer
central part on which the vis a tergo acts with most advantage ;
and in a direction generally parallel to the blue bands, so far as
they are due to inequalities of motion in the horizontal plane.f
My earliest attempts to obtain clear views of the internal forces
acting on a semi-rigid body, impelled by self-contained hydros-
tatic forces, convinced me how little could be founded on the
completeness of any mathematical investigation of them, which
in our present state of knowledge may well be considered as
hopeless ; and reserving to myself the not so difficult task of
extricating at a future time the more important practical laws of
these strains and thrusts, I very carefully avoided, in my first
publication, any allusion to what might be considered as their
actual distribution ; a distribution varying not only from point
to point of the glacier surface, but throughout its thickness, and
most undoubtedly varying also for the same point at different
seasons of the year, or even changing its sign, so that a tension
at one season may become a thrust at another.

I had no reason to repent of this caution, from which I only

* Travels, 1st edit. pp. 178, 370 ; 2d edit. p. 370 and Note.
fSee Philosophical Magazine, May 1845, p. 408.

1846.] THRUSTS AND TENSIONS—FORMATION OF CREVASSES. 151

departed so far in my Seventh Letter on Glaciers, published
subsequently, as to deduce in an approximate manner, from
elementary mechanical laws, the directions of the surfaces of
tearing within such a mass as I had described, upon the simple
supposition that the hydrostatic pressure acting uniformly, the
tendency of motion of any particle will be in the direction of
least resistance when all the resistances are taken into account,
and that the surfaces of rupture will divide particles whose
motions are dissimilar, but will not divide particles whose
motions are alike. I repeat that I had no reason to repent of
my abstinence from theorizing, when I found that a far better
mathematician than myself, taking up the inquiry where I had
left it, and after applying himself for a long time to the exclusive
mechanical considerations which the viscous theory had suggested,
Jeft the subject, as I conceive, little more advanced than he had
found it, and fell into some mistakes and inconsistencies, almost
inseparable from this way of treating a problem which extensive
observation and patient thought can alone disentangle.

Formation of Crevasses. — It has been seen in the third sec-
tion of this paper, that De Saussure, and almost all his succes-
sors, have regarded the crevasses as accidents of glacier motion,
and not essential to it ; and in this view I of course concur.
Nevertheless, the study of crevasses is one of considerable,
though secondary interest, and is very far indeed from being
completed. It requires, among other things, a very sedulous
attention to the state of the glacier at various seasons, and even
whilst covered with snow ; and it requires further a two-fold
classification of crevasses, into those which may be considered
as proper to the mass of the glacier, and those which merely
seam its surface.

I will first speak of the last point.

Though the formation of a crevasse betokens a local dis-
tending force, such a force cannot with any certainty be referred
to the whole depth of the glacier below the point where the
chasm opens. On the contrary, there is a fully greater proba-
bility that under that very spot the ice is compressed. If one

152 VISCOUS THEORY OF GLACIER MOTION. [1846.

cause of a crevasse be, as is universally acknowledged, a pro-
tuberance or inequality in the bed over which the ice is impelled,
for the same reason that a beam, [supported at one end and]
broken by means of weights, is in a state of longitudinal com-
pression below, where its surface is concave, and of distension
above, where its surface is convex, the cracks in the glacier may
be due solely to this last and partial cause. Superficial crevasses
may consequently be occasioned where there is no general dis-
tension of the mass, either (1.) By the shoving of the semi-rigid
glacier as a whole, over a convex declivity; or (2.) From an internal
turgescence arising from hydrostatic pressure, resisted by the
intense friction of the anterior or more advanced parts of the
glacier, which, causing the line of least resistance to be upwards
and forwards, forces the pasty mass to tumefy or increase in thick-
ness, exactly as it has been seen in § 2 [of this paper, page 91],
that sluggish lava streams do in a similar case. But if the tume-
faction be pushed beyond the limits of plasticity of the superior
and more distended -portions, they must burst and assume the
crevassed forms actually observed in the plastic models described
in p. 78. Hence the existence of crevasses not only does not
always result from a state of general distension in the glacier,
but may arise from the precisely contrary condition of great
internal compression. This argument is well illustrated by the
recent observations of M. Agassiz's co-operators on the glacier
of the Aar, whose observations I have elsewhere shown* to be
incompatible with any other view than that of intense longitu-
dinal compression in the mass generally, and yet the surface
abounds in crevasses of the usual form and dimensiens.f

The manner of formation of crevasses generally, including
such as may betoken a real distending force acting on any part
of a glacier throughout its thickness, is not only a most curious
question in itself, but suggests others which a correct theory of
glacier motion can alone answer. If a crevasse once formed

* Ninth Letter on Glaciers. [See p. 70 of this Volume.]

f [This seems too strongly stated. There are few glaciers less crevassed than
that of the Lower Aar, and especially towards the middle of its breadth. Nov. 1858.]

1846.] CHARACTER OF CREVASSES CHANGES WITH THE SEASON. 153

remain a fissure in the ice for ever after, why is the horizontal
projection or ground-plan of the crevasses of a canal-shaped
glacier convex towards the origin of the glacier, and not protu-
berant in the direction of its motion, as the ascertained greater
velocity of the centre would assign ? Why are the crevasses
for the most part vertical and not inclined forwards, or at least
not notably so, on the same account ? Why, if the glacier be
urged downwards by a longitudinal force distending it, do not
the crevasses continually widen in proportion as they are further
from the origin ? These questions seem incapable of a sound
answer except by supposing that the crevasses are, at least in a
great degree, the fresh production of every spring, and arise
from the sudden start which the glacier makes when that
extremity which descends into the valley begins to experience
the thawing effects of returning summer. I should not wish to
speak positively upon what involves a difficult if not impossible
observation, — the state of the glacier with respect to crevasses
whilst still under the winter's covering of snow. But the fact
of the transverse direction of the crevasses, or even their con-
vexity towards the origin, from year to year, seems to admit of
no other explanation. But besides this, I can affirm, from a
careful observation of the crevasses of the Mer de Glace from
June to September in one year, that the changes which they
underwent were such as preclude the possibility of a cre-
vasse of autumn being merely preserved by the snow of winter,
and re-appearing afresh in spring as it had done the previous
one. The thing is impossible, because the character of the cre-
vasse is essentially altered. In order that an autumnal crevasse
may become a spring crevasse, it must be sealed up, annihilated,
and opened again. A glance at the three sections in Plate II.
fig. 4, will illustrate this. No. 1 shows crevasses freshly opened
soon after the snow has quitted the surface of the ice — the
edges are sharp, the sides vertical, the openings so small that
they may be easily stepped across, and in other instances they
are not wider than may admit the blade of a knife. No. 2 shows
the crevasse opened to its widest extent by the acceleration of

154 VISCOUS THEORY OF GLACIER MOTION. [1846.

the motion, by the force of the sun which has altogether wasted
away the side with the southern exposure, and by the copious
drippings of the melting ice and mild rain. No. 3 (which as
well as No. 2 is taken from a sketch on the spot, No; 1 being
done from recollection) shows what I have elsewhere called the
state of collapse of the glacier, which affords the most direct
possible evidence of its plastic condition ; for we there see, not
merely the prominences worn away and blunted by the heat of
summer, but subsiding into the hollows, the crevasses being
choked by the yielding of their sides, and the glacier again re-
sumes a traversable character, only that the plane surface
of spring is changed into irregular undulations preparatory
to a complete amalgamation of the whole glacier into one
mass.*

The collapse is thus described in my Journal of 1842, writ-
ten at the time, and therefore more emphatic and unbiassed
than after my theoretical views had been matured and pub-
lished. " 1842, Sept. 16, Friday. The level (of the Mer de
Glace at the ' Angle ') has sunk since the 9th of August, nine
feet 8J inches. The effect of this immense fall is abundantly
evident in this part of the glacier. On my first visit this time
(i. e., after an absence of a month), on the 10th, I was quite
struck with its shrunk appearance, as I was to-day with the col-
lapsed state of the crevasses. There cannot be a question but
that the glacier had subsided bodily into its bed, and that the
semifused pliancy of its materials causes them to recover a uni-
form and lower level. The crevasses are much less deep than
in July arid August, as at that time they were larger and more
numerous than in June. They are collapsed and (opposite
Trelaporte) almost soldered up ; the edges all rounded and
melted by the sun's heat." The phenomena here described, " the
shrunk appearance," "the semifused pliancy" "the soldered
crevasses," " the rounded edges," convey to the attentive spec-
tator an intuitive conviction of the plasticity of ice at the thaw-

* See Travels, p. 174; and Fourth Letter on Glaciers [pages 27 and 28.]

1846.] DISTENSION AND COMPRESSION ALTERNATE. 155

ing season, which no words can express, no mathematical sym-
bols weave into a demonstration. I can only say that it is
easier to believe than to disbelieve ; and that, sooner or later, it
will, I doubt not, be generally admitted.

Considering the crevasses as chiefly superficial in the normal
glacier (I mean that of which the inclination is not excessive),
it is evident that the formation of the crevasses must depend
mainly upon the configuration of the bed. Where the section of
the bed parallel to the length of the glacier is convex upwards,
there the tension at the surface will cause the crevasses to ex-
pand ; when the bed is concave and the surface is being com-
pressed, the crevasses tend to close. Hence the surface of the
glacier descending, an irregular bed may be alternately in a state
of distension and compression, and the crevasses do not tend to
widen indefinitely, which would be the case if the whole glacier
were distended. This tendency in the crevasses to expand and
contract in accordance with their position is beautifully seen in
viewing the Mer de Glace from a height, as we have recom-
mended. The steep fall opposite Trelaporte shows the expan-
sion of the crevasses, but the comparative level opposite the
little glacier of Charmoz gives it time to recover its solidity by
the general closing of the crevasses under compression. The
careful study of such a scene as this gives a more clear insight
into the glacier phenomena than any other part of the inquiry,
excepting only the measurement of velocities.

Law of Velocities. — To these velocities we now return.
The varying velocities in different glaciers, at different seasons,
and in different parts of the same season, are all in accordance
with the motions of a viscous or plastic body. They depend
upon the slope ; being greatest, ceteris paribus, when the slope
is greatest ; and upon the climate to which the glacier is ex-
posed, being greatest in glaciers which descend into deep val-
leys, and least in those which, though very steep (such as that
of the Schonhorn described in § 6), are placed in so elevated
and therefore dry and cold an atmosphere as to afford insuffi-
cient water to moisten the snowy mass or neve, and which are

156 VISCOUS THEORY OF GLACIER MOTION. [1846.

therefore endowed with very feebly hydrostatic qualities.* This is
demonstrated on the one hand by the extreme smallness of their
motions, and on the other by the insignificant streams of water
to which they give birth even in the height of summer. ' In any
individual glacier the velocity of the parts must (on any theory)
vary with the area of section through which the ice stream has
to pass ; but yet it may happen that the contraction of a valley,
if not accompanied (as is often the case) with an increased slope,
will oppose so great a resistance to the efflux of the mass, that
under intense longitudinal compression its forward motion is re-
tarded, and the condition of uniform discharge is satisfied by the
accumulation of the ice in a vertical direction, the rise of the
surface being necessarily accompanied with a thrust from below
upwards, and a sliding of the particles over one another in that
direction. This appears conclusively to be the case for a great
extent of the lower part of the glacier of the Aar, as already
mentioned, and affords the most direct evidence which could be
desired, that the kind of internal motion necessary for producing
the frontal dip in the veined structure (which arises from tear-
ing or crushing in sliding in the vertical plane)f was correctly
foreseen.

The law of velocities at different points of the axis of a
glacier from its origin towards its termination, must evidently
depend upon the configuration of each particular glacier. It
may be constantly increasing from the origin to the extremity,
it may be diminishing, or it may have alternations of increase
and diminution ; and upon this circumstance the frequency and
magnitude of the crevasses will mainly depend. But the regime

* [This consideration (or rather the observations on which it is founded) proves
convincingly the important part which water plays as a component part of a glacier.
A bed of mere indurated snow or neve, though it is evidently more incoherent than
glacial ice, owing to its dryness, is, as we see, far less mobile than the glacier pro-
per. It acts like sand, transmitting pressure with difficulty (see page 142) ; it is
also, notwithstanding its state of division into granules, intrinsically harder than
ice perfectly lubricated with water, and probably very slightly colder because dry.
See below, page 166, and later in this volume, the paper On some Properties of Ice
near its melting point. — Nov. 1858.]

f Seventh Letter on Glaciers. [Page 59, above.)

1846.] INFLUENCE OF SEASONS ON VELOCITY — ABLATION. 157

of the glacier, by which we mean to express the combination of
circumstances determining its motion, varies from one season of
the year to another, owing not only to the general influence of
heat and cold, but also to the progressive communication of that
influence to portions of the glacier in successive stages of ele-
vation. Evidently the extremity nearest to the valley will
receive the earliest and most violent impression of solar heat,
whilst the middle and upper regions are involved in complete
winter. Partial dilatations must take place in spring, partial con-
densations in the decline of the year ; as is evident from the
consideration that temperatures inferior to freezing do not sen-
sibly affect the motion of the ice (see above, p. 138) which
higher temperatures do, consequently the influence of season
will be chiefly felt in those parts of the glacier where the tem-
perature of the air seldom falls in summer to 32°, whilst the more
stable motion of the higher part acts as a drag or equalizer
upon the whole system. The condition of violent distension
produces crevasses, that of violent compression produces the
frontal dip of the veined structure, or that share of it which is
due to the relative motions in a vertical plane. The longitudi-
nal veins will result whether the axis of the glacier be distended
or compressed. Hence the reason why the frontal dip is diffi-
cultly seen in all the middle region of a glacier which, like the
Mer de Glace, is subject to much extension due to great and
increasing declivity, and to be well seen must be sought for in
the higher parts of the glacier, as above Trelaporte, at the foot
of the Couvercle,* and in glaciers subject to great compression,
as that of La Brenva, the glacier of the Rhone, the Aar, etc.

Ablation of the Surface. — One phenomenon is most satis-
factorily explained by the variations of velocity established and
illustrated in this paper. The collapsed state of the glacier after
the hot summer of 1842, and the absolute lowering of its surface
level by thirty feet in the space of a few months, had struck me
as requiring an energy altogether extraordinary in kind and
degree to restore next spring the level which had been lost, in

* Travels, 2d edit. p. 167.

158 VISCOUS THEORY OF GLACIER MOTION. [1846.

order to allow for an equal ablation the succeeding summer ;
and at first I was disposed to admit so much of the dilatation
theory to be true as would account for the swelling of the sur-
face in a vertical direction by the freezing during winter of the
infiltrated water.* Further reflection convinced me, however,
that this explanation was insufficient and also not required, and
I accordingly concluded, " that the main cause of the restora-
tion of the surface is the diminished fluidity of the glacier in
cold weather, which retards (as we know) the motion of all its
parts, but especially of those parts which move most rapidly in
summer. The disproportion of velocity throughout the length
and breadth of the glacier is therefore less ; the ice more
pressed together and less drawn asunder ; the crevasses are con-
solidated, while the increased friction and viscosity causes the
whole to swell, and especially the inferior parts, which are most
wasted." t I have nothing to add to this explanation, except
that the observation of the motion throughout the whole
year confirms it in every particular. The more elevated por-
tions of the glacier, which during a large portion of the year
are exposed to a mean temperature under 32°, move in a
manner comparatively uniform, the lower extremities undergo
great oscillations in their speed (in the ratio of four or five to
one, see page 129) ; hence the attenuation during the summer
regime, which is owing to the drag taking place downwards in an
excessive degree ; but the winter's cold, equalizing in some
measure the velocity everywhere, brings the plasticity into full
action, fills the crevasses, and swells the surface to its old level.
As it is universally admitted that the glacier proper does
not grow in thickness by snowy accumulations, the important
variations in its level in different years J cannot be ascribed to
the severity of certain seasons increasing the mass of snow

* Fourth Letter. [Page 34 above.]

•f- Travels, p. 386, 2d edit. [See also the very striking analogous case of lava
streams mentioned in § 2 of this paper, page 91.]

\ For instance, it has been seen from Balmat's narrative (p. 134 above), that
in 1845 the glacier attained a much higher level at the Angle than it had done for
three previous years at least, since all the marks of measurements which were cut

1846.] OBJECTIONS TO PLASTICITY CONSIDERED. 159

falling upon it, but rather to the prolongation of the winter cold
into spring and summer, which causes the condensing or accu-
mulating process to be in excess, and therefore the thickness of
the plastic mass to accumulate beyond its due amount.

Thus we have the following phenomena, all independently
observed, reconciled and explained by one hypothesis ; the
general convexity of the crevasses upwards, notwithstanding the
excess of motion in the centre ; the general vertically of the
crevasses, notwithstanding the retardation of the bottom ; the
perfect state of the crevasses every spring succeeding their
visible collapse in autumn ; the ascertained velocity of different
parts of the glacier, and the diversity of the annual changes
which these velocities present ; the seemingly opposed facts
showing the glacier to be subjected to powerful tension, pro-
ducing crevasses, and yet to be under a compression which pro-
duces in some places the frontal dip ; and finally, the renewal
of the level of the ice during winter, which has been lost partly
by superficial melting, but as much or more so by the attenua-
tion and collapse of the glacier during summer. These various
effects of one cause, though they do not embrace all the pheno-
mena of glaciers, certainly include a very remarkable and com-
plicated group of facts.

Plasticity — Veined Structure. — I certainly never expected,
when promulgating the viscous theory, that it would have met
with so much opposition on the ground that the more familiar pro-
perties of ice are opposed to the admission of its plasticity ; and
that the fragility of hand specimens should be considered as
conclusive against the plastic effect of most intense forces acting
on the most stupendous scale upon a body placed in circum-
stances which subject it to a trial, beneath which the most mas-
sive constructions of the pyramid-building ages would sway,
totter, and crumble. In an age when generalizations of the

on the rock in 1842 were concealed : and he attributes this, apparently with reason,
to the extreme lateness and coldness of the spring. [See also the observations on
the glacier of La Brenva in the Twelfth Letter below.]

160 VISCOUS THEORY OF GLACIER MOTION. [184G.

more obvious kinds are no longer proofs of genius and perspi-
cacity, and when popular writers on science delight to startle
their readers by showing how bodies the most dissimilar possess
properties in common ; in an age in which gradations of pro-
perties and organs have been studied with such persevering
sagacity, and in which so many unexpected qualities have been
discovered ; — when iron is classed as a combustible, when metals
are found which float on water and which catch fire on touching
ice, when a pneumatic vacuum is formed and maintained in
vessels five miles long, and whose sides are ripped open twenty
times a day;* — when, moreover the simpler abstractions of former
times are being daily overset, when no body seems to possess
any one property in perfection, and all seem to possess imper-
fectly every quality admitting of degree ; — when adamant is re-
jected from our vocabulary, and softness means only less hard-
ness, and the definition of a perfect fluid is as imaginary as that
of a solid without weight ; — when a vacuum and a plenum are
alike scoffed at, and even the heavenly bodies toil through media
more or less resisting ; — when no substance is admitted to ex-
pand uniformly by heat, when glass may be considered a con-
ductor of electricity, and metals as imperfect insulators; — in
these days, when the barriers of the categories are so completely
beaten down, I had not expected to meet with so determined
an opposition to the proposition that the stupendous aggrega-
tion of freezing water and thawing ice called a glacier, subjected
to the pressure of thousands of vertical feet of its own substance,
might not under these circumstances possess a degree of yield-
ing, moulding, self-adapting power, sufficient to admit of slight
changes of figure in long periods of time. Still less could I
have anticipated that when the plastic changes of form had been
measured and compared, and calculated and mapped, and con-
firmed by independent observers, that we should still have had
men of science appealing to the fragility of an icicle as an un-
answerable argument ! More philosophical surely was the
appeal of the Bishop of Annecy from what we already know to

* [In the Atmospheric Railway.]

1846.] LIMITS OF DUCTILITY OF ICE. 161

what we may one day learn if willing to be taught : " Quand
on agit sur un morceau de glace, qu'on le frappe, on lui trouve
une rigidite qui est en opposition directe avec les apparences
dont nous venons de parler. Peut-etre que les experiences faites
sur de plus qrandes masses donneraient d'autres resultats.* "

JL *s

The " ductility" is indeed not great; the compact ice even
of the slowest moving glaciers bears evidence, in the veined
structure, or " blue bands," to the bruise which it has received
from the all-powerful strain which has acted on it. When the
difference of motions is excessive, or the slope occasions the
speed to be greater than permits the gradual molecular adapta-
tion of the semi-rigid parts to one another, the masses are
broken up and fall more or less tumultuously ; the strain being
then removed by the dislocation, the veined or bruised structure is
invariably extinguished at last. I shall quote a series of ex-
amples of the gradation of phenomena, which I conceive to be
plainly connected by a common cause.

1. In any torrential glacier, such as the Glacier des Bossons,
the upper part of the glaciers of La Brenva, Allalein, or the
Rhone, and many others, the fractures are so numerous that the

* Theorie des Glaciers de la Savoie, p. 84. Quoted in my Travels, p. 367, 2d
edit. Since this paper was read, Mr. Christie, Secretary of the Royal Society, has
kindly communicated to me a very striking remark upon a well-known and easily-
repeated experiment. The experiment is this : If, in the course of a severe winter,
a hollow iron shell be filled with water and exposed to the frost with the fuze-hole
uppermost, a portion of the water expands in freezing, so as to protrude a cylin-
der of ice from the fuze hole ; but if the experiment be continued, the cylinder con-
tinues to grow, inch by inch, in proportion as the central nucleus of water freezes.
" In the first instance,'' says Mr. Christie, " a shell of ice containing water was
formed, no doubt, within the iron shell, and the fuze-hole might be filled by the ex-
pansion of the water in the act of freezing ; so that there may be no reason for
attributing plasticity to the ice as far as this goes ; but the shell of ice once formed,
and the fuze-hole filled with ice, the subsequent rise of the ice must have proceeded
from the ice of the interior shell being squeezed through the narrow orifice. No
thawing took place during the process. Does not this show plasticity even in very
small masses of ice ? " I have also been lately informed, on excellent authority,
that in a new work by a most eminent German mineralogist, the plastic character
of ice in masses is assumed as an admitted fact. In corroboration of what has been
said in the text, I may farther add, that whilst these sheets are passing through the
press, I observe in the Athenaeum (June 20, 1846), an account of a patent process
for moulding solid tin into tubes and other utensils, in the course of which it is stated
that " tin under a pressure of about twenty tons to a circular inch will run accord-
ing to the law of fluids."

M

162 VISCOUS THEORY OF GLACIER MOTION. [1846.

ice descends in blocks, almost as water in a cascade often does
in spray, and hence, the internal strains being destroyed, no
structure is developed, or if previously developed, tends to wear
out.*

2. In a glacier moving torrentially, that is, with frequent
and considerable changes of velocity, but without being divided
into blocks by intersecting crevasses, we find real internal cracks
in the ice, some feet in length, and an inch or more in thick-
ness, marked by the pure frozen water which fills these spaces
in the comparatively opaque whitish ice of which glaciers de-
scending rapidly from the region of the neve are composed-
Such are peculiarly visible in the lower and more accessible
region of the glacier of Bossons ; f perhaps the most instructive
which can be named as showing these infiltrated cracks, which
by their dimensions, direction, and in every other particular,
form a true link between the longitudinal dislocations of a tor-
rential glacier and the perfect veined structure or bruise into
which it passes by imperceptible gradations, including a perfectly
regular development of the frontal dip, where we might expect
it to be well shown, for the observations of page 128 show that
the lowest portion of the glacier of Bossons moves slower than
its middle portion ; there is therefore a manifest longitudinal
compression arising from the friction of the bed.J

3. The next stage is that of the perfect bruise or veined
structure, best seen in the most united and least fissured parts
of glaciers with rocky sides and moving over a moderate slope.
Whatever increases lateral compression (without however ne-
cessitating dislocation), such as the union of two or more
glaciers in one, tends to develope the structure more perfectly.§
Such cases are well seen on several parts of the Mer de Glace,
and of the glacier of Miage.H

* See Third Letter on Glaciers ; page 24 of the present volume.

f See Travels, p. 181.

| The internal rents in the lava of Zafarana referred to in § 2 of this paper, and
figured in Plate I., fig. 8, present a perfect analogy with those of the glacier of Bos-
sons, and appear to he due to the same cause.

g Third Letter [p. 24, above.]

|| See the figures of the structure of the glacier of Miage. Travels, p. 197.

1846.] GRADATIONS OF DISCONTINUITY IN GLACIERS. 163

4. In very wide glaciers, moving with feeble velocities, the
veined structure is slightly developed, except near the sides,
simply because the twist being small the ice is hardly bruised.
Nor can we wonder to find the structure at the distance of
many hundred yards from the sides of a vast slow-moving
glacier of this description, if developed at all, to be complex
and irregular, exhibiting twists such as I have figured in my
Travels, p. 164, and which are peculiarly conspicuous in the
magnificent glacier of Aletsch. This circumstance finds a pre-
cise analogue in the case of a great river, such as the Rhine,
or indeed in any river moving with a very slight inclination ;
the excess of velocity of the central above the lateral parts,
not very great at any rate, is distributed over such a space
that the slightest casual disturbance of the current, from an
irregularity in the bottom or sinuosities of the course, pro-
duces local differences of velocity, occasioning ripples and eddies
in various parts of the breadth. If these ripples and eddies,
in other words, differential motions of adjacent particles, could
be visibly represented by using differently coloured fluids, they
would undoubtedly afford sections exhibiting undulations and
contortions exactly like those which the ice presents in the
cases mentioned above. We claim therefore the apparent
exception as a real proof of our general rule.

5. In the neve proper, no true veined structure is developed ;
first, Because, whilst the mass is snowy, its powdery nature
yields without admitting of a fracture or bruise ; secondly,
Because the true neve has rarely any lateral compression worth
mentioning, being widely spread, and not contained between
steep barriers ; thirdly, Because its motion is altogether very
small; lastly, Because its extreme dryness does not afford water
enough to percolate its substance and there to be frozen ; *
when it does so, it ceases to be neve.

On these grounds I hope that the theory of the veined

* [Or, according to the view which 1 first adopted on a renewed examination of
the glacier in 1846 (a few months after these pages were written), because moisture
and pressure, combined with internal friction, had not moulded the granules of the
snow into a uniform consistence. See Letter Thirteenth, below.]

164 VISCOUS THEORY OF GLACIER MOTION. [1846.

structure, so important to that of glaciers, may be considered
as explaining a number of intimately connected phenomena.

" The glacier struggles between a condition of fluidity and
rigidity."* " A glacier is not a mass of solid ice, but a com-
pound of ice and water more or less yielding, according to its
state of wetness or infiltration." f " The pressure communi-
cated from one portion to the other will not be the whole
pressure of a vertical column of the material equal in height to
the difference of level of the parts of the fluid considered ; the
consistency or mutual support of the parts opposes a certain
resistance to the pressure, and prevents its indefinite trans-
mission. . . A glacier is not coherent ice, but a granular
compound of ice and water." { " When the semi-fluid ice
inclines to solidity during a frost, the motion is checked ; if
its fluidity is increased by a thaw, the motion is instantly
accelerated. . . It is greater in hot weather than in cold,
because the sun's heat affords water to saturate the crevasses."§
Such were the terms in which, within a few months after sug-
gesting the viscous theory, I expressed my opinion of the
influence of the compound structure of the glacier, a mass
composed, not of ice alone, but of ice including water in its
countless capillaries, never frozen || even in winter. The quality
of plasticity or viscosity resulting from the union of a nearly
perfect fluid with an imperfect solid is seen in very numerous
and familiar instances, as for instance in sand, which is itself
devoid of any tenacity until its interstices have been saturated
with just so much water as to cause it to flow ; or in the still
more familiar instance of water-ice prepared for the table, in
which the varying proportion of the solid and fluid ingredient
gives to it every shade of consistency, from a brittle solid to
a liquor, including suspended solid grains. The prodigious
effect of capillary infiltration in determining the motion of even

* Third Letter on Glaciers, August 1842, [p. 23 above],
f Travels, 1st Edit., 1843, p. 175.

j Travels, p. 367., Edit, 1843. $ Ibid, p. 372.

U Ibid, p. 361, 372.

1846.] EFFECT OF WATER IN THE SUBSTANCE OF THE GLACIER. 165

the most solid and ponderous bodies, breaking up their parts,
and giving to the motion of the whole a more or less river-like
character, is seen in the frequent case of land-slips, as for in-
stance that of Goldau. And scarcely less instructive are the
numerous examples, cited in the first section of this paper, of
huge masses of almost cold and brittle lavas being pressed on
with a uniform and graduated motion, by the almost unimpeded
hydrostatical communication of pressure from the yet active
fluid which circulates unseen in their pores. With this analogy
before me, I replied in 1844 in the following terms to the
question, " How far a glacier is to be regarded as a plastic
mass ?" — " 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 semi-fluid body transmitting pressure
horizontally, and whose parts might change their mutual posi-
tion 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 as it is 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 communi-
cates 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 crackling mass of seemingly
rigid ice in which it is, as it were, bound up.*

Now the water in the crevices does not constitue 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

* Sixth Letter, [page 51 above.]

166 VISCOUS THEORY OF GLACIER MOTION. [1846.

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 compres-
sions, 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 re-attachment after
its parts-have suffered a bruise, according to the violence of the
action to which it has been exposed.

The action of warm weather in accelerating the movement
of the glacier is plainly due to the abundance of the water
saturating its pores ; but this may act in two ways — first, by
rendering the frame-work of ice less brittle when it is in the
very act of dissolving by the circulation of water in a per-
fectly fluid state through its pores,* and secondly, and more
particularly, from the hydrostatic effect of gorging a porous mass
with fluid. When an incipient frost dries even momentarily
the surface of the glacier, the vast porous mass begins to drain.
This is a very slow process, owing to the resistance to the
passage of a fluid through very long and complicated canals.
Were it not so, glaciers would be entirely dry after sunset and
in winter, which is not the case. The hydrostatic pressure
within the whole glacier is however sensibly diminished by the
process of drainage ; this is evident from watching the level of
water in a vertical hole of any depth made within the solid ice
of the glacier. After much rain or heat this level is always
higher than after dry cold. In the former case the glacier may
be said to be gorged, the supply of water from the surface
exceeding the power of the drainage to carry it off. The cir-
culating vessels are therefore overcharged. In the latter case
the superficial supply is stopped, the drainage goes on slowly

* This I think is undeniable, from the appearance of the collapsed crevasses
above referred to, notwithstanding the difficulty of imagining any variation in the
sensible heat of water circulating in ice. It is not the only fact in the glacier
theory which seems to require some modification of the commonly received laws of
latent heat at the very limit of congelation and liquefaction. [See later in this
volume, the Sixteenth Letter on Glaciers, and the paper entitled " On some Pro-
perties of Ice near its Melting Point."]

1846.] ADDITION TO MB. CHKlSTIE's OBSERVATION. 167

though uninterruptedly, and the level of the water in the vertical
shaft slowly descends, indicating the diminution of internal
pressure. If it were not for the capillarity of the ducts, it is plain
that no effective hydrostatic pressure would be developed at all ;
the flow being equal to the supply, no part of the vis viva would
be expended in producing internal pressures. With this con-
cluding observation I commit the Plastic or Viscous Theory of
Glaciers to the impartial judgment of those qualified to decide
on its merits in explaining facts, and on the variety of difficult
and complicated considerations which opposed and still oppose
themselves to a complete development of it.

