it patiently, let us found upon it, if we can, a theory of icicles.
255. First, then, you are to know that the _air_ of our atmosphere
is hardly heated at all by the rays of the sun, whether visible or
invisible. The air is highly transparent to all kinds of rays, and
it is only the scanty fraction to which it is _not_ transparent that
expend their force in warming it.
256. Not so, however, with the snow on which the sunbeams fall. It
absorbs the solar heat, and on a sunny day you may see the summits of
the high Alps glistening with the water of liquefaction. The _air_
above and around the mountains may at the same time be many degrees
below the freezing point in temperature.
257. You have only to pass from sunshine into shade to prove this. A
single step suffices to carry you from a place where the thermometer
stands high to one where it stands low; the change being due, not
to any difference in the temperature of the _air_, but simply to the
withdrawal of the thermometer from the direct action of the solar
rays. Nay, without shifting the thermometer at all, by interposing a
suitable screen, which cuts off the sun's rays, the coldness of the
air may be demonstrated.
258. Look now to the snow upon your house roof. The sun plays upon
it, and melts it; the water trickles to the eave and then drops down.
If the eave face the sun the water remains water; but if the eave
do not face the sun, the drop, before it quits its parent snow, _is
already in shadow_. Now the shaded space, as we have learnt, may be
below the freezing temperature. If so the drop, instead of falling,
congeals, and the rudiment of an icicle is formed. Other drops and
driblets succeed, which trickle over the rudiment, congeal upon it in
part and _thicken_ it at the root. But a portion of the water reaches
the free end of the icicle, hangs from it, and is there congealed
before it escapes. The icicle is thus _lengthened_. In the Alps,
where the liquefaction is copious and the cold of the shaded crevasse
intense, the icicles, though produced in the same way, naturally grow
to a greater size. The drainage of the snow after the sun's power is
withdrawn also produces icicles.
259. It is interesting and important that you should be able to
explain the formation of an icicle; but it is far more important
that you should realise the way in which the various threads of what
we call Nature are woven together. You cannot fully understand an
icicle without first knowing that solar beams powerful enough to
fuse the snows and blister the human skin, nay, it might be added,
powerful enough, when concentrated, to burn up the human body itself,
may pass through the air, and still leave it at an icy temperature.
§ 38. _The Bergschrund._
260. Having cleared away this difficulty, let us turn once more to
the crevasses, taking them in the order of their formation. First
then above the névé we have the final Alpine peaks and crests,
against which the snow is often reared as a steep buttress. We have
already learned that both névés and glaciers are moving slowly
downwards; but it usually happens that the attachment of the highest
portion of the buttress to the rocks is great enough to enable it to
hold on while the lower portion breaks away. A very characteristic
crevasse is thus formed, called in the German-speaking portion of the
Alps a _Bergschrund_. It often surrounds a peak like a fosse, as if
to defend it against the assaults of climbers.
261. Look more closely into its formation. Imagine the snow as yet
unbroken. Its higher portions cling to the rocks, and move downwards
with extreme slowness. But its lower portions, whether from their
greater depth and weight, or their less perfect attachment, are
compelled to move more quickly. _A pull_ is therefore exerted,
tending to separate the lower from the upper snow. For a time this
pull is resisted by the cohesion of the névé; but this at length
gives way, and a crack is formed exactly across the line in which
the pull is exerted. In other words, _a crevasse is formed at right
angles to the line of tension_.
§ 39. _Transverse Crevasses._
262. Both on the névé and on the glacier the origin of the crevasses
is the same. Through some cause or other the ice is thrown into a
state of strain, and as it cannot _stretch_ it _breaks_ across the
line of tension. Take for example, the ice-fall of the Géant, or
of the Talèfre, above which you know the crevasses yawn terribly.
Imagine the névé and the glacier entirely peeled away, so as to
expose the surface over which they move. From the Col du Géant we
should see this surface falling gently to the place now occupied by
the brow of the cascade. Here the surface would fall steeply down to
the bed of the present Glacier du Géant, where the slope would become
gentle once more.
