origin of the deposits is this. The intercalated boulder clays bear
no marks of stratification, and are evidently the true unstratified
till formed when the country was covered by ice. But the fact that
these beds are both underlaid and overlaid by stratified deposits
would, on the marine theory, imply not merely the repeated appearance
and disappearance of the ice, but also the repeated submergence and
emergence of the land. If the opinion be correct that the submergences
and emergences of the glacial epoch were due to depressions and
elevations of the land, and not to oscillations of sea-level, then
the difficulty in question is, indeed, a formidable one. But, on the
other hand, if the theory of submergences propounded in Chapters XXIII.
and XXIV. be the true one, the difficulty entirely disappears. The
explanation is as follows, viz., during a cold period of the glacial
epoch, when the winter solstice was in aphelion, the low grounds would
be covered with ice, under which a mass of till would be formed.
After the cold began to decrease, and the ice to disappear from the
plains, the greatest rise of the ocean, for reasons already stated,
would take place. The till covering the low grounds would be submerged
to a considerable depth and would soon be covered over by mud, sand,
and gravel, carried down by streams from the high ground, which, at
the time, would still be covered with snow and ice. In course of time
the sea would begin to sink and a warm and continental period of,
perhaps, from 6,000 to 10,000 years, would follow, when the sea would
be standing at a much lower level than at present. The warm period
would be succeeded by a second cold period, and the ice would again
cover the land and form a second mass of till, which, in some places,
would rest directly on the former till, while in other places it would
be laid down upon the surface of the sands and gravels overlying the
first mass. Again, on the disappearance of the ice the second mass of
till would be covered over in like manner by mud, sand, and gravel, and
so on, while the eccentricity of the earth’s orbit continued at a high
value. In this way we might have three, four, five, or more masses of
till separated by beds of sand and gravel.
It will be seen from Table IV. of the eccentricity of the earth’s
orbit, given in Chapter XIX., that the former half of that long
succession of cold and warm periods, known as the glacial epoch,
was much more severe than the latter half. That is to say, in the
former half the accumulation of ice during the cold periods, and its
disappearance in polar regions during the warm periods, would be
greater than in the latter half. It was probable that it was during
the warm periods of the earlier part of the glacial epoch that the two
buried channels of the Midland valley were occupied by rivers, and that
it was during the latter and less severe part of the glacial epoch that
these channels became filled up with that remarkable series of deposits
which we have been considering.
_Other buried River Channels._—A good many examples of buried river
channels have been found both in Scotland and in England, though none
of them of so remarkable a character as the two occupying the valley
of the Forth and Clyde Canal which have been just described. I may,
however, briefly refer to one or two localities where some of these
occur.
(1.) An ancient buried river channel, similar to the one extending
from Kilsyth to Grangemouth, exists in the coal-fields of Durham,
and is known to miners in the district as the “Wash.” Its course was
traced by Mr. Nicholas Wood, F.G.S., and Mr. E. F. Boyd, from Durham
to Newcastle, a distance of fourteen miles.[289] It traverses, after
passing the city of Durham, a portion of the valley of the Wear, passes
Chester-le-Street, and then follows the valley of the river Team, and
terminates at the river Tyne. And what is remarkable, it enters the
Tyne at a depth of 140 feet below the present level of the sea. This
curious hollow lies buried, like the Scottish one just alluded to,
under an enormous mass of drift, and it is only through means of boring
and other mining operations that its character has been revealed. The
bottom and sides of this channel everywhere bear evidence of long
exposure to the abrading influence of water in motion; the rocky bottom
being smoothed, furrowed, and water-worn. The river Wear of the present
day flows to the sea over the surface of the drift at an elevation of
more than 100 feet above this buried river-bed. At the time that this
channel was occupied by running water the sea-level must have been at
least 140 feet lower than at present. This old river evidently belongs
to the same continental period as those of Scotland.
(2.) From extensive borings and excavations, made at the docks of Hull
and Grimsby, it is found that the ancient bed of the Humber is buried
under more than 100 feet of silt, clay, and gravel. At Hull the bottom
of this buried trough was found to be 110 feet below the sea-level.
And what is most interesting at both these places, the remains of a
submerged forest was found at a depth of from thirty to fifty feet
below the sea-level. In some places two forests were found divided by a
bed of leafy clay from five to fifteen feet thick.
(3.) In the valleys of Norfolk we also find the same conditions
exhibited. The ancient bed of the Yare and other rivers of this
district enter the sea at a depth of more than 100 feet below the
present sea-level. At Yarmouth the surface was found 170 feet thick,
and the deep surface extends along the Yare to beyond Norwich. Buried
forests are also found here similar to those on the Humber.
It is probable that all our British rivers flow into the sea over their
old buried channels, except in cases where they may have changed their
courses since the beginning of the glacial epoch.
(4.) In the Sanquhar Coal Basin, at the foot of the Kello Water, an
old buried river course was found by Mr. B. N. Peach. It ran at right
angles to the Kello, and was filled with boulder clay which cut off the
coal; but, on driving the mine through the clay, the coal was found in
position on the other side.
(5.) An old river course, under the boulder clay, is described by Mr.
Milne Home in his memoir on the Mid-Lothian coal-fields. It has been
traced out from Niddry away in a N.E. direction by New Craighall. At
Niddry, the hollow is about 100 yards wide and between 60 and 70 feet
deep. It seems to deepen and widen as it approaches towards the sea,
for at New Craighall it is about 200 yards wide and 97 feet deep. This
old channel will probably enter the sea about Musselburgh. Like the
channels in the Midland Valley of Scotland already described, it is so
completely filled up by drift that not a trace of it is to be seen on
the surface. And like these, also, it must have belonged to a period
when the sea-level stood much lower than at present.
(6.) At Hailes’ Quarry, near Edinburgh, there is to be seen a portion
of an ancient watercourse under the boulder drift. A short account
of it was given by Dr. Page in a paper read before the Edinburgh
Geological Society.[290] The superincumbent sandstone, he says, has
been cut to a depth of 60 feet. The width of the channel at the surface
varies from 12 to 14 feet, but gradually narrows to 2 or 3 feet at the
bottom. The sides and bottom are smoothed and polished, and the whole
is now filled with till and boulders.
(7.) One of the most remarkable buried channels is that along the
Valley of Strathmore, supposed to be the ancient bed of the Tay. It
extends from Dunkeld, the south of Blairgowrie, Ruthven, and Forfar,
and enters the German Ocean at Lunan Bay. Its length is about 34 miles.
“No great river,” says Sir Charles Lyell, “follows this course, but
it is marked everywhere by lakes or ponds, which afford shell-marl,
swamps, and peat moss, commonly surrounded by ridges of detritus from
50 to 70 feet high, consisting in the lower part of till and boulders,
and in the upper of stratified gravels, sand, loam, and clay, in some
instances curved or contorted.”[291]
“It evidently marks an ancient line, by which, first, a great glacier
descended from the mountains to the sea, and by which, secondly, at
a later period, the principal water drainage of this country was
effected.”[292]
(8.) A number of examples of ancient river courses, underneath the
boulder clay, are detailed by Professor Geikie in his glacial drift of
Scotland. Some of the cases described by him have acquired additional
interest from the fact of their bearing decided testimony to the
existence of inter-glacial warm periods. I shall briefly refer to a few
of the cases described by him.
In driving a trial mine in a pit at Chapelhall, near Airdrie, the
workmen came upon what they believed to be an old river course. At
the end of the trial mine the ironstone, with its accompanying coal
and fire-clay, were cut off at an angle of about 20° by a stiff,
dark-coloured earth, stuck full of angular pieces of white sandstone,
coal, and shale, with rounded pebbles of greenstone, basalt, quartz,
&c. Above this lay a fine series of sand and clay beds. Above these
stratified beds lay a depth of 50 or 60 feet of true boulder clay. The
channel ran in the direction of north-east and south-west. Mr. Russell,
of Chapelhall, informs Professor Geikie that another of the same kind,
a mile farther to the north-west, had been traced in some of the pit
workings.
“It is clear,” says Professor Geikie, “that whatever may be the true
explanation of these channels and basins, they unquestionably belong to
the period of the boulder clay. The Chapelhall basin lies, indeed, in a
hollow of the carboniferous rocks, but its stratified sands and clays
rest on an irregular floor of true till. The old channel near the banks
of the Calder is likewise scooped out of sandstones and shales; but
it has a coating of boulder clay, on which its finely-laminated sands
and clays repose, _as if the channel itself had once been filled with
boulder clay, which was re-excavated to allow of the deposition of the
stratified deposits. In all cases, a thick mantle of coarse, tumultuous
boulder clay buries the whole._”[293]
Professor Geikie found between the mouth of the Pease Burn and St.
Abb’s Head, Berwickshire, several ancient buried channels. One at
the Menzie Cleuch, near Redheugh Shore, was filled to the brim with
boulder clay. Another, the Lumsden Dean, half a mile to the east of
Fast Castle, on the bank of the Carmichael Burn, near the parish church
of Carmichael,—an old watercourse of the boulder clay period—is to
be seen. The valley of the Mouse Water he instances as a remarkable
example.
One or two he found in Ayrshire, and also one on the banks of the Lyne
Water, a tributary of the Tweed.
(9.) In the valley of the Clyde, above Hamilton, several buried river
channels have been observed. They are thus described by Mr. James
Geikie:—[294]
“In the Wishaw district, two deep, winding troughs, filled with sand
and fine gravel, have been traced over a considerable area in the coal
workings.[295] These troughs form no feature at the surface, but are
entirely concealed below a thick covering of boulder clay. They appear
to be old stream courses, and are in all probability the pre-glacial
ravines of the Calder Water and the Tillon Burn. The ‘sand-dyke’ that
represents the pre-glacial course of the Calder Water runs for some
distance parallel to the present course of the stream down to Wishaw
House, where it is intersected by the Calder, and the deposits which
choke it up are well seen in the steep wooded banks below the house
and in the cliff on the opposite side. It next strikes to south-east,
and is again well exposed on the road-side leading down from Wishaw
to the Calder Water. From this point it has been traced underground,
more or less continuously, as far as Wishaw Ironworks. Beyond this
place the coal-seams sink to a greater depth, and therefore cease to
be intersected by the ancient ravine, the course of which, however,
may still be inferred from the evidence obtained during the sinking
of shafts and trial borings. In all probability it runs south, and
enters the old course of the Clyde a little below Cambusnethan House.
Only a portion of the old ravine of the Tillon Burn is shown upon the
Map. It is first met with in the coal-workings of Cleland Townhead
(Sheet 31). From this place it winds underground in a southerly
direction until it is intersected by the present Tillon Burn, a little
north of Glencleland (Sheet 31). It now runs to south-west, keeping
parallel to the burn, and crosses the valley of the Calder just
immediately above the mouth of the Tillon. From this point it can be
traced in pit-shafts, open-air sections, borings, and coal-workings,
by Ravenscraig, Nether Johnstone, and Robberhall Belting, on to the
Calder Water below Coursington Bridge (Sheet 31). It would thus appear
that in pre-glacial times the Calder and the Tillon were independent
streams, and that since glacial times the Calder Water, forsaking its
pre-glacial course, has cut its way across the intervening ground,
ploughing out deep ravines in the solid rocks, until eventually it
united with the Tillon. Similar buried stream courses occur at other
places. Thus, at Fairholme, near Larkhall, as already mentioned (par.
94), the pre-glacial course of the Avon has been traced in pit-shafts
and borings for some distance to the north. Another old course, filled
up with boulder clay, is exposed in a burn near Plotcock, a mile
south-west from Millheugh; and a similar pre-glacial ravine was met
with in the cement-stone workings at Calderwood.[296] Indeed, it might
be said with truth that nearly all the rocky ravines through which the
waters flow, especially in the carboniferous areas, are of post-glacial
age—the pre-glacial courses lying concealed under masses of drift. Most
frequently, however, the present courses of the streams are partly
pre-glacial and partly post-glacial. In the pre-glacial portions the
streams flow through boulder clay, in the post-glacial reaches their
course, as just mentioned, is usually in rocky ravines. The Avon and
the Calder, with their tributaries, afford numerous illustrations of
these phenomena.”
The question naturally arises, When were those channels scooped out?