Edinburgh, Jan. 10, 1846.*

ADDITION TO THE FOOTNOTE at page 161 of the preceding
Paper ON THE PLASTICITY OF ICE ON A SMALL SCALE.

[I take this opportunity of recording, that soon after the
publication of this paper (in 1846) I made some experiments
connected with Mr. Christie's ingenious observation. The
freezing of water was performed in strong glass vessels, so that
the manner of protrusion of the ice could be better examined.
It was conceivable that the expulsion of the ice in the cast-iron
shell through the cylindrical opening was facilitated by the
internal pressure punching out, as it were, successive cylinders
of ice from the spherical shells of ice successively formed, or
that the extrusion was accomplished by a series of cylindric
fractures, and not by the general moulding of the plastic ice
from a wider through a narrower outlet. To show that the
latter and not the former assumption was correct, I took some
greasy matter of a bright red colour, and introducing it by the
finger through the aperture of the strong glass vessel, I anointed
with it the inside of the glass all round the internal orifice or
throat of the aperture, keeping, however, the walls of the aperture

* [Printed by mistake 1845 in the Phil. Trans.]

168 VISCOUS THEORY OF GLACIER MOTION. [184G.

itself, and even the closely adjoining expansion of the interior
surface, carefully and perfectly clean. Yet when the protrusion
of the ice cylinder took place, the surface of it, even at the
height of an inch or two ahove the neck of the bottle, was
smeared with red colour. In one of the specimens the definite
ring of red was, as it were, transferred from its position on the
internal expansion of the bottle to the surface of the compara-
tively narrow cylinder of ice which had been discharged through
the aperture. Now this could not possibly have occurred unless
the plastic substance of the ice had been forced laterally and by
a converging pressure from all sides (up even to the particles in
contact with the interior of the glass), so as to be forced through
the contracted outlet as a tenacious fluid under its own pressure,
or a plastic solid subjected to a considerable force would do
under like circumstances. The Frontispiece, Plate VI.*fig. 1,
taken from a coloured chalk sketch made at the time* from one
of these experiments, will explain more distinctly my meaning.
It shows the impress of the coloured ring transferred from the
comparatively large surface whence it was derived to the cylinder
of small diameter into which it has been compressed. Figure
2 shows the appearance of the protruded ice when partly thawed,
the curved surfaces of air-bubbles indicating the graduated effect
of friction as the distance varied from the glass, which appears
to be consistent only with a molecular plasticity of the ice. In
these experiments the slow progress of congelation of the interior
water, which is the source of the intense pressure, is eminently
favourable to their development, while it also bears some analogy
to the extremely gradual internal movements of a glacier.
Were it attempted to produce by intense pressure acting for a
few minutes what we here produce in many hours, or even some
days,t the effect, though perhaps externally analogous, would be
deficient in the evidence of plasticity. Nov. 1858.]

* [In the winter 1846-7, if I recollect rightly.]

f [Mr. Christie's experiment lasted several days. His letter, which is now
before me, is dated 4th April 1846.J

PLATE 71

Figlp.170

Kg. S. p. 173.

Soiitli

North

Centre

Fig. ^.p. 173.

200fi.

rv

En SchenctLitir EdinburgL

1846.] ON THE DEPRESSION OF THE GLACIER SURFACE. 169

XIV. ELEVENTH LETTER ON GLACIERS.* Addressed
to PROFESSOR JAMESON.

Observations on the Depression of the Glacier Surface — Ablation and Subsidence
distinguished and ascertained — On the Kelative Velocity of the Surface and
Bottom of a Glacier.

MY DEAR SIR — In my Tenth Letter on Glaciers, which
you did me the favour to publish lately, a question was dis-
cussed respecting the apparent depression of the surface of a
glacier. I had already pointed out in the first edition of my
Travels, that several causes combine to produce this depression,
but that observations were wanting to distinguish them. The
causes then enumerated were (if I mistake not) these : — 1. The
actual waste or melting of the ice at its surface. 2. The sub-
sidence of the glacier in its bed, owing to the melting of its
inferior surface, whether by the healj of the earth, or that due
to currents of water. 3. The effect of the drawing out of the
glacier where it is in a state of distension, which tends to reduce
the thickness of the mass of ice ; (when a glacier is violently
compressed the effect will be contrary, or an elevation will
result) ; to which may be added the influence of the slope of
the bed of the glacier, by which, as it moves forward, its abso-
lute elevation is diminished, or the contrary if it ascends. I
had also pointed out a method f by which the first of these
effects, or the absolute ablation of the ice (as it has been termed
by M. Agassiz), might be distinguished from the other two,
namely, by driving a horizontal hole into the wall of a crevasse,
and observing the diminution of the thickness of the stratum of
ice above it. The partial and total effects I have observed in
the following manner, during the present summer, on the Mer

* Edinburgh New Philosophical Journal, October 1846. [The Tenth Letter of
the Series is not reprinted, being mainly controversial, and not containing new
original observations. It will be found in the same Journal for January 1846.]

f Travels, 1st Edition (1843), p. 154.

170 ELEVENTH LETTER ON GLACIERS. [1846.

de Glace of Chamouni. A crevasse, nearly vertical, and of no
great depth, was selected, running in a direction transverse to
the glacier. The most vertical wall (nearest A, Plate VI., fig. 1)
is always the [one] least exposed to the sun, and the waste of its
surface is very small, unless in the case of rain. In this wall a
horizontal hole, C, was bored, to the depth of at least a foot, and
was renewed from time to time. The depth at which this hole
existed below the surface of the glacier was determined by
stretching a string, AB, across the crevasse, and measuring by a
line the vertical height from C to AB. The variation of this
quantity gives the actual fusion of the surface, free from the
errors mentioned in my former letter. It is, of course, very
variable, depending on the weather as well as on the place of
experiment. Opposite the Montanvert, about 200 feet from the
side of the glacier, during the hot weather of July and August
1846, the ablation amounted on an average to 3 '62 inches per
day ; at a higher station between the Angle and Trelaporte
(opposite station Q of the year 1844, see Eighth Letter), it
was only 2*73 inches, the ice being also remarkably clean and
white, and the distance from the western bank of the glacier
553 feet.

The subsidence of the glacier in its bed, or the difference
between the geometrical depression of the surface and the
ablation, was very easily and most accurately obtained in the
following manner : — The theodolite being placed and levelled on
the ice in the neighbourhood of the place of observation (not
necessarily always on the same spot), the height of the horizontal
wire of the telescope above the horizontal hole pierced in the
side of the crevasse was noted by directing the level upon a
measuring tape divided into feet and inches [held vertically by
an assistant], the ring at the extremity of which was passed over
the boring instrument, which was then firmly adjusted in the
horizontal hole. The reading at the telescope gave the height of
the eye at the moment above the hole in question. The level
was then directed against a fixed object on the moraine, where
a cross had been cut in a stone as a point of departure for the

1846.]

ABLATION AND SUBSIDENCE MEASURED.

171

vertical height. The height of the eye above or below the fixed
point was measured, and the sum or the difference (as the case
might be) of this measure and the last gives the difference of the
level of the horizontal hole in the ice, and the mark on the
moraine. The following may serve as an example : —

STATION U, NEAR MONTANVERT.

1846. A
Horizontal hole C below A. B,*

C below Theodolite, .
Cross (+) U below Theodolite,

C below (+)U .

;ug. 1, 4£h. P.M.
Ft. In.
9 3-3

Aug. 3, 6 P.M.
Ft. In.
8 7-0

Difference.
8'3 inches = ablation.

1'4 inches = subsidence.

12 7-2

2 38

11 9'8
1 5-0

10 3-4

10 4-8

Of course the horizontal hole may be renewed as often as
convenient, all that is necessary being to ascertain the difference
of level of the old and new hole.

In the preceding example (which has been selected by
chance), the subsidence bears an unusually small proportion to the
ablation. At the station in question, the average daily ablation
in July and August was 3*62 inches, the average daily subsidence
1*63 inches. The sum of the two, or the geometrical depres-
sion of the surface 5'25 inches, whereof seven-tenths were pro-
duced by ablation, three-tenths by subsidence. These relations,
together with those opposite the old station Q, are shown in one
view in the following table of

MEAN EESULTS.

Slope of
Surface.

Daily Pro-
gression.

Daily
Ablation.

Daily Sub-
sidence.

Geometr.
Depression.

Proportic
Ablation.

)n due to
Subsid.

STATION U.

...

Inches.
18'7f

Inch.
3-62

Inch.
1-63

Inch.
5-25

•69

•31

STATION Q.

2° f

21-2

2-73

0'97

3-70

•74

•26

The last two columns show the effects of the ablation and
subsidence- in hundredth parts of the whole depression.

* See Plate VI. fig. 1.

f Taken from the observation of the neighbouring mark D'2.

172 ELEVENTH LETTER ON GLACISES. [1846.

As we do not know correctly the slope of the bottom or
bed of the glacier, it is impossible to estimate how much of the
subsidence is owing to the declivity. It is probable, however,
that the greater part of it may be thus accounted for. The
amount of geometrical depression agrees well with that ascer-
tained by me in 1842, at the Angle, which is in a position in-
termediate between the stations U and Q, but nearest to Q.
During the height of summer, i. e., from the 26th June to the
28th July, the daily depression was 4*1 inches.*

Relative Velocity of the Surface and Bottom of a Glacier.

The influence of the sides or walls of a glacier in retarding
its motion laterally, was demonstrated by my first observations
in June 1842 ; and the same cause might well be presumed to
influence the motion of the ice in a vertical plane. That the
superficial ice should overflow that which presses on the bottom,
seems a simple corollary, from the fact that the centre of a
glacier flows past its sides. Even admitting the , irregularities
of the bottom to be less than those of the lateral expansions
and contractions of the valley, the enormous pressure on the bed
must generate a friction proportionally great. Some persons
have, however, found so much difficulty in conceiving the fact
of varying velocity in a vertical plane (notwithstanding the
evident analogy of a river), that I was glad to take an unex-
ceptionable opportunity of demonstrating it.

I have already shown, at the close of my Sixth Letter, that
the effect of friction in retarding the rate of motion must be
most sensible nearly in contact with the soil ; and that when

* As some numerical or typographical errors have slipped into the Table of
Depressions of the Level of the Ice at the Angle in 1842 (Travels, p. 154, 2d Edit.)
I take this opportunity of correcting them, after a careful comparison with my
note-books. The observed depression from June 26 to June 30 ought to be 1 ft.
4'5 in., instead of 1 ft. 9'0 in. ; and the daily depression should be the following : —
1842, June 26 — June 30 . . . 4-1 inches.
June 30— July 28 . . . 4'09 „
July 28— Aug. 9 ... 3'92 „
Aug. 9— Sept. 17 ..- . . ; . 3-06 „

1846.] VELOCITY OF SURFACE AND BOTTOM COMPARED. 173

the glacier is of great thickness, the upper part of it (to which
alone we have access) may be expected to move uniformly or
sensibly so, which accounts for the approximate vertically of
most crevasses, during the limited period of their existence.* It
is, therefore, near the contact of a glacier with the subjacent
soil, that the most sensible effect may be looked for. The cir-
cumstances which first suggested themselves to me as the most
favourable for such an observation, were where a glacier emerg-
ing from a gorge, or from between a double mound of its own
formation, falls into a valley, and presents, for some space, lateral
faces of ice, not, indeed, quite vertical, but still very highly in-
clined, and which repose immediately on a bed of rubbish, which,
if not flat, but sloping somewhat towards the centre of the glacier,
might be considered, beyond cavil, as the floor or bed on which
it rests. But a careful examination of several glaciers with a
view to such an experiment convinced me that, even if success-
ful, it would not be conclusive. For it almost invariably hap-
pens, under these circumstances, that the glacier being no longer
confined laterally, tends to scale off by means of fissures parallel
to its length (as in Plate VI. fig. 2) ; and even if these fissures
do not give rise to a sensible sliding of the surfaces, they indicate
the direction of the twist to which the ice is exposed by the more
rapid motion of the centre. To avoid misapprehension, I here
repeat that such a tendency to scale by means of longitudinal
fissures, occurs only where lateral compression is wanting, and
there, consequently, the veined structure is always feebly de-
veloped.

I succeeded, however (though not without difficulty), in
establishing points of observation in the terminal face of the
Glacier des Bois, at Chamouni, whose relative position will be
understood from the sketch of a front view of the glacier, fig.
3, and from the vertical section parallel to the length of the
glacier in fig. 4. It will be seen by fig. 3, that whilst the
lateral parts of the glacier were fissured and scaly, in conse-
quence of the action described in the last paragraph, the central

* [See page 55 above.]

174 ELEVENTH LETTER ON GLACIERS. [1840.

mass was exceedingly compact and uniform. The terminal face
of compact ice was inclined, at the point selected for experi-
ment, at an angle of about 40° to the horizon, and the three
stations (1), (2), (3), were selected one above the other in a
vertical plane passing through this face in a direction which
was judged to be nearly that of the progressive motion of the
ice. Any variation in the motion of these three points could
only be imputed to the effect of the friction of the soil, for by
the progress of the glacier each would pass in succession verti-
cally over the same spot. The position was also unexcep-
tionable in this respect, that the glacier is here subject to no
extraordinary constraint. The sides are free, and the base
rests on a bed of rubbish and debris nearly flat, therefore
offering no fixed barrier to the forward motion of the ice ; the
retardation, if it exist, can only, therefore, be due to the legiti-
mate effect of friction. The glacier, too, is here seen from top
to bottom, for the contact with the soil is only concealed by
the trifling mound of rubbish not many feet in height, shewn
at M in fig. 4, which it presses before it ; the gravel between
M and X being flat, and untouched by the glacier for many
years. The lowest mark (1) was estimated at not less than 4
feet, and not exceeding 12 feet from the real base or soil of
the glacier. The mark (2) was 46 feet vertically above (1),
and No. (3) was 89 feet vertically above No. (2). From the
analogy of the lateral friction of glaciers, and from the phe-
nomena of rivers, it was anticipated that the retardation of (1)
upon (2), and of (2) upon (3), would be sensible, but that the
former would be greatest, which the results confirm.

The progress of each point, as well in direction as in
amount, was rigorously determined by a trigonometrical pro-
cess, reference being had to two fixed stations, one of which
X, seen in fig. 4, was in the original plane of the points ob-
served, the other was 75*525 feet distant to the right hand
[in] fig. 3. The choice of stations was limited by the peculiar
local circumstances, and was not otherwise the most desirable.
The continual fall of blocks which bounded with great velocity

1846.] VELOCITY OF A GLACIER AT DIFFERENT DEPTHS. 175

from the terminal face of the glacier, rendered it necessary to
consult the safety of the observer and the instrument ; and in
order to plant and maintain the wooden pins which marked
the points (1), (2), (3), it was necessary to commence by
laboriously removing the blocks and rubbish from the surface
of the glacier above, whose fall would at every instant have
threatened the safety and even the lives of my assistants.
Two men were laboriously employed for some hours at this
task.

Circumstances prevented me from pursuing these observa-
tions for more than five days, which was to be regretted ; but,
in this time, ample evidence was obtained of the existence
and amount of the effect of friction in retarding the lower
ice. Less than 50 feet of thickness between Nos. (1) and (2)
corresponded to an apparent acceleration of nearly half the
motion at the lower point. The ratios of the motion at (1) and
(2) were, by three independent sets of observations,
1:1-41 ... 1:1-50 'V 1:1-49

the acceleration of (3) upon (2) was (as anticipated) less con-
siderable, and also more difficult of correct estimation, owing to
the greater horizontal distance,* but the following results appear
to be worthy of confidence.

?, a ft

Motion from the 13th Aug., 11 A.M. to> 0 Q7 . 1Q .„„
18th Aug., 3 P.M. .|2'8

Eatios '// .... 1-00 2-46 1-62
Angle (p) made by the motion with

the direction of X . . . 5°-0 8°«3 10°-1

The three points being approximately, 8, 54, and 143 feet above
the bed or floor of the glacier.

These results have been computed by the following formulae,
which may be useful to those desirous of repeating the observa-
tions : —

Let X and x be the two trigonometrical stations from which

* The horizontal distances of the points (1), (2), (3), from X were, at the com-
mencement of the observations, 95*79, 138'0, and 246'8 feet.

176 TWELFTH LETTER ON GLACIERS. [1846.

the motion of the points (1), (2), (3), are observed. Let 6 be
the angle under which X and x are seen from one of these points.
Let^> be the linear transversal movement of the point as seen
from X (deduced from the apparent angular motion and the
known distance of X). Let q be the similar quantity with
respect to x, which will have the same or contrary sign with p,
as the apparent motion from the two stations is in the same or
in contrary directions. Then the total motion of the point ob-
served (which is assumed to be small relatively to the dimen-
sions of the triangle which has X, a?, for two of its corners)
will be

r = Y/^2 + (q cotan Q — p cosec of

and the angle (p) of the direction of motion with the visual line
through X, is found by this equation

q

sm 9 = -
r

I shall take the liberty of addressing you again, as to the
farther observations which I have been able to make, in another
letter. — I remain, etc.

Largs, Ayrshire, 16th Sept. 1846.

TWELFTH LETTER ON GLACIERS. Addressed
to PROFESSOR JAMESON.*

On the Extraordinary Increase of the Glacier of La Brenva from 1842 to 1846—
Its Motion in Winter, as observed by M. Guicharda — Observations on the
Veined Structure in 1846, and Experimentum Crucis respecting its Origin—
with measurements of the motion of the Ice — Analogy from the motion of
the Khone.

MY DEAR SIR — In continuation of the results of my recent
journey, of which I communicated a part in my Eleventh
Letter, I shall first give some account of the phenomena
observed on a fresh visit to the glacier of La Brenva, on the

* Head to the Royal Society of Edinburgh, 7th December 1846. Edinburgh
Philosophical Journal for January 1847.

PLATE W.

Figlp.177

Glacier of k Brenvam 18 42.

Fig 2.^). 177.

Tr 3oTiexflt43.' Audnw Sf EamKx^v.

Glacier of la. Brenva, in 18^6.

1846.] CHANGES IN GLACIER OF LA BRENVA. 177

south side of Mont Blanc. Having undertaken the journey
from Chamouni to Courmayeur and back, for the sole purpose
of examining anew this glacier and that of Miage (of which I
have already given a full account in the tenth chapter of my
Travels in the Alps of Savoy, etc.), I thought myself fortunate
in obtaining some important and unexpected observations, as
well as in again meeting M. Carrel, canon of Aoste, who, in
company with M. L'Eglise, canon of the Great St. Bernard,
very kindly made a visit to Courmayeur, on purpose to join me,
and afterwards accompanied me upon the glaciers.

Upon arriving opposite to the glacier of La Brenva, on the
6th August last, in descending from the Col de la Seigne, I
was first struck by the surprising extension of volume which
it had undergone since my last visit in 1842. This will be
understood by a comparison of the limit of the glacier in 1842,
as sketched in the plan No. II., opposite to page 193 of the first
edition of my Travels, and reproduced in Plate VIII., fig. 1,
accompanying this paper, and the line of its extension in 1846,
marked by a dotted line in the same figure. I wish it to be
understood that the sketches in question, being only taken by
the eye, have no pretension to exact accuracy, but fortunately
the land-marks are sufficiently distinct to prevent the least
dubiety. Thus, for example, the bay or hollow opposite to
C in the sketch, and which was drawn, in 1842, as filled with
the old moraine of 1818, was filled up with ice very nearly, if
not quite, to the level of that moraine, so as to follow the natural
curve of the soil, which it everywhere touched, presenting a
steep wall of nearly unbroken ice, 70 feet in height, facing the
bank. Again, opposite to the remarkable chapel of Notre
Dame de la Guerison, of which I have given an account in my
Travels, with the history of its invasion and ruin by the pro-
gress of the ice in 1818, the ice has risen against the projecting
rock, beneath the old larch tree (seen in Plate IV. of my Tra-
vels, and in Plate VII., fig. 1, accompanying this paper), and
seems to threaten the security of the path, in the same manner
as it did at that time. " The height of this rock " I stated in

N

178 TWELFTH LETTER ON GLACIERS. [1846.

1842, " is now about 300 feet ;" in August 1846 it was scarcely
more than 100 ! and as the glacier towered up to a great
height bej^ond, it had all the appearance of menacing the path-
way, and once more tearing up the limestone rock. But this
will be still more distinctly conceived by referring to Plate VII.,
figures 1 and 2, which shew the size of the glacier in 1846,
contrasted with the view of 1842. Both these sketches were
taken from the same spot (marked A on the ground-plan), and
as I had not on the latter occasion my former drawing for
comparison, the evidence of the increase is the more striking,
and the accuracy of outline of the fixed parts of the landscape
is confirmed. The increase of height and length of the glacier,
as well as its breadth, is here well marked. In consequence
of this great accumulation of ice (accompanied, probably, by
an increased velocity of motion), the surface of the glacier is
exceedingly crevassed, in some places even divided into pin-
nacles, and, generally, incapable of being traversed without
great difficulty, although, in 1842, I could have walked over it
in almost every direction.

The cause of this very surprising increase is a general
one, since most of the glaciers which have been recently ex-
amined, present it in a more or less striking degree ; for
instance, the Glaciers des Bois and Bossons, in the valley of
Chamouni.* The cause is no doubt to be sought partly in the
great fall of snow of the two winters 1843-4 and 1844-5, and
the cold wet summers which followed them. The immediate
effect of the snow is to protect the ice and diminish the annual
ablation. Hence the glacier shoots farther into the valley
before the waste suffices to equalize the supply. In the cold
spring and summer of 1845, this effect appears to have been
most conspicuous, as appears from the decisive observations
of Balmat, which I have elsewhere published.! The swollen

* A very striking evidence of this change has occurred at Chamouni. The
torrent Arveiron, instead of issuing from heneath the bed of the Glacier des Bois,
at its termination, escapes chiefly at a much higher level, and formed, in 1846, a
striking cascade on the west side of the glacier, nearly opposite to the Chapeau.
A similar occurrence is said to have happened about 30 years ago.

f Philosophical Transactions, 1846, [pp. 189, 190J, [and pp. 134, 136, of this
volume.]

PLATE "Till.

1846.] OBSERVATIONS OF M. GUICHABDA. 179

condition of the Mer de Glace of Chamouni in 1846 was
shewn by the fact, that, in spite of the intense and continued
heat, it was much higher opposite to the Angle in the middle
of August, than it was in June 1842, most of the marks
which I then made, for the purpose of estimating the progress
of the glacier, being covered by the moraine at the latter
period. The extremity of all the glaciers of which I have
obtained information, was advancing towards the valley during
the summer of 1846 ; and this was even accelerated by the great
heat of the season. For though it mast increase the ablation
of the surface, and the melting of the terminal face, and thus
diminish the mass of ice, its immediate effect is to fuse the
glacier into a state of pliancy, such as to increase its motion in
a very perceptible manner (as I have established by direct
experiment), and thus discharging its icy burden into the valley
faster than even the increased atmospheric heat is capable of
dissolving it, it spreads with a velocity which, if it could be
supposed continual, could not fail to be alarming. Thus it
appears, from certain observations made at the desire of M.
Carrel, by M. Guicharda, vicar at Courmayeur, that the snout
or extremity of the glacier of La Brenva, has protruded into
the valley no less than 22 metres, or about 60 [70] feet during
the two months of summer, being at the rate of a foot a day.
The result of this advance is, that the glacier is rapidly attaining
the old moraine of 1818 in the bottom of the valley, from
which it is now only about 100 yards distant (see the plan,
Plate VIII. fig. 1), whilst it is tearing or ploughing up the soil
on the southern bank, marked in Plate VII. fig. 2, on the left-
hand side.

The same gentleman, M. Guicharda, has himself made, with
considerable labour, observations intended to test the reality of
the movement of the glacier during winter, which confirm, in
every particular, those which I have already published regarding
the glaciers of Chamouni. The movement appears to be very
regular; but, from the position selected for the measurement
(the terminal face of the glacier), where the friction is most

180 TWELFTH LETTER ON GLACIERS. [1846.

intense, and the movement slow (see Eleventh Letter), and
where the danger from falling stones is such as absolutely to
prevent the continuance of observations during summer, they
are less comparable arid complete than would otherwise have
been the case. Having explained fully to MM. Carrel and
Guicharda my methods of observing, and pointed out a more
convenient site, we may hope that we shall have a continuous
series of trustworthy observations of this most interesting glacier.
In the meantime I publish, with M. Carrel's permission, the
degree of motion of a stake fixed in the ice, near the lowest
extremity of the glacier, and little elevated above the soil.

tion. Daily Metres.

metres. 0'13
0-13
0-113

0-12

0-075
0-15

0-10

The mean daily motion is about 5 English inches, and it
is probable, from the discrepancy of the 5th and 6th observa-
tions, that the measurement of the 30th January was faulty.
If this be so, the general uniformity, as well as smallness of
the motion, is accounted for by the important retardation due to
friction.*

In the absence of an exact geometrical plan of the glacier,
it was important to preserve some accurate record of its exten-
sion at the time of my visit in August 1846, which was done
in the following manner : — The theodolite stationed at the point
B, Plate VIII. figs. 1 and 4, that is, on a promontory of limestone
a little to the west of the chapel of Notre Dame, and close to
the path from Courmayeur to the Allee Blanche. It is also 57

* It is to he noted, that the advance of the glacier of 60 [70] feet in two
months, mentioned in page 179, is not the motion of a point in the ice, but the pro-
trusion of the front of the glacier after the effect due to melting has been deducted.

1845.

h.

h.

Me

Nov.

25,

H

to

Nov.

28,

li

0

•42

...

28,

14

to

Dec.

I,

1J

0

•38

Dec.

1,

1J

to

Dec.

H,

12 j

I

•127

1846

...

H,

12|

to

Jan.

'7,

2

3

•23

1846.

Jan.

7,

2

to

...

30,

llf

1

•735

...

30,

12

to

Feb.

9,

12

1

•58

*

*

Mar.

21,

1

to

April

2,

10

1

•18

1846.] OSCILLATIONS OF GLACIERS— THEIR CAUSES. 181

feet to the eastward of the old larch seen in the views, Plate VII.
From this point, then (which cannot readily be mistaken), the
telescope of the theodolite was directed upon the steeple of the
little church of Entreves, the azimuth marked (286°), and the
telescope was exactly levelled. It was then turned in azimuth
(tracing out the horizontal plane), until it cut the contour of the

Horizon

Fig. 20. Entries.

glacier of La Brenva, which was at an azimuth of 1841°. The
difference of azimuth 101|°. It is plain that if the glacier
advances bodily into the valley, this angle will diminish ; if it
retreats, the angle will increase. Farther, from the same station,
B, the elevation of the highest part of the front of the glacier,
where it crosses the valley, was 11° 37'. To fix the position of the
lowest point of the glacier in the valley, a line joining the vault of
ice from which the river Doire issues, and the door of the chapel
of Notre Dame (observed from below), runs S. 42° W. magnetic.
On the whole, the observations of this year on the sur-
prising extension of this glacier during the short space of four
years under the influence of meteorological circumstances, pecu-
liar, no doubt, but scarcely anomalous, confirm amply the
remarks which I formerly made (Travels, p. 205), on the great
extension which it underwent in 1818. Were the climate of
the years 1844 and 1845 to become permanent, the increase of
all glaciers would evidently be enormous. In* fact, a more
turbid and cloudy atmosphere, with an increase of the usual
precipitations, would suffice to increase glaciers to almost any
extent ; a great degree of dry cold would not produce the
desired effect (viz., an extension which would account for the
erratic phenomenon), a temperate climate being most favourable
to the growth, especially the. progression, of glaciers.

182

TWELFTH LETTER ON GLACIERS.

[1846.

Veined Structure. Glacier of La Brenva.

I must now proceed to notice an observation of a very
interesting kind which my visit of 1846 to this glacier enabled
me to make, and which seems perfectly conclusive as to the
truth of the explanation which I have elsewhere given of the
origin of the veined structure, so important to the correct theory
of glacier motion. The present appears to me to be an experi-
mentum crucis ; I can only hope that it [the peculiarity in
question] may remain in existence long enough to convince all
those who have any remaining doubts on the subject.

The glacier of La Brenva is distinguished by the beauty of
its structure. I have described it in my Travels, p. 202, and
endeavoured to represent it in Plate V. of that work. This
was as it existed in 1842. But now that the ice has risen up
against the promontory B, Plate VIII. fig. 1 [of the present
volume], and has filled up the bay C, the structure opposite to
B has become developed in the most remarkable and beautiful
manner. The plates of green and white ice alternating in the
direction of the lines E D, not close to the foot of the rock at

Fig. 21.

B, but at a little distance, the veins E D running in the direc-
tion of the declivity of the glacier, and beginning to be deve-
loped at E, becoming more and more so towards D, where they
form a tangent to a swelling surface of rock, whose resistance
evidently gives rise to them. It seems clear that all the ice
within the line F E D, or between it and the shore i C B D is
embayed, as it were, and has but little motion, in consequence

1846.] THE VEINED STRUCTURE EXAMINED AND ACCOUNTED FOR. 183

of the intense pressure with which the whole mass of the glacier
is urged against the side of the valley ; but as the glacier is
finally compelled to move in the direction of the declivity from
E towards D, a longitudinal tearing force arises parallel to E D,
and the motion is facilitated by the formation of the veined struc-
ture already mentioned, for of longitudinal crevasses there are
absolutely none. Such crevasses as there are, are transverse to
the blue bands, shewing the usual direction of the tearing force
due to the lateral friction. But they are small and isolated.

Now, in this case, the development of the structure is evi-
dently due to the projection at D, in contact with which it
terminates ; and though the whole glacier is more or less struc-
tured in the manner described and figured in my former work,
this remarkable development gradually ceases [in an upward
direction] about E, where the rush of the ice-current past the
promontory B has ceased to exert so palpable an effect. I can
positively state that no such unusual development of the structure
occurred opposite to B in 1842, and for this reason, that the glacier
lying then more in the trough of the valley, not being violently
pressed against the promontory B, and embayed in the height *
C, the friction and longitudinal tearing force was incomparably
less. I wish I could convey any adequate -idea of the beauty of
the ice for an extent of some hundred feet in length, and for a
comparatively trifling breadth between D and E. It resembled
the greenish-veined marble called by the Italians Cipollmo,
when of the highest perfection, and polished or wetted ; and it
was impossible to resist the wish to carry off slabs, and to per-
petuate it by hand specimens in cabinets.

I did not, however, content myself with arguing the rela-
tive motions of the parts of the ice and embaying of the rest,
from the mere configuration of the ground. But I made the
following experiments, which proved that my first belief was
correct.

The instrument [theodolite] being stationed at B [Plate
VIII. fig. 4], and a transverse visual line established with reference

* [Evidently a misprint for bight, which is the word used in my original notes.]

184 TWELFTH LETTER ON GLACIEKS. [1846

to an object on the farther side of the glacier, two vertical holes
were made at (1) and (2), the first on the nearer, the second on the
farther side of the remarkable veined structure already described.
If that structure was occasioned, as I suspected, by the rapidity
with which the farther portion of the ice was moved past the
nearer portion, such difference of velocity might be expected to
be observed in a marked manner. The mark (1) was about 50
feet from the nearest edge of the glacier, and 103 feet below
the station B. The mark (2) was about 170 feet farther and
60 feet higher. An approximate section of this part of the
glacier, together with the measurements on which it is founded,
is given in Plate VIII. fig. 4, and will serve to compare the state
of matters at any future time. The following table gives the
result of the motions for two days : —

Motion. Motion for 24 hours.