263. Think of the névé moving over such a surface. It descends from
the Col till it reaches the brow just referred to. It crosses the
brow, and must bend down to keep upon its bed. Realise clearly what
must occur. The surface of the névé is evidently thrown into a
state of strain; it breaks and forms a crevasse. Each fresh portion
of the névé as it passes the brow is similarly broken, and thus a
succession of crevasses is sent down the fall. Between every two
chasms is a great transverse ridge. Through local strains upon the
fall those ridges are also frequently broken across, towers of
ice--_séracs_--being the result. Down the fall both ridges and séracs
are borne, the dislocation being augmented during the descent.
264. What must occur at the foot of the fall? Here the slope suddenly
lessens in steepness. It is plain that the crevasses must not only
cease to open here, but that they must in whole or in part close
up. At the summit of the fall, the bending was such as to make the
surface convex; at the bottom of the fall the bending renders the
surface concave. In the one case we have _strain_, in the other
_pressure_. In the one case, therefore, we have the _opening_, and
in the other the _closing_ of crevasses. This reasoning corresponds
exactly with the facts of observation.
265. Lay bare your arm and stretch it straight. Make two ink dots
half an inch or an inch apart, exactly opposite the elbow. Bend your
arm, the dots approach each other, and are finally brought together.
Let the two dots represent the two sides of a crevasse at the bottom
of an ice-fall; the bending of the arm resembles the bending of the
ice, and the closing up of the dots resembles the closing of the
fissures.
266. The same remarks apply to various portions of the Mer de Glace.
At certain places the inclination changes from a gentler to a steeper
slope, and on crossing the brow between both the glacier breaks its
back. _Transverse crevasses_ are thus formed. There is such a change
of inclination opposite to the Angle, and a still greater but similar
change at the head of the Glacier des Bois. The consequence is that
the Mer de Glace at the former point is impassable, and at the latter
the rending and dislocation are such as we have seen and described.
Below the Angle, and at the bottom of the Glacier des Bois, the
steepness relaxes, the crevasses heal up, and the glacier becomes
once more continuous and compact.
§ 40. _Marginal Crevasses._
267. Supposing, then, that we had no changes of inclination, should
we have no crevasses? We should certainly have less of them, but they
would not wholly disappear. For other circumstances exist to throw
the ice into a state of strain, and to determine its fracture. The
principal of these is the more rapid movement of the centre of the
glacier.
268. Helped by the labours of an eminent man, now dead, the late Mr.
Wm. Hopkins, of Cambridge, let us master the explanation of this
point together. But the pleasure of mastering it would be enhanced
if we could see beforehand the perplexing and delusive appearances
accounted for by the explanation. Could my wishes be followed out, I
would at this point of our researches carry you off with me to Basel,
thence to Thun, thence to Interlaken, thence to Grindelwald, where
you would find yourself in the actual presence of the Wetterhorn and
the Eiger, with all the greatest peaks of the Bernese Oberland, the
Finsteraarhorn, the Schreckhorn, the Monch, the Jungfrau, at hand.
At Grindelwald, as we have already learnt, there are two well-known
glaciers--the Ober Grindelwald and the Unter Grindelwald glaciers--on
the latter of which our observations should commence.
269. Dropping down from the village to the bottom of the valley, we
should breast the opposite mountain, and with the great limestone
precipices of the Wetterhorn to our left, we should get upon a path
which commands a view of the glacier. Here we should see beautiful
examples of the opening of crevasses at the summit of a brow, and
their closing at the bottom. But the chief point of interest would
be the crevasses formed at the _side_ of this glacier--the _marginal
crevasses_, as they may be called.
270. We should find the side copiously fissured, even at those places
where the centre is compact; and we should particularly notice that
the fissures would neither run in the direction of the glacier, nor
straight across it, but that they would be _oblique_ to it, enclosing
an angle of about 45 degrees with the sides. Starting from the side
of the glacier the crevasses would be seen to point _upwards_; that
is to say, the ends of the fissures abutting against the bounding
mountain would appear to be _dragged down_. Were you less instructed
than you now are, I might lay a wager that the aspect of these
fissures would cause you to conclude that the centre of the glacier
is left behind by the quicker motion of the sides.