To what geological period must those ancient rivers be referred? It
will not do to conclude that those channels must be pre-glacial simply
because they contain boulder clay. Had the glacial epoch been one
unbroken period of cold, and the boulder clay one continuous formation,
then the fact of finding boulder clay in those channels would show that
they were pre-glacial. But when we find undoubted geological evidence
of a warm condition of climate of long continuance, during the severest
part of the glacial epoch, when the ice, to a great extent, must have
disappeared, and water began to flow as usual down our valleys, all
that can reasonably be inferred from the fact of finding till in those
channels, is that they must be older than the till they contain. We
cannot infer that they are older than all the till lying on the face
of the country. The probability, however, is, that some of them are
of pre-glacial and others of inter-glacial origin. That many of these
channels have been used as watercourses during the glacial epoch, or
rather during warm periods of that epoch, is certain, from the fact
that they have been filled with boulder clay, then re-excavated, and
finally filled up again with the clay.
CHAPTER XXX.
THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THEORIES OF
GLACIER-MOTION.
Why the Question of Glacier-motion has been found to be so
difficult.—The Regelation Theory.—It accounts for the
Continuity of a Glacier, but not for its Motion.—Gravitation
proved by Canon Moseley insufficient to shear the Ice
of a Glacier.—Mr. Mathew’s Experiment.—No Parallel
between the bending of an Ice Plank and the shearing
of a Glacier.—Mr. Ball’s Objection to Canon Moseley’s
Experiment.—Canon Moseley’s Method of determining the Unit
of Shear.—Defect of Method.—Motion of a Glacier in some
Way dependent on Heat.—Canon Moseley’s Theory.—Objections
to his Theory.—Professor James Thomson’s Theory.—This
Theory fails to explain Glacier-motion.—De Saussure and
Hopkins’s “Sliding” Theories.—M. Charpentier’s “Dilatation”
Theory.—Important Element in the Theory.
The cause of the motion of glaciers has proved to be one of the most
difficult and perplexing questions within the whole domain of physics.
The main difficulty lies in reconciling the motion of the glacier with
the physical properties of the ice. A glacier moves down a valley
very much in the same way as a river, the motion being least at the
sides and greatest at the centre, and greater at the surface than at
the bottom. In a cross section scarcely two particles will be moving
with the same velocity. Again, a glacier accommodates itself to the
inequalities of the channel in which it moves exactly as a semifluid
or plastic substance would do. So thoroughly does a glacier behave
in the manner of a viscous or plastic body that Professor Forbes was
induced to believe that viscosity was a property of the ice, and that
in virtue of this property it was enabled to move with a differential
motion and accommodate itself to all the inequalities of its channel
without losing its continuity just as a mass of mud or putty would do.
But experience proves that ice is a hard and brittle substance far
more resembling glass than putty. In fact it is one of the most brittle
and unyielding substances in nature. So unyielding is a glacier that
it will snap in two before it will stretch to any perceptible extent.
This is proved by the fact that crevasses resulting from a strain on
the glacier consist at first of a simple crack scarcely wide enough to
admit the blade of a penknife.
All the effects which were considered to be due to the viscosity of
the ice have been fully explained and accounted for on the principle
of fracture and regelation discovered by Faraday. The principle of
regelation explains why the ice moving with a differential motion and
accommodating itself to the inequalities of its channel is yet enabled
to retain its continuity, but it does not account for the _cause_ of
glacier motion. In fact it rather involves the question in deeper
mystery than before. For it is far more difficult to conceive how the
particles of a hard and brittle solid like that of ice can move with
a differential motion, than it is to conceive how this may take place
in the case of a soft and yielding substance. The particles of ice
have all to be displaced one over another and alongside each other,
and as those particles are rigidly fixed together this connection must
be broken before the one can slide over the other. _Shearing-force_,
as Canon Moseley shows, comes into play. Were ice a plastic substance
there would not be much difficulty in understanding how the particles
should move the one over the other, but it is totally different when
we conceive ice to be a solid and unyielding substance. The difficulty
in connection with glacier-motion is not to account for the continuity
of the ice, for the principle of regelation fully explains this, but
to show how it is that one particle succeeds in sliding over the over.
The principle of regelation, instead of assisting to remove this
difficulty, increases it tenfold. Regelation does not explain the cause
of glacier-motion, but the reverse. It rather tends to show that a
glacier should not move. What, then, is the cause of glacier-motion?
According to the regelation theory, gravitation is the impelling
cause. But is gravitation sufficient to _shear_ the ice in the manner
in which it is actually done in a glacier?
I presume that few who have given much thought to the subject of
glacier-motion have not had some slight misgivings in regard to the
commonly received theory. There are some facts which I never could
harmonize with this theory. For example, boulder clay is a far looser
substance than ice; its shearing-force must be very much less than
that of ice; yet immense masses of boulder clay will lie immovable for
ages on the slope of a hill so steep that one can hardly venture to
climb it, while a glacier will come crawling down a valley which by
the eye we could hardly detect to be actually off the level. Again, a
glacier moves faster during the day than during the night, and about
twice as fast during summer as during winter. Professor Forbes, for
example, found that the Glacier des Bois near its lower extremity moved
sometimes in December only 11·5 inches daily, while during the month
of July its rate of motion sometimes reached 52·1 inches per day. Why
such a difference in the rate of motion between day and night, summer
and winter? The glacier is not heavier during the day than it is
during the night, or during the summer than it is during the winter;
neither is the shearing-force of the great mass of the ice of a glacier
sensibly less during day than night, or during summer than winter;
for the temperature of the great mass of the ice does not sensibly
vary with the seasons. If this be the case, then gravitation ought to
be as able to shear the ice during the night as during the day, or
during the winter as during the summer. At any rate, if there should
be any difference it ought to be but trifling. It is true that, owing
to the melting of the ice, the crevices of the glacier are more gorged
with water during summer than winter; and this, as Professor Forbes
maintains,[297] may tend to make the glacier move faster during the
former than the latter season. But the advocates of the regelation
theory cannot conclude, with Professor Forbes, that the water favours
the motion of the glacier by making the ice more soft and plastic. The
melting of the ice, according to the regelation theory, cannot very
materially aid the motion of the glacier.
The theory which has led to the general belief that the ice of a
glacier is sheared by the force of gravity appears to be the following.
It is supposed that the only forces to which the motion of a glacier
can be referred are _gravitation_ and _heat_; but as the great mass
of a glacier remains constantly at the same uniform temperature it
is concluded to be impossible that the motion of the glacier can be
due to this cause, and therefore of course it must be attributed to
gravitation, there being no other cause.
That gravitation is insufficient to shear the ice of a glacier has been
clearly demonstrated by Canon Moseley.[298] He determined by experiment
the amount of force required to shear one square inch of ice, and found
it to be about 75 lbs. By a process of calculation which will be found
detailed in the Memoir referred to, he demonstrated that to descend
by its own weight at the rate at which Professor Tyndall observed the
ice of the Mer de Glace to be descending at the Tacul, the unit of
shearing force of the ice could not have been more than 1·31931 lbs.
Consequently it will require a force more than 34 times the weight of
the glacier to shear the ice and cause it to descend in the manner in
which it is found to descend.
It is now six years since Canon Moseley’s results were laid before the
public, and no one, as far as I am aware, has yet attempted to point
out any serious defect in his mathematical treatment of the question.
Seeing the great amount of interest manifested in the question of
glacier-motion, I think we are warranted to conclude that had the
mathematical part of the memoir been inconclusive its defects would
have been pointed out ere this time. The question, then, hinges on
whether the experimental data on which his calculations are based
be correct. Or, in other words, is the unit of shear of ice as much
as 75 lbs.? This part of Mr. Moseley’s researches has not passed
unquestioned. Mr. Ball and Mr. Mathews, both of whom have had much
experience among glaciers, and have bestowed considerable attention on
the subject of glacier-motion, have objected to the accuracy of Mr.
Moseley’s unit of shear. I have carefully read the interesting memoirs
of Mr. Mathews and Mr. Ball in reply to Canon Moseley, but I am unable
to perceive that anything which they have advanced materially affects
his general conclusions as regards the commonly received theory. Mr.
Mathews objects to Canon Moseley’s experiments on the grounds that
extraneous forces are brought to bear upon the substance submitted
to operation, and that conditions are thus introduced which do not
obtain in the case of an actual glacier. “It would throw,” he says,
“great light upon our inquiry if we were to change this method of
procedure and simply to observe the deportment of masses of ice under
the influence of no external forces but the gravitation of their own
particles.”[299] A plank of ice six inches wide and 2⅜ inches in
thickness was supported at each end by bearers six feet apart. From the
moment the plank was placed in position it began to sink, and continued
to do so until it touched the surface over which it was supported. Mr.
Mathews remarks that with this property of ice, viz., its power to
change its form under strains produced by its own gravitation, combined
with the sliding movement demonstrated by Hopkins, we have an adequate
cause for glacier-motion. Mr. Mathews concludes from this experiment
that the unit of shear in ice, instead of being 75 lbs., is less than
1¾ lbs.
There is, however, no parallel between the bending of the ice-plank and
the shearing of a glacier. Mr. Mathews’ experiment appears to prove too
much, as will be seen from the following reply of Canon Moseley:—
“Now I will,” he says, “suggest to Mr. Mathews a parallel experiment
and a parallel explanation. If a bar of wrought iron 1 inch square and
20 feet long were supported at its extremities, it would _bend_ by its
weight alone, and would therefore shear. Now the weight of such a
rod would be about 67 lbs. According to Mr. Mathews’s explanation in
the case of the ice-plank, the unit of shear in wrought-iron should
therefore be 67 lbs. per square inch. It is actually 50,000 lbs.”[300]
Whatever theory we may adopt as to the cause of the motion of glaciers,
the deflection of the plank in the way described by Mr. Mathews
_follows as a necessary consequence_. Although no weight was placed
upon the plank, it does not necessarily follow that the deflection
was caused by the weight of the ice alone; for, according to Canon
Moseley’s own theory of the motion of glaciers by heat, the plank
ought to be deflected in the middle, just as it was in Mr. Mathews’s
experiment. A solid body, when exposed to variations of temperature,
will expand and contract transversely as well as longitudinally. Ice,
according to Canon Moseley’s theory, expands and contracts by heat.
Then if the plank expands transversely, the upper half of the plank
must rise and the lower half descend. But the side which rises has
to perform work against gravity, whereas the side which descends has
work performed upon it by gravity; consequently more of the plank will
descend than rise, and this will, of course, tend to lower or deflect
the plank in the middle. Again, when the plank contracts, the lower
half will rise and the upper half will descend; but as gravitation,
in this case also, favours the descending part and opposes the rising
part, more of the plank will descend than rise, and consequently
the plank will be lowered in the middle by contraction as well as
by expansion. Thus, as the plank changes its temperature, it must,
according to Mr. Moseley’s theory, descend or be deflected in the
middle, step by step—and this not by gravitation alone, but chiefly
by the motive power of heat. I do not, of course, mean to assert that
the descent of the plank was caused by heat; but I assert that Mr.
Mathews’s experiment does not necessarily prove (and this is all that
is required in the meantime) that gravitation alone was the cause of
the deflection of the plank. Neither does this experiment prove that
the ice was deflected without shearing; for although the weight of the
plank was not sufficient to shear the ice, as Mr. Mathews, I presume,
admits, yet Mr. Moseley would reply that the weight of the ice,
assisted by the motive power of heat, was perfectly sufficient.
I shall now briefly refer to Mr. Ball’s principal objections to Canon
Moseley’s proof that a glacier cannot shear by its weight alone. One
of his chief objections is that Mr. Moseley has assumed the ice to be
homogeneous in structure, and that pressures and tensions acting within
it, are not modified by the varying constitution of the mass.[301]
Although there is, no doubt, some force in this objection (for we have
probably good reason to believe that ice will shear, for example, more
easily along certain planes than others), still I can hardly think that
Canon Moseley’s main conclusion can ever be materially affected by this
objection. The main question is this, Can the ice of the glacier shear
by its own weight in the way generally supposed? Now the shearing force
of ice, take it in whatever direction we may, so enormously exceeds
that required by Mr. Moseley in order to allow a glacier to descend by
its weight only, that it is a matter of indifference whether ice be
regarded as homogeneous in structure or not. Mr. Ball objects also to
Mr. Moseley’s imaginary glacier lying on an even slope and in a uniform
rectangular channel. He thinks that an irregular channel and a variable
slope would be more favourable to the descent of the ice. But surely
if the work by the weight of the ice be not equal to the work by the
resistance in a glacier of uniform breadth and slope, it must be much
less so in the case of one of irregular shape and slope.
That a relative displacement of the particles of the ice is involved
in the motion of a glacier, is admitted, of course, by Mr. Ball; but
he states that the amount of this displacement is but small, and that
it is effected with extreme slowness. This may be the case; but if the
weight of the ice be not able to overcome the mutual cohesion of the
particles, then the weight of the ice cannot produce the required
displacement, however small it may be. Mr. Ball then objects to Mr.
