CHIEFLY SCIENTIFIC.
Aber im stillen Gemach entwirft bedeutende Zirkel
Sinnend der Weise, beschleicht forschend den schaffenden Geist,
Prueft der Stoffe Gewalt, der Magnete Hassen und Lieben,
Folgt durch die Luefte dem Klang, folgt durch den Aether dem Strahl,
Sucht das vertraute Gesetz in des Zufalls grausenden Wundern,
Sucht den ruhenden Pol in der Erscheinungen Flucht.
Schiller.
ON LIGHT AND HEAT.
(1.)
[Sidenote: THEORIES OF LIGHT.]
What is Light? The ancients supposed it to be something emitted by the
eyes, and for ages no notion was entertained that it required time to
pass through space. In the year 1676 Roemer first proved that the light
from Jupiter's satellites required a certain time to cross the earth's
orbit. Bradley afterwards found that, owing to the velocity with which
the earth flies through space, the rays of the stars are slightly
inclined, just as rain-drops which descend vertically appear to meet us
when we move swiftly through the shower. In Kew Gardens there is a
sun-dial commemorative of this discovery, which is called the
_aberration of light_. Knowing the velocity of the earth, and the
inclination of the stellar rays, Bradley was able to calculate the
velocity of light; and his result agrees closely with that of Roemer.
Celestial distances were here involved, but a few years ago M. Fizeau,
by an extremely ingenious contrivance, determined the time required by
light to pass over a distance of about 9000 yards; and his experiment is
quite in accordance with the results of his predecessors.
But what is it which thus moves? Some, and among the number Newton,
imagined light to consist of particles darted out from luminous bodies.
This is the so-called Emission-Theory, which was held by some of the
greatest men: Laplace, for example, accepted it; and M. Biot has
developed it with a lucidity and power peculiar to himself. It was first
opposed by the astronomer Huyghens, and afterwards by Euler, both of
whom supposed light to be a kind of undulatory motion; but they were
borne down by their great antagonists, and the emission-theory held its
ground until the commencement of the present century, when Thomas Young,
Professor of Natural Philosophy in the Royal Institution, reversed the
scientific creed by placing the Theory of Undulation on firm
foundations. He was followed by a young Frenchman of extraordinary
genius, who, by the force of his logic and the conclusiveness of his
experiments, left the Wave-Theory without a competitor. The name of this
young Frenchman was Augustin Fresnel.
Since his time some of the ablest minds in Europe have been applied to
the investigation of this subject; and thus a mastery, almost
miraculous, has been attained over the grandest and most subtle of
natural phenomena. True knowledge is always fruitful, and a clear
conception regarding any one natural agent leads infallibly to better
notions regarding others. Thus it is that our knowledge of light has
corrected and expanded our knowledge of _heat_, while the latter, in its
turn, will assuredly lead us to clearer conceptions regarding the other
forces of Nature.
I think it will not be a useless labour if I here endeavour to state, in
a simple manner, our present views of light and heat. Such knowledge is
essential to the explanation of many of the phenomena referred to in the
foregoing pages; and even to the full comprehension of the origin of the
glaciers themselves. A few remarks on the nature of sound will form a
fit introduction.
[Sidenote: NATURE OF SOUND.]
It is known that sound is conveyed to our organs of hearing by the air:
a bell struck in a vacuum emits no sound, and even when the air is thin
the sound is enfeebled. Hawksbee proved this by the air-pump; De
Saussure fired a pistol at the top of Mont Blanc,--I have repeated the
experiment myself, and found, with him, that the sound is feebler than
at the sea level. Sound is not produced by anything projected through
the air. The explosion of a gun, for example, is sent forward by a
motion of a totally different kind from that which animates the bullet
projected from the gun: the latter is a motion of _translation_; the
former, one of _vibration_. To use a rough comparison, sound is
projected through the air as a push is through a crowd; it is the
propagation of a _wave_ or _pulse_, each particle taking up the motion
of its neighbour, and delivering it on to the next. These aerial waves
enter the external ear, meet a membrane, the so-called tympanic
membrane, which is drawn across the passage at a certain place, and
break upon it as sea-waves do upon the shore. The membrane is shaken,
its tremors are communicated to the auditory nerve, and transmitted by
it to the brain, where they produce the impression to which we give the
name of sound.
[Sidenote: CAUSE OF MUSIC.]
In the tumult of a city, pulses of different kinds strike irregularly
upon the tympanum, and we call the effect _noise_; but when a succession
of impulses reach the ear _at regular intervals_ we feel the effect as
_music_. Thus, a vibrating string imparts a series of shocks to the air
around it, which are transmitted with perfect regularity to the ear, and
produce a _musical note_. When we hear the song of a soaring lark we may
be sure that the entire atmosphere between us and the bird is filled
with pulses, or undulations, or waves, as they are often called,
produced by the little songster's organ of voice. This organ is a
vibrating instrument, resembling, in principle, the reed of a clarionet.
Let us suppose that we hear the song of a lark, elevated to a height of
500 feet in the air. Before this is possible, the bird must have
agitated a sphere of air 1000 feet in diameter; that is to say, it must
have communicated to 17,888 tons of air a motion sufficiently intense to
be appreciated by our organs of hearing.
[Sidenote: CAUSE OF PITCH.]
Musical sounds differ in _pitch_: some notes are high and shrill, others
low and deep. Boys are chosen as choristers to produce the shrill
notes; men are chosen to produce the bass notes. Now, the sole
difference here is, that the boy's organ vibrates _more rapidly_ than
the man's--it sends a greater number of impulses per second to the ear.