No. (1.) No. (2.) No. (1.) No. (2.)

1846. Hour. inch. inch. inch. inch.

Augusts, 9iA.M. 5.7 15.7fi

9,

3-7 11-25 4-55 13-85

Sums, 9-4 27-0 Means, 4-90 14'2

consequently the velocity of motion had increased in the space
of 170 feet traversed by the veined structure, in the ratio of no
less than 29 to 10.

I also examined the condition of motion of the embayed
ice in the position C on the ground-plan, of which a section
(on a scale much larger than the last) is given in fig. 3. It
is made through the visual line, or in the direction C, C 2 of the
plan [p. 182]. Two pins were fixed at C 1 by excavating a niche
in the nearly vertical face of the ice ; one was placed vertically,
the other horizontally. At C 2, 40 feet higher and 69 feet more
distant, a vertical pin was placed. The diagonal from C to C 1
was also accurately measured with a line. After more than 24
hours' interval, it was found that the two marks at C 1 had not
moved towards the right hand, or in the direction of motion of
the glacier by the smallest perceptible quantity • and that [the]

1846.] ANALOGY FROM THE RIVER RHONE. 185

mark 0 2 had advanced by only a small fraction of an inch
(0*2) ; but on the other hand the vertical pin at C 1 had ap-
proached the station C ~by two inches ; shewing that the motion
is here entirely directed outwards and upwards against the bank
at C, exactly as an hydrostatic pressure acting on a plastic mass
would occasion. The result was the more interesting because
it was altogether unexpected. It had not occurred to me that
the embaying of the ice could be so complete as to leave no
appreciable effect due to the drag of the central ice in the direc-
tion of the declivity.

The precise analogy of the phenomena now described to
what obtain when the motion of a stream of water is interrupted
by lateral obstacles, will suggest itself to almost every one j but
to take an illustration which is not imaginary, I have given, in
the annexed figure, a sketch of what occurs in the course of

Fig. 22.

the river Rhone at the bridge near Sierre. The course of the
current is indicated by the arrows, the pier of the bridge b
embays the water at c, where it is whirled about by the tan-
gential action of the current, but does not escape ; the wavy lines,
e d, indicate the ripple produced by the friction of the central
past the lateral portions of the stream, the disturbance from
which is propagated towards the shore in the waves/ All
that is here described may be still better seen in a sluggish
stream, and if it be covered with any kind of scum, it will be
torn up into longitudinal shreds between e and d, exactly cor-
responding to the position of the icy bands. The nature of
the force producing these lines of scum is also well shewn by

186 THIRTEENTH LETTER ON GLACIERS. [1846.

the rotatory motion acquired by any floating patches of foam,
which is always in a direction forwards and towards the side of
the stream to which it is nearest ; thus, in the preceding figure,
it would be in the direction of the hands of a watch; — I re-
main, etc.

EDINBURGH, 24th October 1846.

XVI. THIRTEENTH LETTER ON GLACIERS.

Addressed to Professor JAMESON.*

Acceleration of the Surface Motion of Glaciers confirmed — Velocity of the Mer de
Glace in the Summer of 1846 at Stations C, D, P, and R — Velocity of the
Glacier of Talefre, near its point of discharge — Discovery of a Knapsack ten
years buried in the Ice, and deduction of the motion — The Glacier of Nant
Blanc visited, and its Motion determined — The Glacier of Miage revisited, and
motion measured — On the Conversion of the Neve into Ice — Congelation of
Infiltrated Water not necessary to produce the Veined Structure — An Attempt
to explain the apparent rejection of Stones from the Glacier — Ridges of Ice in
certain parts of Glaciers due to the bruising effect of intense Pressure — Three
Orders of discontinuity, Ridges, Crevasses, Veined Structure.

My dear Sir — Since the completion of my Twelfth Letter,
I have observed, in the Comptes Rendus for 26th October, an
account of a communication made to the Academy of Sciences
by M. Martins, of an experiment on the relative motion of the
surface and inferior part of a glacier, on which I have also made
the experiments detailed in my Eleventh Letter (dated 16th
September), and published on 1st October in The New Edin-
burgh Philosophical Journal. My experiments, it will be recol-
lected, were made at three points of the terminal face of the
Glacier des Bois, and proved, as I had long ago anticipated, that
the superficial ice has by much the most rapid motion. MM.
Dollfuss and Martins arrive at the same result, establishing the
desired identity with the motion of rivers ; but their experiment
being made, not on the terminal face, but only on the steep

* Read before the Royal Society of Edinburgh, 21 st December 1846. Published
in Edinburgh Philosophical Journal, January 1847.

1846.] RELATIVE MOTIONS OF SURFACE AND BOTTOM. 187

lateral face of a glacier, is open to the possible objections which
I anticipated in my former letter, and which I carefully endea-
voured to avoid (see p. 173 of the Eleventh Letter, and fig. 2 of
Plate VI.), so that even the most scrupulous might be, if possible,
satisfied. It is pleasing to me to find that the French observers
corroborate my result ; but I must remark that they obtained it a
fortnight later, and my publication preceded theirs nearly a month.

I shall not seem to insist too much upon the collusiveness
of this observation, when it is recollected that an opponent of
the Viscous Theory has virtually staked the question of the cause
of glacier motion upon such an experiment (notwithstanding that
I think he has attached to it an undue importance), in the fol-
lowing passage : —

« * * The claims of the two theories (the sliding theory
and that of plastic motion) will undoubtedly be determined by
other means. The observations required are such as shall de-
termine, as far as possible, the motions of the upper and lower
surfaces of a glacier. We may never hope to have access to the
bottom of a glacier in its deeper portions, but at the extremities
of glaciers the amount of sliding may easily be ascertained, as
well as at many other points, probably, if sought for, along their
flanks ; fissures also, of considerable depth, are not unfrequently
met with, in which the deviation from verticality, if it exist,
might be easily determined ; and though the evidence thus ob-
tained might not afford positive demonstration with respect to
the deepest portions of a glacier, still, should it all concur in
showing an approximate equality in the motions of the upper and
lower surfaces, every candid and impartial mind must admit, I
conceive, the sliding in preference to the viscous theory; but if,
on the contrary, it should be proved that the velocity of the
upper bears a large ratio to that of the lower surface, the claims
of the latter theory must be at once admitted."* It is very
fortunate that independent observers on different glaciers should
have arrived, in ignorance of each other's results, at conclusions
which permit only the alternative favourable to the viscous

* Mr. Hopkins, in London Philosophical Magazine, March 1845, p. 250.

188 THIRTEENTH LETTER ON GLACIERS. [1846.

theory to be adopted ; the observations having been made, too,
one at " the extremity," the other " along the flanks," as sug-
gested in this passage.

I shall now proceed to conclude the account of the most
material observations which I had an opportunity of making last
summer ; and those which remain refer chiefly to three points :
(1.) The rate of motion of glaciers generally, and especially
during the summer of 1846 ; and, (2.) To the conversion of the
Neve into the icy condition ; (3.) As to the apparent ejection of
stones upon the surface.

(1.) RATES OF MOTION.
A. At points previously observed.

It is very interesting to compare the ANNUAL motion of
glaciers in different years, and also the comparative motions of
a given point, at the same season, in different years. I have
obtained several such results on the Mer de Glace of Chamouni,
all of which tend to show a marked increase of the rates of mo-
tion during the two years which have elapsed since my last
measures were made.

Thus, at station C, the Pierre Platte, opposite to the pro-
montory of Tacul,* from the 19th August 1844, to 21st July
1846 (701 days), the advance was 622 feet, or 10'65 inches
daily.

The following little Table shews the relation of this to
former years : —

1842-3.t

1843-4.+

1844-6.

Daily Motion in INCHES,
Annual Motion in FEET,

8-56
260-4

9-47
288-3

10-65
323-8

The mean velocity was, therefore, about a fourth part
greater in 1845-6 than in 1843-4.

The following Table shews the daily motion, in inches, of

* See Travels in the Alps, 2d Edition, pp. 92 and 135, and Map annexed,
f [The numbers for 1842-3 and 1843-4 were inadvertently transposed in the
former impression.]

1846.

VELOCITIES OF GLACIER OPPOSITE MONTANVEIIT.

189

a series of transverse stations nearly opposite to the Montan-
vert, and marked with the numbers D 2, D 5, D 6, D 3, since
the commencement of my observations.* The first in order
was about 100 yards from the western shore, D 5, was 130
yards farther, D 6 was 75 yards farther, and on the axis of
the " dirt-bands ; " D 3 was 60 yards farther, near the medial
moraine, and about 350 yards from the eastern side of the
glacier.

MEAN DAILY MOTIONS.

D2.

D 5.

D 6.

D3.

Inch.

Inch.

Inch.

Inch.

1842, Aug. 1-9,

16-6

24-7

1843, Sept. 11-13, .

16-0

1844, Aug. 17-22, .

17-4

23-3

23-7

239

1846, July 22— Aug. 3, .

18-7

25-4

26-8

26-5 f

RELATIVE DAILY MOTIONS, D 2 BEING = 1.

D 2.

D 5. D 6.

D 3.

1842, .
1844, .
1846, .

1-000
1-000
1-000

1-332 1-356
1-339 1-362
1-358 1 1'433 J:

1*367
1-374
1-417 t

i

It will be seen that both the absolute and relative motions
were markedly greater in 1846 than in the previous years ;
and that this also agrees with the annual motions at station C
given above.

A block, on the ice opposite Les Fonts (between Montan-
vert and the Angle), marked with the letter P on the 18th
September 1843, moved 486 feet from that date to 9th
August 1844 (or 331 days), being 17*62 inches daily, or 536
feet annually. The same block, re-examined on the 16th July
1846, had moved 776 feet since the 9th August 1844, or only

* Travels, 2d Edition, pp. 137-140.

f From 25th July to 3d August only.

J [These numbers were incorrectly calculated in this letter as originally printed.]

190 THIRTEENTH LETTER ON GLACIERS. [1846.

13*2 inches daily, or 402 feet annually. The retardation must
be attributed to the block having been thrown almost upon the
moraine, as happened to the block D 7 under similar circum-
stances.* To avoid such a mischance again, I marked the
position of a fine block now near the centre of the Mer de
Glace opposite to Les Fonts. It was painted with the letter
V on both sides, and its position determined with reference to
a point on the pathway between the two Fonts, to which it
was exactly opposite on the 30th July 1846, when the observa-
tions were made. The mark is a cross with the letter V on a
small granite block solidly placed on the left of the pathway.
The block on the ice is 760 feet distant from the west shore.

By reference to my Eighth Letter, published in this
Journal, and to the fuller details in the London Philosophical
Transactions for 1846, on the Plasticity of Glacier Ice, it will
be recollected that I made observations at a station marked Q
between the Angle and Trelaporte.f This station was 300 feet
from the west moraine. The following observations were made
last summer in the same part of the glacier, at a distance of 553
feet from the moraine : —

Daily Motion.
Inches.

July 23— July 30, 21'5

July 30— Aug. 3, .... . 20'7
Aug. 3— Aug. 14, . . - . . . 21-1

A very fine block of granite, marked R in oil paint on three
sides, about the same distance as Q from the west moraine (300
feet), had its position fixed on the 26th August 1844, and was
found this year to have moved in 693 days, to 20th July 1846,
1108 feet, or at the rate of no less than 19'2 inches daily, or
533-6 feet annually, a remarkably rapid motion, but which was
carefully verified by two concordant measures. The rate of
motion of this block (engaged as it was in the great crevasses
near the Angle), from the 23d to the 30th July 1846, was 11
feet 4*5 inches, or 20'7 inches daily.

* See Fifth Letter on Glaciers [p. 36 above.]
f [See page 105 above, and the sketch map in Plate III.]

1846.] MOTION OF ICE OF GLACIER OF TALEFRE. 191

B. At points not before observed.

I had long wished to ascertain the motion of the ice, which
issues from the very remarkable basin-shaped glacier of Talefre.
That an oval circus or amphitheatre, whose length may be
roughly stated at 4000 yards, its breadth at 2000, entirely filled
with snow and ice (excepting the rocky island of the Jardin),
should disgorge itself by an icy stream through a chasm less
than 700 yards wide in its broadest part, appears to me, as I
have elsewhere stated at large,* a plain demonstration of the
viscous theory of glaciers ; the outlet in this case acting pre-
cisely as a stream does to a lake of indefinite extent, as a mere
overflow, or trop-plem, a fact only consistent with the quasi-
fluid motion of the mass so discharged.

The great distance of the Talefre from habitable spots renders
it inconvenient for prolonged experiments ; but having last sum-
mer twice spent the greater part of a day in its neighbourhood, I
have been enabled to determine with exactness its rate of motion.

The part which I selected for experiment will be under-
stood from the little portion of a map contained in Plate VIII.
fig. 2, on the same scale as my large map of the Mer de Glace.
The station marked W, near the foot of the Aiguille du Moine,
is a considerable block of granite, a few yards to the left of the
usual pathway ascending to the Jardin ; and my point of obser-
vation was permanently marked (as usual) by a cross cut in the
top of the stone, and painted red with oil colour, and the
letter W by its side. It is not many minutes' walk short
of the spot where travellers bound for the Jardin usually
enter on the glacier of Talefre. Hence it will, I hope, be easily
recovered. It is about 8700 feet above the sea. From it my
mark on the Tacul (station B of my map) could be distinctly
seen with the telescope ; and a transverse line across the glacier
of Talefre, in which marks were placed on the 24th July 1846,
made an angle, with the direction of B, equal to 98° 30'. The

* London Philosophical Magazine, May 1845, page 415. Compare the map of
the Mer de Glace in Travels in the Alps of Savoy. The outlet of the glacier is
seen in Plate VIII. fig. 2, accompanying the present paper.

192 THIRTEENTH LETTER ON GLACIERS. [1846.

position of this line of stations is shown distinctly in the small
map. It will be seen that it was near the spot where the con-
traction of the gorge and the icy stream was greatest, in fact,
in the throat of the funnel. The inclination or slope of the
glacier in the direction of its motion was inconsiderable, espe-
cially near the centre and the southern side ; and the transverse
crevasses, which are here just beginning to be numerous, are
evidently owing to the extended influence of the steep ice-fall,
which is not many hundred feet in advance of the transverse
line selected. The glacier of Talefre, higher up, is remarkably
compact and united ; the crevasses few and inconsiderable, par-
ticularly on the northern side. The crevasses, as they present
themselves, are convex towards the origin or basin of the glacier,
and are here, as in other cases, perpendicular to the veined
structure so far as developed, and which was quite correctly
laid down in my map of 1842 ; the structural bands converging
towards the outlet like filaments of water towards the contracted
vein of a spout. The curvature of the crevasses (seen in piano]
appeared to me to have a point of contrary flexure, as shewn
in the drawing, dividing them into two loops.

Three stations were selected, as shewn in the plan. The
central one, marked (2.), was placed on the moraine descending
from the Jardin. No. (1.), was nearly midway between it and
the northern shore at station W. Here there is a hollow in
the surface of the glacier, which was thickly covered with snow
in the end of July. At (2.) and (3.) the ice was quite bare
and more level, but at the same time more crevassed, — the
crevasses being remarkably well defined, narrow, rather deep,
and rectilinear, or slightly curved, not uneven. No. (3.) was 1068
feet beyond (2.) No. (1.) was 533 feet northwards from No.
(2.), and was 687 feet distant from the northern shore. These
distances were ascertained partly by direct measurement, partly
trigonometrically. These three points were fixed in the trans-
verse visual line from W on the 24th July, by forming three
perfectly round and vertical holes, no less than five feet deep,
by means of an iron jumper.

1846.] MOTION OF GLACIER OF TALEFRE — CURIOUS INCIDENT. 193

Being examined on the 31st July, after a lapse of six days
and seventeen hours, the motions were found to have been —

No. (1.) No. (2.) No. (3.)

Ft. In. Ft. In. Ft. In.
July 24, 5 P.M., to July 31, 10 A.M. 7 2-5 8 9-0 7 2'5

Or daily in INCHES . . 12-9 15-7 12'9

By a remarkable coincidence, the 1st and 3d stations had been
so symmetrically chosen, as to have precisely the same velocity.
The middle point, as usual, moved fastest.

This velocity will be considered to be large when we recollect
the great height of the glacier above the sea, and the small incli-
nation of its surface at this place. On the other hand, it is a
natural consequence of the theory which regards this narrow
outlet as the overflow of a vast reservoir, in which snow has
peculiar facility in accumulating.*

T shall now mention a truly remarkable circumstance connected
with the velocity of the discharge of the icy stream which empties
the basin of Talefre. I was apprised by David Couttet, on the
23d July last, on his return from looking for crystals on the
moraines of the Glacier of Talefre, that he had found opposite
to Les Egralets (see the map), or where the ice of the Talefre
joins the Glacier de Lechaud, some fragments of a knapsack
lying on the ice, which he at once recognised to be the same
as had been lost by a guide some years before, who fell into a
crevasse, where he had nearly perished, in conducting a traveller
to the Jardin. Couttet rightly believed that the determination
of the motion of the ice in the interval, since the loss of the
knapsack, would be a matter of interest to me.

At the same time, the seeming improbability of recovering
so destructible a material as a bag or knapsack, made of cloth,
after remaining for ten years in the bowels of a glacier, was so
great as to make me resolve to investigate the matter thoroughly
whilst on the spot.

The next day I went to the Glacier of Talefre, accompanied

* [And, perhaps, it might be added, of the proximity of the ice-fall in front,
diminishing the frontal resistance.]

0

194 THIRTEENTH LETTER ON GLACIERS. [184G.

by Couttet, Balmat, and another, and Couttet led us straight
to the spot marked in the small map (Plate VIII. fig. 2), with
the words " knapsack found," a little higher up than where the
usual path to the Jardin by the ascent of " Les Egralets," leads
from the ice to the ascent of the rock on the left. Just where
the ice of the Talefre incorporates itself with that of Lechaud,
and at the distance of 158 feet from the face of rock which
bounds the glacier, we found several fragments of strong blue
and white cotton stuff (now shown to the Society) very much
worn by friction, but by no means rotten, and not discoloured,
with portions of very strong straps and loops partly attached to
it of figured tape, which had formed the attachments and shoulder
straps (bretelles) of the knapsack. Beside them were pieces (or
a piece) of bottle glass. Balmat immediately confirmed Couttet's
recollection of them as parts of the identical bag lost by Jullien
Devouassou ten years before.

Now, to explain how Couttet and Balmat were in a position
to speak so positively to the identity of the fragments, I must
observe, that Couttet was then, and has been ever since, lessee
of the pavilion at Montanvert, and that the knapsack in question
was his property, and was left at the Montanvert, for the express
purpose of carrying provisions for travellers, who then, much more
rarely than at present, visited the Jardin. Auguste Balmat, my
guide, was at that time servant to David Couttet, and kept the
pavilion under him, and had very often carried on his back this
very knapsack. The figured stripe of green and purple on the
shoulder straps was very marked, and could not easily be mis-
taken. The testimony of these two men was therefore per-
fectly authentic, but I verified it by questioning the very Jullien
Michel Devouassou himself, who is still a guide at Chamouni,
and who, on seeing the fragments, offered to verify them upon
oath. The accident occurred thus : — On the 29th July 1836
(or ten years all but five days from my recovering the frag-
ments), Devouassou accompanied a stranger to the Jardin,
taking, as usual, the knapsack from the Montanvert, with a
supply of bread, cheese, and wine. They arrived without acci-

1846.] KNAPSACK LOST AND RECOVERED AFTER TEN YEARS. 195

dent at the top of the Couvercle, between the Aiguille du Moine
and the Glacier du Talefre ; and when at the point marked in
the accompanying sketch with the words " Knapsack lost," the
guide, to shorten the way, attempted to take an oblique course
to the Jardin, instead of following the usual track on firm
ground round the foot of the Aiguille du Moine, and then turn-
ing sharply to the right, so as to make the passage of the gla-
cier as short as possible. The ice on which he ventured was
partly covered with snow, as is almost always the case there in
July, and near the edge it was also full of concealed fissures.
Into one of these the guide suddenly dropped, leaving the
astonished traveller alone in this wilderness of rocks and ice.
After vainly calling to his guide and obtaining no answer, he
left the place in despair, and returned to the Montanvert by the
way he had come. Devouassou, however, having reached the
bottom of the crevasse but little hurt, managed, by the aid of
his pocket-knife, to cut steps in the walls of ice, and, finally,
with great exertion and suffering, to raise himself to the sur-
face, and make his escape, leaving behind him his knapsack, of
which, of course, he had first disembarrassed himself. Astonish-
ing fact ! that the yet undecayed vestiges, together with a part
of even the very bottle which formed his burden, should be
brought to light on the surface of the ice after ten years' fric-
tion and onward movement ! I took pains to measure angles
with my theodolite at each point, which enabled me to project
them with tolerable accuracy on the map, as shown in the
Plate ; and I ascertained by the barometer the difference of
level of the two points, and also their elevation above the sea.

Height above
the Sea.

English Feet.

Knapsack lost, Glacier du Talefre, 29th July 1836, 8657
Knapsack found, Glacier du Lechaud, 24th July 1846, 7512

Difference of level, . 1145

Horizontal distance moved over, . . . 4300 feet.

Declivity, . . . 14° 55'

Annual motion, . v 430 feet.

196 THIRTEENTH LETTER ON GLACIERS. [1840.

This motion harmonises sufficiently well with the numbers
found in page 193, if we allow for the acceleration manifestly
due to the rapid declivity which commences immediately below
station W.

This interesting discovery forms a curious pendant to the
history of De Saussure's ladder, believed to have been found
in fragments opposite Trelaporte, as fully detailed in my Travels,
page 86 ; but this new case is much better ascertained in all
its particulars.

I may here mention, as the subject suggests it, another
proof of the extraordinarily conservative power of the ice even
upon the most seemingly destructible bodies. On the 20th July
1846, 1 found on the surface of the ice at station Q (opposite the
Glacier of Charmoz), all within a few feet of each other, four of the
small wooden pins which I had left there in making the expe-
riments in August 1844, described in my Eighth Letter, a
quantity of the thin string, and pieces of the birch broom then
used, and even a quantity of the loose straw which I used to
wrap round my shoes in cold weather. These were neither
decomposed nor blown away by the winds, nor had they fallen
into any of the numerous crevasses, but were lying on the sur-
face of the glacier (a very different surface, of course from that
on which they had really been left), as if they had been in use
at most but a few weeks before.*

Glacier du Nant Blanc. — This is a small glacier descending
from the foot of the Aiguille de Dru, exactly opposite the
Montanvert. I visited it for the first time in 1846. Its form

* [From its close analogy to the instances above mentioned of the power of the
glacier to preserve substances buried in its mass, and to restore them years later to
open day, I may here record a still more recent instance : — On the 25th September
1842, I lost a geological hammer, having a very peculiar shape, in a "moulin" of
great depth, on the level part of the glacier de 1'Lechaud, opposite to the ice cascade
of Talefre. It was recovered in the summer of 1857 by Mr. Alfred Wills at a point
" not far below the Tacul," and was at once recognised by Balmat, who accompanied
him, and who was with me when I lost it. Mr. Wills having kindly shown me the
hammer, I had no difficulty in bearing testimony to its identity. Indeed, anticipat-
ing its recovery, I had made a sketch of it in the journal in which I recorded its
loss. I should add, that the iron of the hammer was not rusted, nor the wood
decayed. January 1859.]

1846.] MOTION OF GLACIER OF NANT BLANC, 197

and position will be understood from a reference to the general
Map of the Mer de Glace. It is shaped somewhat like a tongue,
and is steep, without being much crevassed or very uneven, at
least in its middle region, where it is about 1000 feet broad,
where cattle are every year compelled to traverse it ; yet even
there it has an inclination of 18° or 20° near its centre. This
glacier terminates at a considerable height above the Mer de
Glace, and gives rise to the Nant Blanc, a torrent which gives
its name to the glacier, and whose volume and appearance is
subject to remarkable changes, depending upon the state of the
weather. It [the glacier] is supported on a vast pile of blocks
which it has brought down, and which forms its moraine ; and
whilst on the left bank the ice overhangs the moraine, on the
right, the latter being elevated in a mound, forms a barrier to
the former.* This glacier has a very small and crevassed nev£,
taking its origin amongst the rugged rocks between the Aiguilles
of Dru and Bochard. The veined structure of the ice of the
glacier is perfectly normal, exhibiting the parabolic loops with
the frontal dip inwards. Its general form resembles that of the
Glacier des Bossons.

I stationed my theodolite on the northern moraine, and
fixed a station towards the centre of the glacier, or rather on
the axis of the structural bands, which usually intimates the
point of swiftest motion. The inclination of the glacier was
here, as already mentioned, 18° or 20°. Notwithstanding this
great declivity, the mean motion from August 4 to August 15,
was only 9*9 inches daily.

Glacier du Miage. — In order to complete here the observa-
tions on the velocity of motion of different glaciers, I shall
include those made also on the south side of Mont Blanc. I
had intended to have made a considerable number of experi-
ments on the remarkable Glacier du Miage, fully described in
my Travels, Chap. X., but I found it less suitable than I ex-
pected. I renewed, however, my survey of all the middle
region of the glacier ; and I ascertained the motion of its

* [See Plate VIII. fig. 6.]

198 THIRTEENTH LETTER ON GLACIERS. [1846.

central part, between the two medial moraines at its issue into
the valley from between the colossal summits of the Aiguilles
Rouges and Mont Broglia ; in fact, at a point near the letter r
of the word " Glacier " on the eye-sketch of a ground-plan of
the glacier in my published volume. From the 7th to the 10th
August, I found the daily motion to be 9'9 inches ; or the very
same as that just given for the Glacier de Nant Blanc, than
which, two cannot be more dissimilar. But the smallness of
the motion in the latter case is due to the great height and
very small nM or reservoir ; in the former, to the exceedingly
slight declivity and enormous frontal resistance to the exit of
the glacier into the Allee Blanche, already encumbered with its
ponderous mass and debris.

Whilst speaking of the Glacier du Miage, I may add, that
I found on the most westerly of the medial moraines, dark blue
lias-schist, which undoubtedly comes from the very axis of this
granitic chain, where it has not hitherto been suspected to exist.
Also, that a repeated examination of the curious " ancient mo-
raines," forming semicircular embankments towards the Lake of
Combal, described in my Travels, p. 194, and figured in the
eye-sketch, has convinced me that these cannot be of so old a
date as I then believed ; but are due to a temporary outbreak
of the glacier at some not very remote period, when the accu-
mulation of pressure of the ice has been such, that an overflow
took place laterally, which deposited these moraines in succes-
sion as it retired, and the glacier retreated to its present limit.
I found an exactly analogous case in the Glacier des Bois, at
Chamouni ; which, at some period not on record, has thrown
out an arm between the Chapeau and the Cote du Piget.

Having fully described the measures of the Glacier of La
Brenva in my last letter, I shall not here return to them.

Thus I have been able to add the velocity of four additional
glaciers to the small number previously ascertained.

1846.] THE CONVERSION OF THE NEVE INTO ICE. 199

(2.) ON THE CONVERSION OF THE NEVE INTO ICE.

I shall now add a few observations tending to throw light on
two of the most obscure glacial phenomena, first, the conversion
of the snow of the Neve into pure ice, and, secondly, on the
apparent ejection of stones from the surface of the glacier.

On the first point, I made some interesting observations
on the higher part of the Glacier du Geant, where the still
snowy ice, marked with the horizontal annual strata, is shoved
violently down the steep, which occasions the scene of desolate
confusion between the Aiguille Noire and the rock called Le
Petit Rognon. The structure of the interior of the embryo
glacier is here perfectly disclosed by the prodigious vertical
rents which make the scene a true giant's staircase ; the ice-
falls succeeding one another at regulated intervals, which appear
to correspond to the renewal of each summer's activity in these
realms of almost perpetual frost, when a swifter motion occasions
a more rapid and wholesale projection of the mass over the
steep, thus forming curvilinear terraces like vast stairs, which
appear afterwards, by consolidation, to form the remarkable
protuberant wrinkles on the surface of the Glacier du Geant
described in my Fifth Letter.0 But the point which at present
concerns us is this, that, according to the best observations
which I could make, the stratified appearance of the Neve dis-
appeared at a depth inconsiderable compared to the vast vertical
sections there exposed, and the interior of the mass was granular,
and without structure or bands of any kind.

I drew the very same conclusion from an attentive survey
of the Glacier de Talefre, which is peculiarly calculated to throw

* It is not unimportant for travellers to be aware that in seasons like 1846,
of unusual warmth and activity amongst the glaciers, the dislocation and pre-
cipitous subsidence of the tabular masses of the Neve is occasionally so complete
as absolutely to debar a passage, at least without the help of a ladder. This
was the case when I ascended the Glacier du Geant on the 14th August last, a
snow bridge by which some travellers had effected a passage a fortnight before
having wasted away. Had travellers at that time crossed the Col du Geant
from Courmayeur to Chamouni, they might have found their descent, if not
impracticable, at least most perilous, and to return from such a distance would
have been an almost equally distressing alternative. On this account it is most
advisable to make this passage from the side of Chamouni.

200 THIRTEENTH LETTER ON GLACIERS. [1846.

light upon this question, and I shall state the result of iny
observations there nearly in the words in which I recorded
them immediately after they were made. A little higher up
than the usual passage of the glacier to the Jardin, or near the
upper limit of the sketch, Plate VIII. fig. 2, the annual layers of
the N&ve are seen. But these wholly disappear further down ;
the great body of the ice in the hollow of the basin exhibits
little sign of structure ; only near the lateral and medial moraines
the structure is icy and vertically veined, elsewhere it is decidedly
snowy, with hardly any trace of bands either vertical or horizontal.
This is true even as far down as the line of stations W in the
figure. The general disposition of the structural lines where
they exist (which, it is to be recollected, are almost vertical), is
seen in the same figure [to be] perpendicular to the crevasses.
From these facts I conclude, first, That the vertical structure
is too close to the original strata of the Neve to allow of the sup-
position that these have all of a sudden turned up vertically in
some parts of the glacier, and disappeared in the remainder.
2dly, That where the vertical bands are not developed in the
higher glacier, the structure remains snowy and undefined.
3 dly, That the conversion into ice is simultaneous, and in this case
identical with the formation of the Hue lands. £thly, That these
bands are formed where the pressure is most intense, and where
the differential motion of the parts is a maximum, that is, near
the walls of the glacier ; but being once formed, it still continues,
at least for a time, to be observed under the medial moraine, and
this may even be traced throughout the ice-fall of the Talefre.

I am satisfied then (and it is only after long doubt that I
venture this confident expression), that the conversion of snow
into ice is due to the effects of pressure upon the loose and
porous structure of the former ; that the very first effect is to
annihilate the annual strata of the Neve, and that the most
rapid glacification is effected by the kneading or working of the
parts upon one another, by the differential motions which the
semi-fluid law of glacier progression occasions, and which also
necessarily takes place under intense pressure.