Moseley’s method of determining the unit of shear on this ground:—The
shearing of the ice in a glacier is effected with extreme slowness;
but the shearing in Canon Moseley’s experiment was effected with
rapidity; and although it required 75 lbs. to shear one square inch of
surface in his experiment, it does not follow that 75 lbs. would be
required to shear the ice if done in the slow manner in which it is
effected in the glacier. “In short,” says Mr. Ball, “to ascertain the
resistance opposed to very slow changes in the relative positions of
the particles, so slight as to be insensible at short distances, Mr.
Moseley measures the resistance opposed to rapid disruption between
contiguous portions of the same substance.”
There is force in this objection; and here we arrive at a really weak
point in Canon Moseley’s reasoning. His experiments show that if we
want to shear ice quickly a weight of nearly 120 lbs. is required; but
if the thing is to be done more slowly, 75 lbs. will suffice.[302] In
short, the number of pounds required to shear the ice depends, to a
large extent, on the length of time that the weight is allowed to act;
the longer it is allowed to act, the less will be the weight required
to perform the work. “I am curious to know,” says Mr. Mathews, when
referring to this point, “what weight would have sheared the ice
if a _day_ had been allowed for its operation.” I do not know what
would have been the weight required to shear the ice in Mr. Moseley’s
experiments had a day been allowed; but I feel pretty confident that,
should the ice remain unmelted, and sufficient time be allowed,
shearing would be produced without the application of any weight
whatever. There are no weights placed upon a glacier to make it move,
and yet the ice of the glacier shears. If the shearing is effected by
weight, the only weight applied is the weight of the ice; and if the
weight of the ice makes the ice shear in the glacier, why may it not
do the same thing in the experiment? Whatever may be the cause which
displaces the particles of the ice in a glacier, they, as a matter of
fact, are displaced without any weight being applied beyond that of
the ice itself; and if so, why may not the particles of the ice in
the experiment be also displaced without the application of weights?
Allow the ice of the glacier to take its own time and its own way, and
the particles will move over each other without the aid of external
weights, whatever may be the cause of this; well, then, allow the ice
in the experiment to take its own time and its own way, and it will
probably do the same thing. There is something here unsatisfactory.
If, by the unit of shear, be meant the pressure in pounds that must
be applied to the ice to break the connection of one square inch of
two surfaces frozen together and cause the one to slip over the other,
then the amount of pressure required to do this will depend upon the
time you allow for the thing being done. If the thing is to be done
rapidly, as in some of Mr. Moseley’s experiments, it will take, as he
has shown, a pressure of about 120 lbs.; but if the thing has to be
done more slowly, as in some other of his experiments, 75 lbs. will
suffice. And if sufficient time be allowed, as in the case of glaciers,
the thing may be done without any weight whatever being applied to the
ice, and, of course, Mr. Moseley’s argument, that a glacier cannot
descend by its weight alone, falls to the ground. But if, by the unit
of shear, be meant not the _weight_ or _pressure_ necessary to shear
the ice, but the amount of _work_ required to shear a square inch of
surface _in a given time or at a given rate_, then he might be able
to show that in the case of a glacier (say the Mer de Glace) the work
of all the resistances which are opposed to its descent at the _rate_
at which it is descending is greater than the work of its weight, and
that consequently there must be some cause, in addition to the weight,
urging the glacier forward. But then he would have no right to affirm
that the glacier would not descend by its weight only; all that he
could affirm would simply be that it could not descend by its weight
alone at the _rate_ at which it is descending.
Mr. Moseley’s unit of shear, however, is not the amount of work
performed in shearing a square inch of ice in a given time, but the
amount of _weight_ or _pressure_ requiring to be applied to the ice
to shear a square inch. But this amount of pressure depends upon the
length of time that the pressure is applied. Here lies the difficulty
in determining what amount of pressure is to be taken as the real unit.
And here also lies the radical defect in Canon Moseley’s result. Time
as well as pressure enters as an element into the process. The key to
the explanation of this curious circumstance will, I think, be found in
the fact that the rate at which a glacier descends depends in some way
or other upon the amount of heat that the ice is receiving. This fact
shows that heat has something to do in the shearing of the ice of the
glacier. But in the communication of heat to the ice _time_ necessarily
enters as an element. There are two different ways in which heat may be
conceived to aid in shearing the ice: (1.) we may conceive that heat
acts as a force along with gravitation in producing displacement of the
particles of the ice; or (2.) we may conceive that heat does not act as
a force in pushing the particles over each other, but that it assists
the shearing processes by diminishing the cohesion of the particles of
the ice, and thus allowing gravitation to produce displacement. The
former is the function attributed to heat in Canon Moseley’s theory
of glacier-motion; the latter is the function attributed to heat in
the theory of glacier-motion which I ventured to advance some time
ago.[303] It results, therefore, from Canon Moseley’s own theory, that
the longer the time that is allowed for the pressure to shear the
ice, the less will be the pressure required; for, according to his
theory, a very large proportion of the displacement is produced by the
motive power of heat entering the ice; and, as it follows of course,
other things being equal, the longer the time during which the heat
is allowed to act, the greater will be the proportionate amount of
displacement produced by the heat; consequently the less will require
to be done by the weight applied. In the case of the glacier, Mr.
Moseley concludes that at least thirty or forty times as much work is
done by the motive power of heat in the way of shearing the ice as is
done by mere pressure or weight. Then, if sufficient time be allowed,
why may not far more be done by heat in shearing the ice in his
experiment than by the weight applied? In this case how is he to know
how much of the shearing is effected by the heat and how much by the
weight? If the greater part of the shearing of the ice in the case of a
glacier is produced, not by pressure, but by the heat which necessarily
enters the ice, it would be inconceivable that in his experiments the
heat entering the ice should not produce, at least to some extent, a
similar effect. And if a portion of the displacement of the particles
is produced by heat, then the weight which is applied cannot be
regarded as the measure of the force employed in the displacement, any
more than it could be inferred that the weight of the glacier is the
measure of the force employed in the shearing of it. If the weight
is not the entire force employed in shearing, but only a part of the
force, then the weight cannot, as in Mr. Moseley’s experiment, be taken
as the measure of the force.
How, then, are we to determine what is the amount of force required to
shear ice? in other words, how is the unit of shear to be determined?
If we are to measure the unit of shear by the weight required to
produce displacement of the particles of the ice, we must make sure
that the displacement is wholly effected by the weight. We must be
certain that heat does not enter as an element in the process. But
if time be allowed to elapse during the experiment, we can never
be certain that heat has not been at work. It is impossible to
prevent heat entering the ice. We may keep the ice at a constant
temperature, but this would not prevent heat from entering the ice and
producing molecular work. True that, according to Moseley’s theory
of glacier-motion, if the temperature of the ice be not permitted
to _vary_, then no displacement of the particles can take place
from the influence of heat; but according to the molecular theory of
glacier-motion, which will shortly be considered, heat will aid the
displacement of the particles whether the temperature be kept constant
or not. In short, it is absolutely impossible in our experiments to
be certain that heat is not in some way or other concerned in the
displacement of the particles of the ice. But we can shorten the time,
and thus make sure that the amount of heat entering the ice during the
experiments is too small to affect materially the result. We cannot in
this case say that all the displacement has been effected by the weight
applied to the ice, but we can say that so little has been done by heat
that, practically, we may regard it as all done by the weight.
This consideration, I trust, shows that the unit of shear adopted by
Canon Moseley in his calculations is not too large. For if in half an
hour, after all the work that may have been done by heat, a pressure of
75 lbs. is still required to displace the particles of one square inch,
it is perfectly evident that if no work had been done by heat during
that time, the force required to produce the displacement could not
have been less than 75 lbs. It might have been more than that; but it
could not have been less. Be this, however, as it may, in determining
the unit of shear we cannot be permitted to prolong the experiment for
any considerable length of time, because the weight under which the
ice might then shear could not be taken as the measure of the force
which is required to shear ice. By prolonging the experiment we might
possibly get a unit smaller than that required by Canon Moseley for
a glacier to descend by its own weight. But it would be just as much
begging the whole question at issue to assume that, because the ice
sheared under such a weight, a glacier might descend by its weight
alone, as it would be to assume that, because a glacier shears without
a weight being placed upon it, the glacier descends by its weight alone.
But why not determine the unit of shear of ice in the same way as we
would the unit of shear of any other solid substance, such, as iron,
stone, or wood? If the shearing force of ice be determined in this
manner, it will be found to be by far too great to allow of the ice
shearing by its weight alone. We shall be obliged to admit either
that the ice of the glacier does not shear (in the ordinary sense of
the term), or if it does shear, that there must, as Canon Moseley
concludes, be some other force in addition to the weight of the ice
urging the glacier forward.
The fact that the rate of descent of a glacier depends upon the amount
of heat which it receives, proves that heat must be regarded either as
a cause or as a necessary condition of its motion; what, then, is the
necessary relationship between heat and the motion of the glacier? If
heat is to be regarded as a cause, in what way does the heat produce
motion? I shall now briefly refer to one or two theories which have
been advanced on the subject. Let us consider first that of Canon
Moseley.
_Canon Moseley’s Theory._—He found, from observations and experiments,
that sheets of lead, placed upon an inclined plane, when subjected to
variations of temperature, tend to descend even when the slope is far
less than that which would enable it to slide down under the influence
of gravitation. The cause of the descent he shows to be this. When the
temperature of the sheet is raised, it expands, and, in expanding, its
upper portion moves up the slope, and its lower portion down the slope;
but as gravitation opposes the upward and favours the downward motion,
more of the sheet moves down than up, and consequently the centre
of gravity of the sheet is slightly lowered. Again, when the sheet
is cooled, it contracts, and in contracting the upper portion moves
downwards and the lower portion upwards, and here again, for the same
reason, more of the sheet moves downwards than upwards. Consequently,
at every change of temperature there is a slight displacement of the
sheet downwards. “Now a theory of the descent of glaciers,” says
Canon Moseley, “which I have ventured to propose myself, is that they
descend, as the lead in this experiment does, by reason of the passage
into them and the withdrawal of the sun’s rays, and that the dilatation
and contraction of the ice so produced is the proximate cause of their
descent, as it is of that of the lead.”[304]
The fundamental condition in Mr. Moseley’s theory of the descent of
solid bodies on an incline, is, not that heat should maintain these
bodies at a high temperature, but that the temperature should vary.
The rate of descent is proportionate, not simply to the amount of
heat received, but to the extent and frequency of the variations of
temperature. As a proof that glaciers are subjected to great variations
of temperature, he adduces the following:—“All alpine travellers,” he
says, “from De Saussure to Forbes and Tyndall, have borne testimony
to the intensity of the solar radiation on the surfaces of glaciers.
‘I scarcely ever,’ says Forbes, ‘remember to have found the sun more
piercing than at the Jardin.’ This heat passes abruptly into a state
of intense cold when any part of the glacier falls into shadow by an
alteration of the position of the sun, or even by the passing over it
of a cloud.”[305]
Mr. Moseley is here narrating simply what the traveller feels, and
not what the glacier experiences. The traveller is subjected to great
variations of temperature; but there is no proof from this that the
glacier experiences any changes of temperature. It is rather because
the temperature of the glacier is not affected by the sun’s heat that
the traveller is so much chilled when the sun’s rays are cut off. The
sun shines down with piercing rays and the traveller is scorched; the
glacier melts on the surface, but it still remains “cold as ice.” The
sun passes behind a cloud or disappears behind a neighbouring hill; the
scorching rays are then withdrawn, and the traveller is now subjected
to radiation on every side from surfaces at the freezing-point.
It is also a necessary condition in Mr. Moseley’s theory that the heat
should pass easily into and out of the glacier; for unless this were
the case sudden changes of temperature could produce little or no
effect on the great mass of the glacier. How, then, is it possible that
during the heat of summer the temperature of the glacier could vary
much? During that season, in the lower valleys at least, everything,
with the exception of the glacier, is above the freezing-point;
consequently when the glacier goes into the shade there is nothing
to lower the ice below the freezing-point; and as the sun’s rays do
not raise the temperature of the ice above the freezing-point, the
temperature of the glacier must therefore remain unaltered during that
season. It therefore follows that, instead of a glacier moving more
rapidly during the middle of summer than during the middle of winter,
it should, according to Moseley’s theory, have no motion whatever
during summer.