In like manner, a short string emits a higher note than a long one,
because it vibrates more quickly. The greater the number of vibrations
which any instrument performs in a given time, the higher will be the
pitch of the note produced. The reason why the hum of a gnat is shriller
than that of a beetle is that the wings of the small insect vibrate more
quickly than those of the larger one. We can, with suitable
arrangements, make those sonorous vibrations visible to the eye;[A] and
we also possess instruments which enable us to tell, with the utmost
exactitude, the number of vibrations due to any particular note. By such
instruments we learn that a gnat can execute many thousand flaps of its
little wings in a second of time.
[Sidenote: NATURE OF LIGHT.]
In the study of nature the coarser phenomena, which come under the
cognizance of the senses, often suggest to us the finer phenomena which
come under the cognizance of the mind; and thus the vibrations which
produce sound, and which, as has been stated, can be rendered visible to
the eye by proper means, first suggested that _light_ might be due to a
somewhat similar action. This is now the universal belief. A luminous
body is supposed to have its atoms, or molecules, in a state of intense
vibration. The motions of the atoms are supposed to be communicated to
a medium suited to their transmission, as air is to the transmission of
sound. This medium is called the _luminiferous ether_, and the little
billows excited in it speed through it with amazing celerity, enter the
pupil of the eye, pass through the humours, and break upon the retina or
optic nerve, which is spread out at the back of the eye. Hence the
tremors they produce are transmitted along the nerve to the brain, where
they announce themselves as _light_. The swiftness with which the waves
of light are propagated through the ether, is however enormously greater
than that with which the waves of sound pass through the air. An aerial
wave of sound travels at about the rate of 1100 feet in a second: a wave
of light leaves 192,000 miles behind it in the same time.
[Sidenote: CAUSE OF COLOUR.]
Thus, then, in the case of sound, we have the sonorous body, the air,
and the auditory nerve, concerned in the phenomenon; in the case of
light, we have the luminous body, the ether, and the optic nerve. The
fundamental analogy of sound and light is thus before us, and it is
easily remembered. But we must push the analogy further. We know that
the white light which comes to us from the sun is made up of an infinite
number of coloured rays. By refraction with a prism we can separate
those rays from each other, and arrange them in the series of colours
which constitute the solar spectrum. The rainbow is an imperfect or
_impure_ spectrum, produced by the drops of falling rain, but by prisms
we can unravel the white light into pure red, orange, yellow, green,
blue, indigo, and violet. Now, this spectrum is to the eye what the
gamut is to the ear; each colour represents a note, and _the different
colours represent notes of different pitch_. The vibrations which
produce the impression of red are _slower_, and the waves which they
produce are _longer_, than those to which we owe the sensation of
violet; while the vibrations which excite the other colours are
intermediate between these two extremes. This, then, is the second grand
analogy between light and sound: _Colour answers to Pitch_. There is
therefore truth in the figure when we say that the gentian of the Alps
sings a shriller note than the wild rhododendron, and that the red glow
of the mountains at sunset is of a lower pitch than the blue of the
firmament at noon.
[Sidenote: LENGTH OF ETHEREAL WAVES.]
These are not fanciful analogies. To the mind of the philosopher these
waves of ether are almost as palpable and certain as the waves of the
sea, or the ripples on the surface of a lake. The length of the waves,
both of sound and light, and the number of shocks which they
respectively impart to the ear and eye, have been the subjects of the
strictest measurement. Let us here go through a simple calculation. It
has been found that 39,000 waves of red light placed end to end would
make up an inch. How many inches are there in 192,000 miles? My youngest
reader can make the calculation for himself, and find the answer to be
12,165,120,000 inches. It is evident that, if we multiply this number by
39,000, we shall obtain the number of waves of red light in 192,000
miles; this number is 474,439,680,000,000. _All these waves enter the
eye in one second_; thus the expression "I see red colour," strictly
means, "My eye is now in receipt of four hundred and seventy-four
millions of millions of impulses per second." To produce the impression
of violet light a still greater number of impulses is necessary; the
wave-length of violet is the 1/57500th part of an inch, and the number
of shocks imparted in a second by waves of this length is, in round
numbers, six hundred and ninety-nine millions of millions. The other
colours of the spectrum, as already stated, rise gradually in pitch from
the red to the violet.
A very curious analogy between the eye and ear may here be noticed. The
range of seeing is different in different persons--some see a longer
spectrum than others; that is to say, rays which are obscure to some are
luminous to others. Dr. Wollaston pointed out a similar fact as regards
hearing; the range of which differs in different individuals. Savart has
shown that a good ear can hear a musical note produced by 8 shocks in a
second; it can also hear a note produced by 24,000 shocks in a second;
but there are ears in which the range is much more limited. It is
possible indeed to produce a sound which shall be painfully shrill to
one person, while it is quite unheard by another. I once crossed a Swiss
mountain in company with a friend; a donkey was in advance of us, and
the dull tramp of the animal was plainly heard by my companion; but to
me this sound was almost masked by the shrill chirruping of innumerable
insects which thronged the adjacent grass; my friend heard nothing of
this, it lay quite beyond his range of hearing.