1846.] GLACIER ICE THE RESULT OF PRESSURE AND COHESION. 201

The belief which I formerly (in common, probably, with
most other persons) entertained, that snow could not pass
into pellucid ice without being first melted and then frozen,
was part of the chemical prejudice that molecular actions
cannot take place except in the liquid state, a prejudice now
disappearing, as the subjoined note, on the very competent
authority of M. Gay Lussac, shews.* The crystalline forces
act on the snowy granules when brought into close contact by
pressure, and the imprisoned air is then distributed in the
direction of the lines of tearing, in the form of layers of regular
globules, just as in the case of the banded lavas which have
been so well described by Mr. Darwm.f Bishop Rendu, whom
I had the pleasure of visiting at Annecy, remarked a familiar
circumstance which illustrates the same thing. We often see,
in the coldest weather, that opaque snow is converted into trans-
lucent ice by the sliding of boys on its surface ; friction and
pressure alone, without the slightest thaw, effect the change,
which must take place still more readily in the glacier, where
the mass is, during a great part of the year, kept on the very
border of thawing, by the ice-cold water which infiltrates it.
In this condition, molecular attachment amongst the granules
must be comparatively easy, and the opacity disappears in pro-
portion as optical contact is attained. Most evidently, also, the
icy structure is first induced near the sides of the glacier where
the pressure and working of the interior of the ice, accompanied
with intense friction, comes into play, and the multitudinous
incipient fissures occasioned by the intense strain, are reunited
by the simple effects of time and cohesion.:):

* " II n'est plus permis aujourdhui d'avoir une foi aveugle au principe si banale-
ment repete des anciens Chimistes, corpora non agunt nisi soluta. II est certain,
au contraire, que tous les corps, solides, liquides, et aeriformes, agissent les uns sur
les autres, mais que, des trois etats des corps, 1'etat solide est le moins favorable a
1'exercice de I'afnnite." — Anndles de Chimieet de Physique, Juin 1846, p. 231.

f On Volcanic Islands, and in Philosophical Magazine, April 1845.

| A very remarkable peculiarity is observed in some of the glaciers of Switzer-
land, which distinguishes them from the more rapid and precipitous ones of Savoy.
The glaciers of the Aar, Rhone, and Great Aletsch, exhibit a degree of crystalline
structure which I have nowhere else observed ; broad, laminated, crystalline plates

202 THIRTEENTH LETTER ON GLACIERS. [1846.

We are therefore relieved from the difficulty of accounting
for the cold which would be necessary to freeze the infiltrated
water which was at one time believed necessary to explain the
conversion of the nSve into proper ice. This would be liable
to most of the objections urged against the Dilatation Theory.

(3.) ON THE FIRST APPEARANCE OF STONES (ERRATICS) ON THE
SURFACE OF GLACIERS.

I shall conclude this letter with a few very partial observations
upon what 1 must consider as one of the phenomena of glaciers,
still obscure as to its explanation, although most familiar as a
fact, — I mean their supposed tendency to reject impurities, and
the undoubted fact that stones are always found near or upon
the surface of the ice. It is strange that it should not have
occurred to every one who sought to explain the appearance of
stones on the surface by the ablation of the ice, that in order to
arrive there at all, the blocks must previously have been im-
bedded in the virgin ice, where popular belief, and, generally
speaking, more accurate observations also, give them no place.
Yet, in the thousands of crevasses which an active observer
passes and examines in a week, how few cases of imbedded
stones in these vast vertical sections are ever visible ? I might
almost ask whether they are ever seen, except in the neigh-
bourhood of the sides of a glacier, i.e., under the lateral mo-
raines ; and even there how rare !

The object of the present memorandum is, on the one hand,
to direct attention anew to the apparently perpetual paradox
which the glaciers present in this respect, and which, I am per-
suaded, offers something yet for careful observation, and, on the
other, to give what seems to me incontrovertible evidence, that
what I have so seldom seen myself must yet exist, and that

(uot unlike, in general size and polish, to those of hypersthene in hypersthene rock),
shew a development of crystallizing force which must evidently be the effect of long
time, and probably of comparatively slow motion. The reflection of the sun or
moon-beam from these plates gives to the glaciers I have mentioned a dazzling effect,
which I have not observed on the glaciers of the Pennine Chain. [I have since
noticed a similar structure in the Nygaard Glacier in Norway.]

1846.] HOW BLOCKS ARRIVE ON THE SURFACE OF GLACIERS. 203

stones are actually extruded, although in a peculiar manner,
from pure ice, or at least exposed by the ablation of the surface.

In my Travels, p. 241, referring to the excessively gradual
development of the moraine of La Noire on the Mer de
Glace of Chamouni, I stated my belief, that this moraine was
buried in a fold between two glaciers, one of which had over-
flowed the other, and that, as the upper glacier decayed away,
the rocky fragments were strewed on the surface. A fresh ex-
amination of the same localities leaves me in the same want of
direct proof of this fact, but the difficulty of explaining it other-
wise, makes me suppose my former view correct.

The extruded stones on the Glacier du Nant Blanc, near
Chamouni (alluded to in the first part of this letter), present a
very remarkable appearance, imperfectly shown in Plate VIII.
fig. 6. The right bank of this glacier (that which appears on
the left of the figure as seen from a distance) is at first bounded
by rocky summits, but in the lower part of its course by a
mound-like moraine of the usual form. The surface-blocks can
only be derived from the precipices near the origin. Yet they
do not even appear on the surface opposite to the rocks, but
only opposite to the moraine ; and they increase in number and
quantity towards the lower end of the glacier, where they
almost blacken the surface of the right side, the left side remaining
almost clean. It is difficult to believe that this accumulation is
not due to the gradual denudation of the blocks by the melting
of the ice in which they have been, in some way or other, im-
bedded ; but it is scarcely less difficult to admit that, having
fallen from the rocks above the Neve, they should have remained
unperceived in the ice during all the intermediate space.

To take another example. The glacier of the Rhone is
distinguished by the extraordinary purity of its surface, and the
consequent absence of lateral moraines. But this general free-
dom from stones on the surface is subject to one exception,
which is remarkable : — Stones begin to appear at the surface on
the terminal slope at a considerable height. How came they
there ? Not a stone the size of the fist can be seen on the sur-

204 THIRTEENTH LETTER ON GLACIERS. [1846.

face farther up ; and, in examining a number of the crevasses, I
could not see any engorged in the ice. The explanation seems
to be, that these stones are actually introduced into the ice by
friction at the bottom of the glacier, and forced upwards by the
action of the frontal resistance which produces the frontal dip of
the veined structure, and they are finally dispersed on the sur-
face by the melting of the ice. What is here supposed to occur
is illustrated by an ideal section of the glacier of the Rhone,
Plate VIII. fig. 7, where the curves of ejection are identical with
those of forced separation, causing the frontal dip of the veined
structure ; and this view is confirmed by what I have often
observed, particularly on the Glacier of Bossons, that the veined
structure in contact with the lateral moraines becomes soiled,
and that dirt and stones may be traced along the course of the
structural bands from the moraine to a considerable depth in the
ice. The action there is in the horizontal plane, what we here
suppose to take place in the vertical, and which the now esta-
blished retardation of the lower strata permits us to assume as
exactly a similar action. I have no doubt that a similar expla-
nation applies to the glacier of the Nant Blanc, and to other
glaciers.*

Before closing this already too long letter, I wish to record
an observation already made by me in 1844, but which I hesi-
tated to publish because the sketch representing it was made
from memory, and not upon the spot ; but I have now verified
it both in the same and another locality. It is represented in
Plate VIII. fig. 5, where b d is part of the wall of the glacier
which is about to turn at a considerable angle with its former
direction (it is at the well-known part of the Mer de Glace named
IS Angle). I knew, from long experience, that the ice here pre-

* It will be seen that this explanation will give an elevatory force to the ice
containing blocks similar to that which De Charpentier (Essai sur les Glaciers,
§ 25) ascribed to the expansion of the frozen water. It will also be seen that it
renders a perfect account of the " veins of the debris of rocks '' in glaciers, particu-
larly near their lower extremities, which that ingenious author has attempted to
account for (unsatisfactorily, I think) by the transporting action of streams of water.
—(Ibid, £ 27.)

1846.J THREE ORDERS OF DISCONTINUITY IN GLACIERS. 205

sents, year after year, compact ridges, such as a b, c d, parallel
to one another, and separated by a mass of crevasses which it
is, generally speaking, impossible to cross ; and any one who
attempts to traverse the more crevassed portions of the glacier
without attending to this peculiarity, will infallibly lose his way.
But, as the drawing explains, the direction of these ridges by no
means corresponds with that of the individual crevasses, whose
grouping subdivides the glacier as I have now described. The
crevasses may be nearly transverse to the glacier, whilst the
systems of crevasses form an angle of perhaps 30° with the
transverse line. The veined structure again cuts the crevasses
at right angles, so that these may be regarded as three orders of
discontinuity, or tearing surfaces, which occur in systems, that
is, regularly repeated at nearly uniform intervals over great por-
tions of the glacier surface. I have this year succeeded, for the
first time, in laying down on a map an approximation to the
various and complex systems of crevasses which traverse the
Mer de Glace, and I have found a repetition of this phenomenon
of a series of discontinuous but parallel fissures ranged along a
line or axis oblique to their direction, to recur at several points
where the strain is very violent. Let it be remarked, too, that
where the violence of the pressure opens a system of such
fissures to relieve it, the bands, or system of surfaces of mole-
cular discontinuity, disappear, or are less well developed. I
remain, etc.

12th December 1846.

XVII. FOURTEENTH LETTER ON GLACIERS.

Addressed to Professor JAMESON.

On the Variation of the Motion at different Seasons. Observations on the Glacier
of the Aar, considered and compared with the Author's results.

My Dear Sir — I am led to request you to reprint in the
Edinburgh Philosophical Journal, part of the 9th section of my
paper on the Viscous Theory of Glacier Motion from the Philo-

206 FOURTEENTH LETTER ON GLACIERS. [1847.

sophical Transactions for 1846,* for the following reason : —
There has lately appeared in the BibliothZque Universelle de
Geribve, a paper by M. Collomb, in which it is maintained, on
the authority of the experiments of M. Dollfuss on the Glacier of
the Aar, that the velocity of progress of that glacier is the same
at all seasons and in all weathers ; and that previous observers
who arrived at different conclusions on that glacier were in
error. It may be so. The Glacier of Aar is, as I have else-
where shewn,f in very peculiar conditions of mechanical con-
straint ; but it is important that it should be shewn by the
testimony of by far the completest series of observations which
has yet been published, and which extends over the entire year,
that this independence of the velocity upon external circum-
stances is not at least the general rule.

The passage in M. Collomb's paper, to which I have alluded,
may be thus translated : — " It results from the examination of
the registers of M. Dollfuss, which contain several thousand
observations, that the Glacier of the Lower Aar moves with per-
fect regularity ; — no sudden starts nor pauses ; the movement
of any particular point taken on the surface of the glacier, whe-
ther in the centre or at the edges, is very slow, uniform, and
independent of atmospheric influences. (He goes on to say,
that, as it is the fastest in the centre of the glacier, and almost
nothing at the edge.) Fine weather or rain, dryness of the air
or humidity, cold or heat, day or night, or different seasons,
have no influence upon the velocity of any particular point of
the surface of the glacier.

" The observers of previous years believed, on the other
hand, that the general march of great glaciers was in intimate
connection with the accompanying state of the atmosphere.
They believed, for instance, that the movement was retarded in
dry and cold weather, and accelerated by moisture and rain.

* [Already printed at page 125, etc.]

t Ninth Letter on Glaciers, Ed. Phil. Jour., vol. xxxviii., p. 332. [See above,
p. 68.] I may add, that M. Martins gives a similar account of the results of Mr.
Dollfuss' experiments in the Comptes Rendus, 26th October 1846.

1847.] OBSERVATIONS ON THE AAR GLACIER. 207

The observations of M. Dollfuss, which will, without doubt, be
published one day in detail, have proved the contrary, and have
shewn that a glacier, like a semifluid body, urged by a mechani-
cal force, moves on without being influenced by the state of the
surrounding medium"*

Now, Sir, the last passage, the italics of which are in the
original, conveys to us, in the first place, the pleasing informa-
tion that M. Collomb, and probably M. Dollfuss also, have ac-
cepted the viscous theory of glacier motion. The word " semi-
fluid" applied to a glacier, is now, notwithstanding its seeming
harshness, an adopted word. But I must enter an earnest pro-
test against the supposed discovery of the uniformity of the
motion and its entire independence of atmospheric circumstances,
being assumed to add any probability to the viscous theory, as
the phraseology of the preceding extract seems to infer. On
the contrary, if there be any glacier which does not present the
law of variable velocity, which I established for the first time,
on the Mer de Glace of Chamouni in 1842, and which has since
been found on the Glacier of Bossons, on the Glacier of Grin-
del wald, and was supposed to have been found on the Lower
Glacier of the Aar, such a glacier affords a proof the less in
favour of viscous or semifluid theory.

It is on this account, Sir, that I desire that the readers of
the scientific journals may be made fully aware of the amount of
the evidence by which, in some glaciers at least, the direct con-
nection between the movements of the glacier and the condi-
tions of temperature and moisture have been established ; and
it is for this object that I crave a few pages of your Journal,
for an extract from a more elaborate and less accessible paper.

Before quitting the subject, I wish to add, that I concur
with M. Collomb in desiring the full publication of M. Doll-
fuss' results on the Glacier of the Aar, which, latterly at least,

* Bibliotheque Universelle publiee 15 Decembve 1846, p. 212, note. The pas-
sage in italics stands thus in the original, — " Un glacier comme un corps semifluide
pousse par une force mecaiiique, marcJieen avnnt nans se laisser inflwncer par Vetat
du milieu ambiant.^

208 FOURTEENTH LETTER ON GLACIERS. [1847.

I know to have been made with excellent instruments, and, it
may be hoped, with due care. Until such a publication takes
place, we must be permitted to withhold our final assent from
a conclusion which is in contradiction to observations on at least
three other glaciers, and to former observations on the same one.
Should it prove correct in the particular case of the Glacier of
the Aar, I suspect that it can only arise from a curious balance
of two opposing influences occasioned by the peculiar circum-
stances of restraint under which that glacier moves, and pro-
bably may apply to only a very limited portion of its surface.

But I repeat, we must wait for the observations. The
public cannot but receive with distrust, reports of conclusions
drawn from unpublished observations so repeatedly contradict-
ing one another, as those which have been furnished by ob-
servers on this same Glacier of the Aar. Not to go farther
back, within four years we have had four positive statements of
fact said to have been deduced from observation, three of which
are irreconcileable. First, we were told that the glacier was
absolutely quiescent in winter, and moved onwards during the
summer months only.* It was next admitted, that in winter
there is also motion, only the summer motion is greatly in ex-
cess.f Next year (1844) we were told that the variation of
velocity is not confined to summer and winter, but is extended
to every variety of meteorological condition, in fact, is left on
the basis on which I had already placed it (as regarded the
Mer de Glace of Chamouni) two years previously. In the mid-
dle of summer, but during cold and snowy weather, which lasted
nine days, the velocity of the Glacier of the Aar fell below the
mean velocity of the same point for the entire year ; whilst
during the succeeding sixteen days of fine weather, the daily
average increased in amount by exactly one-half, and rose con-

* Agassiz, 1842. — " Ce que je puis annoncer positivement des a present, c'est
quele glacier est immobile en hiver." Letter to M. Arago, dated from the Glacier
of the Aar, Comptes Sendus, 8th August J842. Compare Ech'n. Phil. Jour., 1842,
vol. xxxiii., p. 253, 254.

f Agassiz, 1 843. — " Le mouvement est beaucoup plus accelere en ete qu'en
hiver." Bulletin de la Societe des Sciences Naturelle de Neufchatel, 8th Nov. 1843.

1847.] OBSERVATIONS ON THE GLACIER OF THE AAR. 209

siderably above the annual average.* Now (1846) we are de-
sired to consider these opinions and deductions to be erroneous,
as well as the measured distances ascertained by M. Wild, who
has hitherto been believed to be a competent surveyor, and who
had found the advance of the points of his triangulation on the
glacier to be more than one-half more rapid during summer than
in the remainder of the year.f The opinion of MM. Dolfuss,
Martins, and Collomb, is, that seasons and weather make no
difference whatever on the motions of the glacier of the Aar !

From these conflicting results, it is plain that there have
been reckless assertions made, and also observations unworthy
of confidence. It is very much to be desired, for the credit of
all the parties who have had a share in the experiments on the
glacier of the Aar, that the source of these discordances should

be investigated. I remain, etc.

*

EDINBURGH, 12th March 1847.

[The observations on the motion of the Mer de Glace, re-
ferred to in the preceding letter, and which were printed at its
close in the Edinburgh Philosophical Journal, being textually
taken from the paper in the Philosophical Transactions for 1846,
and already printed at page 125, etc. of the present volume, are
of course omitted.]

* Comptes JRendus, 9th Dec. 1844, p. 1301. — " L'avancement [journalierj etait
loin ,d'etre uniforme, il variait considerablement suivant les conditions atmosphe-
riques." Compare Ninth Letter [p. 73 above].

f Bull, de la Soc. de Neufchatel, No. 1. — From the results there mentioned, I
give the comparative motion of seven points of the glacier during fifty-seven days of
summer, and for the same length of time taken from the average of the remainder

of the year.

Summer. Other Seasons.

Feet. Feet.

50-2 33'4

54-8 34-9

47'9 27'9

47-1 29-6

35-0 26-3

25-5 16-5

18-3 11-4

210 FIFTEENTH LETTER ON GLACIERS. [1848.

XVIII. FIFTEENTH LETTER ON GLACIERS.
Addressed to the Rev. DR. WHEWELL.*

OBSERVATIONS ON THE ANALOGIES DERIVED FROM MUD- SLIDES ON A
LARGE SCALE, AND FROM SOME PROCESSES IN THE ARTS IN FAVOUR

OF THE VISCOUS THEORY OF GLACIERS.

Land slips as observed by M. Collin — Mr. Milward on the Phenomena of a large
Mud-Slide at Malta, and on the Cause of the Dirt-Bands and Wrinkles of
Glaciers — Connection with Frontal Dip — Surfaces of Detrusion in Iron Turn-
ings, attended with Vertical Accumulation of Material.

My dear Sir — It is considerably more than a year since
you did me the favour to communicate to me the interesting
drawing and remarks by your friend Mr. Milward, on a mud-
slide on a large scale, which had come under his observation
at Malta, and which led him to notice some interesting ana-
logies with the structure of glaciers. Again, last August, you
communicated some farther reflections and observations by
Mr. Milward, and you invited me to send any remarks on
the same subject which occurred to me, to be communicated,
along with Mr. Milward's papers, to the meeting of the Bri-
tish Association. I sent you, on the llth of August, a let-
ter, the chief parts of which I shall embody in this one, but
which was not read at Swansea, in consequence of the pres-
sure of business in the Geological Section, which barely ad-
mitted (as I afterwards heard) of Mr. Milward's papers being
read, and consequently no discussion took place. Since that
time, Mr. Milward, before returning to Malta, was kind
enough to place his papers at my disposal, which I then of-
fered to Professor Jameson for publication in his Journal, which
he accepted, and now allows me to add my remarks on the
same subject, which I address to you, as having been the intro-
ducer of Mr. Milward's facts, and as having first desired my
opinion with regard to them.

The phenomena presented by mud-slides on a large scale,
are not now studied quite for the first time. About two

* Edinburgh New Philosophical Journal for January 1849.

PLATE IX.

I. Versailles S

g. 2 . Reservoir of Gercey p3io?£imcLT
p.211.

A D S

Pig. 3.^.215.

/iffib

Pigure

reading.

Sketct. of a Staving of MaHeaHe Iron
Snowing plaoies of detrusion .

\

It gams a lead. .

. i.Anewnead fo.rms.

5. A second crease.

Pig 7 The Snavrng deTel0pea..p.219.

1848.] ANALOGIES DERIVED FROM LAND-SLIPS. 211

years ago, I obtained a French work, entitled, " Recherches
Experimentales sur les Glissements spontanes des terrains
argileux, par Alexandre Collin, Ingenieur des Pouts et Chaus-
sees." Paris, 1846, 4to. This interesting work, illustrated by
plates, contains no allusion to the subject of glaciers; — the pheno-
mena of mud-slides being considered solely in an engineering point
of view. The principal object of the work is to investigate the
form of the surface of sliding, which separates the solid from the
moving soil of railway cuttings, embankments, and the like.
That subject is not particularly connected with the one before us ;
but M. Collin has, at the same time, presented us with excel-
lent and detailed sections of land-slips, the mere inspection of
which recalls forcibly the outline of glaciers, and, although evi-
dently unaware of my theory of the latter, his remarks confirm,
in a very satisfactory way, several of my anticipations respecting
the internal movements of viscous bodies. Thus, in the trans-
verse section of an embankment of the Paris and Versailles Rail-
way (Rive gauche) [copied in Plate IX. fig. 1., of the present
volume], we find the original declivity of forced earth, denoted
by the dotted line, remodelled by the slide over the surface DFE
into the bulged form FGH, which recalls at once the terminal
section of a glacier. In fig. 2, again, we have a similar pheno-
menon, observed at Cercey in Burgundy, where the mass has been
more solid, the swelling of the surface less continuous, and trans-
verse crevasses, exactly like those of a glacier, have opened.
The length of the talus or declivity, before sliding, was about
26 metres, or 85 English feet, in the first case, and 24 metres,
or 79 English feet, in the second. M. Collin also measured
daily, for more than two months, the horizontal advance of
the lower extremity of the earth-slide of Cercey, and likewise
the perpendicular fall of its upper extremity. These results,
which are the only ones of the kind which 1 have met with,
are highly interesting, as shewing the continuity and general
regularity of this very small motion in a mass which could not
be called fluid, in any ordinary sense of the word, since we
are told that there was not the slightest trace of any exudation

212 FIFTEENTH LETTER ON GLACIERS. [1848.

of water from the reservoir, of which the mass in question
formed the embankment, and that " the absence of continued
rain during the period of observation, singularly favoured the
regularity of the descent."0 But the best proof of the solidity
of the material (a clayey soil, near the canal of Burgundy) is,
that it admitted of being cut to permanent slopes of 30°, and
even of 45°. The amount of horizontal movement of the lower
end of the land-slip increased gradually during the first three
weeks, and soon after ceased entirely ; but the top of the
slip continued to move during the whole continuance of the
observations. This fact was confirmed by independent observa-
tions on a subsequent slip.f It follows, therefore, as a mathe-
matical necessity, that the central parts of the slip being thus
compressed, must either have discharged themselves laterally,
or been heaped up vertically. An inspection of the change of
figure of the displaced matter FGH, in fig. 1, which originally
had the section EADF, plainly shews that the loosened earth
was heaped up by the frontal resistance near H. — that the
posterior parts of the mass overrode, the anterior ones ; in short,
gave rise to the upward and forward internal sliding motion, to
which I ascribed, in the glacier, the phenomenon of the frontal
dip of the veined structure (Travels in the Alps, 2d edit. p. 164).
This condensation or swelling (boursoufflement] was noticed by
M. Collin as characterising earth-slides, and the actual " ascen-
sional movement" of the parts, due to the ^wm-hydrostatic
pressure, when a solid obstacle resisted the progress of the
stream, is also admitted by him :{ of course, a mass of the
mud or earth, of sufficient weight to produce an intense friction
on a level or on a small declivity, would produce the same
effect. In these observations, the daily motion of the terminal
part of the land-slip varied from one-hundredth of a metre
(0-4 inch) to a metre and one-third (4J feet), but this last
motion seems to have been the result of a sudden concussion ;
the steadiest motion was from half an inch to four inches daily. §

* Collin, Glissements spontane?, p. 50. f Ibid, p. 54.

t Ibid, p. 47, and Plate XI.

% My friend Mr. John Thomson (of Glasgow), in a short note with which he

1848.]

MR. MILWARD S OBSERVATIONS.

213

Having now attempted to do justice to M. Collin's interest-
ing observations, I pass on to the papers of Mr. Milward.*
This gentleman, being acquainted generally with the analogies
lately attempted to be established between viscous bodies and
glaciers, at once directed his attention to the peculiarities of
surface of the great mud-slide which he witnessed at Malta. In
the stream of the first slide he observed, under a favourable
light, curved bands, alternately dark and light coloured, which,
like their analogues, the dirt-bands of glaciers, are best seen
from a height, and when the light falls obliquely. On close
inspection, these bands were found to be composed (superficially)
of smooth, fine mud, and of rough, coarse mud alternately, the
latter being somewhat the higher of the two. In a second case
of a mud-slide, he found that the smoothness of the mud was a
superficial phenomenon due to the settling of the more fluid part
in slight depressions existing between " the rough bands, which
were raised from a foot-and-a-half to two feet, so as to form ridges,
or waves, or wrinkles, swelling and falling over." The sketch given
[by Mr. M.] of these " wrinkles" is shewn in fig. 3 of Plate IX.

Rocks.

Rocks.
Fig. 23. PLAN or GLACIER DU GEANT.

has favoured me, states that, in his experience of making railway embankments
(in Leicestershire), he has found concentric waves or wrinkles pressed out of the
soft clay of the embankment, in proportion as the load of earth increases. [In the
course of the last ten years I have continued to receive numerous additional con-
firmations from several correspondents.]

* [In the Edinburgh Philosophical Journal for January 1849.]

214 FIFTEENTH LETTER ON GLACIERS. [1848.

Mr. Milward was not aware that the analogous phenomenon
was discovered by me in glaciers as far back as 1843, and
A

|Fig. 24. SECTION OF GLACIER DU GEANT.

described in my Fifth Letter on Glaciers,* where I have
given the preceding plan and section representing them. It
must be as satisfactory to Mr. Milward as it is pleasing to
me, to find that his shrewd conjectures as to the probability
of their discovery, although based solely on the analogy of
viscous fluids, are thus perfectly confirmed. They were, in
fact, discovered in the place and at the time that Mr. Milward
supposed they would be, and they were already designated
by the very term he uses, " wrinkles," years before he wrote
of them. " It will be useless," he observes, " to look for
them at the lower parts of glaciers, as they will have disappeared
under the effects of atmospheric and other action. . . It is,
therefore, to the head of the glacier, where the true glacial
structure commences, that we are to look for such ridges : the
best time, also, will be at the commencement of summer, after
the disappearance of the snow, and before the confusion of the
surface occasioned by the sun's influence." t In point of fact, it
will be found, from the reference above, that the glacier wrinkles
were observed on the Glacier du Geant, at the upper part of the

* Edinburgh New Philosophical Journal, 1844, p. 117, and pages 40 and 41
of the present volume, where these figures occur.

f [An Attempt to illustrate the Origin of "Dirt-bands in Glaciers." By A. Milward,
Esq. Edinburgh New PhilosophicalJournal, Jan. 1849. The remarkable insight into
phenomena which this paper displays, and its intimate connection with the subject
of the present Letter, induces me to print Mr. Milward's contribution as an Appen-
dix to the present volume ; the rather as it might easily be overlooked in the volu-
minous journal in which it first appeared. The account of the Mud-Slide at Malta
(which is not reprinted) is also well worthy of being read. The writer of these con-
tributions to the theory of Glaciers is now unfortunately deceased. He died about
two years ago in the neighbourhood of Clifton, where I had first the advantage of
making his acquaintance in 1852. His very bad health must have prevented him,
for a great many years from exploring a glacier, if indeed he ever had that advantage.
His deductions are therefore, in every respect, the more remarkable. — Dec. 1858.]

1848.] MECHANICAL ORIGIN OF CREASES OR WRINKLES. 215

Mer de Glace ; — and that the season was that of the disappear-
ance of the winter snow, which lay in wreaths between the
ridges, thus perfectly defining their contour.

With respect to the origin of these wrinkles in the mud-
slide, Mr. Milward has, I think, very justly rejected the expla-
nation of them formed on a supposed alternation of beds or strata
of different texture, the existence of such beds being entirely
hypothetical, and, considering the manner in which such a mud-
slide is formed, utterly improbable. In looking for an explana-
tion of the analogous phenomena in glaciers, it would, therefore,
be wise to try to find one which should hold good in a mass of
uniform consistence, and rather to look for the less compact
structure of the ice beneath the dirt-bands as an effect of the
same cause which produces the wrinkles, than as the cause
itself. I believe that the phenomena of ridges or wrinkles is a
general one, depending on the toughness of a semifluid or semi-
solid mass forcibly compelled to advance or extend itself; that
the periodicity, or repetition of the wrinkles at nearly regular
intervals, is due to mechanical causes alone, and to no variation
of internal consistence.

Having successfully imitated these wrinkles in my experi-
ments, I think that I am able so far to account for them. Although
neither mud nor plaster is capable of retaining the internal veined
structure of the frontal dip, which bears evidence of the direc-
tion of the slide being such as I have stated, that evidence is
to be found in the glaciers themselves, in certain cases of lava
streams, which I have elsewhere described,* and in Professor
Gordon's beautiful experiment with brittle pitch in motion.f

A body soft enough to convey a pressure partly hydro-
static, or one acting in any direction, — if it be tenacious enough,
— always tends to crease, or have its surface near the point of
pressure pushed upwards and forwards relatively to the surface
farther off. A heavy weight laid on a tough paste will raise a
wrinkle round it, but at some distance ; farther off in proportion
to its toughness. So railway embankments raised on a boggy

* Phil. Trans. 1836. f phil- Mag., 1845. vol. xxvi. p. 206.

216 FIFTEENTH LETTER ON GLACIERS. [1848.

bottom force out a crease, or two or three successive parallel
creases, on either side ; and, I daresay, you recollect that one
effect of the great land-slip at Lyme, some years ago, was to
elevate a ridge of shingle above the level of the sea at some dis-
tance from the shore. A succession of equidistant wrinkles
will be formed whenever different parts of a plastic body are
subjected in succession to a pressure violent enough to produce
detrusion, or sliding amongst the particles. Thus, if a very
viscid fluid is poured in a gradual stream upon a flat surface, so
that it may spread uniformly, a succession of circular creases
is formed in consequence of the hydrostatic, pressure from the
heaped-up centre becoming sufficient to overcome, for a moment,
the viscosity at a certain distance from the centre ; as the cir-
cumference rises in a crease, the centre falls, the central pres-
sure suddenly diminishes, and if the stream continue to be
poured uniformly upon the centre, although the circles will
expand slightly, it will not be until a sufficient head is again
raised to overcome suddenly the viscosity of the fluid that a
new wrinkle is formed in exactly the same relative position as
the first.* Treacle, mortar, tar, and similar bodies, usually
present such creases when poured out. What has now been
said of a viscid mass spreading uniformly from a centre applies
equally to one confined in a trough with parallel sides, if con-
stantly fed from one end. A succession of waves are thus
formed, as in Mr. Milward's mud-slides, or as in a glacier.
They become confounded or not at a distance from the origin ;
that depends entirely on the rate of motion of the stream at
different points, which again depends chiefly upon the declivity
of its bed.

These wrinkles or creases, then, do occur at regular inter-
vals, even in bodies perfectly homogeneous, and, under external
circumstances, perfectly uniform. The intervals of such waves
depend, in these cases, solely upon the physical qualities of
tenacity, specific gravity, etc., of the body, and the more or less

* The steps of this process are attempted to be illustrated by the curves in Plate
IX. fig. 4.

1848.] ORIGIN OF THE WRINKLES ON THE GLACIER. 217

ample stream which furnishes it. We perceive here nothing
like an annual recurrence ; and this circumstance at first puzzled
me, the intervals between the dirt-bands of the Mer de Glace
(which are evidently the same with the wrinkles) being, as ob-
served by me at the very time of their first discovery, so nearly
consistent with what I supposed to be the annual motion of the
ice stream, and which was afterwards -confirmed by direct
experiment, as scarcely to allow us to suppose the coincidence
fortuitous.