271. This indeed was the conclusion drawn by M. Agassiz from this
very appearance, before he had measured the motion of the sides and
centre of the glacier of the Unteraar. Intimately versed with the
treatment of mechanical problems, Mr. Hopkins immediately deduced
the obliquity of the lateral crevasses from the quicker flow of the
centre. Standing beside the glacier with pencil and note-book in
hand, I would at once make the matter clear to you thus.
[Illustration]
272. Let A C, in the annexed figure, be one side of the glacier, and
B D the other; and let the direction of motion be that indicated
by the arrow. Let S T be a transverse slice of the glacier, taken
straight across it, say to-day. A few days or weeks hence this
slice will have been carried down, and because the centre moves more
quickly than the sides it will not remain straight, but will bend
into the form S' T'.
273. Supposing T _i_ to be a small square of the original slice near
the side of the glacier. In its new position the square will be
distorted to the lozenge-shaped figure T' _i'_. Fix your attention
upon the diagonal T _i_ of the square; in the lowest position this
diagonal, _if the ice could stretch_, would be lengthened to T' _i'_.
But the ice does not stretch; it breaks, and we have a crevasse
formed at right angles to T' _i'_. The mere inspection of the diagram
will assure you that the crevasse will point obliquely _upwards_.
274. Along the whole side of the glacier the quicker movement of the
centre produces a similar state of strain; and the consequence is
that the sides are copiously cut by those oblique crevasses, even at
places where the centre is free from them.
275. It is curious to see at other places the transverse fissures
of the centre uniting with those at the sides, so as to form great
curved crevasses which stretch across the glacier from side to
side. The convexity of the curve is turned _upwards_, as mechanical
principles declare it ought to be. (See sketch on opposite page.)
But if you were ignorant of those principles, you would never infer
from the aspect of these curves the quicker motion of the centre. In
landslips, and in the motion of partially indurated mud, you may
sometimes notice appearances similar to those exhibited by the ice.
[Illustration: SKETCH OF CURVED CREVASSES: THE GLACIER MOVES FROM
LEFT TO RIGHT.]
§ 41. _Longitudinal Crevasses._
276. We have thus unravelled the origin of both transverse and
marginal crevasses. But where a glacier issues from a steep and
narrow defile upon a comparatively level plain which allows it room
to expand laterally, its motion is in part arrested, and the level
portion has to bear the thrust of the steeper portions behind. Here
the line of thrust is in the direction of the glacier, while the
direction at right angles to this is one of tension. Across this
latter the glacier breaks, and _longitudinal crevasses_ are formed.
277. Examples of this kind of crevasse are furnished by the lower
part of the Glacier of the Rhone, when looked down upon from the
Grimsel Pass, or from any commanding point on the flanking mountains.
§ 42. _Crevasses in relation to Curvature of Glacier._
278. One point in addition remains to be discussed, and your present
knowledge will enable you to master it in a moment. You remember at
an early period of OUT researches that we crossed the Mer de Glace
from the Chapeau side to the Montanvert side. I then desired you to
notice that the Chapeau side of the glacier was more fissured than
either the centre or the Montanvert side (75). Why should this be so?
Knowing as we now do that the Chapeau side of the glacier moves more
quickly than the other; that the point of maximum motion does not
lie on the centre but far east of it, we are prepared to answer this
question in a perfectly satisfactory manner.
279. Let A B and C D, in the diagram opposite, represent the two
curved sides of the Mer de Glace at the Montanvert, and let _m n_ be
a straight line across the glacier. Let _o_ be the point of maximum
motion. The mechanical state of the two sides of the glacier may be
thus made plain. Supposing the line _m n_ to be a straight elastic
string with its ends fixed; let it be grasped firmly at the point
o by the finger and thumb, and drawn to _o'_, keeping the distance
between _o'_ and the side C D constant. Here the length, _n o_ of
the string would have stretched to _n o'_, and the length _m o_ to
_m o'_ and you see plainly that the stretching of the short line, in
comparison with its length, is greater than that of the long line in
comparison with its length. In other words, the strain upon _n o'_ is
greater than that upon _m o'_; so that if one of them were to break
under the strain, it would be the short one.
[Illustration: _Montanvert_]
280. These two lines represent the conditions of strain upon the
two sides of the glacier. The sides are held back, and the centre
tries to move on, a strain being thus set up between the centre and
sides. But the displacement of the point of maximum motion through
the curvature of the valley makes the strain upon the eastern ice
greater than that upon the western. The eastern side of the glacier
is therefore more crevassed than the western.