A third and most important analogy between sound and light is now to be
noted; and it will be best understood by reference to something more
tangible than either. When a stone is thrown into calm water a series of
rings spread themselves around the centre of disturbance. If a second
stone be thrown in at some distance from the first, the rings emanating
from both centres will cross each other, and at those points where the
ridge of one wave coincides with the ridge of another the water will be
lifted to a greater height. At those points, on the contrary, where the
ridge of one wave crosses the furrow of another, we have both
obliterated, and the water restored to its ordinary level. Where two
ridges or two furrows unite, we have a case of _coincidence_; but where
a ridge and a furrow unite we have what is called _interference_. It is
quite possible to send two systems of waves into the same channel, and
to hold back one system a little, so that its ridges shall coincide
with the furrows of the other system. The "interference" would be here
complete, and the waves thus circumstanced would mutually destroy each
other, smooth water being the result. In this way, by the addition of
motion to motion, _rest_ may be produced.
[Sidenote: LIGHT ADDED TO LIGHT MAKES DARKNESS.]
In a precisely similar manner two systems of sonorous waves can be
caused to interfere and mutually to destroy each other: thus, by adding
sound to sound, _silence_ may be produced. Two beams of light also may
be caused to interfere and effect their mutual extinction: thus, by
adding light to light, we can produce _darkness_. Here indeed we have a
critical analogy between sound and light--_the_ one, in fact, which
compels the most profound thinkers of the present day to assume that
light, like sound, is a case of undulatory motion.
We see here the vision of the intellect prolonged beyond the boundaries
of sense into the region of what might be considered mere imagination.
But, unlike other imaginations, we can bring ours to the test of
experiment; indeed, so great a mastery have we obtained over these
waves, which eye has not seen, nor ear heard, that we can with
mathematical certainty cause them to coincide or to interfere, to help
each other or to destroy each other, at pleasure. It is perhaps possible
to be a little more precise here. Let two stones--with a small distance
between them--be dropped into water at the same moment; a system of
circular waves will be formed round each stone. Let the distance from
one little crest to the next following one be called _the length of the
wave_, and now let us inquire what will take place at a point equally
distant from the places where the two stones were dropped in. Fixing our
attention upon the ridge of the first wave in each case, it is manifest
that, as the water propagates both systems with the same velocity, the
two foremost ridges will reach the point in question at the same
moment; the ridge of one would therefore coincide with the ridge of the
other, and the water at this point would be lifted to a height greater
than that of either of the previous ridges.
[Sidenote: COINCIDENCE AND INTERFERENCE.]
Again, supposing that by any means we had it in our power to retard one
system of waves so as to cause the first ridge of the one to be exactly
one wave length behind the first ridge of the other, when they arrive at
the point referred to. It is plain that the first ridge of the retarded
system now falls in with the second ridge of the unretarded system, and
we have another case of coincidence. A little reflection will show the
same to be true when one system is retarded any number of _whole
wave-lengths_; the first ridge of the retarded system will always, at
the point referred to, coincide with a _ridge_ of the unretarded system.
But now suppose the one system to be retarded only _half a wave-length_;
it is perfectly clear that, in this case the first ridge of the retarded
system would fall in with the first _furrow_ of the unretarded system,
and instead of coincidence we should have interference. One system, in
fact, would tend to make a hollow at the point referred to, the other
would tend to make a hill, and thus the two systems would oppose and
neutralize each other, so that neither the hollow nor the hill would be
produced; the water would maintain its ordinary level. What is here said
of a single half-wave-length of retardation, is also true if the
retardation amount to any _odd_ number of half-wave-lengths. In all such
cases we should have the ridge of the one system falling in with the
furrow of the other; a mutual destruction of the waves of both systems
being the consequence. The same remarks apply when the point, instead of
being equally distant from both stones, is an even or an odd number of
semi-undulations farther from the one than from the other. In the former
case we should have coincidence, and in the latter case interference,
at the point in question.
[Sidenote: LIQUID WAVES.]
To the eye of a person who understands these things, nothing can be more
interesting than the rippling of water under certain circumstances. By
the action of interference its surface is sometimes shivered into the
most beautiful mosaic, shifting and trembling as if with a kind of
visible music. When the tide advances over a sea-beach on a calm and
sunny day, and its tiny ripples enter, at various points, the clear
shallow pools which the preceding tide had left behind, the little
wavelets run and climb and cross each other, and thus form a lovely
_chasing_, which has its counterpart in the lines of light converged by
the ripples upon the sand underneath. When waves are skilfully generated
in a vessel of mercury, and a strong light reflected from the surface of
the metal is received upon a screen, the most beautiful effects may be
observed. The shape of the vessel determines, in part, the character of
the figures produced; in a circular dish of mercury, for example, a
disturbance at the centre propagates itself in circular waves, which
after reflection again encircle the centre. If the point of disturbance
be a little removed from the centre, the intersections of the direct and
reflected waves produce the magnificent chasing shown in the annexed
figure (16), which I have borrowed from the excellent work on Waves by
the Messrs. Weber. The luminous figure reflected from such a surface is
exceedingly beautiful. When the mercury is lightly struck by a glass
point, in a direction concentric with the circumference of the vessel,
the lines of light run round the vessel in mazy coils, interlacing and
unravelling themselves in the most wonderful manner. If the vessel be
square, a splendid mosaic is produced by the crossing of the direct and
reflected waves. Description, however, can give but a feeble idea of
these exquisite effects;--
"Thou canst not wave thy staff in the air,
Or dip thy paddle in the lake,
But it carves the brow of beauty there,
And the ripples in rhymes the oar forsake."
[Sidenote: CHASING PRODUCED BY WAVES.]
[Illustration: Fig. 16. Chasing produced by waves.]
[Sidenote: EFFECT OF RETARDATION.]