The following, written fifteen years ago by Professor Forbes on this
very point, is most conclusive:—“But how stands the fact? Mr. Moseley
quotes from De Saussure the following _daily ranges_ of the temperature
of the air in the month of July at the Col du Géant and at Chamouni,
between which points the glacier lies:
°
At the Col du Géant 4·257 Réaumur.
At Chamouni 10·092 〃
And he assumes ‘the same mean daily variation of temperature to obtain
throughout the length’ [and depth?] ‘of the Glacier du Géant which De
Saussure observed in July at the Col du Géant.’ But between what limits
does the temperature of the air oscillate? We find, by referring to
the third volume of De Saussure’s ‘Travels,’ that the mean temperature
of the coldest hour (4 A.M.) during his stay at the Col du Géant was
33°·03 Fahrenheit, and of the warmest (2 P.M.) 42°·61 F. So that even
upon that exposed ridge, between 2,000 and 3,000 feet above where the
glacier can be properly said to commence, the air does not, on an
average of the month of July, reach the freezing-point at any hour
of the night. Consequently the _range of temperature attributed to
the glacier is between limits absolutely incapable of effecting the
expansion of the ice in the smallest degree_.”[306]
Again, during winter, as Mr. Ball remarks, the glacier is completely
covered with snow and thus protected both from the influence of
cold and of heat, so that there can be nothing either to raise the
temperature of the ice above the freezing-point or to bring it below
that point; and consequently the glacier ought to remain immovable
during that season also.
“There can be no doubt, therefore,” Mr. Moseley states, “that the
rays of the sun, which in those alpine regions are of such remarkable
intensity, find their way into the depths of the glacier. They are
a _power_, and there is no such thing as the loss of power. The
mechanical work which is their equivalent, and into which they are
converted when received into the substance of a solid body, accumulates
and stores itself up in the ice under the form of what we call
elastic force or tendency to dilate, until it becomes sufficient to
produce actual dilatation of the ice in the direction in which the
resistance is weakest, and by its withdrawal to produce contraction.
From this expansion and contraction follows of necessity the descent
of the glacier.”[307] When the temperature of the ice is below
the freezing-point, the rays which are absorbed will, no doubt,
produce dilatation; but during summer, when the ice is not below the
freezing-point, no dilatation can possibly take place. All physicists,
so far as I am aware, agree that the rays that are then absorbed go to
melt the ice, and not to expand it. But to this Mr. Moseley replied
as follows:—“To this there is the obvious answer that radiant heat
does find its way into ice as a matter of common observation, and
that it does not melt it except at its surface. Blocks of ice may be
seen in the windows of ice-shops with the sun shining full upon them,
and melting nowhere but on their surfaces. And the experiment of the
ice-lens shows that heat may stream through ice in abundance (of which
a portion is necessarily stopped in the passage) without melting it,
except on its surface.” But what evidence is there to conclude that
if there is no melting of the ice in the interior of the lens there
is a portion of the rays “necessarily stopped” in the interior? It
will not do to assume a point so much opposed to all that we know of
the physical properties of ice as this really is. It is absolutely
essential to Mr. Moseley’s theory of the motion of glaciers, during
summer at least, that ice should continue to expand after it reaches
the melting-point; and it has therefore to be shown that such is the
case; or it need not be wondered at that we cannot accept his theory,
because it demands the adoption of a conclusion contrary to all our
previous conceptions. But, as a matter of fact, it is not strictly true
that when rays pass through a piece of ice there is no melting of the
ice in the interior. Experiments made by Professor Tyndall show the
contrary.[308]
There is, however, one fortunate circumstance connected with Canon
Moseley’s theory. It is this: its truth can be easily tested by direct
experiment. The ice, according to this theory, descends not simply
in virtue of heat, but in virtue of _change of temperature_. Try,
then, Hopkins’s famous experiment, but keep the ice at a _constant
temperature_; then, according to Moseley’s theory, the ice will not
descend. Let it be observed, however, that although the ice under this
condition should descend (as there is little doubt but it would),
it would show that Mr. Moseley’s theory of the descent of glaciers
is incorrect, still it would not in the least degree affect the
conclusions which he lately arrived at in regard to the generally
received theory of glacier-motion. It would not prove that the ice
sheared, in the way generally supposed, by its weight only. It might be
the heat, after all, entering the ice, which accounted for its descent,
although gravitation (the weight of the ice) might be the impelling
cause.
According to this theory, the glacier, like the sheet of lead, must
expand and contract as one entire mass, and it is difficult to
conceive how this could account for the differential motion of the
particles of the ice.
_Professor James Thomson’s Theory._—It was discovered by this physicist
that the freezing-point of water is lowered by pressure. The extent
of the lowering is equal to ·0075° centigrade for every atmosphere
of pressure. As glacier ice is generally about the melting-point,
it follows that when enormous pressure is brought to bear upon any
given point of a glacier a melting of the ice at that particular spot
will take place in consequence of the lowering of the melting-point.
The melting of the ice will, of course, tend to favour the descent
of the glacier, but I can hardly think the liquefaction produced by
pressure can account for the motion of glaciers. It will help to
explain the giving way of the ice at particular points subjected to
great pressure, but I am unable to comprehend how it can account for
the general descent of the glacier. Conceive a rectangular glacier of
uniform breadth and thickness, and lying upon an even slope. In such a
glacier the pressure at each particular point would remain constant,
for there would be no reason why it should be greater at one time than
at another. Suppose the glacier to be 500 feet in thickness; the ice
at the lower surface of the glacier, owing to pressure, would have its
melting-point permanently lowered one-tenth of a degree centigrade
below that of the upper surface; but the ice at the lower surface would
not, on this account, be in the fluid state. It would simply be ice at
a slightly lower temperature. True, when pressure is exerted the ice
melts in consequence of the lowering of the melting-point, but in the
case under consideration there would, properly speaking, be no exertion
of pressure, but a constant statical pressure resulting from the weight
of the ice. But this statical condition of pressure would not produce
fluidity any more than a statical condition of pressure would produce
heat, and consequently motion could not take place as a result of
fluidity. In short, motion itself is required to produce the fluidity.
I need not here wait to consider the sliding theories of De Saussure
and Hopkins, as they are now almost universally admitted to be
inadequate to explain the phenomena of glacier-motion, seeing that they
do not account for the displacement of the particles of the ice over
one another.
According to the dilatation theory of M. Charpentier, a glacier is
impelled by the force exerted by water freezing in the fissures of the
ice. A glacier he considers is full of fissures into which water is
being constantly infiltrated, and when the temperature of the air sinks
below the freezing-point it converts the water into ice. The water, in
passing into ice, expands, and in expanding tends to impel the glacier
in the direction of least resistance. This theory, although it does not
explain glacier-motion, as has been clearly shown by Professor J. D.
Forbes, nevertheless contains one important element which, as we shall
see, must enter into the true explanation. The element to which I refer
is the expansive force exerted on the glacier by water freezing.
CHAPTER XXXI.
THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE
MOLECULAR THEORY.
Present State of the Question.—Heat necessary to the Motion of
a Glacier.—Ice does not shear in the Solid State.—Motion
of a Glacier _molecular_.—How Heat is transmitted through
Ice.—Momentary Loss of Shearing Force.—The _Rationale_
of Regelation.—The Origin of “Crevasses.”—Effects of
Tension.—Modification of Theory.—Fluid Molecules crystallize
in Interstices.—Expansive Force of crystallizing Molecules
a Cause of Motion.—Internal molecular Pressure the chief
Moving Power.—How Ice can excavate a Rock Basin.—How Ice can
ascend a Slope.—How deep River Valleys are striated across.—A
remarkable Example in the Valley of the Tay.—How Boulders can
be carried from a lower to a higher Level.
The condition which the perplexing question of the cause of the descent
of glaciers has now reached seems to be something like the following.
The ice of a glacier is not in a soft and plastic state, but is solid,
hard, brittle, and unyielding. It nevertheless behaves in some respects
in a manner very like what a soft and plastic substance would do if
placed in similar circumstances, inasmuch as it accommodates itself
to all the inequalities of the channel in which it moves. The ice of
the glacier, though hard and solid, moves with a differential motion;
the particles of the ice are displaced over each other, or, in other
words, the ice shears as it descends. It had been concluded that the
mere weight of the glacier is sufficient to shear the ice. Canon
Moseley has investigated this point, and shown that it is not. He has
found that for a glacier to shear in the way that it is supposed to
do, it would require a force some thirty or forty times as great as
the weight of the glacier. Consequently, for the glacier to descend,
a force in addition to that of gravitation is required. What, then,
is this force? It is found that the rate at which the glacier descends
depends upon the amount of heat which it is receiving. This shows that
the motion of the glacier is in some way or other dependent upon heat.
Is heat, then, the force we are in search of? The answer to this, of
course, is, since heat is a force necessarily required, we have no
right to assume any other till we see whether or not heat will suffice.
In what way, then, does heat aid gravitation in the descent of the
glacier? In what way does heat assist gravitation in the shearing of
the ice? There are two ways whereby we may conceive the thing to be
done: the heat may assist gravitation to shear, by pressing the ice
forward, or it may assist gravitation by diminishing the cohesion of
the particles, and thus allow gravitation to produce motion which it
otherwise could not produce. Every attempt which has yet been made
to explain how heat can act as a force in pushing the ice forward,
has failed. The fact that heat cannot expand the ice of the glacier
may be regarded as a sufficient proof that it does not act as a force
impelling the glacier forward; and we are thus obliged to turn our
attention to the other conception, viz., that heat assists gravitation
to shear the ice, not by direct pressure, but by diminishing the
cohesive force of the particles, so as to enable gravitation to push
the one past the other. But how is this done? Does heat diminish the
cohesion by acting as an expansive force in separating the particles?
Heat cannot do this, because it cannot expand the ice of a glacier;
and besides, were it to do this, it would destroy the solid and firm
character of the ice, and the ice of the glacier would not then, as
a mass, possess the great amount of shearing-force which observation
and experiment show that it does. In short it is because the particles
are so firmly fixed together at the time the glacier is descending,
that we are obliged to call in the aid of some other force in addition
to the weight of the glacier to shear the ice. Heat does not cause
displacement of the particles by making the ice soft and plastic; for
we know that the ice of the glacier is not soft and plastic, but
hard and brittle. The shearing-force of the ice of the moving glacier
is found to be by at least from thirty to forty times too great to
permit of the ice being sheared by the mere force of gravitation;
how, then, is it that gravitation, without the direct assistance of
any other force, can manage to shear the ice? Or to put the question
under another form: heat does not reduce the shearing-force of the ice
of a glacier to something like 1·3193 lb. per square inch of surface,
the unit required by Mr. Moseley to enable a glacier to shear by
its weight; the shearing-force of the ice, notwithstanding all the
heat received, still remains at about 75 lbs.; how, then, can the
glacier shear without any other force than its own weight pushing it
forward? _This is the fundamental question; and the true answer to it
must reveal the mystery of glacier-motion._ We are compelled in the
present state of the problem to admit that glaciers do descend with
a differential motion without any other force than their own weight
pushing them forward; and yet the shearing-force of the ice is actually
found to be thirty or forty times the maximum that would permit of the
glacier shearing by its weight only. _The explanation of this apparent
paradox will remove all our difficulties in reference to the cause of
the descent of glaciers._
There seems to be but one explanation (and it is a very obvious
one), viz. that the motion of the glacier is _molecular_. The ice
descends molecule by molecule. The ice of a glacier is in the hard
crystalline state, but it does not descend in this state. Gravitation
is a constantly acting force; if a particle of the ice lose its
shearing-force, though but for the moment, it will descend by its
weight alone. But a particle of the ice will lose its shearing-force
for a moment if the particle loses its crystalline state for the
moment. The passage of heat through ice, whether by conduction or by
radiation, in all probability is a molecular process; that is, the
form of energy termed heat is transmitted from molecule to molecule
of the ice. A particle takes the energy from its neighbour A on the
one side and hands it over to its neighbour B on the opposite side.
But the particle must be in a different state at the moment it is in
possession of the energy from what it was before it received it from
A, and from what it will be after it has handed it over to B. Before
it became possessed of the energy, it was in the crystalline state—it
was ice; and after it loses possession of the energy it will be ice;
but at the moment that it is in possession of the passing energy is
it in the crystalline or icy state? If we assume that it is not, but
that in becoming possessed of the energy, it loses its crystalline form
and for the moment becomes water, all our difficulties regarding the
cause of the motion of glaciers are removed. We know that the ice of a
glacier in the mass cannot become possessed of energy in the form of
heat without becoming fluid; _if it can be shown that the same thing
holds true of the ice particle, we have the key to the mystery of
glacier-motion_. A moment’s reflection will suffice to convince any one
that if the glacier ice in the mass cannot receive energy in the form
of heat without melting, the same must hold true of the ice particles,
for it is inconceivable that the ice in the mass could melt and yet
the ice particles themselves remain in the solid state. It is the
solidity of the particles which constitutes the solidity of the mass.