But an easy experiment establishes the analogy perfectly.
If the stream of plastic matter already supposed be not uniformly
supplied, but arrive in gushes, every such overflow, by the rapid
rise of the head, throws off a wrinkle in the most regular manner.
So that, for example, on examining those plaster models, for-
merly repeatedly described by me, in which cupfuls of white
and of blue plaster of Paris were alternately poured down an in-
clined channel, each separate flow was found to constitute a
wave or crease. In a glacier, especially in its higher regions,
the difference of summer and of winter velocity is sufficient to
produce what may be called (relatively) a gush ; and I suppose
that the wrinkles are formed in most glaciers at the foot of the
steeps of the neve (as Mr. Mil ward also believes), where a pres-
sure d tergo is produced by the heat of the short summer, suf-
ficient to overcome the incalculable resistance which a mass of
half-frozen snow, hundreds of feet thick and hundreds of yards
wide, presents, to be squeezed and moulded, after the manner
of a semifluid, into a convex wrinkle. Of the fact there is no
doubt. Each wrinkle, then, is nothing else than a local swel-
ling, such as those figured by M. Collin, taking place at the
moment when the upward and forward force due to the quasi-
hydrostatic pressure of the mass becomes insupportable, and
gives rise to the forced separation of the cohering substance by
countless fissures, constituting the frontal dip of the veined
structure of the glacier, whose position, taken in connection with
the wrinkles, is shewn in Plate IX. fig. 5.

I farther beg leave to direct your attention to a very

218 FIFTEENTH LETTER ON GLACIERS. [1848.

curious illustration of these views which I lately noticed. In
the mechanical turning or planing of malleable iron, the spiral
shavings have a structure which is truly remarkable, and shews
convincingly that the effect of a steady pressure upon a semi-
solid or plastic body, is really such as to produce not merely
wrinkles or creases on the surface, in the usual wave-like form,
advanced in the centre, and withdrawn or retarded at the sides;
but that the shaving has its particles squeezed upwards and
forwards, as I have maintained that the mass of ice is, in con-
sequence of the intense frontal resistance, and when the tenacity
of the metal is pushed to its utmost limit of endurance, detrusion
takes place at intervals sensibly equal, as in one of the speci-
mens herewith sent, — being nothing else than the wrinkles
exaggerated, and the bruise producing the veined structure
pushed to an actual separation. (See Plate IX. fig. 6). These
specimens (and such may be found in the workshop of almost
any machine-maker) * have the higher degree of interest, be-
cause the surface of detrusion makes so very large an angle with
the line of pressure. This process of heaping up by internal
sliding of the parts of a semifluid mass was pointed out by me,
I believe, for the first time, as applicable not only to very tena-
cious bodies, but even to streams no more viscid than common
water. But I concluded that when the frontal resistance (due
to friction and cohesion) becomes very great, the planes of least
resistance may assume an inclination of 60° or more, a notion
which has been treated as practically untenable by a mathema-
tical critic of my theory, whilst he admits that it is theoretically
possible. The iron shavings in question demonstrate the truth

* They are not so common as I supposed when I wrote this ; they are princi-
pally to be found when coarse planings are made from iron of not the very best
quality, and not lubricated with water. The finest iron is too plastic. On men-
tioning recently these observations to Mr. James Naysmith of Manchester, he
stated it to me, as a fact familiar to practical men, that, in turning a cylinder three
feet in circumference, the shaving, owing to frontal condensation, and the over-
riding of the parts, is perhaps only two and a half feet long. How perfect the
analogy with what I have always maintained to be the mechanism of the glacier !
It is this thickening, amounting to one-fifth part, which compensates during winter
the summer's waste.

1848.] CURIOUS ANALOGIES IN PLANED IRON. 219

and feasibility of my anticipation. There is no difficulty in de-
termining the exact line of pressure, for it is obviously that in
which the tool is made to act, or it is mathematically parallel to
the flat side of the shaving itself, if we suppose it straightened.
(Fig. 7.) * In one of the specimens now before me, the planes
of detrusion, or frontal dip, make an angle, as nearly as can be
estimated, of seventy degrees, with the base or line of pressure.
From i\\Q fibrous appearance of the whole mass, I have little
doubt that it is traversed by numberless fissures or flaws parallel
to the planes of actual sliding, flaws which might probably be
made evident by immersing the whole in dilute acid.

Time does not allow me to add more. Some may consider
these approximations and analogies trifling, but I persuade my-
self that you will not do so, being well aware how much has
resulted in the progress of science from the patient study of
minute facts not obviously related to one another. It is some
pleasure to me to persuade myself that my speculations upon
the cause of the motion of glaciers have had some slight in-
fluence in drawing attention to the loose manner in which
bodies have hitherto been classified as solid and fluid, rigid,
flexible, or plastic. On the one hand, attention is directed to
the way in which stress or strain is exerted upon masses, and
modified by their internal constitution in a way which no theory
not embracing an expression of that constitution founded on
experience can possibly represent. On the other hand, the im-
perfect views which practical men have entertained as to the
manner in which intense strains affect materials of certain kinds
and in certain forms, are apparently about to undergo a con-
siderable revolution. I remain, my dear Sir, yours very truly.

EDINBURGH, 2d December 1848.

* [The analogy of this figure to fig. 7 of Plate I., representing the effect of
vertical detrusion on a lava stream of Mount Etna, is remarkable.]

220 SIXTEENTH LETTER ON GLACIERS. [i860.

XIX. SIXTEENTH LETTER ON GLACIERS. Addressed
to PROFESSOR JAMESON.*

Observations on the Movement of the Mer de Glace down to 1850. Observations
by Balmat, at different seasons, in continuation of those formerly detailed. On
the gradual passage of Ice into the Fluid State ; observations of M. Person-
Notice of an undescribed Pass of the Alps, from Chamouni to Orsieres by the
Glaciers of Tour and Salena.

My dear Sir — Having had the good fortune once more
to spend a few (though very few) days amongst the glaciers of
Chamouni last summer, I avail myself of your kind permission
to carry forward the account of my observations, which has
now, for a period of eight years, been regularly communicated
to the readers of your Journal. As my stay was limited by
imperative engagements to little more than a week, I was pre-
vented from undertaking a continuous series of observations on
the movement of the ice. I was fortunate, however, in obtain-
ing materials for the correction and extension of certain parts
of my Map of the Mer de Gflace, which were deficient in my
former observations, especially as to the exact form of the basin
of the great Glacier du Gtant, which I had only visited once
before, on occasion of the passage of the Col of that name in
1842. This year I traversed again all the difficult part of that
glacier, and took angles with the theodolite from the upper part
of the basin, immediately under the Aiguille du Geant. But
as these observations can have little interest until reduced into
the form of a corrected edition of the Map, I shall say nothing
of them here.f

It will be recollected by some of your readers that a re-
markable stone called " La Pierre platte,"J: was one of the

* Edinburgh New Philosophical Journal for January 1851.

-j- [These corrections were introduced into the new edition of my Map of the Mer
de Glace (including also the Glacier of Bossons), accompanying the small abridg-
ment of my " Travels," published in 1855 by Messrs. Black under the title of " Tour
of Mont Blanc and of Monte Rosa."]

| Lying on the surface of the Glacier de LGchaud (in the upper part of the
Mer de Gkice), and carried along by the motion of the ice. It is marked C in my
Map of the Glaciers.

1850.] STATIONS C AND V ON THE MEB DE GLACE. 221

earliest points whose position was ascertained by me in 1842.
Its daily motion was watched by me during that summer,* and
its annual motion was ascertained by renewed observations in
1843, 1844, 1846, and again this year. I measured the dis-
tance along the ice from the original position of the " Pierre
platte" on the 27th June 1842 (ascertained by reference to
fixed marks on the rocks) to its position on the 12th July 1850.
and found it to be 2520 feet. But of this distance, 1212 feet
had been travelled at my previous observation on the 21st July
1846, leaving 1308 feet during the last four years against 1212
in the first four. When more accurately stated and compared,
the mean annual and daily motions will stand as follows : —

1842-3. 1843-4. 1844-6. 1846-50.

Daily motion, in INCHES, -8'56 9'47 10'65 1081

Annual motion, in FEET, 260'4 288'3 323'8 328'8

We cannot infer, with absolute certainty, that the slight increase
of velocity here noticed since 1844 is due to a change in the
conditions of the glacier (although I believe that the recurrence
of several snowy seasons and the very marked increase of the
volume and extent of the glacier during these years would pro-
duce such an effect), because it has moved nearly half a mile
from its position when first observed, and the part of the glacier
on which it now lies may be subject to different accelerating
and retarding causes.

It is mentioned in my Thirteenth Letter, page 4, that I
marked a fine solitary block towards the centre of the Mer de
Glace, opposite " Les Fonts," with the letter V in 1846, and
that I took angles for fixing its place with reference to the ad-
jacent rocks. It was then about 760 feet distant from the west
bank. I had little difficulty in recognizing the block in 1850,
although it had travelled a great distance, and was considerably
lower than the Montanvert. It had preserved its parallelism
to the shore, for I found it at almost the same distance from
the west bank as at first ; and by measuring carefully along the
side of the glacier, I estimated its progress in four years, from

* Travels in the Alps of Savoy, 2d edition, p. 139, 140.

SIXTEENTH LETTER ON GLACIERS. [1850.

30th July 1846 to 13th July 1850, at 3255 feet. This gives,
for the mean motion in 365 days, 822*8 feet, or the mean daily
motion 2 7 '05 inches, which is remarkably large. Its position
is very near the point of one of the " dirt-bands," but a little
nearer the western bank. It lies, however, on the band.

I shall now give the sequel of my guide Auguste Balmat's
observations on the motion of the Glacier des Bois (the outlet
of the Mer de Glace), and of the Glacier des Bossons, since the
period to which the table in my Fourteenth Letter extends,* which
will be found to embrace continuous observations, by periods of
a few weeks from the 2d October 1844 to the 21st November
1845. They were continued in like manner until the 19th
February 1846, when they were interrupted by Balmat's illness,
which was accompanied by inflammation of the eyes. But in
October of the same year they were resumed, and were con-
tinued without intermission until the end of June 1848, em-
bracing altogether a period of nearly four years, with only eight
months' intermission. It is necessary to observe that the
station on the glacier of Bossons was altogether changed after
the above mentioned interruption, being transferred from the
west to the east side (in the same region of the glacier), and it
was 340 feet from the bank. The station on the Glacier des
Bois was almost unchanged [?], and was about 280 feet from the
north bank, between the Cote du Piget and the acclivity of the
Chapeau. I have added a column giving the mean of the tem-
peratures of the several periods of observation, carefully calcu-
lated from the published observations at Geneva and the great
St. Bernard, on the same principle as I have fully explained in
my Fourteenth Letter above referred to.f The comparisons of
the temperature and the rate of motion lead to conclusions
similar to those which I have drawn in that paper from the
earlier observations, the general observation always holding that
the acceleration in spring is in a greater proportion to the tem-
perature than at any other season of the year, on account of

* [Borrowed from the paper ip the Philosophical Transactions, reprinted at
page 128 of this volume.] f [See rather p. 131 above.]

1850.] CONFIDENCE DUE TO BALMAT's OBSERVATIONS. 223

the great influence of the melting snows in imparting fluidity to
the glacier masses. I do not mean that the comparison leads
always to consistent results. I do not think that the causes
of the comparative acceleration of one glacier and retardation
of another have yet been clearly brought out, though I conceive
that accurate local observations, combined with such measure-
ments, would gradually but surely unveil them. Nor do I
mean to affirm that measurements made with so much labour
and trouble, and under circumstances even of personal danger
at certain seasons of the year, are irreproachable in point of
accuracy. I think it even probable that oversights have oc-
curred ; but I have very strong reason for confiding in the
absolute fidelity with which the observations have been made
and transmitted to me. Circumstances have transpired since
my last publication which increase this confidence ; and I should
be ungrateful if I did not once more publicly acknowledge,
whilst giving to the world the sequel of observations made
under such circumstances that their resumption is scarcely pro-
bable, the lasting obligations which I owe to the zeal, fidelity,
and disinterestedness of my worthy though humble friend and
guide.

224

SIXTEENTH LETTER ON GLACIERS.

[1850.

TABLE shewing the mean daily motion in inches of the Glaciers of Chamouni,
deduced from Balmaf s Observations, and continued from [page 128].

Intervals of Observation.

Mean Daily Motion in Eng. inches.

Temp.
Centigrade
of Air*

Remarks.

Bois,
No. I.

Bois,
No. II.

Bossons,
No. I.

Bossons,
No. II.

west side

1845.fNov. 16 to Dec. 16

14-0

10-9

30-2

6-4

— 1-47

fDec. 16 to Jan. 19

12-0

5-7

18'8

10-0

— 4-19

1846/f-Jan. 19 to Feb. 19

16-1

5-1

16-9

13-0

— 0-16

(Observations interrupted by Balmat's illness.)

east side

Oct. 12 to Nov. 19

21-8

10'8

1-65

16th Oct. Snow

Nov. 19 to Dec. 20

24-0

13-1

— 4-41

at Montanvert.

Dec. 20 to Jan. 18

24-5

12-8

— 5-88

1847. Jan. 18 to March 4

31-5

14-5

— 4-82

Vast quantity of

March 4 to April 12
April 12 to May 14

34-5
37-3

13-9
19-7

— 1-08
3-10

snow. Destruc-
tive avalanches.

May 14 to July 2
July 2 to July 23
July 23 to Aug. 16
Aug. 16 to Sept. 9

34-2
30-5
34-0
44-7

22-6
23-1
25-8
23-5

9'97
13-88
11-89
9-65

Snow disappeared
on Bossons, 2d
week of May ; on
Bois, 3d week of
May.

Sept. 9 to Sept. 28

37-7

22-6

7-95

Sept. 28 to Oct. 18

32-2

21-5

5-34

Oct. 18 to Nov. 6

30-7

14-5

3-41

Nov. 6 to Nov. 27

30-2

10-7

0-24

Nov. 27 to Jan. 10

24-4

10-5

— 3-74

1848. Jan. 10 to Feb. 19

26-5

14-5

— 5-79

Feb. 19 to April 1

23-5

12'6

— 0-64

April 1 to May 3

33-8

18-8

4-93

May 3 to June 6

35-3

17-6

8-68

June 6 to June 30

43'8

17-6

11-57

In my former Letters I have taken occasion to mention
experiments and observations which have occurred from time
to time of a nature to confirm the fundamental hypothesis of
the quasi fluidity of the ice of glaciers on the great scale, and
I cannot doubt that these incidental remarks have tended to
diminish the natural incredulity with which that theory was at
first received in some quarters. I have now to cite a fact of

* Mean of Geneva and Great St. Bernard.

f [There is much reason to think that Balmat, in entering or transcribing these
observations, has put those on the Glacier du Bois in place of those on the Glacier
des Bossons, and vice versa. It would farther appear that the renewed station on
the Glacier des Bois, after the interruption of the observations, did not exactly
coincide with the old one, as it shows a smaller annual fluctuation of velocity. See
also note to page 139.]

1850.] M. PERSON'S OBSERVATIONS ON THE FUSION OF ICE. 225

the same kind 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
fact, viz. : — That ice does not pass abruptly from the solid to
the fluid state : That it begins to soften at a temperature of 2°
centigrade below its thawing point ; that consequently between
28°-4 and 32° of Fahrenheit, 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.
M. Person deduces this from the examination of the heat
requisite to liquify ice at different temperatures. The following
sentences contain his conclusions in his own words : — " II
parait d'apres mes experiences que le ramollissement qui
precede la fusion, est circonscrit dans line intervalle d'environ
2 degres. La glace est done un des corps dont la fusion est
la plus nette ; mais cependant le passage de 1'etat solide a
,1'etat liquide s'y fait encore par degres, et non par un saut
brusque."*

Now it appears very clearly from M. Agassiz' thermome-
trical experiments, and from my own observations, that from
28° to 32° Fahrenheit is the habitual temperature of the great
mass of a glacier ; that the most rigorous nights propagate an
intense cold to but a very small depth ; and I am perfectly
convinced that in the middle and lower regions of glaciers
which are habitually saturated with water in summer, the
interior is little, if at all, reduced below the freezing point,
even by the prolonged cold of winter ; it would be contrary to
all just theories of the propagation of heat if it were otherwise,
when we recollect [that] the enormous mass of snow which such
glaciers bear during the coldest months of the year is a cover-
ing sufficient to prevent any profound congelation in common
earth ; and admitting that ice is probably a better conductor
of heat than the ground, it is quite, incredible that a thickness
of many hundred feet of ice, saturated with fluid water, should
be reduced much below the freezing point, or should even

* Comptfs Sendus, 29th April 1850.
Q

226 SIXTEENTH LETTER ON GLACIERS. [i860.

be frozen throughout. And that it is not [so frozen], the
striking testimony of the continued stream of water issuing all
winter from under the ice can hardly fail to convince us ; still
more, the circumstance mentioned in my Fourteenth Letter,
that even in the month of February the source of the Arveiron
becomes whitish and dirty as in summer, before a change of
weather, proving (as I have there remarked) that " in the
middle of winter a temporary rise of temperature over the
higher glacier regions (which is the precursor of bad weather)
not only produces a thaw there, but finds the usual channels
still open for transmitting the accumulated snow-water."*

It thus appears quite certain that ice, under the circum-
stances in which we find it in the great bulk of glaciers, is in
a state more or less softened even in winter ; and that, during
nearly the whole summer, whilst surrounded by air above 32°,
and itself at that temperature, it has acquired a still greater
degree of plasticity, due to the latent heat which it has then
absorbed.f

I have mentioned that the observations of this and some
previous summers have enabled me to extend the survey of
the valley of Chamouni beyond the limits to which my Map
was originally confined. I have also obtained a great number
of approximate altitudes of all the highest summits of the chain
of Mont Blanc, from the extended base which the distance from
the Mont Breven to the Croix de Flegere (above 15,400 feet)
has afforded me. But the results are as yet only partially
calculated. I have also made some additions to our knowledge
of the geography of the eastern part of the chain of Mont
Blanc, by examining the Glacier of La Tour in its whole
extent, which proved the configuration of the mountains to be
different from what has been represented on all the maps and
models which I have seen. The Glaciers of Argentiere and

* [See p. 133, note, of this volume.]

f [See note to Philosophical Transactions, 1846, p. 209 (p. 166 of this volume),
and also the paper following this one.]

1850.] PASS OF THE COL DU TOtJfc. 227

Le Tour are separated throughout by a rocky ridge, but the
Glaciers of Le Tour and Trient all but unite at their highest
parts, and the main chain is prolonged with scarcely a break in
the north-east direction, sending off only a spur towards the
Col de Balme, which, perhaps from being the political boundary
of Savoy and Switzerland, has been represented generally on
an exaggerated scale. What surprised me most, was the great
elevation of the axis of the chain at the head of the Glaciers of
La Tour and Trient. I found it barometrically to be 4044 feet
above the chalet of the Col de Balme, which, from five com-
parisons made with the observatory at Geneva, is 7291 English
feet, or 2220 metres above the sea, a result agreeing closely
with the recent measurement by M. Favre, which is 2222
metres. Adding this result to the former, we obtain 11,335
English feet for the height of the granitic axis at the lowest
point between the Glaciers of La Tour and Salena on the side
of the Swiss Yal Ferret. By a single direct barometrical com-
parison with Geneva, I obtained 11,284 English feet above
the sea, or 140 feet higher than the Col du Geant. I was suc-
cessful in traversing the Glacier of Salena to Orsieres the
same day, a pass which has not before been described, and
which has this interest, in addition to the singular wildness of
the scenery, that it includes those regions of beautiful crystal-
lized protogine, here in situ, which have been known to geolo-
gists hitherto chiefly from the numerous moraines which they
form in the valleys of Ferret and of the Rhone, and especially
the majority of the blocks of Monthey, which have been
derived, according to M. de Buch, entirely from this region of
the Alps.*

* [See a fuller account of this pass in my account of "Norway and its
(Glaciers," etc., and also in the "Tour of Mont Blanc," etc., p. 301.]

228 ON THE PROPERTIES OF ICE NEAR ITS MELTING POINT. [1858.

XX. ON SOME PROPERTIES OF ICE NEAR ITS
MELTING POINT.*

During the last month of March I made some experiments
on the properties of ice near its melting point, with particular
reference to those of Mr. Faraday, published in the Athenaeum
and Literary Gazette for June 1850,t to which attention has
been more lately called by Dr. Tyndall and Mr. Huxley in
relation to the phenomena of glaciers.

Owing to indisposition, 1 have been obliged to leave my
experiments for the present incomplete. But I am desirous,
before the session of the Royal Society closes, to place on record
some facts which I have observed, and also some conclusions
which I deduce from these and other recent experiments and
discussions.

Mr. Faraday's chief fact, to which the term " regelation "
has been more lately applied, is this, that pieces of ice, in a
medium above 32°, when closely applied, freeze together, and
flannel adheres apparently by congelation to ice under the same
circumstances.

1. These observations I have confirmed. But I have also
found that metals become frozen to ice when they are surrounded
by it, or when they are otherwise prevented from transmitting
heat too abundantly. Thus a pile of shillings being laid on a
piece of ice in a warm room, the lowest shilling, after becoming
sunk in the ice, was found firmly attached to it.

2. Mere contact without pressure, is sufficient to produce
these effects. Two slabs of ice, having their corresponding sur-
faces ground tolerably flat, were suspended in an inhabited room

* From the Proceedings of the Koyal Society of Edinburgh. 19th April 1858.

f [The abstract of this communication by Mr. Faraday, so far as it relates to
the properties referred to above not being otherwise conveniently accessible, is
reprinted in Appendix II. to the present volume.]

1858.] GRADATION OF TEMPERATURE IN FREEZING WATER. 229

upon a horizontal glass rod passing through two holes in the
plates of ice, so that the plane of the plates was vertical. Con-
tact of the even surfaces was obtained by means of two very
weak pieces of watch-spring. In an hour and a half the cohe-
sion was so complete, that, when violently broken in pieces,
many portions of the plates (which had each a surface of 20
or more square inches) continued united. In fact, it appeared
as complete as in another experiment where similar surfaces
were pressed together by weights. I conclude that the effect
of pressure in assisting " regulation" is principally or solely due
to the larger surfaces of contact obtained by the moulding of the
surfaces to one another.

3. Masses of strong ice, which had already for a long time
been floating in unfrozen water-casks, or kept for days in a
thawing state, being rapidly pounded, showed a temperature of
0°-3 Fahrenheit below the true freezing point, shown by deli-
cate thermometers (both of mercury and alcohol), carefully
tested by long immersion in a considerable mass of pounded ice
or snow in a thawing state.

4. Water being carefully frozen into a cylinder several
inches long, with the bulb of a thermometer in its axis, and the
cylinder being then gradually thawed, or allowed to lie for a
considerable time in pounded ice at a thawing temperature,
showed also a temperature decidedly inferior to 32°, not less, I
think, than 0°*35 Fahrenheit.

I think that the preceding results are all explicable on the
one admission, that Person's view of the gradual liquefaction
of ice is correct (Comptes Rendus, 1850, vol. xxx. p. 526),* or
that ice gradually absorbs latent heat from a point very sensibly
lower than the zero of the centigrade scale.

* Quoted by me in 1851, in my sixteenth letter on Glaciers [p. 225 above].—
[See also the anticipation of the fact in my paper of 1846, note to page 156 of this
volume. Even in 1851 I was unacquainted with Mr. Faraday's experiment, the
publication of which was, so far as I know, confined to the journals quoted at page
228. As I was in Germany at the time, they naturally escaped my notice. To
facilitate reference to Mr. Faraday's- observation, I have, as already mentioned,
thought it well to reprint the report of it in the Appendix.]

230 ON THE PROPERTIES OF ICE NEAR ITS MELTING POINT. [1858.

I. This explains the permanent lower temperature of the
interior of ice.

Let AB be the surface of a block of ice contained in water

d

Ice

N

M

Water

Water

M'

L1

Fig. 25.

at what is called a freezing temperature. That temperature is
marked by the level of the line QP above some arbitrary zero.
LM is, in like manner, the permanent but somewhat lower tem-
perature possessed by the interior of the ice. The space, partly
water, partly ice, or partaking of the nature of each, MNOP,
has a temperature which varies from point to point, the portion
NO corresponding to what may be called the physical surface
of the ice between AB and ab, which is " plastic ice," or " viscid
water," having the most rapid variation of local temperature.

II. Such a state of temperature, though it is in one
sense permanent, is so by compensation of effects. Bodies of
-different temperatures cannot continue so without interaction.
The water must give off heat to the ice, but it spends it in an
insignificant thaw at the surface, which therefore wastes even
though the water be what is called ice cold, or having the tempe-
rature of a body of water inclosed in a cavity of ice.*

* I incline to think that water, in these circumstances, may, though surrounded
by ice, have a fixed temperature somewhat higher than what is called 32°. But I
have not yet had an opportunity of verifying the conjecture.

(My idea is, that the invasion of cold from the surrounding ice is spent in
producing a very gradual " regelation " in the water which touches the ice, leaving
the interior water in possession of its full dose of latent heat, and also of a tempera-
ture which may slightly exceed 32°. By similar reasoning, a small body of ice, in-
closed in a large mass of water, will preserve its proper internal temperature beloio
32° ; but, instead of regelation taking place, the surface is being gradually thawed.
This is the case contemplated in the paragraph of the text to which this note refers.)

N.B.- — The words in brackets were added to this note during printing. 13th
May 1858.

1858.] " BEGELATION " EXPLAINED ON M. PERSON'S PRINCIPLES. 231

This waste has yet to be proved ; but I have little doubt
of it ; and it is confirmed by the wasting action of superficial
streams on the ice of glaciers, though other circumstances may
also contribute to this effect.

III. The theory explains " regelation." For let a second
plane surface of ice A'B' be brought up to nearly physical contact
with the first surface AB. There is a double film of " viscid
water " isolated between two ice surfaces colder than itself.
The former equilibrium is now destroyed. The films AB la and
A'B' ~b'af were kept in a liquid or semi-liquid state by the heat
communicated to them by the perfect water beyond. That is
now removed, and the film in question has ice colder than itself
on both sides. Part of the sensible heat it possesses is given to
the neighbouring strata which have less heat than itself, and
the intercepted film of water in the transition state becomes
more or less perfect ice.

Even if the second surface be not of ice, provided it be a
bad conductor, the effect is practically the same. For the film
of water is robbed of its heat on one hand by the colder ice, and
the other badly-conducting surface cannot afford warmth enough
to keep the water liquid.

This effect is well seen by the instant freezing of a piece
of ice to a worsted glove even when on a warm hand. But
metals may act so, provided they are prevented from conveying
heat by surrounding them with ice. Thus, as has been shewn,
metals adhere to melting ice.

EDINBURGH, 19th April 1858.

GENERAL VIEW OF GLACIAL PHENOMENA. 233

XXI. ON GLACIERS IN GENERAL.*

GLACIER is a name given to masses of ice which descend from
snowy mountains into the adjacent valleys, where they attain a
level often far below the upper limit of the surrounding vegeta-
tion. The following are the synonymes for a glacier in some
different languages and dialects. In French, glacier / German,
gletscher ; Italian, ghiacciaja ; Tyrolese, fern; in Carinthia,
kass ; in the Vallais, biegno ; in part of Italy, vedretto ; in
Piedmont, ruize / in the Pyrenees, serneille ; in Norway, iisbrce
or iisbrede ; in Lapland, geikna or jegna ; in Iceland, jokull or
fall-jokull.

The characteristic appearance of a glacier can be nowhere
better studied than in Switzerland and Savoy. The icy mass
of the glacier of Bossons at Chamouni — which descends imme-
diately from the highest part of Mont Blanc, but lies, summer
and winter, in the valley at a height above the sea of no more
than 3500 English feet (the height of perpetual snow being about
9000 feet), where it is embosomed amongst luxuriant wood, and
is almost in contact with corn-fields — exhibits a spectacle which
none who have once seen it can forget, and which attracts more
interest and curiosity the more carefully it is considered. The
lower glacier of Grindelwald, descending to 3400 feet, is another
familiar example of the same phenomenon. In the Arctic
regions true glaciers also exist, which, descending the valleys
(often of great width and little inclination), enter the sea, and,
breaking off, supply the floating ice-islands or icebergs which
frequently drift into comparatively low latitudes. These glaciers
do not essentially differ from those of alpine countries.

* Being the article "Glacier'' in the 8th edition of the Encyclopaedia Britan-
nica, published in 1855.

234 ON GLACIERS IN GENERAL.

The diminution of temperature as we ascend the slopes of
mountains, as indicated by successive zones of vegetation, and
finally by the occurrence of perpetual snow, is stated and ex-
plained in other articles,* and therefore may be assumed here.
Thus, in the high mountains of the Andes and Himalaya, be-
tween the tropics, the commencement of perpetual snow is
found at from 15,000 to 18,000, or even 19,000 feet, according
to circumstances ; whilst in southern Europe the level is from
8000 to 9000 feet, and in Norway from 5500 to 3000 feet, ac-
cording to the latitude and the distance from the sea. It was
first shown by Baron Humboldt and Von Buch that the limit of
perpetual snow depends principally on the temperature of the
summer, and not upon that of the whole year.

It has been already explained that an accumulation of snow,
even frozen snow, does not constitute properly a glacier. A
glacier is a mass of ice, having its origin in the hollows of
mountains where perpetual snow accumulates, but which makes
its way down towards the lower valleys, where it gradually
melts, and it terminates exactly where the melting, due to the
contact of the warmer air, earth, and rain of the valley — compen-
sates for the bodily descent of the ice from the snow reservoirs
of the higher mountains. From this it is evident, without
any formal measurements, that a GLACIER is ICE IN MOTION

UNDER GRAVITY.f

Geographical Distribution of Glaciers. — Glaciers are not
peculiar to any country or region of the earth- It may be that
there are extensive snowy mountains wholly devoid of them, as
is supposed to be the case in tropical South America ; but even
this exception requires confirmation. There are peculiarities in
the form of mountains, and still more in climate, which, as we shall
see, favour the formation of glaciers, or may even totally pre-
vent it.

* Encyclopaedia JBritannica, articles "Climate," "Physical Geography," and
" Botany," Part III. See also Johnston's Physical Atlas, article on " Glaciers," by
the present writer.

f [The words "under gravity," which give greater precision to the definition by
excluding icebergs, are added in this reprint.]

GLACIEKS OF THE HYMALAYA. 235

Although it is only of late years that glaciers have been
generally acknowledged to exist in the Himalaya, the descrip-
tions given many years ago by Captain Hodgson of the source
of the Ganges could leave no doubt as to the fact on the mind
of any one familiar with the glaciers of the Alps. " The
Bhagiruttee or Ganges," he wrote in 1817, " issues from under

a very low arch at the foot of the grand snow-bed

Over the debouche the mass of snow is perfectly perpendicular,
and from the bed of the stream to the summit, we estimate the
thickness at little less than 300 feet of solid frozen snow, pro-
bably the accumulation of ages. It is in layers of some feet
thick, each seemingly the remains of a fall of a separate year."*
The level of the source of the Ganges is 12,900 feet, and the
chief error of this description is in the interchange of the word
snow for ice, and in the absence of a clear perception that the
ice could not have always lain there, some thousand feet below
the snow-line, but must have travelled progressively down the val-
ley, producing the phenomena of rents and superficial rubbish-
heaps which Captain Hodgson describes in another paragraph.