Now, all that we have said regarding the retardation of the waves of
water, by a whole undulation and a semi-undulation, is perfectly
applicable to the case of light. Two luminous points may be placed near
to each other so as to resemble the two stones dropped into the water;
and when the light of these is properly received upon a screen, or
directly upon the retina, we find that at some places the action of the
rays upon each other produces darkness, and at others augmented light.
The former places are those where the rays emitted from one point are an
_odd_ number of semi-undulations in advance of the rays sent from the
other; the latter places are those where the difference of path
described by the rays is either nothing, or an _even_ number of
semi-undulations. Supposing _a_ and _b_ (Fig. 17) to be two such
sources of light, and S R a screen on which the light falls; at a point
_l_, equally distant from _a_ and _b_, we have _light_; at a point _d_,
where _a d_ is half an undulation longer than _b d_, we have darkness;
at _l'_, where _a l'_ is a whole wave-length, or two semi-undulations,
longer than _b l'_, we again have light; and at a point _d'_, where the
difference is three semi-undulations, we have darkness; and thus we
obtain a series of bright and dark spaces as we recede laterally from
the central point _l_.
[Illustration: Fig. 17. Diagram explanatory of Interference.]
Let a bit of tin foil be closely pasted upon a piece of glass, and the
edge of a penknife drawn across the foil so as to produce a slit.
Looking through this slit at a small and distant light, we find the
light spread out in a direction at right angles to the slit, and if the
light looked at be _monochromatic_, that is, composed of a single
colour, we shall have a series of bright and dark bars corresponding to
the points at which the rays from the different points of the slit
alternately coincide and interfere upon the retina. By properly
drawing a knife across a sheet of letter-paper a suitable slit may also
be obtained; and those practised in such things can obtain the effect by
looking through their fingers or their eyelashes.
[Illustration: INTERFERENCE SPECTRA, PRODUCED BY DIFFRACTION.
Fig. 18. _To face_ p. 235.]
[Sidenote: CHROMATIC EFFECTS.]
But if the light looked at be white, the light of a candle for example,
or of a jet of gas, instead of having a series of bright and dark bars,
we have the bars _coloured_. And see how beautifully this harmonizes
with what has been already said regarding the different lengths of the
waves which produce different colours. Looking again at Fig. 17 we see
that a certain obliquity is necessary to cause one ray to be a whole
undulation in advance of the other at the point _l'_; but it is
perfectly manifest that the obliquity must depend upon the length of the
undulation; a long undulation would require a greater obliquity than a
short one; red light, for example, requires a greater obliquity than
blue light; so that if the point _l'_ represents the place where the
first bar of red light would be at its maximum strength, the maximum for
blue would lie a little to the left of _l'_; the different colours are
in this way separated from each other, and exhibit themselves as
distinct fringes when a distant source of white light is regarded
through a narrow slit.
By varying the shape of the aperture we alter the form of the chromatic
image. A circular aperture, for example, placed in front of a telescope
through which a point of white light is regarded, is seen surrounded by
a concentric system of coloured rings. If we multiply our slits or
apertures the phenomena augment in complexity and splendour. To give
some notion of this I have copied from the excellent work of M. Schwerd
the annexed figure (Fig. 18) which represents the gorgeous effect
observed when a distant point of light is looked at through two gratings
with slits of different widths.[B] A bird's feather represents a
peculiar system of slits, and the effect observed on properly looking
through it is extremely interesting.
[Sidenote: COLOURS OF THIN FILMS.]
There are many ways by which the retardation necessary to the production
of interference is effected. The splendid colours of a soap-bubble are
entirely due to interference; the beam falling upon the transparent film
is partially reflected at its outer surface, but a portion of it enters
the film and is reflected at its _inner_ surface. The latter portion
having crossed the film and returned, is retarded, in comparison with
the former, and, if the film be of suitable thickness, these two beams
will clash and extinguish each other, while another thickness will cause
the beams to coincide and illuminate the film with a light of greater
intensity. From what has been said it must be manifest that to make two
red beams thus coincide a thicker film would be required than would be
necessary for two blue or green beams; thus, when the thickness of the
bubble is suitable for the development of red, it is not suitable for
the development of green, blue, &c.; the consequence is that we have
different colours at different parts of the bubble. Owing to its
compactness and to its being shaded by a covering of debris from the
direct heat of the sun, the ice underneath the moraines of glaciers
appears sometimes of a pitchy blackness. While cutting such ice with my
axe I have often been surprised and delighted by sudden flashes of
coloured light which broke like fire from the mass. These flashes were
due to internal rupture, by which fissures were produced as thin as the
film of a soap-bubble; the colours being due to the interference of the
light reflected from the opposite sides of the fissures.
If spirit of turpentine, or olive oil, be thrown upon water, it speedily
spreads in a thin film over the surface, and the most gorgeous
chromatic phenomena may be thus produced. Oil of lemons is also
peculiarly suited to this experiment. If water be placed in a tea-tray,
and light of sufficient intensity be suffered to fall upon it, this
light will be reflected from the upper and under surfaces of the film of
oil, and the colours thus produced may be received upon a screen, and
seen at once by many hundred persons. If the oil of cinnamon be used,
fine colours are also obtained, and the breaking up of this film
exhibits a most interesting case of molecular action. By using a kind of
varnish, instead of oil, Mr. Delarue has imparted such tenacity to these
films that they may be removed from the water on which they rest and
preserved for any length of time. By such films the colours of certain
beetles, and of the wings of certain insects, may be accurately
imitated; and a rook's feather may be made to shine with magnificent
iridescences. The colours of tempered metals, and the beautiful
metallochrome of Nobili are also due to a similar cause.