If the particles lose their solid form the mass loses its solid form,
for the mass has no other solidity than that which is possessed by the
particles.
The correctness of the conclusion, that the weight of the ice is
not a sufficient cause, depends upon the truth of a certain element
taken for granted in the reasoning, viz. that the _shearing-force_ of
the molecules of the ice remains _constant_. If this force remains
constant, then Canon Moseley’s conclusion is undoubtedly correct,
but not otherwise; for if a molecule should lose its shearing-force,
though it were but for a moment, if no obstacle stood in front of the
molecule, it would descend in virtue of its weight.
The fact that the shearing-force of a mass of ice is found to be
constant does not prove that the same is the case in regard to the
individual molecules. If we take a mass of molecules in the aggregate,
the shearing-force of the mass taken thus collectively may remain
absolutely constant, while at the same time each individual molecule
may be suffering repeated momentary losses of shearing-force. This is
so obvious as to require no further elucidation. The whole matter,
therefore, resolves itself into this one question, as to whether or not
the shearing-force of a crystalline molecule of ice remains constant.
In the case of ordinary solid bodies we have no reason to conclude that
the shearing-force of the molecules ever disappears, but in regard to
ice it is very different.
If we analyze the process by which heat is conducted through ice, we
shall find that we have reason to believe _that while a molecule of
ice is in the act of transmitting the energy received (say from a
fire), it loses for the moment its shearing-force if the temperature of
the ice be not under_ 32° F. If we apply heat to the end of a bar of
iron, the molecules at the surface of the end have their temperatures
raised. Molecule A at the surface, whose temperature has been raised,
instantly commences to transfer to B a portion of the energy received.
The tendency of this process is to lower the temperature of A and raise
that of B. B then, with its temperature raised, begins to transfer
the energy to C. The result here is the same; B tends to fall in
temperature, and C to rise. This process goes on from molecule to
molecule until the opposite end of the bar is reached. Here in this
case the energy or heat applied to the end of the bar is transmitted
from molecule to molecule under the form of _heat or temperature_.
The energy applied to the bar does _not change its character; it
passes right along from molecule to molecule under the form of heat or
temperature_. But the nature of the process must be wholly different if
the transferrence takes place through a bar of ice at the temperature
of 32°. Suppose we apply the heat of the fire to the end of the bar
of ice at 32°, the molecules of the ice cannot possibly have their
temperatures raised in the least degree. How, then, can molecule A
take on, _under the form of heat_, the energy received from the fire
without being heated or having its _temperature_ raised? The thing is
impossible. The energy of the fire must appear in A under a different
form from that of heat. The same process of reasoning is equally
applicable to B. The molecule B cannot accept of the energy from A
under the form of heat; it must receive it under some other form. The
same must hold equally true of all the other molecules till we reach
the opposite end of the bar of ice. And yet, strange to say, the last
molecule transmits in the form of heat its energy to the objects
beyond; for we find that the heat applied to one side of a piece of ice
will affect the thermal pile on the opposite side.
The question is susceptible of a clear and definite answer. When
heat is applied to a molecule of ice at 32°, the heat applied
does not raise the temperature of the molecule, it is consumed in
work against the cohesive forces binding the atoms or particles
together into the crystalline form. The energy then must exist in
the dissolved crystalline molecule, under the statical form of an
affinity—crystalline affinity, or whatever else we may call it. That is
to say, the energy then exists in the particles as a power or tendency
to rush together again into the crystalline form, and the moment they
are allowed to do so they give out the energy that was expended upon
them in their separation. This energy, when it is thus given out again,
assumes the dynamical form of heat; in other words, the molecule gives
out _heat_ in the act of freezing. The heat thus given out may be
employed to melt the next adjoining molecule. The ice-molecules take
on energy from a heated body by melting. That peculiar form of motion
or energy called heat disappears in forcing the particles of the
crystalline molecule separate, and for the time being exists in the
form of a tendency in the separated particles to come together again
into the crystalline form.
But it must be observed that although the crystalline molecule, when
it is acting as a conductor, takes on energy under this form from the
heated body, it only exists in the molecule under such a form during
the moment of transmission; that is to say, the molecule is melted, but
only for the moment. When B accepts of the energy from A, the molecule
A instantly assumes the crystalline form. B is now melted; and when C
accepts of the energy from B, then B also in turn assumes the solid
state. This process goes on from molecule to molecule till the energy
is transmitted through to the opposite side and the ice is left in its
original solid state. This, as will be shown in the Appendix, is the
_rationale_ of Faraday’s property of regelation.
This is no mere theory or hypothesis; it is a necessary consequence
from known facts. We know that ice at 32° cannot take on energy from
a heated body without melting; and we know also equally well that a
slab of ice at 32°, notwithstanding this, still, as a mass, retains its
solid state while the heat is being transmitted through it. This proves
that every molecule resumes its crystalline form the moment after the
energy is transferred to the adjoining molecule.
This point being established, every difficulty regarding the descent
of the glacier entirely disappears; for a molecule the moment that
it assumes the fluid state is completely freed from shearing-force,
and can descend by virtue of its own weight without any impediment.
All that the molecule requires is simply room or space to advance in.
If the molecule were in absolute contact with the adjoining molecule
below, it would not descend unless it could push that molecule before
it, which it probably would not be able to do. But the molecule
actually has room in which to advance; for in passing from the solid
to the liquid state its volume is diminished by about 1/10, and it
consequently can descend. True, when it again assumes the solid form
it will regain its former volume; but the question is, will it go back
to its old position? If we examine the matter thoroughly we shall find
that it cannot. If there were only this one molecule affected by the
heat, this molecule would certainly not descend; but all the molecules
are similarly affected, although not all at the same moment of time.
Let us observe what takes place, say, at the lower end of the glacier.
The molecule A at the lower end, say, of the surface, receives heat
from the sun’s rays; it melts, and in melting not only loses its
shearing-force and descends by its own weight, but it contracts also.
B immediately above it is now, so far as A is concerned, at liberty to
descend, and will do so the moment that it assumes the liquid state. A
by this time has become solid, and again fixed by shearing-force; but
it is not fixed in its old position, but a little below where it was
before. If B has not already passed into the fluid state in consequence
of heat derived from the sun, the additional supply which it will
receive from the solidifying of A will melt it. The moment that B
becomes fluid it will descend till it reaches A. B then is solidified
a little below its former position. The same process of reasoning is
in a similar manner applicable to every molecule of the glacier. Each
molecule of the glacier consequently descends step by step as it melts
and solidifies, and hence the glacier, considered as a mass, is in a
state of constant motion downwards. The fact observed by Professor
Tyndall that there are certain planes in the ice along which melting
takes place more readily than others will perhaps favour the descent of
the glacier.
We have in this theory a satisfactory explanation of the origin of
“crevasses” in glaciers. Take, for example, the transverse crevasses
formed at the point where an increase in the inclination of the glacier
takes place. Suppose a change of inclination from, say, 4° to 8° in
the bed of the glacier. The molecules on the slope of 8° will descend
more rapidly than those above on the slope of 4°. A state of tension
will therefore be induced at the point where the change of inclination
occurs. The ice on the slope of 8° will tend to pull after it the mass
of the glacier moving more slowly on the slope above. The pull being
continued, the glacier will snap asunder the moment that the cohesion
of the ice is overcome. The greater the change of inclination is, the
more readily will the rupture of the ice take place. Every species of
crevasse can be explained upon the same principle.[309]
This theory explains also why a glacier moves at a greater rate during
summer than during winter; for as the supply of heat to the glacier is
greater during the former season than during the latter, the molecules
will pass oftener into the liquid state.
As regards the denuding power of glaciers, I may observe that, though
a glacier descends molecule by molecule, it will grind the rocky bed
over which it moves as effectually as it would do did it slide down in
a rigid mass in the way generally supposed; for the grinding-effect
is produced not by the ice of the glacier, but by the stones, sand,
and other materials forced along under it. But if all the resistances
opposing the descent of a glacier, internal and external, are overcome
by the mere weight of the ice alone, it can be proved that in the case
of one descending with a given velocity the amount of work performed
in forcing the grinding materials lying under the ice forward must be
as great, supposing the motion of the ice to be molecular in the way
I have explained, as it would be supposing the ice descended in the
manner generally supposed.
Of course, a glacier could not descend by means of its weight as
rapidly in the latter case as in the former; for, in fact, as Canon
Moseley has shown, it would not in the latter case descend at all; but
assuming for the sake of argument the rate of descent in both cases to
be the same, the conclusion I have stated would follow. Consequently
whatever denuding effects may have been attributed to the glacier,
according to the ordinary theory, must be equally attributable to it
according to the present explanation.
This theory, however, explains, what has always hitherto excited
astonishment, viz., why a glacier can descend a slope almost
horizontal, or why the ice can move off the face of a continent
perfectly level.
This is the form in which my explanation was first stated about
half-a-dozen years ago.[310] There is, however another element
which must be taken into account. It is one which will help to cast
additional light on some obscure points connected with glacial
phenomena.
Ice is evidently not absolutely solid throughout. It is composed of
crystalline particles, which, though in contact with one another, are,
however, not packed together so as to occupy the least possible space,
and, even though they were, the particles would not fit so closely
together as to exclude interstices. The crystalline particles are,
however, united to one another at special points determined by their
polarity, and on this account they require more space; and this in
all probability is the reason, as Professor Tyndall remarks, why ice,
volume for volume, is less dense than water.
“They (the molecules) like the magnets,” says Professor Tyndall, “are
acted upon by two distinct forces; for a time, while the liquid is
being cooled, they approach each other, in obedience to their general
attraction for each other. But at a certain point new forces, some
attractive some repulsive, _emanating from special points_ of the
molecules, come into play. The attracted points close up, the repelled
points retreat. Thus the molecules turn and rearrange themselves,
demanding as they do so more space, and overcoming all ordinary
resistance by the energy of their demand. This, in general terms, is an
explanation of the expansion of water in solidifying.”[311]
It will be obvious, then, that when a crystalline molecule melts, it
will not merely descend in the manner already described, but capillary
attraction will cause it to flow into the interstices between the
adjoining molecules. The moment that it parts with the heat received,
it will of course resolidify, as has been shown, but it will not
solidify so as to fit the cavity which it occupied when in the fluid
state. For the liquid molecule in solidifying assumes the crystalline
form, and of course there will be a definite proportion between the
length, breadth, and thickness of the crystal; consequently it will
always happen that the interstice in which it solidifies will be too
narrow to contain it. The result will be that the fluid molecule
in passing into the crystalline form will press the two adjoining
molecules aside in order to make sufficient room for itself between
them, and this it will do, no matter what amount of space it may
possess in all other directions. The crystal will not form to suit the
cavity, the cavity must be made to contain the crystal. And what holds
true of one molecule, holds true of every molecule which melts and
resolidifies. This process is therefore going on incessantly in every
part of the glacier, and in proportion to the amount of heat which the
glacier is receiving. This internal molecular pressure, resulting from
the solidifying of the fluid molecules in the interstices of the ice,
acts on the mass of the ice as an expansive force, tending to cause the
glacier to widen out laterally in all directions.
Conceive a mass of ice lying on a flat horizontal surface, and
receiving heat on its upper surface, say from the sun; as the heat
passes downwards through the mass, the molecules, acting as conductors,
melt and resolidify. Each fluid molecule solidifies in an interstice,
which has to be widened in order to contain it. The pressure thus
exerted by the continual resolidifying of the molecules will cause the
mass to widen out laterally, and of course as the mass widens out it
will grow thinner and thinner if it does not receive fresh acquisition
on its surface. In the case of a glacier lying in a valley, motion,
however, will only take place in one direction. The sides of the
valley prevent the glacier from widening; and as gravitation opposes
the motion of the ice up, and favours its motion down the valley, the
path of least resistance to molecular pressure will always be down
the slope, and consequently in this direction molecular displacement
will take place. Molecular pressure will therefore produce motion in
the same direction as that of gravity. In other words, it will tend to
cause the glacier to descend the valley.