For many years after 1817 the glaciers of the Himalaya, if
mentioned at all, were so under the false name of snow-beds,
and their relations to physical geography were wholly neglected.
This arose from the imperfect education of those clever men
who have at different times explored our Indian possessions,
who being chiefly bred in that remote land, had little acquaint-
ance with the scientific literature of Europe, and still less with
its physical features. Scarcely any of our Himalayan travellers
had previously visited the Alps.

It is since 1840 that we have acquired more correct infor-
mation as to the glaciers of India. Mr. Vigne, in his interesting
Travels in Kashmir, has described the perfectly characteristic
features of the glaciers of some of the sources of the river
Indus occurring in the territory of Little Thibet, about lat.
35°. Colonel Madden and Captain Richard Strachey directed

* Asiatic Researches, vol. xiv., quoted in Captain B. Strachey's paper cited
below.

236 ON GLACIERS IN GENERAL. "

attention to the glaciers of the central Himalaya (Kumaon) at
the source of the rivers Pindur and Kuphinee, in lat. 30° 20'?
at the level of 11,300 and 12,000 feet respectively, the height
of the snow-line being there about 15,000 feet.* The pheno-
mena and mode of progression of these glaciers, as noted by
Captain R. Strachey, appear to be identical with those which
we shall presently describe as characteristic of those of Europe.
Farther in the interior of the same chain, Dr. Thomas Thomson
has lately described f numerous glaciers filling valleys of the
central Himalaya (particularly that on the north side of the
Bardar or Umasi pass, lat. 33°, 20', long. 76J° E.), .which pro-
bably exceeds in size any other yet described. Dr. Joseph
Hooker, in his interesting Himalayan Journals, has described
in detail the glaciers of the eastern portion of the same range
in the territories of Sikkim and Nepaul, where the gigantic
mountain Kinchinjunga rears its head to 28,178 feet above the
sea, whence the ice descends (he states) in one unbroken mass
of 14,000 feet of vertical height to the source 'of the Thlonok
river. Both Dr. Thomson and Dr. Hooker concur in ascribing
to the Himalayan glaciers a formerly much greater extension
towards the plains of India, which has left geological evidence
of, their former sojourn in the lower valleys, in the masses of
transported rock and rubbish there accumulated. Thus, instead
of glaciers being rare or unexampled phenomena in the east,
as was at one time supposed, we find them developed on a
scale commensurate with that of the stupendous mountains with
which they are connected, and that from one end to the other
of the Himalayan range.

Passing over the less important glaciers of the Caucasus and
Altai, .we come to the glaciers of Europe, which are principally
confined to two great mountainous districts, the Alps and the
high lands of Norway.

>.. Referring for minute topographical details to the works
which have been published more particularly in connection with

* Journal of the Asiatic Society of Bengal, vol. xvi. part 2.

f Western Himalaya and Tibet, etc. by Thomas Thomson, M.D., London, 1852.

GLACIERS OF THE ALPS AND OF NORWAY. 237

the subject, and with those countries,* we may state generally,
that wherever (in Europe) any. considerable area of mountainous
country rises above the snow-line, there glaciers are found in
more or less abundance. In the Alps this level is, on an average,
about 7200 feet, including glaciers of all descriptions (Schlagint-
weit) . The great glaciers have of course the lowest mean level.
Of these there are, on the same authority, sixty in the whole
Alpine chain. Glaciers commence on the south-western pro-
longation of the chain in the region of Mont Pelvoux and Monte
Viso (lat. 45°), and they extend on the N.E. to the Gross
Glockner in Carinthia. The best known and most important
glacier-bearing groups in the interval are those of Mont Blanc,
Monte Rosa, the Bernese Alps (Finsteraarhorn and Jungfrau),
and the Oertler Spitz in the Tyrol. The most considerable in-
dividual glaciers are the Mer de Glace of Chamouni, the Gorner
Glacier near Zermatt (Monte Rosa), the Lower Glacier of the
Aar (Bernese Oberland), the Aletsch Glacier and Glacier of the
Rhone (Vallais), and the Pasterzeii Glacier (Carinthia). Of
these, the first, third, and last, have been made the subjects of
the most careful surveys and observations.

In Great Britain no mountain fully attains the height of the
snow-line, consequently there are no glaciers. But patches of
snow, with a more or less icy structure, remain through the
summer in the clefts of some of the Scottish hills. Geological
appearances, however, strongly indicate the former existence of
glaciers, especially in Scotland and Wales.

In Norway we find two principal groups of glacier-bearing
mountains — those in the Bergenstift and those within the arctic
circle. The former were well described by M. Durocher.f
The present writer, in a recent work,J has detailed his observa-
tions on most of them, has given an enumeration of all the
known glaciers of Norway, and has compared their conditions
and structure with those of the glaciers of the Alps. Of the

* Gniner's and De Saussure's Travels in the Alps; Forbes's Travels in the
Alps of /Savoy; Agassiz, Etudes sur les Glaciers; Hugi, Alpenreise; Schlagint-
weit, Untersuchungen ; and Johnston's Physical Atlas.

f Annales des Mines, 4th series, torn. xii.

| Forbes's Norway and its Glaciers visited in 1851.

238 ON GLACIERS IN GENERAL.

Bergen group, those of Justedal are the best known, and
probably the best worth visiting. Justedal is connected with
the inmost ramification of the intricate Sognefiord. On the
Fjaerlandsfiord, another branch of the same inlet, two important
glaciers are found, one of which terminates only 105 feet above
the sea level, in lat. 61°. The Hardanger fiord, somewhat
farther south, presents one fine glacier, the Bondhuusbrae. The
more northern group of Norwegian glaciers commences at
Fondal, just within the limits of the arctic circle, where nume-
rous glaciers descend almost to the sea level. About lat. 70°,
on the promontory of Lyngen, are several glaciers, and in the
neighbouring Jokulsfiord is one which is stated actually to enter
the sea, and to break off in miniature icebergs. About the
North Cape the mountains are not sufficiently high to afford
any perpetual snow.

Iceland, with a summer temperature far inferior to that of
Norway, abounds in glaciers, which, however, have not been
very particularly described. Those of Swina-fels and Holaar
are stated to be large and characteristic.

The glaciers of Spitzbergen have been minutely described
by Dr. Scoresby* and by M. Martins.f They appear to be
essentially of the same nature, and subject to the same laws as
those of Switzerland, but modified by the depression of the
snow-line, and the extreme shortness of the summer. The
texture is less icy, the rate of progression probably slower. As
the superficial fusion is not great, they descend in vast sheets
into the waters of the sea (as at Magdalena Bay), where they
form icebergs. The western coasts of Greenland appear to
offer the same phenomena, but on a grander scale.

The interior of arctic North America has too even a surface,
and perhaps too dry and rigorous a climate, to present glaciers
in perfection.

In South America, about lat. 47°, where the climate is one
of the worst in the world, numerous glaciers, resembling probably
those of Iceland, have been described by Captain King and Mr.

* Arctic Regions, vol. i. f Bibliotheque Universelle, 1840.

GENERAL PHENOMENA OF ALPINE GLACIERS. 239

Darwin. Sir James Clark Ross, in his antarctic voyage, has
described and represented by admirable views the stupendous
icy barriers which fringe the coast of the inhospitable southern
continent.

After reviewing the descriptions of glaciers in all regions of
the world, we recur to those of the Alps as presenting all the
characteristic features of glaciers in perfect development, and
under circumstances the most convenient for study.

General Phenomena of Alpine Glaciers. — The manner in
which a glacier protrudes itself into a valley, far below the level
of perpetual snow, has been already mentioned. The inference
being obvious, that since it is continually melting (during sum-
mer) in all its parts, yet retains its general form and place, the
waste below must be supplied by the continual advance of the
glacier forwards and downwards, we shall consider in the mean-
time the motion of the glacier as an established fact to which
we shall afterwards devote a separate and special discussion.
It very frequently happens that the termination of the greater
glaciers takes place in an alluvial flat in the bottom of a large
alpine valley (as in the glaciers of the Mer de Glace, Brenva,
Rhone, Lower Aar, and those of Grindelwald). From a vault
in the green-blue ice, more or less perfectly formed each summer,
the torrent issues, which represents the natural drainage of the
valley, derived partly from land-springs, partly from the fusion
of the ice. That of the Arveiron, near Chamouni, is perhaps
the best known, but almost every glacier possesses such a vault.

Most usually the glacier terminates amidst a wilderness of
stones borne down upon its surface and deposited by its fusion.
Sometimes these blocks are heaped up in mounds called moraines,
which, when in front of the lower end of a glacier, are called
its terminal moraines, and mark in a characteristic and certain
manner the greatest limit of extension which the glacier has at
any one time attained. Sometimes a glacier is seen to have
withdrawn very far within its old limits, leaving a prodigious
barren waste of stones in advance of it, which, being devoid of
soil, nourishes not one blade of grass. At other times the

240 ON GLACIERS IN GENERAL.

glacier pushes forward its margin beyond the limit which it has
ever before reached (at least within the memory of man), tears
up the ground with its icy ploughshare, and shoves forward the
yielding turf in wrinkled folds, uprooting trees, moving vast
rocks, and scattering the walls of dwelling-houses in fragments
before its irresistible onward march.*

The lower end of a glacier is usually steep ; sometimes
with a dome-shaped unbroken outline, more frequently broken
up by intersecting cracks into prismatic masses which the con-
tinued action of the sun and rain sharpen into pyramids, often
assuming (as in the Glacier of Bossons at Chamouni) grotesque
or beautiful forms.

The united or crevassed condition of the glacier generally
depends almost entirely on the slope of its bed. If it inclines
rapidly, numerous transverse fissures are formed owing to the
imperfect yielding of the ice during its forced descent along its
uneven channel. These cracks often extend for hundreds of yards,
and may be hundreds of feet in depth ; but their greatest depth
is not accurately known, since they are rarely quite vertical.
In many cases, however, the crevasses are comparatively few in
number, and the glacier may be readily traversed in all direc-
tions. This is especially the case if a glacier of considerable
dimensions meet with any contraction in its course. The ice
is embayed and compressed, and its slope lessens, just as in the
case of a river when it nears a similar contraction preceding a
fall. Such level and generally traversable spaces may be found
about the middle regions of the Mer de Glace, the Lower Glacier
of Grindelwald, the Lower Glacier of the Aar, and in many other
cases. The last-named glacier is perhaps the most remarkably
even and accessible of any in Switzerland. The slope of its
surface is in many places only 3°. The Pasterzen glacier in
Carinthia is even less inclined. It is in such portions of a

• Such a sudden and disastrous increase took place in many of the glaciers of
Switzerland and Savoy in 1818 (occasioning the catastrophe of the Val de Bagnes),
and in those of the Bergenstift in Norway ahout 1740. The retreat of a glacier far
within its old moraines is well exemplified in most of the glaciers of the latter
country, and especially in that of Nygaard.

MOULINS — GRAVEL CONES — GLACIER TABLES. 241

glacier that we commonly find internal cascades, or " moulins."
These arise from the superficial water of a glacier being col-
lected into a considerable mass by a long course over its un-
broken surface, and then precipitated with violence into the first
fissure it meets with. The descending cascade keeps open its
channel, which finally loses the form of a fissure, presenting
that of an open shaft, often of immense depth.

Nearly connected in their origin with the internal cascades
are the gravel cones, occasionally seen on the surface of glaciers,
which appear to be formed in this way : — a considerable amount
of earthy matter derived by the superficial water-runs from the
moraine, accumulates in heaps in the inequalities of the ice, or
at the bottom of the " moulins." As the glacier surface wastes
by the action of the sun and rain, these heaps are brought to
the surface, or rather the general surface is depressed to their
level. If the earthy mass be considerable, the ice beneath is
protected from the radiation of the sun and from the violent
washing of the rain ; it at length protrudes above the general
level of the glacier, and finally forms a cone which appears to
be entirely composed of gravel, but is in fact ice at the heart,
with merely a protecting cover of earthy matter. These singu-
lar cones are very well seen on the Glacier of the Aar, but on
most others they are comparatively rare.

The similar protective action of large stones detached from
the moraines and lying on the surface of the ice often produces
the striking phenomenon of glacier tables. Stones of any consi-
derable size almost invariably stand upon a slightly elevated
pillar of ice ; but when they are broad and flat they occasionally
attain a height of six and even of twelve feet above the general
level. A striking instance has been described and drawn in my
Travels in the Alps of Savoy.

To this peculiar tendency of glaciers apparently to elevate
heavy and opaque bodies above their surface — in reality to have
their surface depressed beneath them — is no doubt mainly owing
the striking, and at first sight perplexing, fact, that stones or
dirt are scarcely ever seen imbedded in the massive ice of

H

242 ON GLACIERS IN GENERAL.

glaciers. The Swiss peasants attribute to them an intrinsic
power of rejecting impurities. The fact is, that year by year,
and month by month, fresh thicknesses of virgin ice become
revealed by the fusion of the surface. That ice, -formed in
the highest mountain hollows, never was or could be impure.
The rocks and earth have fallen upon the surface since ; and,
by the conditions which we have mentioned, once there, there
they remain. Even those blocks which fall into the crevasses
are usually arrested at no great depth, and by the general
lowering of the glacier-surface, soon attain its level.

The superficial waste of a glacier is thus a very important
phenomenon. Owing to it the body of the ice has its vertical
thickness rapidly diminished during the heats of summer, and,
as we have already intimated, the lower end of a glacier has its
position determined by the amount of this waste. Suppose a
glacier to move along its bed at the rate of 300 feet per annum,
and imagine (merely for the sake of illustration) its yearly
superficial waste to be 20 feet : then the thickness of the glacier
will diminish by 20 feet for every 300 feet of its length, or at
the rate of 360 feet per mile ; * so that the longitudinal section of
a glacier has the form of a wedge ; and however enormous its
original thickness, after a certain course we must at length come
to the thin end of the wedge, and that the more rapidily as the
causes of melting increase towards the lower extremity. These
causes are indeed so various that it is difficult to estimate them
with accuracy. We have (1) the direct solar heat ; (2) the
contact of warm air ; (3) the washing of rain. All these causes
act on the surface, and produce the ablation of the surface.
Besides these, the ice of the glacier wastes somewhat beneath
by the contact of the soil and the washing of the inferior streams.
This may be called its subsidence. Further, the natural slope of
the rocky bed of the glacier causes any point of the surface to
stand absolutely lower each day in consequence of the progres-
sive motion. These three causes united produce the geometrical
depression of the surface. I have elsewhere shown how the

* [ More correctly 352 feet per mile.]

SUBSIDENCE — MORAINES—SLOPE OF GLACIERS. 243

several effects may usually be distinguished by observation.*
During the height of summer, near the Montanvert, I found the
daily average ablation to be 3'62 inches, the daily subsidence to be
1*63 inches. Seven-tenths of the geometrical depression are due,
therefore, to the former cause, and three-tenths to the latter. This
is a very large amount, and it is certain that during the colder
period of the year, and whilst the glacier is covered with snow,
the subsidence is not only suspended, but that the glacier recruits
in thickness a portion of its waste during the season of summer
and autumn. To this subject we shall again return.

One point about moraines we have not yet mentioned. As
we ascend any considerable glacier we almost invariably observe
several parallel trails of debris extending throughout its length,
and not mixing with one another. These medial moraines may
in all cases be traced to a rocky promontory where two tributary
glaciers have united. The rocky masses detached by frost and
rain which have rolled upon the margin of the confluent glaciers
are borne along by the progress of each to the point of union.
But where the icy streams unite the trails of rock do so also ;
and being continually retained on the surface by the causes we
have mentioned, float, as it were, down the middle of the com-
mon glacier, preserving throughout the distinctive character of
their origin. Four such medial moraines may readily be traced
to their sources on the great Glacier of Chamouni ; but the
grandest specimen of a medial moraine is that on the Glacier of
the Lower Aar, effectively represented in one of the plates in
M. Agassiz' work (Etudes sur les Glaciers).

The middle region of the great glaciers of the Alps extends
from the level of about 6000 to 8000 feet above the sea. The
inclination is usually there most moderate — say from 2J° to 66.
But this is not invariably the case. Beyond 8000 feet we reach
the snow line. The snow line is a fact as definite on the surface
of a glacier as on that of a mountain, only in the former case it

* Eleventh Letter on Glaciers. Edinburgh PkilosophicalJournal, 1846. [Re-
printed in the present volume.] Observations of the ablation of the ice have also been
made by MM. Martins and Agassiz. The amount, of course, depends materially
on the elevation and exposure of the glacier as well as on the weather and season.

244 ON GLACIERS IN GENERAL.

occurs at a somewhat lower level. It cannot be too distinctly
understood that the fresh snow annually disappears from the
glacier proper. Where it ceases entirely to melt, it of course
becomes incorporated with the glacier. We hav'e therefore
arrived at the region where the glacier forms; everywhere
below it only wastes. This snowy region of the glacier is
called in French neve ; in German, firn. As we ascend the
glacier it passes gradually from the state of ice to the state of
snow. The superficial layers become more snowy and white, in
fact, nearly pure snow ; the deeper ones have more colour and
consistence, and break on the large scale into vast fragments,
which at Chamouni are called seracs. The neve moves as the
glacier proper does, and it is fissured by the inequalities of the
ground over which it passes. These fissures are less regular
than those of the lower glacier. They are often much wider,
in fact, of stupendous dimensions, and being often covered with
treacherous snowy roofs, constitute one of the chief dangers of
glacier travelling. The constitution of the neve" may be well
studied on the Glacier du Geant, a tributary of the Mer de
Glace.

The mountain-clefts in which large glaciers lie, usually
expand in their higher portions (in conformity with the ordinary
structure of valleys) into extensive basins in which snow is per-
petual, and which therefore contain the nev6, the true origin and
material of the glacier, which is literally the overflow of these
snowy reservoirs. The amount of overflow, or the discharge of
the glacier — upon which depends the extent of its prolongation
into the lower valleys — depends in its turn on the extent of the
neve or. collecting reservoir. Glaciers with small reservoirs, of
necessity perish soon. Their thickness is small, and consequently
the wedge of the glacier soon thins out. Such glaciers are com-
mon in confined clefts of the higher mountains. Being destitute
of reservoirs, they soon terminate abruptly. Such are the gla-
ciers of the second order described by De Saussure. They are
exceedingly numerous in all glacier-bearing chains of mountains,
but from their comparative smallness and inaccessibility, they

STRUCTURE OF GLACIER ICE — DIRT-BANDS. 245

usually attract but little attention. Their elope is often very
great— from 20° to 40°.

Structure of Glacier Ice, and Dirt-Sands. — The ice of the
glacier proper has a very peculiar structure, quite distinct from
the stratification of the snow on the neve (the relics of its mode
of deposit), and one which requires special notice. When we
examine the appearance of the ice in the wall of an ordinary
crevasse (especially if it be tolerably near the side of the glacier)
we are struck with the beautiful vertically laminated structure
which it commonly presents, resembling delicately- veined mar-
ble, in shades varying from bluish-green, through green, to
white. It sometimes resembles the marble called in Italy cipol-
lino. When we trace the direction of the planes constituting
the laminated structure, by observing them on the surface of the
glacier (where they are usually well seen after rain, or in the
channels of superficial water-runs), we find that where best
developed (or not very far from the sides of the glacier) these
laminae are nearly parallel to the sides, but rather incline from
the shore to the centre of the ice stream as we follow the
declivity of the glacier. '

The general out-crop of the veined structure may best be
seized at a glance by means of a correlative phenomenon thus
described by the present writer, who first observed it : — " On
the evening of the 24th July (1842), the day following my
descent from the Col du Geant, I walked up the hill of Charmoz
bo a height of 600 or 700 feet above the Montanvert, or 1000
feet above the level of the glacier. The tints of sunset were
3ast in a glorious manner over the distant mountains, whilst the
glacier was thrown into comparative shadow. This condition
3f half-illumination is far more proper for distinguishing feeble
shades of colour on a very white surface like that of a glacier
than the broad day. Accordingly, whilst revolving in my mind,
during this evening's stroll, the singular problems of the ice
world, my eye was caught by a very peculiar appearance of the
surface of the ice, which I was certain that I now saw for the
first time. It consisted in a series of nearly hyperbolic brown-

246 ON GLACIERS IN GENERAL.

ish bands on the glacier, the curves pointing downwards, and
the two branches mingling indiscriminately with the (lateral)
moraines, presenting an appearance of waves some hundred feet
apart, and having, opposite to the Montanvert, the form which
I have attempted to show upon the map,* where they are repre-
sented in the exact figure and number in which they occur. . .

I was satisfied, from the general knowledge which I

then had of the ' veined structure ' of the ice, that these coloured
bands probably followed that direction." t Farther examination
confirmed this conjecture, and showed that these superficial dis-
coloration s in the form of excessively elongated hyperbolas are
due to the recurrence (at intervals of some hundred feet along
the course of the glacier) of portions of ice in which the veined
structure is more energetically developed than elsewhere, and
where, by the decomposition of the softer laminae, portions of
sand and dirt become entangled in the superficial ice, and give
rise to the phenomena of dirt-bands, which thus at a distance
display (though in a manner requiring some attention to discover)
the exact course of this singular structure on the surface of the
glacier. The annexed figure 26 displays the superficial form of
the dirt-bands, and the course of the structural laminae projected

fig. 26. Fig. 27. Mg. 28.

horizontally. Fig. 27 shows an ideal transverse section of
the glacier ; and Fig. 28 another vertical section parallel to its
length. These three sections in rectangular planes will serve
to give a correct idea of the course of this remarkable structure
within the ice, but a more popular conception will be formed of
it from the imaginary sections of a canal-shaped glacier in the
annexed woodcut, Fig. 29. The structure of the compound gla-
cier, originally double, becomes gradually single ; and the frontal

* Map of the Mer de Glace of Chamouni, etc., in Forbes s Travels in the Alps,
or in the Tour of Mont Blanc, etc.

f Travels in the Alps of Savoy, etc., 2d edit., p. 162.

COURSE OF VEINED STRUCTURE — MOTION OF GLACIERS. 247

dip of the laminae at the loop of the horizontal curves, which in
the upper region of the glacier is nearly vertical, gradually slopes
forwards, until, at the lower termination, it has a very slight dip
inwards, or, indeed, may be reversed, and fall outwards and
forwards. The general form of a structural lamina of a glacier
rudely resembles that of a spoon.

This structure and the accompanying dirt-bands have been
recognised by different observers in almost all glaciers, including
those of Norway and of India. The interval between the dirt-
bands has been shown in the case of the Mer de Glace (and
therefore probably in other cases) to coincide with annual rate
of progression, and in the higher parts of the glacier (towards the
neve) to be accompanied by wrinkles or inequalities of the sur-
face, which are well marked by the snow lying in them during
the period of its partial disappearance.*

The Motion of Glaciers, and its causes. — The most charac-
teristic and remarkable feature of glaciers is their motion down-

wards from the neve towards the lower valley. The explanation

* Fifth Letter on Glaciers. Edinburgh Philosophical Journal, 1844 [reprinted
in the present volume] ; and Travels in the}Alps, 2d edit.

248 ON GLACIERS IN GENERAL.

of it is by far the most important application of mechanical
physics connected with the subject.

Obvious as the fact itself must appear by what has been
already stated, manifest confusion has obtained in .the minds of
intelligent persons regarding it. Thus Ebel, in his well-known
Swiss guide-book, affirms the motion of the glaciers of Cha-
mouni to be 14 feet, and those of Grindelwald 25 feet in a year ;
quantities which, if they have any meaning, must refer to the
apparent advance of the lower termination of those glaciers
into the valley, which therefore only indicate the difference of
the real motion, and of the waste in any particular season, and
which may become null, or even negative, if the summer be
more than usually warm. The peasants, however — who are
inevitably made aware of the progressive motion of the ice by
observing the progressive advance of conspicuous blocks on its
surface — commonly ascribe to the glaciers the more correct
measure of several hundred feet per annum.

M. Hugi, of Soleure, measured, with some accuracy, year by
year, the progress of a conspicuous block on the glacier of the
Aar, which he found to be 2200 feet in nine years, or about
240 feet per annum.* M. Agassiz continued some of these
annual measures, but only in a rough way, by causing his
guides to reckon the distance of a block on the moraine by
lengths of a pole or rod from a fixed rock some thousand feet
off. These measures appear not to have been altogether trust-
worthy.

The principal theories to account for the progressive motion
of glaciers which were prevalent previous to 1842, may be briefly
characterised as De Saussure's and De Charpentier's, though
each had been maintained in times long antecedent by the
earlier Swiss writers. The first may for brevity be called the
gravitation theory, the latter the dilatation theory. Both sup-
pose that the motion of the ice takes place by its sliding bodily
over its rocky bed, but they differ as to the force which urges

* Agassiz, Etudes surles Glaciers, p. 150.

GRAVITATION THEORY — DILATATION THEORY. 249

it over the obstacles opposed by friction and the irregularities
of the surface on which it moves.

The following quotation from De Saussure explains his
views with his usual precision : — " These frozen masses, carried
along by the slope of the bed on which they rest, disengaged by
the water (arising from their fusion owing to the natural heat
of the earth) from the adhesion which they might otherwise
contract to the bottom — sometimes even elevated by the water
— must gradually slide and descend along the declivity of the
valleys or mountain slopes (croupes) which they cover. It is
this slow but continual sliding of the icy masses (des glaces) on
their inclined bases which carries them down into the lower
valleys, and which replenishes continually the stock of ice in
valleys warm enough to produce large trees and rich harvests."*
Very sufficient objections have been urged against this theory.
It is evident that De Saussure considered a glacier as an accu-
mulation of icy fragments, instead of a great and continuous
mass, throughout which the fissures and crevasses bear a small
proportion to the solid portion ; and that he has attributed to
the subglacial water a kind and amount of action for which
there exists no sufficient or even probable evidence. The main
objection, however, is this, that a sliding motion of the kind
supposed, if it commence, must be accelerated by gravity, and
the glacier must slide from its bed in an avalanche. The small
slope of most glacier-valleys, and the extreme irregularity of
their bounding walls, are also great objections to this hypothesis.

The Dilatation theory ingeniously meets the difficulty of the
want of a sufficient moving power to drag or shove a glacier
over its bed, by calling in the well-known force with which
water expands on its conversion into ice. The glacier being
traversed by innumerable capillary fissures, and being in sum-
mer saturated with water in all its parts, it was natural to
invoke the freezing action of the night to convert this water
into ice, and by the amount of its expansion to urge the glacier
onwards in the direction of its greatest slope. In answer to

* Voyages dans les Alps, sec. 535.

250 ON GLACIERS IN GENERAL.

this, it is sufficient to observe, in the first place, that during the
height of summer the portions of those glaciers which move
fastest are never reduced below the freezing point, and that
even in the most favourable cases of nocturnal radiation pro-
ducing congelation at the surface, it cannot (by well-known
laws of conduction) penetrate above a few inches into the in-
terior of the glacier. Again, the ascertained laws of glacier
motion are (as will be immediately seen) entirely adverse to this
theory, as it is always accelerated by hot weather and retarded
by cold, yet does not cease even in the depth of winter.*

It is singular how slow observers were to perceive the im-
portance to the solution of the problem of glacier motion of
ascertaining with geometrical precision the amount of motion
of the ice, not only from year to year, but from day to day ;
whether constant or variable at the same point, whether con-
tinuous or by starts ; if variable, on what circumstances it
depended, and in what manner it was affected at different
points of the length and breadth of a glacier.

This method of studying the question was taken up by the
writer of this paper. His observations were commenced on the
Mer de Glace of Chamouni, in June 1842. Between the 26th
and 27th of that month the motion of the ice opposite a point
called the " Angle" was found, by means of a theodolite, to be
16'5 inches in 26 hours ; between the 27th and 28th, 17'4
inches in 25J hours ; and from about 6 A.M. to 6 P.M. on the
28th the motion was 9'5 inches, or 17'5 inches in 24 hours ;
whilst even the proportional motion during an hour and a half
was observed. No doubt could therefore remain that the mo-
tion of the ice is continuous and tolerably uniform — in short, that
it does not move by jerks. He also ascertained about the same
time that the motion of the ice is greatest towards the centre of
a glacier and slower at the sides, contrary to an opinion then
maintained on high authority. He next found that the rate of

* The fullest exposition of the Dilatation Theory is to be found in De Char-
pentier, Essai sur les Glaciers (1841), and AgaSsiz, Etudes snr les Glaciers
(1840).

MOTION OF A GLACIER — ITS LAWS. 251

motion varied at different points of the length of the same
glacier, being on the whole greatest where the inclination of its
surface is greatest. As the season advanced, he observed
notable changes in the rate of motion of the same part of the
ice, and connected it by a very striking direct relation with the
temperature of the air. These facts were established during the
summer of 1842, and promptly published. [See the first four
Letters on Glaciers, reprinted in the present volume.] By means
of occasional observations during the following winter and spring
by his guide, Auguste Balmat of Chamouni, and by a more full
comparison of the entire motion of a glacier for twelve months
with its motion during the hot season of the year, another gene-
rally received error was rectified : the motion of the glacier
continues even in winter, and it has a very perceptible ratio to
the summer motion. Last of all, it was found that the surface
of a glacier moves faster than the ice nearer the bottom or bed.*

These and some minor laws of motion are undoubted ex-
pressions of the way in which glaciers move ; and having been
successively confirmed by succeeding observers, seem to admit
of but one expression in the form of an approximate theory,
" A glacier is an imperfect fluid or a viscous body, which is
urged down slopes of a certain inclination by the mutual
pressure of its parts."f The analogy subsisting between the
motion of a glacier and that of a river (which is a viscous
fluid ; — were it not so, its motion would be widely different)
will be best perceived by stating more precisely its laws of
motion in order.

1. Each portion of a glacier moves, not indeed with a constant
velocity, but in a continuous manner, and not by sudden sub-
sidences with intervals of repose. This, of course, is character-
istic also of a river.

2. The ice in the middle part of the glacier moves much
faster than that near the sides or banks ; also the surface moves
faster than the bottom. Both these facts obtain in the motion

* Eleventh Letter. Edinburgh Philosophical Journal, 1846. [Reprinted in
this volume.]

f Travels in the Alps, p. 365.

252 ON GLACIERS IN GENERAL.

of a river in consequence of the friction of the fluid on its banks,
and in consequence^ also of that internal friction of the fluid
which constitutes its viscosity.

Thus, in the case of a glacier, at four stations of the Mer
de Glace, distant respectively from the west shore of the glacier

100 230 305 365 yds.,
the relative velocities were . . I'OOO 1'332 1-356 1'367.

3. The variation of velocity (as in a river) is most rapid
near the sides, whilst the middle parts move nearly uniformly.
This and the preceding laws are also fully brought out by the
subsequent experiments of M. Agassiz on the Glacier of the Aar,
and of MM. Schlagintweit on the Pasterzen Glacier.*

4. The variation of velocity of a glacier from the sides to
the middle, varies nearly in proportion to the absolute velocity
of the glacier, whether that absolute velocity change in the same
place in consequence of change of season, or between one point
and another of the length of the same glacier, depending on its
declivity. (See (5) and (6) below.) These facts, clearly brought
out in my observations of 1842, present a striking analogy to the
phenomena of rivers, as observed by Dubuat.f

5. The glacier, like a stream, has its pools and its rapids.
Where it is embayed by rocks it accumulates, its declivity
decreases,! and its velocity at the same time. When it passes
down a steep, issuing by a narrow outlet, its velocity increases.
Thus the approximate declivities of the inferior, middle, and
superior region of the Mer de Glace (taken in the direction of

its length) are 15° 4£° 8°

and the relative velocities are as the

numbers 1-398 -574 -925 §

* See Mousson, Gletscher der Jetzt-zeit, p. 126.

f Traite cC Hydraulique, arts. 37, 49, 65.