[Sidenote: DIFFRACTION.]
These colours are called the colours of _thin plates_, and are
distinguished in treatises on optics from the coloured bars and fringes
above referred to, which are produced by _diffraction_, or the bending
of the waves round the edge of an object. One result of this bending,
which is of interest to us, was obtained by the celebrated Thomas Young.
Permitting a beam of sunlight to enter a dark room through an aperture
made with a fine needle, and placing in the path of the beam a bit of
card one-thirtieth of an inch wide, he found the shadow of this card, or
rather the line on which its shadow might be supposed to fall, always
_bright_; and he proved the effect to be due to the bending of the waves
of ether round the two edges of the card, and their coincidence at the
other side. It has, indeed, been shown by M. Poisson, that the centre of
the shadow of a small circular opaque disk which stands in the way of a
beam diverging from a point is exactly as much illuminated as if the
disk were absent. The singular effects described by M. Necker in the
letter quoted at page 178 at once suggest themselves here; and we see
how possible it is for the solar rays, in grazing a distant tree, so to
bend round it as to produce upon the retina, where shadow might be
expected, the impression of a tree of light.[C] Another effect of
diffraction is especially interesting to us at present. Let the seed of
lycopodium be scattered over a glass plate, or even like a cloud in the
air, and let a distant point of light be regarded through it; the
luminous point will appear surrounded by a series of coloured rings, and
when the light is intense, like the electric or the Drummond light, the
effect is exceedingly fine.
[Sidenote: CLOUD IRIDESCENCE, ETC., EXPLAINED.]
And now for the application of these experiments. I have already
mentioned a series of coloured rings observed around the sun by Mr.
Huxley and myself from the Rhone glacier; I have also referred to the
cloud iridescences on the Aletschhorn; and to the colours observed
during my second ascent of Monte Rosa, the magnificence of which is
neither to be rendered by pigments nor described in words. All these
splendid phenomena are, I believe, produced by diffraction, the vesicles
or spherules of water in the case of the cloud acting the part of the
sporules in the case of the lycopodium. The coloured fringe which
surrounds the _Spirit of the Brocken_, and the spectra which I have
spoken of as surrounding the sun, are also produced by diffraction. By
the interference of their rays in the earth's atmosphere the stars can
momentarily quench themselves; and probably to an intermittent action of
this kind their twinkling, and the swift chromatic changes already
mentioned, are due. Does not all this sound more like a fairy tale than
the sober conclusions of science? What effort of the imagination could
transcend the realities here presented to us? The ancients had their
spheral melodies, but have not we ours, which only want a sense
sufficiently refined to hear them? Immensity is filled with this music;
wherever a star sheds its light its notes are heard. Our sun, for
example, thrills concentric waves through space, and every luminous
point that gems our skies is surrounded by a similar system. I have
spoken of the rising, climbing and crossing of the tiny ripples of a
calm tide upon a smooth strand; but what are they to those intersecting
ripples of the "uncontinented deep" by which Infinity is engine-turned!
Crossing solar and stellar distances, they bring us the light of sun and
stars; thrilled back from our atmosphere, they give us the blue radiance
of the sky; rounding liquid spherules, they clash at the other side, and
the survivors of the tumult bear to our vision the wondrous cloud-dyes
of Monte Rosa.
FOOTNOTES:
[A] The vibrations of the air of a room in which a musical instrument is
sounded may be made manifest by the way in which fine sand arranges
itself upon a thin stretched membrane over which it is strewn; and
indeed Savart has thus rendered visible the vibrations of the tympanum
itself. Every trace of sand was swept from a paper drum held in the
clock-tower of Westminster when the Great Bell was sounded. Another way
of showing the propagation of aerial pulses is to insert a small gas jet
into a vertical glass tube about a foot in length, in which the flame
may be caused to burn tranquilly. On pitching the voice to the note of
an open tube a foot long, the little flame quivers, stretches itself,
and responds by producing a clear melodious note of the same pitch as
that which excited it. The flame will continue its song for hours
without intermission.
[B] I am not aware whether in his own country, or in any other, a
recognition at all commensurate with the value of the performance has
followed Schwerd's admirable essay entitled 'The Phenomena of
Diffraction deduced from the Theory of Undulation.'
[C] I think, however, that the strong irradiation from the glistening
sides of the twigs and branches must also contribute to the result.
[Sidenote: RADIANT HEAT.]
(2.)
Thus, then, we have been led from Sound to Light, and light now in its
turn will lead us to _Radiant Heat_; for in the order in which they are
here mentioned the conviction arose that they are all three different
kinds of motion. It has been said that the beams of the sun consist of
rays of different colours, but this is not a complete statement of the
case. The sun emits a multitude of rays which are perfectly
non-luminous; and the same is true, in a still greater degree, of our
artificial sources of illumination. Measured by the quantity of heat
which they produce, 90 per cent. of the rays emanating from a flame of
oil are obscure; while 99 out of every 100 of those which emanate from
an alcohol flame are of the same description.[A]
[Sidenote: OBSCURE RAYS.]
In fact, the visible solar spectrum simply embraces an interval of rays
of which the eye is formed to take cognizance, but it by no means marks
the limits of solar action. Beyond the violet end of the spectrum we
have obscure rays capable of producing chemical changes, and beyond the
red we have rays possessing a high heating power, but incapable of
exciting the impression of light. This latter fact was first established
by Sir William Herschel, and it has been amply corroborated since.