281. Here indeed resides the difficulty of getting along the eastern
side of the Mer de Glace: a difficulty which was one reason for our
crossing the glacier opposite to the Montanvert. There are two convex
sweeps on the eastern side to one on the western side, hence on the
whole the eastern side of the Mer de Glace is most riven.
§ 43. _Moraine-ridges, Glacier Tables, and Sand-Cones._
282. When you and I first crossed the Mer de Glace from Trélaporte
to the Couvercle, we found that the stripes of rocks and rubbish
which constituted the medial moraines were ridges raised above the
general level of the glacier to a height at some places of twenty or
thirty feet. On examining these ridges we found the rubbish to be
superficial, and that it rested upon a great spine of ice which ran
along the back of the glacier. By what means has this ridge of ice
been raised?
283. Most boys have read the story of Dr. Franklin's placing bits of
cloth of various colours upon snow on a sunny day. The bits of cloth
sank in the snow, the dark ones most.
284. Consider this experiment. The sun's rays first of all fall
upon the upper surface of the cloth and warm it. The heat is then
conducted through the cloth to the under surface, and the under
surface passes it on to the snow, which is finally liquefied by the
heat. It is quite manifest that the quantity of snow melted will
altogether depend upon the amount of heat sent from the upper to the
under surface of the cloth.
285. Now cloth is what is called a bad conductor. It does not permit
heat to travel freely through it. But where it has merely to pass
through the thickness of a single bit of cloth, a good quantity of
the heat gets through. But if you double or treble or quintuple the
thickness of the cloth; or, what is easier, if you put several pieces
one upon the other, you come at length to a point where no sensible
amount of heat could get through from the upper to the under surface.
286. What must occur if such a thick piece, or such a series of
pieces of cloth, were placed upon snow on which a strong sun is
falling? The snow round the cloth is melted, but that underneath the
cloth is protected. If the action continue long enough the inevitable
result will be, that the level of the snow all round the cloth will
sink, and the cloth will be left behind perched upon an eminence of
snow.
287. If you understand this, you have already mastered the cause of
the moraine-ridges. They are not produced by any swelling of the ice
upwards. But the ice underneath the rocks and rubbish being protected
from the sun, the glacier right and left melts away and leaves a
ridge behind.
288. Various other appearances upon the glacier are accounted for
in the same way. Here upon the Mer de Glace we have flat slabs of
rock sometimes lifted up on pillars of ice. These are the so-called
_Glacier Tables_. They are produced, not by the growth of a stalk of
ice out of the glacier, but by the melting of the glacier all round
the ice protected by the stone. Here is a sketch of one of the Tables
of the Mer de Glace.
[Illustration]
289. Notice moreover that a glacier table is hardly ever set square
upon its pillar. It generally leans to one side, and repeated
observation teaches you that it so leans as to enable you always to
draw the north and south line upon the glacier. For the sun being
south of the zenith at noon pours its rays against the southern end
of the table, while the northern end remains in shadow. The southern
end, therefore, being most warmed does not protect the ice underneath
it so effectually as the northern end. The table becomes inclined,
and ends by sliding bodily off its pedestal.
290. In the figure opposite we have what maybe called an ideal
Table. The oblique lines represent the direction of the sunbeams,
and the consequent tilting of the table here shown resembles that
observed upon the glaciers.
291. A pebble will not rise thus: like Franklin's single bit of
cloth, a dark-coloured pebble sinks in the ice. A spot of black mould
will not rest upon the surface, but will sink; and various parts of
the Glacier du Géant are honeycombed by the sinking of such spots of
dirt into the ice.
[Illustration]
292. But when the dirt is of a thickness sufficient to protect the
ice the case is different. Sand is often washed away by a stream from
the mountains, or from the moraines, and strewn over certain spaces
of the glacier. A most curious action follows: the sanded surface
rises, the part on which the sand lies thickest rising highest.