The lateral expansion of the ice from internal molecular pressure
explains in a clear and satisfactory manner how rock-basins may be
excavated by means of land-ice. It also removes the difficulties
which have been felt in accounting for the ascent of ice up a steep
slope. The main difficulty besetting the theory of the excavation of
rock-basins by ice is to explain how the ice after entering the basin
manages to get out again—how the ice at the bottom is made to ascend
the sloping sides of the basin. Pressure acting from behind, it has
been argued by some; but if the basin be deep and its sides steep, this
will simply cause the ice lying above the level of the basin to move
forward over the surface of the mass filling it. This conclusion is,
however, incorrect. The ice filling the basin and the glacier overlying
it are united in one solid mass, so that the latter cannot move over
the former without shearing; and although the resistance to motion
offered by the sloping sides of the basin may be much greater than the
resistance to shear, still the ice will be slowly dragged out of the
basin. However, in order to obviate this objection to which I refer,
the advocates of the glacial origin of lake-basins point out that
the length of those basins in proportion to their depth is so great
that the slope up which the ice has to pass is in reality but small.
This no doubt is true of lake-basins in general, but it does not hold
universally true. But the theory does not demand that an ice-formed
lake-basin cannot have steep sides. We have incontestable evidence that
ice will pass up a steep slope; and, if ice can pass up a steep slope,
it can excavate a basin with a steep slope. That ice will pass up a
steep slope is proved by the fact that comparatively deep and narrow
river valleys are often found striated across, while hills which stood
directly in the path of the ice of the glacial epoch are sometimes
found striated _upwards_ from their base to their summit. Some striking
examples of striæ running up hill are given by Professor Geikie in
his “Glacial Drift of Scotland.” I have myself seen a slope striated
upwards so steep that one could not climb it.
A very good example of a river valley striated across came under
my observation during the past summer. The Tay, between Cargill
and Stanley (in the centre of the broad plain of Strathmore), has
excavated, through the Old Red Sandstone, a channel between 200 and
300 feet in depth. The channel here runs at right angles to the path
taken during the glacial epoch by the great mass of ice coming from
the North-west Highlands. At a short distance below Cargill, the trap
rising out of the bed of the river is beautifully ice-grooved and
striated, at right angles to the stream. A trap-dyke, several miles in
length, crosses the river about a mile above Stanley, forming a rapid,
known as the Linn of Campsie. This dyke is _moutonnée_ and striated
from near the Linn up the sloping bank to the level of the surrounding
country, showing that the ice must have ascended a gradient of one in
seven to a height of 300 feet.
From what has been already stated in reference to the resolidifying of
the molecules in the interstices of the ice, the application of the
molecular theory to the explanation of the effects under consideration
will no doubt be apparent. Take the case of the passage of the
ice-sheet across a river valley. As the upper surface of the ice-sheet
is constantly receiving heat from the sun and the air in contact with
it, there is consequently a transferrence of heat from above downwards
to the bottom of the sheet. This transferrence of heat from molecule
to molecule is accompanied by the melting and resolidifying of the
successive molecules in the manner already detailed. As the fluid
molecules tend to flow into adjoining interstices before solidifying
and assuming the crystalline form, the interstices of the ice at the
bottom of the valley are constantly being filled by fluid molecules
from above. These molecules no sooner enter the interstices than they
pass into the crystalline form, and become, of course, separated from
their neighbours by fresh interstices, which new interstices become
filled by fluid molecules, which, in turn, crystallize, forming fresh
interstices, and so on. The ice at the bottom of the valley, so long as
this process continues, is constantly receiving fresh additions from
above. The ice must therefore expand laterally to make room for these
additions, which it must do unless the resistance to lateral expansion
be greater than the force exerted by the molecules in crystallizing.
But a resistance sufficient to do this must be enormous. The ice at the
bottom of the valley cannot expand laterally without passing up the
sloping sides. In expanding it will take the path of least resistance,
but the path of least resistance will always be on the side of the
valley towards which the general mass of the ice above is flowing.
It has been shown (Chapter XXVII.) that the ice passing over Strathmore
must have been over 2,000 feet in thickness. An ice-sheet 2,000 feet
in thickness exerts on its bed a pressure of upwards of 51 tons per
square foot. When we reflect that ice under so enormous a pressure,
with grinding materials lying underneath, was forced by irresistible
molecular energy up an incline of one in seven, it is not at all
surprising that the hard trap should be ground down and striated.
We can also understand how the softer portions of the rocky surface
over which the ice moved should have been excavated into hollow basins.
We have also an explanation of the transport of boulders from a lower
to a higher level, for if ice can move from a lower to a higher level,
it of course can carry boulders along with it.
The bearing which the foregoing considerations of the manner in which
heat is transmitted through ice have on the question of the cause of
regelation will be considered in the Appendix.
APPENDIX.
I.
OPINIONS EXPRESSED PREVIOUS TO 1864 REGARDING
THE INFLUENCE OF THE ECCENTRICITY OF THE EARTH’S
ORBIT ON CLIMATE.[312]
M. DE MAIRAN.
M. de Mairan, in an article in the _Memoirs of the Royal Academy of
France_[313] “On the General Cause of Heat in Summer and Cold in
Winter, in so far as depends on the internal and permanent Heat of the
Earth,” makes the following remarks on the influence of the difference
of distance of the sun in apogee and perigee:—
“Cet élément est constant pour les deux solstices; tandis que les
autres (height of the sun and obliquity of his rays) y varient à raison
des latitudes locales; et il y a encore cela de particulier, qu’il
tend à diminuer la valeur de notre été, et à augmenter celle de notre
hiver dans l’hémisphère boréal où nous sommes, et tout au contraire
dans l’austral. Remarquons cependant que de ces mêmes distances, qui
constituent ce troisième élément, naît en partie un autre principe
de chaleur tout opposé, et qui semble devoir tempérer les effets du
précédent; sçavoir, la lenteur et la vitesse réciproques du mouvement
annuel apparent, en vertu duquel et du réel qui s’y mêle, le soleil
emploie 8 jours de plus à parcourir les signes septentrionaux.
C’est-à-dire, que le soleil passe 186½ jours dans notre hémisphère, et
seulement 178½ dans l’hémisphère opposé. Ce qui, en général, ne peut
manquer de répandre un pen plus de chaleur sur l’été du premier, et un
peu moins sur son hiver.”
MR. RICHARD KIRWAN.
“Œpinus,[314] reasoning on astronomical principles, attributes the
inferior temperature of the southern hemisphere to the shorter abode of
the sun in the southern tropic, shorter by seven days, which produces
a difference of fourteen days in favour of the northern hemisphere,
during which more heat is accumulated, and hence he infers that the
temperature of the northern hemisphere is to that of the southern, as
189·5 to 175·5, or as 14 to 13.”—_Trans. of the Royal Irish Academy_,
vol. viii., p. 417. 1802.
SIR CHARLES LYELL.
“Before the amount of difference between the temperature of the two
hemispheres was ascertained, it was referred by astronomers to the
acceleration of the earth’s motion in its perihelion; in consequence of
which the spring and summer of the southern hemisphere are shorter by
nearly eight days than those seasons north of the equator. A sensible
effect is probably produced by this source of disturbance, but it is
quite inadequate to explain the whole phenomena. It is, however, of
importance to the geologist to bear in mind that in consequence of the
precession of the equinoxes, the two hemispheres receive alternately,
each for a period of upwards of 10,000 years, a greater share of
solar light and heat. This cause may sometimes tend to counterbalance
inequalities resulting from other circumstances of a far more
influential nature; but, on the other hand, it must sometimes tend to
increase the extreme of deviation, which certain combinations of causes
produce at distant epochs.”—_Principles_, First Edition, 1830, p. 110,
vol. i.
SIR JOHN F. HERSCHEL, BART.
The following, in so far as it relates to the effects of eccentricity,
is a copy of Sir John Herschel’s memoir, “On the Astronomical Causes
which may influence Geological Phenomena,” read before the Geological
Society, Dec. 15th, 1830.—_Trans. Geol. Soc._, vol. iii., p. 293,
Second Series:—
“... Let us next consider the changes arising in the orbit of the earth
itself about the sun, from the disturbing action of the planets. In so
doing it will be obviously unnecessary to consider the effect produced
on the solar tides, to which the above reasoning applies much more
forcibly than in the case of the lunar. It is, therefore, only the
variations in the supply of light and heat received from the sun that
we have now to consider.
“Geometers having demonstrated the absolute invariability of the _mean_
distance of the earth from the sun, it would seem to follow that
the mean annual supply of light and heat derived from that luminary
would be alike invariable; but a closer consideration of the subject
will show that this would not be a legitimate conclusion, but that,
on the contrary, the _mean_ amount of solar radiation is dependent
on the eccentricity of the orbit, and therefore liable to variation.
Without going at present into any geometrical investigations, it will
be sufficient for the purpose here to state it as a theorem, of which
any one may easily satisfy himself by no very abstruse geometrical
reasoning, that ‘_the eccentricity of the orbit varying, the_ total
_quantity of heat received by the earth from the sun in one revolution
is inversely proportional to the_ minor _axis of the orbit_.’ Now since
the major axis is, as above observed, invariable, and therefore, of
course, the absolute length of the year, it will follow that the _mean
annual_ average of heat will also be in the same inverse ratio of the
_minor_ axis; and thus we see that the very circumstance which on a
cursory view we should have regarded as demonstrative of the constancy
of our supply of solar heat, forms an essential link in the chain of
strict reasoning by which its variability is proved.
“The eccentricity of the earth’s orbits is actually diminishing, and
has been so for ages, beyond the records of history. In consequence,
the ellipse is in a state of approach to a circle, and its minor
axis being, therefore, on the increase, the annual average of solar
radiation is actually on the _decrease_.
“So far this is in accordance with the testimony of geological
evidence, which indicates a general refrigeration of climate; but when
we come to consider the amount of diminution which the eccentricity
must be supposed to have undergone to render an account of the
variation which has taken place, we have to consider that, in the first
place, a great diminution of the eccentricity is required to produce
any sensible increase of the minor axis. This is a purely geometrical
conclusion, and is best shown by the following table:—
Eccentricity. Minor Axis. Reciprocal or Ratio of
Heat received.
0·00 1·000 1·000
0·05 0·999 1·002
0·10 0·995 1·005
0·15 0·989 1·011
0·20 0·980 1·021
0·25 0·968 1·032
0·30 0·954 1·048
By this it appears that a variation of the eccentricity of the orbit
from the circular form to that of an ellipse, having an eccentricity
of one-fourth of the major axis, would produce only a variation of 3
per cent. on the _mean_ annual amount of solar radiation, and this
variation takes in the whole range of the planetary eccentricities,
from that of Pallas and Juno downwards.
“I am not aware that the limit of increase of the eccentricity of the
earth’s orbit has ever been determined. That it has a limit has been
satisfactorily proved; but the celebrated theorem of Laplace, which
is usually cited as demonstrating that none of the planetary orbits
can ever deviate materially from the circular form, leads to no such
conclusion, except in the case of the great preponderant planets
Jupiter and Saturn, while for anything that theorem proves to the
contrary, the orbit of the earth may become elliptic to any amount.
“In the absence of calculations which though practicable have, I
believe, never been made,[315] and would be no slight undertaking, we
may assume that eccentricities which exist in the orbits of planets,
both interior and exterior to that of the earth, may _possibly_
have been attained, and may be attained again by that of the earth
itself. It is clear that such eccentricities _existing_ they cannot
be incompatible with the stability of the system generally, and that,
therefore, the question of the possibility of such an amount in the
particular case of the earth’s orbit will depend on the particular
data belonging to that case, and can only be determined by executing
the calculations alluded to, having regard to the simultaneous effects
of at least the four most influential planets, Venus, Mars, Jupiter,
and Saturn, _not only on the orbit of the earth, but on those of each
other_. The principles of this calculation are detailed in the article
of Laplace’s work cited. But before entering on a work of so much
labour, it is quite necessary to inquire what prospect of advantage
there is to induce any one to undertake it.