The belief now universally prevalent is, that the rays of heat differ
from the rays of light simply as one colour differs from another. As the
waves which produce red are longer than those which produce yellow, so
the waves which produce this obscure heat are longer than those which
produce red. In fact, it may be shown that the longest waves never reach
the retina at all; they are completely absorbed by the humours of the
eye.
What is true of the sun's obscure rays is also true of calorific rays
emanating from any obscure source,--from our own bodies, for example, or
from the surface of a vessel containing boiling water. We must, in fact,
figure a warm body also as having its particles in a state of vibration.
When these motions are communicated from particle to particle of the
body the heat is said to be _conducted_; when, on the contrary, the
particles transmit their vibrations through the surrounding ether, the
heat is said to be _radiant_. This radiant heat, though obscure,
exhibits a deportment exactly similar to light. It may be refracted and
reflected, and collected in the focus of a mirror or of a suitable lens.
The principle of interference also applies to it, so that by adding heat
to heat we can produce _cold_. The identity indeed is complete
throughout, and, recurring to the analogy of sound, we might define
this radiant heat to be light of too low a pitch to be visible.
I have thus far spoken of _obscure_ heat only; but the selfsame ray may
excite both light and heat. The red rays of the spectrum possess a very
high heating power. It was once supposed that the heat of the spectrum
was an essence totally distinct from its light; but a profounder
knowledge dispels this supposition, and leads us to infer that the
selfsame ray, falling upon the nerves of feeling, excites heat, and
falling upon the nerves of seeing, excites light. As the same electric
current, if sent round a magnetic needle, along a wire, and across a
conducting liquid, produces different physical effects, so also the same
agent acting upon different organs of the body affects our consciousness
differently.
FOOTNOTES:
[A] Melloni.
(3.)
[Sidenote: HEAT A KIND OF MOTION.]
Heat has been defined in the foregoing section as a motion of the
molecules or atoms of a body; but though the evidence in favour of this
view is at present overwhelming, I do not ask the reader to accept it as
a certainty, if he feels sceptically disposed. In this case, I would
only ask him to accept it as a symbol. Regarded as a mere physical
image, a kind of paper-currency of the mind, convertible, in due time,
into the gold of truth, the hypothesis will be found exceedingly useful.
All known bodies possess more or less of this molecular motion, and all
bodies are communicating it to the ether in which they are immersed. Ice
possesses it. Ice before it melts attains a temperature of 32 deg. Fahr.,
but the substance in winter often possesses a temperature far below 32 deg.,
so that in rising to 32 deg. it is _warmed_. In experimenting with ice I
have often had occasion to cool it to 100 deg. and more below the freezing
point, and to warm it afterwards up to 32 deg.
If then we stand before a wall of ice, the wall radiates heat to us, and
we also radiate heat to it; but the quantity which we radiate being
greater than that which the ice radiates, we lose more than we gain, and
are consequently chilled. If, on the contrary, we stand before a warm
stove, a system of exchanges also takes place; but here the quantity we
receive is in excess of the quantity lost, and we are warmed by the
difference.
In like manner the earth radiates heat by day and by night into space,
and against the sun, moon, and stars. By day, however, the quantity
received is greater than the quantity lost, and the earth is warmed; by
night the conditions are reversed; the earth radiates more heat than is
sent to her by the moon and stars, and she is consequently cooled.
But here an important point is to be noted:--the earth receives the heat
of the sun, moon, and stars, in great part as _luminous_ heat, but she
gives it out as _obscure_ heat. I do not now speak of the heat reflected
by the earth into space, as the light of the moon is to us; but of the
heat which, after it has been absorbed by the earth, and has contributed
to warm it, is radiated into space, as if the earth itself were its
independent source. Thus we may properly say that the heat radiated from
the earth is _different in quality_ from that which the earth has
received from the sun.
[Sidenote: QUALITIES OF HEAT.]
In one particular especially does this difference of quality show
itself; besides being non-luminous, the heat radiated from the earth is
more easily intercepted and absorbed by almost all transparent
substances. A vast portion of the sun's rays, for example, can pass
instantaneously through a thick sheet of water; gunpowder could easily
be fired by the heat of the sun's rays converged by passing through a
thick water lens; the drops upon leaves in greenhouses often act as
lenses, and cause the sun to burn the leaves upon which they rest. But
with regard to the rays of heat emanating from an obscure source, they
are all absorbed by a layer of water less than the 20th of an inch in
thickness: water is opaque to such rays, and cuts them off almost as
effectually as a metallic screen. The same is true of other liquids, and
also of many transparent solids.
[Sidenote: THE ATMOSPHERE LIKE A RATCHET.]
Assuming the same to be true of gaseous bodies, that they also intercept
the obscure rays much more readily than the luminous ones, it would
follow that while the sun's rays penetrate our atmosphere with freedom,
the change which they undergo in warming the earth deprives them in a
measure of this penetrating power. They can reach the earth, but _they
cannot get back_; thus the atmosphere acts the part of a ratchet-wheel
in mechanics; it allows of motion in one direction, but prevents it in
the other.
De Saussure, Fourier, M. Pouillet, and Mr. Hopkins have developed this
speculation, and drawn from it consequences of the utmost importance;
but it nevertheless rested upon a basis of conjecture. Indeed some of
the eminent men above-named deemed its truth beyond the possibility of
experimental verification. Melloni showed that for a distance of 18 or
20 feet the absorption of obscure rays by the atmosphere was absolutely
inappreciable. Hence, the _total_ absorption being so small as to elude
even Melloni's delicate tests, it was reasonable to infer that
_differences_ of absorption, if such existed at all, must be far beyond
the reach of the finest means which we could apply to detect them.