\ [In the Encyclopaedia, increases, apparently by mistake.]

§ The absolute velocity of a glacier depends upon so many circumstances
besides its declivity, that this law must not be sought to be verified, except under
like conditions. The breadth and depth of a glacier (as of a river) no doubt mate-
rially affect its rate of motion, and its elevation has a not less important influence.
Small lofty glaciers of the second order move slowly over steep inclinations. See
Philosophical Transactions, 1846, p. 177. [Reprinted in the present volume.]

PLASTIC NATURE OF GLACIER ICE. 253

6. A fact not less important than any of the preceding, and
equally well-established, not only by observations of my own,
but by those of succeeding experimenters, is this : that in-
creased temperature of the air favours the motion of the ice ;
and generally whatever tends to increase the proportion of. the
watery to the solid constituents of a glacier, as mild rains, and
especially the thawing of the superficial snow in spring. The
velocity does not, however, descend to nothing even in the
depth of winter. Indeed, in the lower and most accessible
portions of the Mer de Glace (or Glacier des Bois) and the
Glacier des Bossons, the ratio of the winter to the summer
motion is almost exactly 1:2. On endeavouring to establish a
relation between the velocity of the glacier and the temperature
of the ambient air, we find that these two quantities diminish
together in an almost regular manner down to the freezing-
point ; below which the velocity seems to remain constant.*

The circumstances of motion detailed in the six preceding
propositions appear to be reconcileable with the assumption of
what may be called the Viscous or Plastic Theory of glacier
motion, and with that alone.

Plastic Nature of Glacier Ice. — Notwithstanding the ap-
parent paradox of calling a vast mass of coherent ice a semi-
fluid body, there is something about a glacier which almost
inevitably conveys to the mind the idea of a stream. This may
be traced in the descriptions of unscientific tourists, of poets,
and of some of those who have addressed themselves more
seriously to the question of the real nature of these bodies. To
the latter class of observers belong Captain Basil Hall and
Monseigneur Rendu, Bishop of Annecy, who had much more
than hinted at the possibility of a true mechanical connection
between the descent of a glacier and that of a mountain torrent,
or of a stream of lava. But until the actual conditions of
motion were reduced to rule, it was impossible to know how
far the analogy was real or apparent.

* Philosophical Transactions, 1846, p. 191 ; and Edinburgh Philosophical
Journal, 1847 [page 138 of this volume].

254 ON GLACIERS IN GENERAL.

The viscous theory of glaciers, as deduced from observa-
tion by the present writer, though now very generally accepted,
had to struggle with numerous and strongly-urged objections ;
of which the oftenest repeated was, that ice is by its nature a
brittle solid, and not sensibly possessed of any viscous or plastic
quality. In answer to this, it may be urged that the qualities
of solid bodies of vast size, and acted on by stupendous and
long-continued forces, cannot be estimated from experiments on
a small scale, especially if short and violent ; that sealing-wax,
pitch, and other similar bodies mould themselves, with time, to
the surfaces on which they lie, even at atmospheric tempera-
tures, and whilst they maintain, at the same time, the quality
of excessive brittleness under a blow or a rapid change of form ;
that even ice does not pass at once, and per saltum, from the
solid to the liquid state, but absorbs its latent heat throughout
a certain small range of temperature (between 28°'4 and 32° of
Fahrenheit) , which is precisely that to which the ice of glaciers
is actually exposed ; that, after all, a glacier is not a crystalline
solid, like ice, tranquilly frozen in a mould, but possesses a
peculiar fissured and laminated structure, through which water
enters (at least for a great part of the year) into its intrinsic
composition. But, waiving the inferences from all these facts,
the main argument in favour of the view now maintained is
this, that admitting the preceding propositions as to the velocity
of its parts (which no one now contests), the quasi-fluid or
viscous motion of the ice of glaciers is not a theory but a FACT.
A substance which is seen to pour itself out of a large basin
through a narrow outlet without losing its continuity, — the
different parts of which, from top to bottom, and from side to
centre, possess distinct, though related velocities— which moves
over slopes inconsistent with the friction between its surface
and the ground on which it rests — which surmounts obstacles,
and, even if cleft into two streams by a projecting rock, instead
of being thereby anchored as a solid would necessarily be,
reunites its streams below, and retains no trace of the fissure,
leaving the rock an islet in the icy flood — a substance which

ORIGIN OF THE VEINED STRUCTURE OF ICE. 255

moves in such a fashion cannot, in any true sense of the word,
be termed a rigid solid, and must be granted to be ductile,
viscous, plastic, or semifluid, or to possess qualities represented
by any of these terms which we may choose to adopt as least
shocking to our ordinary conception of the brittleness of ice.*

Origin of the Veined Structure of Ice. — Any mechanical
theory of glaciers must be more or less imperfect which does
not explain the remarkable veined or ribboned structure of the
ice, which, with its peculiar course through the interior of the
glacier, has been described at page 245. In applying the
theory of quasi-fluid motion to explain this structure, two great
difficulties are experienced ; first, our confessed ignorance of
the modus operandi of those molecular forces which induce
pervading structures of this kind ; secondly, our imperfect
acquaintance with the laws of motion of semifluids under the
action of gravity, even in simple cases. Nevertheless it seems
possible to prove, partly from admitted mechanical principles,
partly from direct experiments, that the tendency of the motion
is to produce such a structure.

The fundamental idea is this, that the veined or ribboned
structure of the ice is the result of internal forces, by which
one portion of ice is dragged past another in a manner so
gradual as not necessarily to produce large fissures in the ice,
and the consequent sliding of one detached part over another,
but rather the effect of a general bruise over a considerable
space of the yielding body. According to this view, the deli-
cate veins seen in the glacier, often less than a quarter of an
inch wide, have their course parallel to the direction of the
sliding effort of one portion of the ice over another. Amongst
other proofs of this fundamental conception that the veined

* For a fuller reply to the objections which have heen urged against the theory
of the plasticity of glaciers, see Philosophical Transactions, 1846, particularly pp.
162, etc. [and p. 140, etc., of this volume]. The confirmatory observations of MM.
Agassiz and Schlagintweit on other glaciers, and their adoption of views virtually
the same with those maintained above, have proved convincing to a majority of
those who at first rejected a theory apparently opposed to commonly received
notions. See Mousson, Die GUtscher der Jetzt-zeit, who says, p. 162, speaking of
the plastic theory, " Er steht noch heute unangefochten da."

256 ON GLACIERS IN GENERAL.

structure is the external symbol of this forced internal motion
of a body comparatively solid, we may mention one very
striking instance recorded by the present writer, as observed
on the Glacier of La Brenva, on the south side of Mont Blanc,
subsequently to the publication of his principal work on the
subject. In this case the ice of the glacier, forcibly pressed
against the naked rocky face of an opposing hill, is turned into
a new direction ; and in thus shoving and squeezing past a pro-
minence of rock, he observed developed in the ice a " veined
structure " so beautiful, that " it ,was impossible to resist the
wish to carry off slabs, and to perpetuate it by hand specimens."
This perfectly developed structure was visible opposite the
promontory which held the glacier in check, and past which it
struggled, leaving a portion of its ice completely embayed in a
recess of the shore behind it. Starting from this point as an
origin, the veined laminae extended backwards and upwards
into the glacier, but did not spread laterally into the embayed
ice. They could, however, be traced from the shore to some
distance from the promontory into the icy mass. The direction
of lamination exactly coincided with that in which the ice must
have moved if it was shoved past the promontory at all. That
it did so move was made the subject of direct proof, by fixing
two marks on the ice opposite the promontory, one on the
nearer, the other on the farther side of the belt of ice which
had the lamination best developed. The first mark was 50
feet from the shore, and moved at the rate of 4*9 inches daily ;
the other mark was 170 feet farther off, and moved almost
three times faster, or 14'2 inches daily. Throughout this
breadth of 170 feet there was not a single longitudinal crevasse
which might have facilitated the differential motion. A paral-
lelogram of compact ice, only 170 feet wide, was therefore
moving in such a manner, that, whilst one of its sides advanced
only a foot, the other advanced a yard. No solid body, at
least no rigid solid body, can advance in such a manner ; it is
therefore plastic, and the veined structure is unquestionably
the result of the struggle between the rigidity of the ice and

LAMINATION DUE TO DIFFERENTIAL MOTION. 257

the quasi-fluid character of the motion impressed upon it.
That it is so is evident not only from the direction of the
lamin£e, but from their becoming distinct exactly in proportion
to their nearness to the point where the bruise is necessarily
strongest.*

This observation sufficiently illustrates the general fact,
that the veined structure appears most vividly in a direction
parallel to the sides of glaciers, being caused by the friction of
the rocky shore compelling a forced molecular separation of the
middle parts from the side parts of the glacier.

But we have seen (p. 246) that the direction of the hori-
zontal section of these laminae gradually inclines inwards, so as
to form loops on the surface of the glacier. The portion of
these loops next to the shore, which is at first parallel to the
shore, but which gradually inclines towards the axis or middle
of the glacier, is conceived to be owing to the differential
motion of the parts retarded by lateral friction, as in the case
of the Glacier of La Brenva just mentioned. But, moreover,
the opposing resistance of the shore-ice immediately in front
will give a tendency to molecular dislocation in a direction
sloping towards the middle of the glacier, where the current
moves fastest, in consequence of the friction being less. When
we arrive at a distance from the shore comparable to the depth
of the ice, then the friction due to the led of the glacier com-
municated through its plastic layers to the surface combines
with the lateral friction in determining the lamination in a
direction at once upwards and towards the middle ; and when
we reach the middle region of the ice, the lamination takes
place entirely in the vertical plane, completing the spoon-shaped
arrangement of those surfaces of dislocation, of which the form
has already been illustrated at p. 247. The particles are
supposed to be acted on by a force partaking of the nature of

* Twelfth Letter on Glaciers. Edinburgh Philosophical Journal, 1847, where
the details of the observation are illustrated by a figure. [Reprinted in this
volume, p. 182.]

S

258 ON GLACIERS IN GENERAL.

hydrostatic pressure, derived from the ice at a higher level in
the rear of the point in question. Each particle is ready to
move in the direction in which the effective pressure is greatest.
Near the bottom, the frontal resistance arising from the lower
ice in front and itself retarded by friction, is very great, but
so also is the pressure of the superincumbent ice. The motion
of the particle will take place under the joint action of these
two resisting forces. As we approach the surface, the latter
of the two resistances (the weight of the glacier ice) is always
diminishing, and bears a less and less proportion to the former.
Near the surface, therefore, the tendency to slide will be more
and more directly vertical.* This consideration seems adequate
to explain the remarkable phenomenon of the frontal dip, with
its gradual fall as we approach the extremity of the glacier,
where, of course, the horizontal resistance from the ice in advance
becomes nothing.

It has been deduced from M. Agassiz' observations on the
glacier of the Aar (which is remarkable for the uniformity of
its section, and its uniform but small slope), that the ice does
actually undergo a compression from back to front as it forces
its way down the valley ; and as ice is not sensibly compressible,
this diminution of the horizontal area, which any given section
of the glacier (between two vertical transverse planes) exhibits
in successive years, can only be explained by admitting that
the ice accumulates in a vertical direction.!

This fact also corresponds with the convex surface which
the slowly-moving glaciers present. Such a surface is seen in
the precisely analogous case of viscous bodies, such as pitch,
and in clayey land-slips.

It also satisfactorily accounts for the otherwise mysterious

* See Seventh Letter on Glaciers. Edinburgh Philosophical Journal, 1844.
[Reprinted in this volume, p. 56, etc., with figures which it has not been thought
necessary here to repeat. See especially Fig. 15, page 59.]

f Ninth Letter on Glaciers. Edinburgh Philosophical Journal, 1845. ("Re-
printed in this volume.]

RESTORATION OF LEVEL DURING WINTER. 259

way in which, during winter, the glacier recovers the level
which it had lost by ablation during summer (p. 243). When
snow covers the whole surface the motion of all points of the
length of a glacier approaches equality. The higher parts
move relatively faster than the lower,* tend, as it were, to
overtake them, and thus to squeeze the yielding mass in a
vertical direction.

* [By higher is here meant the portion of the glacier towards its origin : — by
lower, the part next its inferior extremity.]

APPENDIX, No. I.

AN ATTEMPT TO ILLUSTRATE THE OEIGIN OF " DIRT-
BANDS " IN GLACIERS. By A. MILWARD, Esq.*

" It will be remembered that Professor Forbes, in his interesting
work on the Glaciers of the Alps, describes the " dirt-bands " to be
nothing more than curved bands of porous ice, the surface of which
affords a readier receptacle to the drift of the glacier. He found the
dirt to be superficial, and merely an indication that the glacier is made
up of two kinds of ice, the one more porous than the other ; so that
the dirt lodges in the one more readily than in the other. The
question to be determined is, how we are to account for the existence
of the different kinds of ice thus regularly alternating. The dirt
being merely accidental to this subject of inquiry, it will be better to
speak of the dirt-bands, and the intervals between them, as the alter-
nating bands of porous and compact ice.f

" The dirt-bands are found to follow the direction of the hyperbolic
curves marked out by the outcrop of the structural planes, known by
the name of the ribbon- structure. The ice forming the dirt- bands is
made up of that structure, in the same way as the other ice ; and
depends, of course, upon those laws in obedience to which the ribbon-
structure originates. For this reason, the curve of the bands is, like
that of the structure, found to be elongated low down the glacier, and
compressed as we approach its source. We have thus only to account for
the existence of bands of different kinds of ice, the form of curve of those
bands being explained in the same way as that of the ribbon-structure.

" It may be observed, that the superior distinctness of the dirt-
bands, as we proceed lower down the glacier, is not necessarily an
evidence that the bands of porous and compact ice are there more
decidedly developed, but only that they are more distinctly apparent.
And this, I imagine, arises from the fact, that the lower ice has been
washed over for years ; and thus the pores have become more dis-
coloured by the deposit of drift than the pores of the corresponding
porous ice above.

* [From the Edinburgh New Philosophical Journal. Jan. 1849.]
f These terms are, of course, only relative.

262 APPENDIX, NO. I.

" It was the remarkable similarity of the alternating bands on the
mud-slide already described, to the ' dirt-bands ' on Professor Forbes'
map of the Mer de Glace, that induced me to take an interest in the
matter, and make a drawing of the phenomenon. In the first instance,
the curved bands were a mystery to me ; and I could not venture to
found any argument on a mere analogy of appearance. The second
mud-slide, however, seemed to shew me another step in the process ;
and, having explained the one from the other, I was led to ask myself
whether the phenomena, to which one class of viscous fluids appeared
to be subject, might not be common to another ; in other words, to a
glacier.

" In our first mud-slide we observe, first, the occurrence of curved
bands ; and, secondly, a difference of consistency in those bands. Our
second mud-slide shews the origin of those curved bands, as far as
mud-slides are concerned, to be the previous existence of ridges or
wrinkles. Turning, on the other hand, to the glacier, we find curved
bands of different consistency and similar appearance, which I have
called relatively bands of porous and compact ice.

" We may then fairly ask, — If, in one species of viscous fluid,
alternate bands are derived from pre-existing ridges, why should not
analogous bands in another species of viscous fluid give rise to a prima
facie presumption, that ridges are to be looked for in an earlier stage
of that viscous fluid also ? Is not the analogy just so far strong
enough as to induce us to examine whether there is any trace of such
ridges or waves ; and, if so, whether their correspondence with the
alternations of porous and compact ice is sufficient to account for the
latter ?

" It is evident that such ridges or waves, if they do exist, must be
very slightly marked, or they would not have been overlooked ; but
then, it is to be remembered, that the difference between the two kinds
of ice is also very slight — in fact, only barely apparent. It will be
useless to look for them at the lower parts of glaciers, as they will
have disappeared under the effects of atmospheric and other action
through the lapse of many years, which will have degraded any exist-
ing ridges, just in the same way as in the case of outcropping strata.

" There is also a tendency to the establishment of an equilibrium as
to elevation, to say nothing of the disturbing effect of the lateral
friction. It is, therefore, towards the head of the glacier, where the
true glacial structure commences, that we are to look for such ridges :
the best time, also, will be at the commencement of summer, after the

MR. MILWARD ON DIRT-BANDS IN GLACIERS. 263

disappearance of the snow, and before the confusion of the surface
occasioned by the sun's influence.

u 1. It has been suggested to me, that in the case of the mud-slide,
there maybe an original difference of consistency in the bands of mud,
which is only increased by the action of the drainage water ; and that
the ridges and intervals are the outcropping of these beds which com-
pose the mud-stream. If there be such a tendency in viscous fluids
to separate into beds of different consistency, such a difference of con-
sistency may exist in glaciers, although the actual ridges may never
be sufficiently developed to be apparent. Such a peculiarity of viscous
structure would account for the bands of porous and compact ice,
whether the ridges be found or not.

" At present, however, we have no proof of such an internal struc-
ture, and may therefore dismiss it.

" 2. If, however, ridges or waves are found to exist, the case be-
comes precisely analogous to that of the mud-slide : The lower extre-
mity of each ridge will be more broken up than the other parts, just
as we see the ridges and lower extremities of the several mud-streams
to be broken up and more porous. From these high and broken
parts, the water will drain and saturate, to a greater extent, the sur-
faces below. The ice thus saturated will become, by the action of
frost, more compact than the rougher ridges. This fact, that satu-
rated ice produces the most compact glacier, appears to have been
already assumed (either from experience or otherwise), in the explanation
which is given of the formation of the transparent blue bands of the
ribbon -structure. An objection to which this explanation is open,
will be found in the very different width of the porous and compact
bands on the glacier ; whereas in the mud-slide they are nearly equal.

" In any case, however, it seems that there is sufficient ground why
we should look for such ridges or waves ; but at the same time, it is
quite possible that they may be proper to a peculiar condition of vis-
cous matter, to which class the glacier does not belong, or it may be
that special resistance and obstruction prevents the development of
such peculiarity in the case of glaciers. If this be so, the direct ana-
logy between the glacier and the mud-slide in this respect vanishes.

" 3. From the manner in which the second mud-slide explains the
first, — depending, it will be remembered on the drainage of water
altering the character of the mud — another deduction may, I think,
be drawn as applicable to glaciers, although not exactly in the same
way. The result to which I am about to draw your attention musf,

264 APPENDIX, NO. I.

I think, have a real existence. It is a result to which the dirt-bands
may be the indication, or it may have passed altogether unobserved.

" We find at the heads of most glaciers, where the neve is passing
into ice, and the body assuming its normal form and construction, that
there are steeper elevations, from which the neve descends, and fre-
quently ice-cascades. The glacier is in these parts, on account of
the abrupt descent, very much broken up, and often impassable.
Now it appears evident, that, at the foot of these slopes, the water
which has passed more quickly down them will accumulate to a greater
extent than if there had been no elevation behind, on account of the
change of inclination. Now, during the summer months, the satura-
tion thus taking place will be greatest, because of the large quantity
of water then coming down. At this period of the year, likewise, the
motion of the glacier is also greatest, and a large advance of the satu-
rated body occurs. This, during the winter frosts, is consolidated,
and formed, I imagine, into more compact ice than would have resulted
from less saturated material.

" On the other hand, in the winter months, that part of the glacier
at the foot of the upper slopes, or ice-cascades, will be less saturated,
as the surface of the whole glacier is then in a state of comparative
rest, in consequence of the diminished effect of the sun's rays in thawing
the surface of the neve. At this time, also, the glacier moves with
far less rapidity ; and so the quantity of glacier in a less saturated
state thus moving on, will be considerably less than that advancing
during the summer. In consequence, also, of its being Jess saturated
with water, it will, after consolidation, be less compact than that which
moved forward during the summer. Viewed in this light, the foot of
the upper slopes, or ice-cascades, may be considered as a kind of
laboratory for the manufacture of alternate bands of compact and porous
ice — the former made during the summer, and the latter in the winter
months. Thus, if my theory be correct, a wide band of comparatively
compact ice, and a narrow band of porous ice, will be annually formed
and added to the glacier.

" If these alternate bands be considered as identical with the porous
and compact bands to which the dirt-bands belong, it follows, that the
porous bands, during the progress down the glacier, become apparent
by the absorption of the drift, which is washed over the surface, and
their distinctness increases with the length of time during which they
have been subjected to the drift.

" Thus the wide compact band answers to the interval between the

MR. MILWARD ON DIRT-BANDS IN GLACIERS. 265

* dirt-bands/ and the narrow porous band to the ' dirt-band ' itself.
The one owes its formation to the summer, the other to the winter.

" It will result also from this theory, that the breadth of the ' dirt-
band ' and interval, taken together, should equal the annual advance
of the glacier. And this appears, from observation, to be the actual
case. We might also expect the relative breadth of the dirt-bands
and interval to approximate towards the proportion of the winter and
summer mean glacial motion. There may, however, be causes, arising
from the positions and proportions of the lower and upper slopes at the
source of the glacier, which would disturb this proportion between the
two bands, even more than they would alter the relation between the
two bands taken together and the annual glacial motion.

" It is with the greatest diffidence that the writer would venture to
submit, that a prima facia case has been made out for three subjects
of inquiry.

" 1st, Whether there are indications of the existence of wide struc-
tural bands (of which the bands on the surface of the mud -slide are
the outcrop) in viscous fluids and glaciers.

" 2d, Whether there are any traces in the upper parts of glaciers of
ridges or waves answering to the ridges occurring on the mud- slide.
And,

" 3<%, Whether the saturation at the foot of the upper slope, which
must theoretically exist, is practically effective, so as to cause the
alternate bands of porous and compact ice, in the manner which I
have endeavoured to describe."

Observations on the preceding communication, and especially on the
cause of the Annual Rings of Glaciers* By Professor FORBES.

" Professor Forbes stated that Mr. Milward's shrewd suspicion of the
bands of ice of different consistence, being accompanied also by wrinkles
or elevations, had been discovered by himself some years before at the
very place and time pointed out as most likely ; and he shewed that,
while there is a tendency in a tenacious viscous fluid to produce wrinkles
under pressure, capable of effecting detrusion even where the supply of
the fluid is uniform, this quality is greatly increased when the supply of
the fluid is by fits, as it is in fact at the head of the glacier, where the
quasi-hydrostatic pressure from behind, combined with the frontal
resistance, produces a thickening, or convex lip or wrinkle.

" He likewise mentioned the analogous instance of the production of
* Proceedings Royal Soc. Edin. 18th Dec. 1848.

266 APPENDIX, NO. II.

equidistant wrinkles in [on] the sides of railway banks from mere
pressure above ; and more particularly in turnings of coarse malleable
iron, where, though the detruding force is constantly equal, still detru-
sion takes place at intervals, forming in the shaving so many wrinkles,
by which frontal resistance, too, it is thickened, and -consequently
shortened, similarly with the mechanism of the glacier."

APPENDIX, No. II.

ME. FAEADAY ON THE PEOPEETIES OF ICE.

" . . . Mr. Faraday then invited attention to the extraordinary
property of ice in solidifying water which is in contact with it. Two
pieces of moist ice will consolidate into one. Hence the property of
damp snow to become compacted into a snow-ball — an effect which
cannot be produced on dry, hard-frozen snow. Mr. Faraday suggested,
and illustrated by a diagram, that a film of water must possess the
property of freezing when placed between two sets of icy particles,
though it will not be affected by a single set of particles. Certain
solid substances, as flannel, will also freeze to an icy surface, though
other substances, as gold leaf, cannot be made to do so. In this
freezing action, latent heat becomes sensible heat, the contiguous
particles must therefore be raised in temperature while the freezing
water is between them. It follows from hence that, by virtue of the
solidifying power at points of contact, the same mass may be freezing
and thawing at the same moment, and even that the freezing process
in the inside may be a thawing process on the outside. Mr. Faraday
then referred to Mr. Thomson's memoirs on the effect of pressure on
the freezing point. Mr. Thomson has shewn that immense pressure
will prevent water from freezing. At 32° ice naturally occupies a
greater volume than that of the water which forms it ; and we may
conceive that, when ice is pressed, the tendency is to give it both the
water bulk and state. In conclusion, Mr. Faraday noticed briefly,
and chiefly by way of suggestion, the molecular condition of ice as
presenting many curious results, and called attention to the strangeness
of striae being formed in a body of such uniform composition as pure
water frozen into ice." — From the Proceedings of the Royal Institu-
tion, reported in the Athenaeum, 15th June 1850, p. 641.

MR. DARWIN ON THE STRUCTURE OF LAVAS. 267

APPENDIX, No. III.

EXTEACTS from LETTERS to PROFESSOR FORBES, on the Analogy
of the Structure of some Volcanic Kocks with that of Glaciers.
By C. Darwin, Esq., F.R.S. With Observations on the same
subject, made by PROFESSOR FORBES.*

" I take the liberty of addressing yon, knowing how much you
are interested on the subject of your discovery of the veined structure
of glacier ice. I have a specimen (from Mr. Stokes's collection) of
Mexican obsidian, which, judging from your description, must resemble,
to a considerable degree, the zoned ice. It is zoned with quite straight
parallel lines, like an agate ; and these zones, as far as I can see
under the microscope, appear entirely due to the greater or lesser
number of excessively minute, flattened air cavities. I cannot avoid
suspecting that in this case, and in many others, in which lava of the
trachytic series (generally of very imperfect fluidity) are laminated,
that the structure is due to the stretching of the mass or stream
during its movement, as in the ice-streams of glaciers. * *

" If the subject of the lamination of volcanic rocks should interest
you, I would venture to ask you to refer to p. 65-72 of my small
volume of * Geological Observations on Volcanic Islands/ f I there

* [Proceedings of the Eoyal Society of Edinburgh, 3d February 1845. There
ought to have been a reference to this paper at page 92 of this volume.]

f The laminated, volcanic rocks of Ascension, consist, as described by Mr. Dar-
win, of excessively thin, quite parallel layers of minute crystals of quartz (deter-
mined by Professor Miller) and diopside ; of atoms of an oxide of iron, and of an
amorphous, black angitic mineral ; and, lastly, of a more or less pure feldspathic
stone, with perfect crystals of feldspar placed lengthways. The following is a
portion of the passage referred to: — " Several causes appear capable of producing
zones of different tension in masses semi-liquified by heat. In a fragment of de-vi-
trified glass I have observed layers of sphserulites, which appeared, from the manner
in which they were abruptly bent, to have been produced by the simple contraction
of the mass in the vessel in which it cooled. In certain dykes on Mount .ZEtna,
described by M. Elie de Beaumont, as bordered by alternating bands of scoriaceous
and compact rock, one is led to suppose that the stretching movement of the sur-
rounding strata, which originally produced the fissures, continued, whilst the injected
rock remained fluid. Guided, however, by Professor Forbes's clear description of
the zoned structure of glacier ice, far the most probable explanation of the laminated
structure of these feldspathic rocks appears to be, that they have been stretched,
whilst slowly flowing onwards in a pasty condition, in precisely the same manner,
as Professor Forbes believes, that the ice of moving glaciers is stretched and fissured.
In both cases, the zones may be compared to those in the finest agates ; in both,

268 APPENDIX, NO. III.

throw out the idea, that the structure in question may perhaps be
explained by your views on the zoned structure of glacier ice, the
layers of less tension being, in the case of the Ascension obsidian-rocks,
rendered apparent, chiefly by the crystalline and concretionary action
superinduced in them, instead of, as in zoned ice, by the congelation
of water. * * * *

" How singular it at first appears, that your discoveries in the
structure of glacier ice should explain the structure, as I fully believe
they will, of many volcanic masses. I, for one, have for years been
quite confounded whenever I thought of the lamination of rocks which
have flowed in a liquified state. Will your views throw any light on
the primary laminated rocks ? The laminse certainly seem very
generally parallel to the lines of disturbance and movement. Believe
me, &c. C. DARWIN."

" To Professor FORBES."

Professor Forbes confirmed the previous remarks by others made
by himself on the specimens transmitted to him by Mr. Darwin, and
on specimens from Lipari and Iceland in the collection of the Royal
Society, as well as by direct observations made by himself on the lava
streams of ^Etna.

they extend in the direction in which the mass has flowed, and those exposed on the
surface are generally vertical. In the ice, the porous laminae are rendered distinct
hy the subsequent congelation of infiltrated water ; in the stony feldspathic lavas
by subsequent crystalline and concretionary action. The fragment of glassy
obsidian in Mr. Stokes's collection, which is zoned with minute air-cells, must
strikingly resemble, judging from Professor Forbes's description, a fragment of the
zoned ice ; and if the rates of cooling and the nature of the mass had been favourable
to its crystallisation, or to concretionary action, we should here have had the finest
parallel zones of different composition and texture. In glaciers, the lines of porous
ice and of minute crevices seem to be due to an incipient stretching, caused by the
central parts of the frozen stream moving faster than the sides and bottom, which
are retarded by friction. Hence, in glaciers of certain form, and towards the lower
end of most glaciers, the zones become horizontal. May we venture to suppose
that, in the feldspathic lavas with horizontal laminae, we see an analogous case. All
geologists who have examined trachytic regions have come to the conclusion, that
the lavas of this series have possessed an exceedingly imperfect fluidity ; . and as it
is evident that only matter thus characterised would be subject to become fissured,
and to be formed into zones of different tensions, in the manner here supposed, we
probably see the reason why augitic lavas, which appear generally to have possessed
a higher degree of fluidity, are not, like the feldspathic lavas, divided into laminae
of different composition and texture. Moreover, in the augitic series, there never
appears to be any tendency to that kind of concretionary action, which, we have
seen, plays an important part in the lamination of rocks of the trachytic series, or,
at least, in rendering that structure apparent."

MR.. GORDON ON THE GLACIER-LIKE MOTION OF PITCH. 269

APPENDIX, No. IV.

ACCOUNT OF AN EXPERIMENT ON STOCKHOLM PITCH,
CONFIRMING THE VISCOUS THEORY OF GLACIERS.
In a Letter from Professor GORDON of Glasgow to Professor J.
D. FORBES of Edinburgh. Communicated by Professor J. D.
Forbes in a Letter to Richard Taylor, Esq.*

To RICHARD TAYLOR, Esq.

My dear Sir — The inclosed communication from Mr. Gordon,
Professor of Civil Engineering in the University of Glasgow, which
he has allowed me to transmit to you for publication, will, I believe,
be found interesting to your readers. The fact that pitch is suscep-
tible of slow fluid motion, whilst it retains the character (in hand
specimens) of a brittle solid, with a conchoidal fracture and glassy
lustre, may assist in resolving the doubts of some impartial persons
who have thought these characters in ice to be incompatible with such
a motion as my theory of glaciers requires, whilst the structural bands
having the frontal dip complete the analogy. — I remain, etc.

JAMES D. FORBES.

EDINBURGH, February 6, 1845.

To PROFESSOR FORBES.

" When you requested me to give you a memorandum of what ap-
peared to me to be the very glacier-like motion and appearance of
Stockholm pitch flowing from a barrel, I considered my observation to
have been too casual to be worth writing, and having foreseen that I
could arrange an experiment at Gateshead in the beginning of the
year, I delayed giving you the memorandum you wished. I had hoped
to have been able to inspect and report on my experiment about this
time, but I cannot go to Gateshead for some time to come, nor have I
had any report of the progress of my pitch glacier since the 6th of
January, when I was informed it had not moved since the day after
I left it on the 28th of December. Your note of yesterday induces
me to offer you the following still perfectly vivid impressions of the
analogy between ice and Stockholm pitch.