“Now it certainly at first sight seems clear that a variation of 3
per cent. only in the mean annual amount of solar radiation, and
that arising from an extreme supposition, does _not_ hold out such a
prospect. Yet it might be argued that the effects of the sun’s heat is
to maintain the temperature of the earth’s surface at its actual mean
height, not above the zero of Fahrenheit’s or any other thermometer,
but above the temperature of the celestial spaces, out of the reach of
the sun’s influence, and what that temperature is may be a matter of
much discussion. M. Fourier has considered it as demonstrated that it
is not greatly inferior to that of the polar regions of our own globe,
but the grounds of this decision appear to me open to considerable
objection.[316] If those regions be really void of matter, their
temperature can only arise, according to M. Fourier’s own view of
the subject, from the radiation of the stars. It ought, therefore,
to be as much inferior to that due to solar radiation, as the light
of a starlight night is to that of the brightest noon day, in other
words it should be very nearly a total privation of heat—almost the
_absolute zero_ respecting which so much difference of opinion exists,
some placing it at 1,000°, some at 5,000° of Fahrenheit below the
freezing-point, and some still lower, in which case a single unit per
cent. in the mean annual amount of radiation would suffice to produce a
change of climate fully commensurate to the demands of geologists.[317]
“Without attempting, however, to enter further into the perplexing
difficulties in which this point is involved, which are far greater
than appear on a cursory view, let us next consider, not the _mean_,
but the _extreme_ effects which a variation in the eccentricity of
the earth’s orbit may be expected to produce in the summer and winter
climates in particular regions of its surface, and under the influence
of circumstances favouring a difference of effect. And here, if I
mistake not, it will appear that an amount of variation, which we need
not hesitate to admit (at least, provisionally) as a possible one, may
be productive of considerable diversity of climate, and may operate
during great periods of time either to mitigate or to exaggerate
the difference of winter and summer temperatures, so as to produce
alternately, in the same latitude of either hemisphere, a perpetual
spring, or the extreme vicissitudes of a burning summer and a rigorous
winter.
“To show this, let us at once take the extreme case of an orbit as
eccentric as that of Juno or Pallas, in which the greatest and least
distances of the sun are to each other as 5 to 3, and consequently the
radiations at those distances as 25 to 9, or very nearly as 3 to 1. To
conceive what would be the _extreme_ effects of this great variation
of the heat received at different periods of the year, let us first
imagine in our latitude the place of the perigee of the sun to coincide
with the summer solstice. In that case, the difference between the
summer and winter temperature would be exaggerated in the same degree
as if three suns were placed side by side in the heavens in the former
season and only one in the latter, which would produce a climate
perfectly intolerable. On the other hand, were the perigee situated
in the winter solstice our three suns would combine to warm us in the
winter, and would afford such an excess of winter radiation as would
probably more than counteract the effect of short days and oblique
sunshine, and throw the summer season into the winter months.
“The actual diminution of the eccentricity is so slow, that the
transition from a state of the orbit such as we have assumed to the
present nearly circular figure would occupy upwards of 600,000 years,
supposing it uniformly changeable—this, of course, would not be the
case; when near the maximum, however, it would vary slower still, so
that at that point it is evident a period of 10,000 years would elapse
without any perceptible change in the state of the data of the case we
are considering.
“Now this adopting the very ingenious idea of Mr. Lyell[318] would
suffice, by reason of the combined effect of the precession of the
equinoxes and the motion of the apsides of the orbit itself, to
transfer the perigee from the summer to the winter solstice, and thus
to produce a transition from the one to the other species of climate in
a period sufficiently great to give room for a material change in the
botanical character of country.
“The supposition above made is an extreme, but it is not demonstrated
to be an impossible one, and should even an approach to such a state
of things be possible, the same consequences, in a mitigated degree,
would follow. But if, on executing the calculations, it should appear
that the limits of the eccentricity of the earth’s orbit are really
narrow, and if, on a full discussion of the very difficult and delicate
point of the actual effect of solar radiation, it should appear that
the mean, as well as the extreme, temperature of our climates would
_not_ be materially affected,—it will be at least satisfactory to
_know_ that the causes of the phenomena in question are to be sought
elsewhere than in the relations of our planet to the system to which
it belongs, since there does not appear to exist any other conceivable
connections between these relations and the facts of geology than
those we have enumerated, the obliquity of the ecliptic being, as we
know, confined within too narrow limits for its variation to have any
sensible influence.”—_J. F. W. Herschel._
The influence which this paper might have had on the question as
to whether eccentricity may be regarded as a cause of changes in
geological climate appears to have been completely neutralized by the
following, which appeared shortly afterwards both in his “Treatise” and
“Outlines of Astronomy,” showing evidently that he had changed his mind
on the subject.
“It appears, therefore, from what has been shown, the supplies of heat
received from the sun will be equal in the two segments, in whatever
direction the line PTQ be drawn. They will, indeed, be described in
unequal times: that in which the perihelion A lies in a shorter, and
the other in a longer, in proportion to their unequal area; but the
greater proximity of the sun in the smaller segment compensates exactly
for its more rapid description, and thus an equilibrium of heat is, as
it were, maintained.
“Were it not for this the eccentricity of the orbit would materially
influence the transition of seasons. The fluctuation of distance
amounts to nearly 1/30th of the mean quantity, and, consequently,
the fluctuation of the sun’s direct heating power to double this, or
1/15th of the whole.... Were it not for the compensation we have just
described, the effect would be to exaggerate the difference of summer
and winter in the southern hemisphere, and to moderate it in the
northern; thus producing a more violent alternation of climate in the
one hemisphere and an approach to perpetual spring in the other. _As it
is, however, no such inequality subsists_, but an equal and impartial
distribution of heat and light is accorded to both.”—“_Treatise of
Astronomy_,” _Cabinet Cyclopædia_, § 315; _Outlines of Astronomy_, §
368.
“The fact of a great change in the general climate of large tracts
of the globe, if not of the whole earth, and of a diminution of
general temperature, having been recognised by geologists, from
their examination of the remains of animals and vegetables of former
ages enclosed in the strata, various causes for such diminution of
temperature have been assigned.... It is evident that the _mean_
temperature of the whole surface of the globe, in so far as it is
maintained by the action of the sun at a higher degree than it would
have were the sun extinguished, must depend on the mean quantity of
the sun’s rays which it receives, or, which comes to the same thing,
on the _total_ quantity received in a given invariable time; and the
length of the year being unchangeable in all the fluctuations of the
planetary system, it follows that the total _annual_ amount of solar
radiation will determine, _cæteris paribus_, the general climate
of the earth. Now, it is not difficult to show that this amount is
inversely proportional to the minor axis of the ellipse described
by the earth about the sun, regarded as slowly variable; and that,
therefore, the major axis remaining, as we know it to be, constant,
and the orbit being actually in a state of approach to a circle,
and consequently the minor axis being on the _increase_, the mean
annual amount of solar radiation received by the whole earth must
be actually on the _decrease_. We have here, therefore, an evident
real cause of sufficient universality, and acting _in the right
direction_, to account for the phenomenon. Its adequacy is another
consideration.”[319]—_Discourse on the Study of Natural Philosophy_,
pp. 145−147 (1830).
SIR CHARLES LYELL, BART.
“_Astronomical Causes of Fluctuations in Climate._—Sir John Herschel
has lately inquired, whether there are any astronomical causes which
may offer a possible explanation of the difference between the actual
climate of the earth’s surface, and those which formerly appear to
have prevailed. He has entered upon this subject, he says, ‘impressed
with the magnificence of that view of geological revolutions, which
regards them rather as regular and necessary effects of great and
general causes, than as resulting from a series of convulsions
and catastrophes, regulated by no laws, and reducible to no fixed
principles.’ Geometers, he adds, have demonstrated the absolute
invariability of the mean distance of the earth from the sun; whence
it would seem to follow that the mean annual supply of light and heat
derived from that luminary would be alike invariable; but a closer
consideration of the subject will show that this would not be a
legitimate conclusion, but that, on the contrary, the _mean_ amount of
solar radiation is dependent on the eccentricity of the earth’s orbit,
and, therefore, liable to variation.
“Now, the eccentricity of the orbit, he continues, is actually
diminishing, and has been so for ages beyond the records of history.
In consequence, the ellipse is in a state of approach to a circle, and
the annual average of solar heat radiated to the earth is actually on
the _decrease_. So far, this is in accordance with geological evidence,
which indicates a general refrigeration of climate; but the question
remains, whether the amount of diminution which the eccentricity may
have ever undergone can be supposed sufficient to account for any
sensible refrigeration.[320] The calculations necessary to determine
this point, though practicable, have never yet been made, and would be
extremely laborious; for they must embrace all the perturbations which
the most influential planets, Venus, Mars, Jupiter, and Saturn, would
cause in the earth’s orbit and in each other’s movements round the sun.
“The problem is also very complicated, inasmuch as it depends not
merely on the ellipticity of the earth’s orbit, but on the assumed
temperature of the celestial spaces beyond the earth’s atmosphere;
a matter still open to discussion, and on which M. Fourier and Sir
J. Herschel have arrived at very different opinions. But if, says
Herschel, we suppose an extreme case, as if the earth’s orbit should
ever become as eccentric as that of the planet Juno or Pallas, a great
change of climate might be conceived to result, the winter and summer
temperatures being sometimes mitigated and at others exaggerated, in
the same latitudes.
“It is much to be desired that the calculations alluded to were
executed, as even if they should demonstrate, as M. Arago thinks highly
probable, that the mean of solar radiation can never be materially
affected by irregularities in the earth’s motion, it would still be
satisfactory to ascertain the point.”—_Principles of Geology_, Ninth
Edition, 1853, p. 127.
M. ARAGO.
“_Can the variations which certain astronomical elements undergo
sensibly modify terrestrial climates?_
“The sun is not always equally distant from the earth. At this time
its least distance is observed in the first days of January, and the
greatest, six months after, or in the first days of July. But, on the
other hand, a time will come when the _minimum_ will occur in July,
and the _maximum_ in January. Here, then, this interesting question
presents itself,—Should a summer such as those we now have, in which
the _maximum_ corresponds to the solar distance, differ sensibly, from
a summer with which the _minimum_ of this distance should coincide?
“At first sight every one probably would answer in the affirmative;
for, between the _maximum_ and the _minimum_ of the sun’s distance
from the earth there is a remarkable difference, a difference in round
numbers of a thirtieth of the whole. Let, however, the consideration of
the velocities be introduced into the problem, elements which cannot
fairly be neglected, and the result will be on the side opposite to
that we originally imagined.
“The part of the orbit where the sun is found nearest the earth, is, at
the same time, the point where the luminary moves most rapidly along.
The demi-orbit, or, in other words, the 180° comprehended betwixt the
two equinoxes of spring-time and autumn, will then be traversed in the
least possible time, when, in moving from the one of the extremities
of this arc to the other, the sun shall pass, near the middle of
this course of six months, at the point of the smallest distance. To
resume—the hypothesis we have just adopted would give, on account of
the lesser distance, a spring-time and summer hotter than they are in
our days; but on account of the greater rapidity, the sum of the two
seasons would be shorter by about seven days. Thus, then, all things
considered, the compensation is mathematically exact. After this it is
superfluous to add, that the point of the sun’s orbit corresponding to
the earth’s least distance changes very gradually; and that since the
most distant periods, the luminary has always passed by this point,
either at the end of autumn or beginning of winter.
“We have thus seen that the changes which take place in the _position_
of the solar orbit, _have no power in modifying the climate of our
globe_. We may now inquire, if it be the same concerning the variations
which this orbit experiences in its _form_....
“Herschel, who has recently been occupying himself with this problem,
in the hope of discovering the explanation of several geological
phenomena, allows that the succession of ages might bring the
eccentricity of the terrestrial orbit to the proportion of that of
the planet Pallas, that is to say, to be the 25/100 of a semi-greater
axis. It is exceedingly improbable that in these periodical changes
the eccentricity of our orbit should ever experience such enormous
variations, and even then these twenty-five hundredth parts (25/100),
would not augment the _mean_ annual solar radiation except by about one
hundredth part (1/100). To repeat, an eccentricity of 25/100 _would not
alter in any appreciated manner the mean thermometrical state of the
globe_....
“The changes of the form, and of the position, of the terrestrial
orbit are mathematically inoperative, or, at most, their influence is
so minute that it is not indicated by the most delicate instruments.
For the explanation of the changes of climates, then, there only
remains to us either the local circumstances, or some alteration in
the heating or illuminating power of the sun. But of these two causes,
we may continue to reject the last. And thus, in fact, all the changes
would come to be attributed to agricultural operations, to the clearing
of plains and mountains from wood, the draining of morasses, &c.
“Thus, at one swoop, to confine, the whole earth, the variations
of climates, past and future, within the limits of the naturally
very narrow influence which the labour of man can effect, would be
a meteorological result of the very last importance.”—pp. 221−224,
_Memoir on the “Thermometrical State of the Terrestrial Globe,” in the
Edinburgh New Philosophical Journal_, vol. xvi., 1834.
BARON HUMBOLDT.