Little peaks and eminences jut forth, and when the distribution
of the sand is favourable, and the action sufficiently prolonged,
you have little mountains formed, sometimes singly, and sometimes
grouped so as to mimic the Alps themselves. The _Sand-Cones_ of the
Mer de Glace are not striking; but on the Görner, the Aletsch, the
Morteratsch, and other glaciers, they form singly and in groups,
reaching sometimes a height of ten or twenty feet.
§ 44. _The Glacier Mills or Moulins._
293. You and I have learned by long experience the character of the
Mer de Glace. We have marched over it daily, with a definite object
in view, but we have not closed our eyes to other objects. It is
from side glimpses of things which are not at the moment occupying
our attention that fresh subjects of enquiry arise in scientific
investigation.
294. Thus in marching over the ice near Trélaporte we were often
struck by a sound resembling low rumbling thunder. We subsequently
sought out the origin of this sound, and found it.
295. A large area of this portion of the glacier is unbroken.
Driblets of water have room to form rills; rills to unite and form
streams; streams to combine to form rushing brooks, which sometimes
cut deep channels in the ice. Sooner or later these streams reach
a strained portion of the glacier, where a crack is formed across
the stream. A way is thus opened for the water to the bottom of the
glacier. By long action the stream hollows out a shaft, the crack
thus becoming the starting-point of a funnel of unseen depth, into
which the water leaps with the sound of thunder.
296. This funnel and its cataract form a glacier Mill or _Moulin_.
297. Let me grasp your hand firmly while you stand upon the edge of
this shaft and look into it. The hole, with its pure blue shimmer,
is beautiful, but it is terrible. Incautious persons have fallen
into these shafts, a second or two of bewilderment being followed by
sudden death. But caution upon the glaciers and mountains ought, by
habit, to be made a second nature to explorers like you and me.
298. The crack into which the stream first descended to form the
moulin, moves down with the glacier. A succeeding portion of the ice
reaches the place where the breaking strain is exerted. A new crack
is then formed above the moulin, which is thenceforth forsaken by the
stream, and moves downward as an empty shaft. Here upon the Mer de
Glace, in advance of the _Grand Moulin_, we see no less than six of
these forsaken holes. Some of them we sound to a depth of 90 feet.
299. But you and I both wish to determine, if possible, the entire
depth of the Mer de Glace. The Grand Moulin offers a chance of doing
this which we must not neglect. Our first effort to sound the moulin
fails through the breaking of our cord by the impetuous plunge of the
water. A lump of grease in the hollow of a weight enables a mariner
to judge of a sea bottom. We employ such a weight, but cannot reach
the bed of the glacier. A depth of 163 feet is the utmost reached by
our plummet.
300. From July 28 to August 8 we have watched the progress of the
Grand Moulin. On the former date the position of the Moulin was
fixed. On the 31st it had moved down 50 inches; a little more than
a day afterwards it had moved 74 inches. On August 8 it had moved
198 inches, which gives an average of about 18 inches in twenty-four
hours. No doubt next summer upon the Mer de Glace a Grand Moulin will
be found thundering near Trélaporte; but like the crevasse of the
Grand Plateau, already referred to (§ 16), it will not be our Moulin.
This, or rather the ice which it penetrated, is now probably more
than a mile lower down than it was in 1857.
§ 45. _The Changes of Volume of Water by Heat and Cold._
301. We have noticed upon the glacier shafts and pits filled with
water of the most delicate blue. In some cases these have been the
shafts of extinct moulins closed at the bottom. A theory has been
advanced to account for them, which, though it may be untenable,
opens out considerations regarding the properties of water that
ought to be familiar to enquirers like you and me.
302. In our dissection of lake ice by a beam of heat (§ 11) we
noticed little vacuous spots at the centres of the liquid flowers
formed by the beam. These spots we referred to the fact that when ice
is melted the water produced is less in volume than the ice, and that
hence the water of the flower was not able to occupy the whole space
covered by the flower.
303. Let us more fully illustrate this subject. Stop a small flask
water-tight with a cork, and through the cork introduce a narrow
glass tube also water-tight. It is easy to fill the flask with water
so that the liquid shall stand at a certain height in the glass tube.
304. Let us now warm the flask with the flame of a spirit-lamp. On
first applying the flame you notice a momentary sinking of the liquid
in the glass tube. This is due to the momentary expansion of the
flask by heat; it becomes suddenly larger when the flame is first
applied.