* [Philosophical Magazine, March 1 845. This paper ought to have been referred
to at page 92.]

270 APPENDIX, NO. IV.

<t

" Allow me in the first place to mention that I read your Travels
in the Alps in May last ; that on the 24th of June I spent almost
twenty hours on the glaciers of the Grindelwald. I went up by the
lower glacier prepared with poles to prove its motion, and actually
observed a progress of above twelve inches in the course of thirteen
hours, from 6 A.M. to 7 P.M. I traced the l dirt-bands ' on the sur-
face. I was let down into several crevasses, one of them to a depth
of thirty feet, and could trace the slaty structure of the ice ; the alter-
nate clear blue thin veins, and the transition to opaque gray, or even
white. I descended from the glacier with a much better appreciation
of the theory of glaciers than I had had, and a strong conviction that
the facts I had observed could not be otherwise accounted for than by
the mechanical theory you have given.

" In passing through Gateshead, in August, a broken-headed barrel
of Stockholm pitch at the wire-rope factory attracted my attention.*

" A mass of Stockholm pitch broken from a [the] barrel in August (at
the time of the observations I am about to mention) presented a dark
brown colour, a glassy lustre, translucent edges. The substance is
fragile, fracture conchoidal and very uniform. A mass which was
brought to me by the workman having charge of this department, and
which he had broken from the end of such a stream as I have repre-
sented coming from the barrel, presented generally the same appear-
ance as a mass broken from an entire barrel, f but had this remarkable
peculiarity, that there were lines — structural lines, whose texture and
colour were different from the general colour of the mass recognizable
on points between any two such structural lines.

" Fig. 2 is an elevation of the stream of pitch, showing pretty nearly
the dimensions and outward appearance of the stream. The striated
slaty structure appears here on the outside, as is more distinctly (in-
tended to be) shown in Fig. 3. There were certain well-defined lines,
and on either side of these, for some little distance, other small lines
or cracks (but not open cracks or fissures), and then a space of smooth
glassy-looking pitch.

." I am strongly impressed with the idea that the structural lines are
a result of the motion, and that they correspond with the veins of
glaciers ; the lines incline most when the surface is steepest, and are

* [The reference to the figures is here omitted. The figures will be found in
the volume of the Philosophical Magazine referred to.]

f " The pitch is fragile at the same time that it flows." — L. G.

MR. BLACKWELL ON THE GLACIERS IN WINTER. 271

very faint and nearly horizontal at *, where the surface of the stream
is nearly so too. I left Gateshead without having an opportunity of
getting a sectional view of this stream. I can get no real Stockholm
pitch in Glasgow, else I should have made the experiment you have
incited me to attempt here. — I am, etc.

" LEWIS GORDON."
" GLASGOW, January 31, 1845."

APPENDIX, No. V.

EXTRACTS from a LETTER from E. BLACKWELL, Esq., containing
Observations on the Movement of Glaciers of Chamouni in Winter.
Communicated with remarks by Professor FORBES.*

" The accessibility of the glaciers, even up to a considerable height,
is at this season a question of mere physical force. I have made
within the last few days two excursions into the region of perpetual
snow. The first of these was on the 6th of January, and was to the
summit of the glacier of Blaitiere, several hundred feet above the point
where I had noted the line of the neve in September and October ;
the second was on the 13th, when I succeeded in reaching the junction
of the glaciers of Bossons and Tacconaz, near the Grands Mulcts.
This junction is exactly at the commencement of the neve, as I re-
marked between the months of August and October, on six different
occasions, when I passed there on my way to and from Mont Blanc,
the D6me de Goute, etc. In both these expeditions I was struck by
the excessive power of the sun ; the greater apparent warmth, even
in the shade, as compared to the valley of Chamouni ; and the sudden
chill which followed sunset. There was also much less snow at these
heights than in the valley, and I have no hesitation in saying that in
winter very little snow falls upon the higher summits. The snow-falls
in the valley are invariably brought by a low creeping fog, which
comes up from Sallanches. It seldom overtops the Col de Voza, and
the Aiguilles appear bright and sunny in the gaps of the cloud. It is
in spring and autumn that these higher peaks are powdered by every
storm ; now the dispersing clouds leave them as dark as before they
gathered. I fancy this winter is unusually cold ; every one is crying

* [Proceedings of the Royal Society of Edinburgh, 5th February 1855.]

272 APPENDIX, NO. V.

out, and complaining that the potatoes are frozen in deep cellars. I
have seen Reaumur's thermometer at — 25° at 5|- in the afternoon, and
I think it may reasonably be supposed that it may have fallen to
— 30° during the night ; wine has frozen on my table before a fire.
In the woods the trees crack with the intense frost, and "there is from
2J to 3 feet of snow in the valley without drifts ; on the glacier of
Blaitiere there is only from 1 to 2 feet,

" In spite of all this cold the glaciers advance steadily. The glacier
de Blaitiere, terminating above the line of trees, pushes its moraine in
front of it, and seems to be on the increase.* Now this is a very
shallow glacier, and, as I have said, covered with but little snow. Is
it possible that infiltrated water can have any action whatever under
such circumstances ?

11 1 will here state a few results of careful observation, and I hope
that, even should they appear strange, you will yet consider them
worthy of confidence. I have no theodolite, but I have a prismatic
compass, and will take the bearings of various points from my stations,
should you deem it advisable.

'* The torrent of Bossons has been quite dry ever since the beginning
of November, and I have profited by this circumstance to endeavour
to determine the motion of the ice within the vault, nearly in contact
with the ground. I believe it is usually supposed that the reason why
the termination of a glacier seems stationary in summer, is that there
the waste predominates over the supply. It seemed to me, therefore,
that in winter, when there is actually no waste — the torrent being
perfectly dry, and its sub-glacial bed even dusty — the end of the
glacier ought to be thrust forward into the valley by the pressure
behind. I accordingly, with some little difficulty, fixed a station on
the ridge or back of the glacier, near the lower extremity ; the result is,
that the ice there is nearly stationary. This is doubtless a clue to the
assertions of some authors, "that the glacier is stationary in winter;"
— they only looked at the end. What becomes, then, of the ice con-
tinually descending from above ? Does it not go to thicken the whole
mass, accumulating behind the more rigid portion below, as water
behind a dam ? I have no space to add more at present, but will
write again if I have your approval of my proceedings. Meanwhile
I have fixed (yesterday) an intermediate station, for the purpose of
determining where this comparative immobility begins. I have noted
my observations, and kept a register of weather, etc. I give one
* [See note to next page-1

MR, BLACKWELL ON THE GLACIERS IN WINTER. 273

observation to show the difference between the middle and lower
glaciers : —

From December 28 to January 11 — 14 days.

Middle glacier (somewhat above where it is usually crossed).
Centre, 14 ft. 7 in. (fourteen feet seven inches).
Side, 11 ft. 6 in. (eleven feet six inches).
Lower glacier during the same period.

Ridge, 1 ft. 7 in. (one foot seven inches).
Interior of vault, 0 ft. 2 in. (two inches).

Observations on Mr. Blackwelfs Letter, by Professor Forbes*

" The cold described (— 25° to 30° of Reaumur— 241° to — 35^°
of Fahrenheit) — appears so excessive as to be unlikely ; I have there-
fore written to enquire if the thermometer could be depended on.

" It is highly satisfactory that the superficial velocity of the glacier
of Bossons — about a foot in twenty-four hours — coincides closely with
the measurements of my guide, Auguste Balmat, some years since,
on the same glacier, at the same season.

" With respect to the ice of the glacier of Blaitiere,' which is above
the level of trees — probably at least 7000 feet above the sea — being
still in motion, it merely confirms the deductions long ago made by
me as to the continuity of glacier motion even in winter. And as
to the apparent paradox of water remaining uncongealed in the fissures
of the ice at this season, though I have nowhere affirmed the presence
of liquid water to be a sine qua non to the plastic motion of glaciers,
it would be difficult to assert positively that it is everywhere frozen in
the heart of a glacier even in the depth of winter. Heat, we know,
penetrates a glacier (up to 32° and no further), not only by conduc-
tion, but much more rapidly by the percolation of water ; but cold
penetrates solely by conduction, and that according to the same law
as in solid earth, though it may be more rapidly. Now, it is known
that at a depth of 24 or 25 feet in the ground, the greatest summer
heat has only arrived at Christmas. A similar retardation in the" effects
of cold must occur in glaciers. Not a particle of water detained in the
capillary fissures can be solidified until its latent heat has been withdrawn.

" The contrast the writer draws between the glaciers of Blaitiere
and Bossons, the latter of which is some thousand feet lower in point
of level, is curious and instructive. The former, he says, appears the
more active, and is pushing forwards its moraine y-j- whilst the latter,

* [Proc. Roy. Soc. Edin., as above.]

f [Even this observation is not without some ambiguity. For no direct measures
of the motion of the ice seem to have been made, and the appearance of pushing the
moraine might be due to movements which bad taken place some time previously.]

T

274 APPENDIX, NO. V.

at its lower extremity, and in contact with the ground, is scarcely
moving at all.

" There is nothing of which we know less than the cause of the
seemingly capricious advance and retreat of the extremities of glaciers
at the same time, and under, seemingly, the same circumstances.

" In the present case, I will only mention as a possible explanation,
that the glacier of Blaitiere probably possesses a continuous slope, from
its middle and higher region down to its lower extremity. But the
Bossons, after its steep descent from Mont Blanc, proceeds a long way
on a comparatively level embankment, which at an early period it
cast up of its own debris, and in which it has dug itself a hollow bed
in which it nestles. The angular slope of the bottom in contact with
the soil is very probably much less than in the case of the glacier of
Blaitiere. Now, when winter has dried up the percolating water, the
viscosity of the mass may be insufficient to drag it over the less slope,
although it carries it over the greater. That the motion of the ice
close to the ground should be nearly nothing, whilst the more super-
ficial part of the glacier over-rides it by its plasticity, is as a separate
fact quite in accordance both with theory and previous observation.

" But as the snout, or lower end of the glacier of Bossons, is almost
stationary, whilst the middle region is moving at the rate of a foot a
day, Mr. Blackwell very pertinently asks, ' What becomes, then, of
the ice continually descending from above ? Does it not go to thicken
the whole mass, accumulating behind the more rigid portion below, as
water behind a dam ? ' I answer, undoubtedly ; and he will find this
explanation given ten years ago in my Travels in the Alps (2d edit.,
p. 386). Speaking of the superficial waste of the glaciers in summer
and autumn, and the manner in which it is repaired before the ensuing
spring, I there observed, ' The main cause of the restoration of the
surface is the diminished fluidity of the glacier in cold weather, which
retards (as we know) the motion of all its parts, but especially of those
parts which move most rapidly in summer. The disproportion of
velocity throughout the length and breadth of the glacier is therefore
less, the ice more pressed together, and less drawn asunder ; the cre-
vasses are consolidated, while the increased friction and viscosity
causes the whole to swell, and especially the inferior parts, which are
the most wasted." — (See also Seventh Letter on Glaciers.*

* [Page 60 of this volume.]

INDEX.

A, Station on the Mer de Glace marked,
37, 125.

Aar Glacier, 2 ; motion of the, 68, etc. ; ob-
servations on the motion of its surface and
bottom, 186 ; additional observations on
the velocity of, 206, 209, note ; different
reports do hot agree, 208

Ablation of surface of glaciers, 157 ; its
amount ascertained, 170, 242.

Agassiz, Professor, quoted, 2, 55, 68, 98, 208,
243, 250, note.

Air bubbles in glaciers, 201 ; in lavas, 267.

Aletsch, motion of glacier of, 15, 62.

'• Angle" on the Mer de Glace, ridges of ice
near, 205.

Annual motion of different parts of the Mer
de Glace, 123, 222. See also the letters
indicating the various stations, and the
word Motion,

Annual rings of glaciers, 25, 217.

Ascensional movement of semifluid particles,
212. See Intumescence and Level.

Auldjo, Mr., quoted, 83.

Balmat, his observations on the velocity of
glaciers at different seasons, 126, etc., 224.

Bos-Neves or snow-beds, 76.

Bischoff, M., quoted, 98.

Blackwell, Mr,, his observations at Cha-
mouni during winter, 271.

Bois, Glacier des, its motion, 126, etc.,
222, etc.

Bossons, Glacier of, appearance of blocks
on the, 204 ; its motion, 126, etc., 222,
etc. ; rents in, frozen up, 162.

Brenva, Glacier of La, revisited in 1846, 176,
etc. ; great increase since 1842, 177 ; its
cause, 178; records of its extent in 1846,
181 ; its remarkable veined structure, 19,
182, 256 ; traced to its mechanical cause,
182, etc. ; movement of the ice tested in
different directions, 184, 185.

Bruising of ice produces the veined struc-
ture, and results from its fragility or im-
perfect plasticity, xvi, 47, 51, 53, 161, 162,
166, 201.

C, Station on the Mer de Glace marked, its
motion, 36, 38, 123, 188, 221.

Cadell, Mr. W. A., quoted, 82.

Charpeutier. See De Cfiarpentier.

Christie, Mr., quoted, 161, note ; his experi-
ment on the plasticity of ice on a small
scale, ib. ; the experiment varied, 167.

Col du Tour, 227.

Collapse of the parts of a glacier at the
close of summer, 27, 28, 1 54.

Collin, M., on land-slips, quoted, 211.

Collomb, M., quoted, 206.

Cones, gravel, 241.

Congelation of water in glaciers, xix, 23,
34, 47, 137, 163 ; congelation not neces-
sary to convert snow into pellucid ice,
200-202.

I Conservative power of the ice, 196.
| Conversion of the Neve into ice, xx, 53, 163,
199 ; simultaneous and identical with the
formation of veined structure, 200 ; due
to pressure and increased by differential
motion, 200 ; commences near the sides
of the glacier, 201.

Convex surface of glaciers, 49, 60, 152 ; of
land-slips, 211.

" Corpora non agunt nisi soluta" an ex-
ploded doctrine, 201.

Crease or wrinkle in semifluid bodies, 215,
216.

Crevasses of glaciers perpendicular .to veined
structure on -he Glacier of the Rhone, 7,
81 ; their probable origin, 8 ; their vertica-
lity and transverse direction reconciled
with the facts of motion, 30, 54 ; crevasses
in lava streams, 45 ; crevasses in glaciers,
their distribution and extent, 143 ; their
size and importance often overrated,
143-5 ; their direction, 145 and note,
192 ; intersecting crevasses comparatively
uncommon, 146 ; length of, 147 ; do not
materially assist progression, 147 ; may
be due to general or to local distension,
151, 152; often renewed and sealed up
by collapse, 153 ; crevasses of the Mer de
Glace, 205 ; of glaciers in general, 240.

D, Station marked at Montanvert, its mo-
tion, 36, 124, 189.

Darwin, Mr., quoted, 92, 201, 267.

De Charpentier's theory of glaciers, 15, 34,
248.

Depression of level of a glacier, 27, 33 ; due
to several causes, 169 ; the effects sepa-
rated, 170 ; geometrical depression, 171 ;
correction of some results in volume of
" Travels," 172, note.

De Saussure's theory of glaciers, xxi, 15 ;

denned, 95, 249 ; modified, 101.
! Detrusive force, 141.

i Detrusion of semifluids, 216 ; in the metals,
218, 219.

Dilatation theory, 15, 32, 249.

276

INDEX.

" Dirt- bands," discovery of, 21, 25, 39 ;
dirt-bands described, 245; their analo-
gues in mud-slides, 213.

Discontinuity of glaciers, three orders of,
205.

Dollfuss and Martins, MM., their observa-
tions on the glacier of the Aar, 186, 206.

Ductility of ice not great, 161 ; its grada-
tions, 161, 162. See Fragility, Plasticity.

Edinburgh Review, article on Glaciers in

the, xxviii, 10.
Egralets, Les, 193.
Elie *de Beaumont, M., quoted, 48, 49,

84, 87.
Erratics, their appearance on the surface of

glaciers, 60, 202 ; often difficult to explain.

203.

Escher, M., quoted, 15.
Etna, lavas of Mount, 88.
Experiments on the flow of plastic bodies,

77 ; on the plasticity of ice on the great

scale, 103, etc. ; on the small scale, 161,

note, 167.
Extrusion of stones from glaciers, 60, 202.

Facts of glacier motion, 11, 35, 251, etc.

Faraday, Mr., quoted, xiii, xxiii, 228 ; on
some properties of ice, 266.

Firn, 244.

Flames from Vesuvius, 44, note.

Fluid motion with friction, 23, 31, 35, 78,
etc.

Fragility of ice not inconsistent with the
theory of plasticity, 35, 47, 51, 53, 160 ;
diminished by impending fusion, 166,
225 ; it is succeeded by reattachment of
the surfaces, 161, 201.

Freezing point of water, liable to some gra-
dation, 229.

Friction of semifluids, 212. See Fluid.

Frontal dip, 19, 24, 41, 46, 59, 70, 159, 204 ;
its analogue in lava streams, 90 ; in pitch,
93, 270 ; in mud-slides, 212, 217 ; frontal
dip illustrated in metal turnings, 219.

Frontal resistance, 59, 71, 145, note, 204;
in mud -slides, 212, 217; in metals,
218.

G,* Station on Mer de Glace marked, 144,
note.

Gay Lussac, quoted, 201.

Geant, Glacier du, its ice-fall, 199 ; wrinkles
on, 40, 199 ; difficulty of traversing, 199,
note.

Glaciers, passim. In great glaciers the win-
ter's cold does not penetrate the entire
mass, 137 ; a glacier not a mass of frag-
ments, 141, 142.

Glaciers, general account of, 233 ; names
for in different languages, 233 ; of the
Himalaya, 235; of Norway, 237; of
Spitzbergen, 238 ; of South America,
238 ; of the Alps, 237, 239 ; laws of mo-
tion recapitulated, 251-3.

Glaciers of the second order, 244; their
veined structure, 21 ; their motion, 67,

146. For individual glaciers, see their

respective names.

Gordon, Professor, quoted, 92, 141,215,269.
Gravitation theory, 248.
Grindelwald, lower Glacier of, 41. Frontal

dip, and wrinkles upon, ibid.
Gruner, quoted, 95.
Guicharda, M., his observations on the

Glacier of La Brenva, 179, 180.

Hall, Captain, quoted, 83.

Hammer, one lost and found on the glacier,

196, note.

Himalaya, glaciers of the, 235.
Hopkins, Mr., quoted, 62, 102, 103, 187.
Hugi, M., quoted, 248.
Huxley, Mr., quoted, xv, 228.

Ice, on some properties of, near its melting
point, 228, 266 ; gradual fusion of, accom-
panied by softening, 35, 154, 166, note.

Illustrations of the viscous theory of glacier
motion, 77, etc.

Intumescence of lava, 47 ; cause of the, 91 ;
of glaciers, 156, 158 ; of mud-slides, 212.

Knapsack, history of one lost and recovered
on the glacier, 193, etc.

Land-slips, their analogies to glaciers, 211 ;
described by M. Collin, 211, etc. ; land-
slip at Lyme-Regis, 216.

Lava streams, phenomena of, 44, etc. ; mo-
raines of, 48; their analogy to glaciers,
82 ; intumescence of, 47, 91 ; veined struc-
ture in, 46, 92, 267.

Lava, velocity of, 85, 93.

Letters on glaciers, first, 9 ; second, 13 :
third, 17; fourth, 26; fifth, 35; sixth,
43 ; seventh, 56 ; eighth, 61 ; ninth, 68 ;
eleventh, 169; twelfth, 176; thirteenth,
186 ; fourteenth, 205 ; fifteenth, 210 ;
sixteenth, 220.

Level of glaciers. See Depression.

Level recovered during winter, 28 ; at first
supposed to be due to congelation, 34 ;
afterwards, more correctly, to frontal
resistance acting on the plastic mass,
especially during winter, 60, 156, 158,
218, note, 259 ; changes of level in diffe-
rent years, 37, 158, 177.

Lines of tearing-, 201.

Longitudinal fissures exceptional, 147, 162.

Lyme-Regis, land-slip at, 216 ; wrinkles
formed by, ibid.

Martins, M., quoted, 99, 101, 209, 238.
Mer de Glace of Chain ouni, experiments on

its motion, 11, 123, 188, 221, et passim ;

change of level in different years, 37,

158; ablation, 169, etc. ; subsidence, 170;

relative motion of surface and bottom,

172 ; old moraine of, 42.
Mer de Glace. See also Glacier des Bois,

and Letters denoting the various stations.
Miage, Glacier of, 83; its velocity, 198;

its moraines, 198.

INDEX.

277

Milward, Mr., quoted, 210, 212; on the
wrinkles of mud-slides, 214 ; of glaciers,
261 ; notice respecting him, 214, note.

Models, plastic, xxviii, 47, 58, 77, etc.

Molecular attachment of granules of ice or
snow under pressure, 201.

Moraine, 239 ; medial, 243.

Moraine, ancient extension on the left bank
of the Mer de Glace, 42 ; moraines of
lava streams, 48 ; of Glacier du Miage.
198 ; of Glacier of Nant Blanc, 203 ; of
the Glacier of the Rhone, 203.

Motion of glaciers, 11, 15 ; by day and by
night, 11 ; theories of the, 15 ; faster at
centre than at sides, 12, 35, etc.; faster
at top than bottom, 24, 54, 172, 186 ; in
winter, 25, 36, 73, 129, etc., 179, 224 ; mo-
tion of glacier of the second order, 67, 74,
119 ; of the glacier of the Aar, 69, 206.

Motion of Glaciers, effects of season on, 73,
129, 208, 222, etc. ; dependence on weather,
132 ; on temperature, 138 ; does not simul-
taneously increase and decrease at dif-
ferent points of the same glacier, 135, 139,
and note ; motion equable, 148 ; argument
thence derived in favour of plasticity,
148 ; causes affecting its amount, 155-7.

Motion of glaciers, facts of, recapitulated,
250.

Moulins, permanence of their position, 29,
37.

Mousson, M., quoted, 252, 255.

Mud-slides, analogies to glaciers, 210 ; that
at Malta described by Mr. Milward, 210.

Nant Blanc, Glacier du, 196 ; its structure
and velocity, 197 ; its moraines, 203.

Naysmith, Mr. J., quoted, 218, note.

Neve defined, 244 ; on the mode of its con-
version into ice, 53, 163, 199 ; stratification
of, 199.

Norway, glaciers of, 237.

Optical contact of icy particles induced by

pressure, 201.
Oscillations of Glacier of La Brenva, 177, etc.

P, station so marked on the Mer de Glace,
124, 189.

Pass, undescribed, across the chain of Mont
Blanc, 226.

Parameter, 19.

Pasterzen Glacier, 240.

Permanence of the u moulins " and other
accidents on the glacier surface, 29, 37.

Person, M., on the fusion of ice, xxiii, 225-
229.

Pitch, a brittle yet plastic substance, 93,
141, 215.

Plastic or viscous theory stated and sup-
ported, 23, 31, 35, 44, 51, 62 ; summary
of evidence for it, 140, etc. ; plastic theory
recapitulated, 253.

Plasticity, a quality which admits of grada-
tion, 53, 161, etc. ; experiments on the
motion of plastic bodies, 47, 58, 77 ; plasti-
city of the ice of glaciers, 35, 62, 64, 147 ;

experiments on the great scale in 1844, 74,
103, etc.; desirable to be repeated, 118;
plasticity deduced from equability of the
motion, 148; from the collapse of the cre-
vasses, 154 ; general arguments for, 159 ;
plasticity of ice exemplified on a small
scale, 161, note, 167 ; increased at a thaw-
ing temperature, 166, and note ; illustrated
by the local movements of the ice of the
Glacier of La Brenva, 184, 185 ; confirmed
by the retardation of the lower surface of
a glacier, 187 ; illustrated by mud-slides,
210 ; by land slips, 211 ; by metal-turn-
ing, 219.

Plasticity of ice near 32° due to the gradual
absorption of latent heat, 225, 230.

Plasticity of ice, some objections to it con-
sidered, 254.

Q, station on the Mer de Glace so marked,
106, 190.

R, station on the Mer de Glace so marked,
190.

Ramond quoted, 98.

" Regelation," fact so called, 228 ; accounted
for by the progressive evolution of latent
heat, 231.

Rendu, Bishop, quoted, 62, 84, 160, 201.

Restoration of level of a glacier effected
during winter. See Level.

Rhone, observations on the, at Sierre, 185.

Rhone, Glacier of the, compared to " a pail-
ful of mortar poured out," 6; lines of
structure on, 7 ; appearance of blocks on
the, 203.

Ribboned structure. See Veined Structure.

Ridges of ice oblique to the axis of the
glacier near the " Angle," 205.

Rigidity, a relative term, 23, 35, 160, etc.

Ripples in moving water, 58, 80, 185.

Saussure. See De Saussure.

Schlagintweit, MM., quoted, xv, 237.

Schonhorn, Glacier of the, 75, 120.

Scoresby, Dr., quoted, 238.

Scrope, Mr., quoted, 85.

Seasons, effect of, on glaciers, 27, 36, 73,

129, etc., 224; on the Glacier of the Aar,

208.
Second order, motion of glaciers of the, 119,

244. See Glaciers.
Semifluid. See Fluid Motion.
Serac, a word used by De Saussure, 146,

note.

Slags, veined structure in, 82.
Slaty cleavage of rocks compared to veined

structure in glacier ice, 8.
Snow line, 243 ; on glaciers, 244.
Softening of ice while thawing. See fee.
Sound of a glacier in motion, 66.
Starke, Mrs., quoted, 50.
Structure in the ice of glaciers. See Veined

Structure.

Studer, M., quoted, 99.
Subsidence of a glacier distinguished from

ablation, 170. See Depression.

278

INDEX.

Surface of a glacier moves faster than its
bottom, stated as probable, 24, 54-56;
proved by experiment, 172, etc. ; con-
firmed on "Glacier of the Aar, 186; favour-
able to the plastic theory, 187.

Tables, glacier, 241.

Talefre, Glacier of, 38 ; issue of ice through
the contracted gorge of the glacier so
called, 39, 142 ; its motion determined,
191, etc. ; veined structure, 192, 200.

Temperature at Geneva and St. Bernard,
131, 224.

Temperature, its effects on the movement
of glaciers, 35, 73, 138, 206, 222.

Temperature of freezing water, 229.

Tensions and thrusts in the interior of a
glacier, 149 ; vary from point to point of
the same glacier at different seasons, 150 ;
tensions and thrusts alternate, 155.

Terminal part of a glacier moves compara-
tively slowly, 130, and note; its move-
ment in winter, 130, note, 179, 272 ; re-
tarded by the friction of the soil, 173.

Theories of glacier motion, De Saussure's,
15, 31, 95, 249; De Charpentier's, 15, 32,
etc., 248. See Plastic Theory.

Thomson, Mr. James, quoted, xiv, xxiv.

Thomson, Mr. John, quoted, 212, note.

Thomson, Prof. William, quoted, xiv, xxiv.

Torrential motion of some glaciers, 147 ;
destroys veined structure, 24, 161-2.

Tour, Le, Glacier and Col of, 227.

Tyndall, Dr., quoted, xiv, etc., 228

U, station so marked on the Mer de Glace,17l.

V, station so marked on the Mer de Glace,
221.

Veined structure of glaciers, observed by
the author in 1841 on the Glaciers of the
Aar and Rhone, 1, etc.; extends to a
great depth, 3; pervades glaciers from
the neve to their termination, 4; moat
developed near moraines, 5 ; endures for
more than one season, 5 ; the direction
of the veins traced over the glacier, 5, 19,
etc. ; produce a false appearance of hori-
zontal stratification of the glacier at its
lower end, 5; apparently perpendicular
to lines of greatest pressure, 6 ; structure
of the Glacier of the Rhone, 6 ; also per-

?endicular to lines of fissure or crevasses,
, 145, 192 ; not due to stratification or
sedimentary deposition, 8 ; resembles
slaty structure of some rocks, 8, 92 ; in
the Glacier of La Brenva, 19 ; course of
the veins generally in glaciers of different
forms, 20 ; in glaciers of the second order,
21; theory of, 23; strongly developed
where glaciers unite, 24 ; tends to disap-
pear where the glacier becomes crevassed,
24, 161 ; renewed from time to tune, 29,
39 ; cuts the medial moraines, 39 ; analo-
gous appearances in lava, 46, etc. ; results
from the limit of perfect plasticity being
exceeded, 35, 47, 51, 53, 161, 166, 201;

forms of veined structure minutely con-
sidered, 56, etc. ; subjected to critical ob-
servation on the Mer de Glace, 64, etc. ;
theory of veined structure illustrated by
plastic models, 79 ; perpendicular to
crevasses, 79, 81.

Veined structure, produced in situ, 29, 39, 42,
105 ; a consequence of imperfect plasticity,
162 ; disappears in glaciers moving torren-
tially, 161-2 ; and in the centre of wide
glaciers with low velocities, 163 ; remark-
able development in 1846 on the glacier
of La Brenva, 182 ; its cause explained,
182, etc. ; experimentum cruets regarding
it, 183, 184 ; analogy from the phenomena
of rivers, 185 ; veined structure on Gla-
cier of Talefre, 192, 200 ; perpendicular
to crevasses there, 192 ; accompanies the
conversion of the neve into ice, 53, 200 ; its
continuance under medial moraines, 200 ;
occurs where pressure is most intense,
200.

Veined structure described generally, 245;
course of, 246, 247 ; theorv of, recapitu-
lated, 255-8. ^

Velocity of glaciers ; maximum not attained
simultaneously at all parts of a glacier,
or on different glaciers, 135, 139, and
note; its dependence on temperature, 138.

Velocity of glacier at the ice-fall of Talefre,
195 ; of Glacier du Nant Blanc, 197.

Velocity of glaciers, causes which in-
fluence it. See Motion and Surface, and
the Letters denoting different stations on
the Mer de Glace ; also under the names
of other glaciers.

Velocity of lava, 85-93.

Vesuvius, flow of lava within the crater of,
44 ; lavas of Vesuvius, 87.

Viscous or plastic theory of glacier motion
illustrated, 77, etc. ; theory recapitulated,
253. See Plastic theory.

W, station on the Mer de Glace (Glacier of
Talefre) so marked, 191, etc., 200.

Waste, superficial, of glaciers, 33, 60, 241.
See Subsidence and Level.

Water in the crevices of glaciers, its effect,
164-166; presumed effect of its congela-
tion in winter, 34; this view corrected,
60, 156, 158, 218, note, 259.

Water, ripple in, 58, 80, 185.

Weather, effects of, on the motion of gla-
ciers, 33, 73, 132, 137, 166.

Weather at Chamouni 1844-5, 133.

Williamson, Mr., quoted, 110, note.

Winter, glaciers move in, 25, 36, 129, etc.,
224, 273 ; glaciers still contain water dur-
ing, 137; movement of Glacier of La
Brenva in, 179.

Wrinkles of the surface of the ice discovered,
39; probably coincident with the dirt-
bands, 40 ; on the Glacier of Grindelwald,
41 ; their probable origin, 199 ; their
analogues in mud-slides, 213, 214, 216 ;
coincident with dirt-bands, 216, 217 ; Mr.
MilM-ard's theory of, 261.