[Sidenote: DIFFERENCES OF ABSORPTION BY GASES.]
This exclusion of one of the three states of material aggregation from
the region of experiment was, however, by no means satisfactory; for our
right to infer, from the deportment of a solid or a liquid towards
radiant heat, the deportment of a gas, is by no means evident. In both
liquids and solids we have the molecules closely packed, and more or
less chained by the force of cohesion; in gases, on the contrary, they
are perfectly free, and widely separated. How do we know that the
interception of radiant heat by liquids and solids may not be due to an
arrangement and comparative rigidity of their parts, which gases do not
at all share? The assumption which took no note of such a possibility
seemed very insecure, and called for verification.
My interest in this question was augmented by the fact, that the
assumption referred to lies, as will be seen, at the root of the glacier
question. I therefore endeavoured to fill the gap, and to do for gases
and vapours what had been already so ably done for liquids and solids by
Melloni. I tried the methods heretofore pursued, and found them
unavailing; oxygen, hydrogen, nitrogen, and atmospheric air, examined by
such methods, showed no action upon radiant heat. Nature was dumb, but
the question occurred, "Had she been addressed in the proper language?"
If the experimentalist is convinced of this, he will rest content even
with a negative; but the absence of this conviction is always a source
of discomfort, and a stimulus to try again.
The principle of the method finally applied is all that can here be
referred to; and it, I hope, will be quite intelligible. Two beams of
heat, from two distinct sources, were allowed to fall upon the same
instrument,[A] and to contend there for mastery. When both beams were
perfectly equal, they completely neutralized each other's action; but
when one of them was in any sensible degree stronger than the other, the
predominance of the former was shown by the instrument. It was so
arranged that one of the conflicting beams passed through a tube which
could be exhausted of air, or filled with any gas; thus varying at
pleasure the medium through which it passed. The question then was,
supposing the two beams to be equal when the tube was filled with air,
will the exhausting of the tube disturb the equality? The answer was
affirmative; the instrument at once showed that a greater quantity of
heat passed through the vacuum than through the air.
The experiment was so arranged that the effect thus produced was very
large as measured by the indications of the instrument. But the action
of the simple gases, oxygen, hydrogen, and nitrogen, was incomparably
less than that produced by some of the compound gases, while these
latter again differed widely from each other. Vapours exhibited
differences of equal magnitude. The experiments indeed proved that
gaseous bodies varied among themselves, as to their power of
transmitting radiant heat, just as much as liquids and solids. It was in
the highest degree interesting to observe how a gas or vapour of perfect
transparency, as regards light, acted like an opaque screen upon the
heat. To the eye, the gas within the tube might be as invisible as the
air itself, while to the radiant heat it behaved like a cloud which it
was almost impossible to penetrate.
[Sidenote: SELECTED HEAT.]
Applying the same method, I have found that from the sun, from the
electric light, or from the lime-light, a large amount of heat can be
selected, which is unaffected not only by air, but by the most energetic
gases that experiment has revealed to me; while this same heat, when it
has its _quality_ changed by being rendered obscure, is powerfully
intercepted. Thus the bold and beautiful speculation above referred to
has been made an experimental fact; the radiant heat of the sun does
certainly pass through the atmosphere to the earth with greater
facility than the radiant heat of the earth can escape into space.
[Sidenote: POSSIBLE HEAT OF NEPTUNE.]
It is probable that, were the earth unfurnished with this atmospheric
swathing, its conditions of temperature would be such as to render it
uninhabitable by man; and it is also probable that a suitable atmosphere
enveloping the most distant planet might render it, as regards
temperature, perfectly habitable. If the planet Neptune, for example, be
surrounded by an atmosphere which permits the solar and stellar rays to
pass towards the planet, but cuts off the escape of the warmth which
they excite, it is easy to see that such an accumulation of heat may at
length take place as to render the planet a comfortable habitation for
beings constituted like ourselves.[B]
But let us not wander too far from our own concerns. Where radiant heat
is allowed to fall upon an absorbing substance, a certain thickness of
the latter is always necessary for the absorption. Supposing we place a
thin film of glass before a source of heat, a certain percentage of the
heat will pass through the glass, and the remainder will be absorbed.
Let the transmitted portion fall upon a second film similar to the
first, a smaller percentage than before will be absorbed. A third plate
would absorb still less, a fourth still less; and, after having passed
through a sufficient number of layers, the heat would be so _sifted_
that all the rays capable of being absorbed by glass would be abstracted
from it. Suppose all these films to be placed together so as to form a
single thick plate of glass, it is evident that the plate must act upon
the heat which falls upon it, in such a manner that the major portion is
absorbed _near the surface at which the heat enters_. This has been
completely verified by experiment.
[Sidenote: COLD OF UPPER ATMOSPHERE.]
Applying this to the heat radiated from the earth, it is manifest that
the greatest quantity of this heat will be absorbed by the lowest
atmospheric strata. And here we find ourselves brought, by
considerations apparently remote, face to face with the fact upon which
the existence of all glaciers depends, namely, the comparative coldness
of the upper regions of the atmosphere. The sun's rays can pass in a
great measure through these regions without heating them; and the
earth's rays, which they might absorb, hardly reach them at all, but are
intercepted by the lower portions of the atmosphere.[C]
Another cause of the greater coldness of the higher atmosphere is the
expansion of the denser air of the lower strata when it ascends. The
dense air makes room for itself by pushing back the lighter and less
elastic air which surrounds it: _it does work_, and, to perform this
work, a certain amount of heat must be consumed. It is the consumption
of this heat--its absolute annihilation as heat--that chills the
expanded air, and to this action a share of the coldness of the higher
atmosphere must undoubtedly be ascribed. A third cause of the difference
of temperature is the large amount of heat communicated, _by way of
contact_, to the air of the earth's surface; and a fourth and final
cause is the loss endured by the highest strata through radiation into
space.