“The question,” he says, “has been raised as to whether the increasing
value of this ellipticity is capable during thousands of years of
modifying to any considerable extent the temperature of the earth,
in reference to the daily and annual quantity and distribution of
heat? Whether a partial solution of the great geological problem of
the imbedding of tropical vegetable and animal remains in the now
cold zones may not be found in these _astronomical_ causes proceeding
regularly in accordance with eternal laws?... It might at the first
glance be supposed that the occurrence of the perihelion at an opposite
time of the year (instead of the winter, as, is now the case, in
summer) must necessarily produce great climatic variations; but, on the
above supposition, the sun will no longer remain seven days longer in
the northern hemisphere; no longer, as is now the case, traverse that
part of the ecliptic from the autumnal equinox to the vernal equinox,
in a space of time which is one week shorter than that in which it
traverses the other half of its orbit from the vernal to the autumnal
equinox.
“The difference of temperature which is considered as the consequence
to be apprehended from the turning of the major axis, _will on the
whole disappear_, principally from the circumstance that the point of
our planet’s orbit in which it is nearest to the sun is at the same
time always that over which it passes with the greatest velocity....
“As the altered position of the major axis is capable of exerting
only a very _slight influence upon the temperature of the earth;_ so
likewise the _limit_ of the probable changes in the elliptical form of
the earth’s orbit are, according to Arago and Poisson, so narrow that
these changes could _only very slightly_ modify the climates of the
individual zones, and that in very long periods.”[321]—_Cosmos_, vol.
iv., pp. 458, 459. Bohn’s Edition. 1852.
SIR HENRY T. DE LA BECHE.
“Mr. Herschel, viewing this subject with the eye of an astronomer,
considers that a diminution of the surface-temperature might arise from
a change in ellipticity of the earth’s orbit, which, though slowly,
gradually becomes more circular. No calculations having yet been made
as to the probable amount of decreased temperature from this cause,
it can at present be only considered as a possible explanation of
those geological phenomena which point to considerable alterations in
climates.”—_Geological Manual_. Third Edition. 1833. p. 8.
PROFESSOR PHILLIPS.
“_Temperature of the Globe._—_Influence of the Sun._—No proposition is
more certain than the fundamental dependence of the temperature of the
surface of the globe on the solar influence.
“It is, therefore, very important for geologists to inquire whether
this be variable or constant; whether the amount of solar heat
communicated to the earth is and has always been the same in every
annual period, or what latitude the laws of planetary movements permit
in this respect.
“Sir John Herschel has examined this question in a satisfactory manner,
in a paper read to the Geological Society of London. The total amount
of solar radiation which determines the general climate of the earth,
the year being of invariable length, is inversely proportional to
the minor axis of the ellipse described by the earth about the sun,
regarded as slowly variable; the major axis remaining constant and
the orbit being actually in a state of approach to a circle, and,
consequently, the minor axis being on the increase, it follows that
the mean annual amount of solar radiation received by the whole earth
must be actually on the decrease. The limits of the variation in the
eccentricity of the earth’s orbit are not known. It is, therefore,
impossible to say accurately what may have been in former periods of
time, the amount of solar radiation; it is, however, certain that
if the ellipticity has ever been so great as that of the orbit of
Mercury or Pallas, the temperature of the earth must have been sensibly
higher than it is at present. But the difference of a few degrees of
temperature thus occasioned, is of too small an order to be employed
in explaining the growth of tropical plants and corals in the polar
or temperate zones, and other great phenomena of Geology.”—_From A
Treatise on Geology_, p. 11, _forming the article under that head in
the seventh edition of the Encyclopædia Britannica_. 1837.
MR. ROBERT BAKEWELL.
“A change in the form of the earth’s orbit, if considerable, might
change the temperature of the earth, by bringing it nearer to the
sun in one part of its course. The orbit of the earth is an ellipsis
approaching nearly to a circle; the distance from the centre of the
orbit to either focus of the ellipsis is called by astronomers ‘the
eccentricity of the orbit.’ This eccentricity has been for ages slowly
decreasing, or, in other words, the orbit of the earth has been
approaching nearer to the form of a perfect circle; after a long period
it will again increase, and the possible extent of the variation has
not been yet ascertained. From what is known respecting the orbits of
Jupiter and Saturn, it appears highly probable that the eccentricity of
the earth’s orbit is confined within limits that preclude the belief
of any great change in the mean annual temperature of the globe ever
having been occasioned by this cause.”—_Introduction to Geology_, p.
600. 1838. Fifth Edition.
MRS. SOMERVILLE.
“Sir John Herschel has shown that the elliptical form of the earth’s
orbit has but a trifling share in producing the variation of
temperature corresponding to the difference of the seasons.”—_Physical
Geography_, vol. ii., p. 20. Third Edition.
MR. L. W. MEECH, A.M.
“Let us, then, look back to that primeval epoch when the earth
was in aphelion at midsummer, and the eccentricity at its maximum
value—assigned by Leverrier near to ·0777. Without entering into
elaborate computation, it is easy to see that the extreme values
of diurnal intensity, in Section IV., would be altered as by the
multiplier ((1 ± _e_)/(1 ± _e′_))^2, that is 1 - 0·11 in summer, and 1
+ 0·11 in winter. This would diminish the midsummer intensity by about
9°, and increase the midwinter intensity by 3° or 4°; the temperature
of spring and autumn being nearly unchanged. But this does not appear
to be of itself adequate to the geological effects in question.
“It is not our purpose, here, to enter into the inquiry whether the
atmosphere was once more dense than now, whether the earth’s axis
had once a different inclination to the orbit, or the sun a greater
emissive power of heat and light. Neither shall we attempt to speculate
upon the primitive heat of the earth, nor of planetary space, nor of
the supposed connection of terrestrial heat and magnetism; nor inquire
how far the existence of coal-fields in this latitude, of fossils,
and other geological remains, have depended upon existing causes. The
preceding discussion seems to prove simply that, under the present
system of physical astronomy, the sun’s intensity could never have been
materially different from what is manifested upon the earth at the
present day. _The causes of notable geological changes must be other
than the relative position of the sun and earth, under their present
laws of motion._”—_“On the Relative Intensity of the Heat and Light of
the Sun.” Smithsonian Contributions to Knowledge_, vol. ix.
M. JEAN REYNAUD.
“La révolution qui pourrait y causer les plus grands changements
thermométriques, celle qui porte l’orbite à s’élargir et à se rétrécir
alternativement et, par suite, la planète à passer, aux époques de
périhélie, plus ou moins près du soleil, embrasse une période de plus
de cent mille années terrestres et demeure comprise dans de si étroites
limites que les habitants doivent être à peine avertis que la chaleur
décroît, par cette raison, depuis une haute antiquité et décroîtra
encore pendant des siècles en variant en même temps dans sa répartition
selon les diverses époques de l’année.... Enfin, le tournoiement de
l’axe du globe s’empreint également d’une manière particulière sur
l’ètablissement des saisons qui, à tour de rôle, dans chacun des deux
hémisphères, deviennent graduellement, durant une période d’environ
vingt-cinq mille ans, de plus en plus uniformes, ou, à l’inverse, de
plus en plus dissemblables. C’est actuellement dans l’hémisphère boréal
que règne l’uniformité, et quoique les étés et les hivers y tendent,
dès à présent, à se trancher de plus en plus, il ne paraît pas douteux
que la modération des saisons n’y produise, pendant longtemps encore,
des effets appréciables. En résumé, de tous ces changements il n’en est
donc aucun ni qui suive un cours précipité, ni qui s’élève jamais à des
valeurs considérables; ils se règlent tous sur un mode de développement
presque insensible, et il s’ensuit que les années de la terre, malgré
leur complexité virtuelle, se distinguent par le constance de leurs
caractères non-seulement de ce qui peut avoir lieu, en vertu des mêmes
principes, dans les autres systèmes planétaires de l’univers, mais
même de ce qui s’observe dans plusieurs des mondes qui composent le
nôtre.”—_Philosophie Religieuse: Terre et Ciel._
M. ADHÉMAR.
Adhémar does not consider the effects which ought to result from a
change in the eccentricity of the earth’s orbit; he only concerns
himself with those which, in his opinion, arise from the present amount
of such eccentricity. He admits, of course, that both hemispheres
receive from the sun equal quantities of heat per annum; but, as
the southern hemisphere has a winter longer by 168 hours than the
corresponding season in the northern hemisphere, an accumulation of
heat necessarily takes place in the latter, and an accumulation of
cold in the former. Adhémar also measures the loss of heat sustained
by the southern hemisphere in a year by the number of hours by which
the southern exceeds the northern winter. “The south pole,” he says,
“loses in one year more heat than it receives, because the total
duration of its nights surpasses that of the days by 168 hours; and the
contrary takes place for the north pole. If, for example, we take for
unity the mean quantity of heat which the sun sends off in one hour,
the heat accumulated at the end of the year at the north pole will be
expressed by 168, while the heat lost by the south pole will be equal
to 168 times what the radiation lessens it by in one hour; so that at
the end of the year the difference in the heat of the two hemispheres
will be represented by 336 times what the earth receives from the sun
or loses in an hour by radiation,”[322] and at the end of 100 years the
difference will be 33,600 times, and at the end of 1,000 years 336,000
times, or equal to what the earth receives from the sun in 38½ years,
and so on during the 10,000 years that the southern winter exceeds in
length the northern. This, in his opinion, is all that is required to
melt the ice off the arctic regions, and cover the antarctic regions
with an enormous ice-cap. He further supposes that in about 10,000
years, when our northern winter will occur in aphelion and the southern
in perihelion, the climatic conditions of the two hemispheres will be
reversed; that is to say, the ice will melt at the south pole, and the
northern hemisphere will become enveloped in one continuous mass of
ice, leagues in thickness, extending down to temperate regions.
This theory, as shown in Chapter V., is based upon a misconception
regarding the laws of radiant heat. The loss of heat sustained by the
southern hemisphere from radiation, resulting from the greater length
of the southern winter, is vastly over-estimated by M. Adhémar, and
could not possibly produce the effects which he supposes. But I need
not enter into this subject here, as the reader will find the whole
question discussed at length in the chapter above referred to. By far
the most important part of Adhemar’s theory, however, is his conception
of the submergence of the land by means of a polar ice-cap. He appears
to have been the first to put forth the idea that a mass of ice placed
on the globe, say, for example, at the south pole, will shift the
earth’s centre of gravity a little to the south of its former position,
and thus, as a physical consequence, cause the sea to sink at the
north pole and to rise at the south. According to Adhémar, as the one
hemisphere cools and the other grows warmer, the ice at the pole of the
former will increase in thickness and that at the pole of the latter
diminish.
The sea, as a consequence, will sink on the warm hemisphere where the
ice is decreasing and rise on the cold hemisphere where the ice is
increasing. And, again, in 10,000 years, when the climatic conditions
of the two hemispheres are reversed, the sea will sink on the
hemisphere where it formerly rose, and rise on the hemisphere where it
formerly sank, and so on in like manner through indefinite ages.
Adhémar, however, acknowledges to have derived the grand conception
of a submergence of the land from the shifting of the earth’s centre
of gravity from the following wild speculation of one Bertrand, of
Hamburgh:—
“Bertrand de Hambourg, dans un ouvrage imprimé en 1799 et qui a
pour titre: _Renouvellement périodique des Continents_, avait déjà
émis cette idée, que la masse des eaux pouvait être alternativement
entraînée d’un hémisphère à l’autre par le déplacement du centre de
gravité du globe. Or, pour expliquer ce déplacement, il supposait que
la terre était creuse et qu’il y avait dans son intérieur un gros noyau
d’aimant auquel les comètes par leur attraction communiquaient un
mouvement de va-et-vient analogue à celui du pendule.”—_Révolutions de
la Mer_, p. 41.
The somewhat extravagant notions which Adhémar has advanced in
connection with his theory of submergence have very much retarded
its acceptance. Amongst other remarkable views he supposes the polar
ice-cap to rest on the bottom of the ocean, and to rise out of the
water to the enormous height of twenty leagues. Again, he holds that
on the winter approaching perihelion and the hemisphere becoming warm
the ice waxes soft and rotten from the accumulated heat, and the sea
now beginning to eat into the base of the cap, this is so undermined
as, at last, to be left standing upon a kind of gigantic pedestal. This
disintegrating process goes on till the fatal moment at length arrives,
when the whole mass tumbles down into the sea in huge fragments which
become floating icebergs. The attraction of the opposite ice-cap, which
has by this time nearly reached its maximum thickness, becomes now
predominant. The earth’s centre of gravity suddenly crosses the plain
of the equator, dragging the ocean along with it, and carrying death
and destruction to everything on the surface of the globe. And these
catastrophes, he asserts, occur alternately on the two hemispheres
every 10,000 years.—_Révolutions de la Mer_, pp. 316−328.