FOOTNOTES:
[A] The opposite faces of a thermo-electric pile.
[B] See a most interesting paper on this subject by Mr. Hopkins in the
Cambridge 'Transactions,' May, 1856.
[C] See M. Pouillet's important Memoir on Solar Radiation. Taylor's
Scientific Memoirs, vol. iv. p. 44.
ORIGIN OF GLACIERS.
(4.)
[Sidenote: THE SNOW-LINE.]
Having thus accounted for the greater cold of the higher atmospheric
regions, its consequences are next to be considered. One of these is,
that clouds formed in the lower portions of the atmosphere, in warm and
temperate latitudes, usually discharge themselves upon the earth as
rain; while those formed in the higher regions discharge themselves upon
the mountains as snow. The snow of the higher atmosphere is often melted
to rain in passing through the warmer lower strata: nothing indeed is
more common than to pass, in descending a mountain, from snow to rain;
and I have already referred to a case of this kind. The appearance of
the grassy and pine-clad alps, as seen from the valleys after a wet
night, is often strikingly beautiful; the level at which the snow turned
to rain being distinctly marked upon the slopes. Above this level the
mountains are white, while below it they are green. The eye follows this
_snow-line_ with ease along the mountains, and when a sufficient extent
of country is commanded its regularity is surprising.
The term "snow-line," however, which has been here applied to a local
and temporary phenomenon, is commonly understood to mean something else.
In the case just referred to it marked the place where the supply of
solid matter from the upper atmospheric regions, during a single fall,
was exactly equal to its consumption; but the term is usually understood
to mean the line along which the quantity of snow which falls _annually_
is melted, and no more. Below this line each year's snow is completely
cleared away by the summer heat; above it a residual layer abides,
which gradually augments in thickness from the snow-line upwards.
[Sidenote: MOUNTAINS UNLOADED BY GLACIERS.]
Here then we have a fresh layer laid on every year; and it is evident
that, if this process continued without interruption, every mountain
which rises above the snow-line must augment annually in height; the
waters of the sea thus piled, in a solid form, upon the summits of the
hills, would raise the latter to an indefinite elevation. But, as might
be expected, the snow upon steep mountain-sides frequently slips and
rolls down in avalanches into warmer regions, where it is reduced to
water. A comparatively small quantity of the snow is, however, thus got
rid of, and the great agent which Nature employs to relieve her
overladen mountains is the glaciers.
Let us here avoid an error which may readily arise out of the foregoing
reflections. The principal region of clouds and rain and snow extends
only to a limited distance upwards in the atmosphere; the highest
regions contain very little moisture, and were our mountains
sufficiently lofty to penetrate those regions, the quantity of snow
falling upon their summits would be too trifling to resist the direct
action of the solar rays. These would annually clear the summits to a
certain level, and hence, were our mountains high enough, we should have
a superior, as well as an inferior, snow-line; the region of perpetual
snow would form a belt, below which, in summer, snowless valleys and
plains would extend, and above which snowless summits would rise.
(5.)
[Sidenote: WHITE AND BLUE ICE.]
At its origin then a glacier is snow--at its lower extremity it is ice.
The blue blocks that arch the source of the Arveiron were once powdery
snow upon the slopes of the Col du Geant. Could our vision penetrate
into the body of the glacier, we should find that the change from white
to blue essentially consists in the gradual expulsion of the air which
was originally entangled in the meshes of the fallen snow. Whiteness
always results from the intimate and irregular mixture of air and a
transparent solid; a crushed diamond would resemble snow; if we pound
the most transparent rock-salt into powder we have a substance as white
as the whitest culinary salt; and the colourless glass vessel which
holds the salt would also, if pounded, give a powder as white as the
salt itself. It is a law of light that in passing from one substance to
another possessing a different power of refraction, a portion of it is
always reflected. Hence when light falls upon a transparent solid mixed
with air, at each passage of the light from the air to the solid and
from the solid to the air a portion of it is reflected; and, in the case
of a powder, this reflection occurs so frequently that the passage of
the light is practically cut off. Thus, from the mixture of two
perfectly transparent substances, we obtain an opaque one; from the
intimate mixture of air and water we obtain foam; clouds owe their
opacity to the same principle; and the condensed steam of a locomotive
casts a shadow upon the fields adjacent to the line, because the
sunlight is wasted in echoes at the innumerable limiting surfaces of
water and air.
[Sidenote: AIR-BUBBLES IN ICE.]
The snow which falls upon high mountain-eminences has often a
temperature far below the freezing point of water. Such snow is _dry_,
and if it always continued so the formation of a glacier from it would
be impossible. The first action of the summer's sun is to raise the
temperature of the superficial snow to 32 deg., and afterwards to melt it.
The water thus formed percolates through the colder mass underneath, and
this I take to be the first active agency in expelling the air
entangled in the snow. But as the liquid trickles over the surfaces of
granules colder than itself it is partially deposited in a solid form on
these surfaces, thus augmenting the size of the granules, and cementing
them together. When the mass thus formed is examined, the air within it
is found as _round bubbles_. Now it is manifest that the air caught in
the irregular interstices of the snow can have no tendency to assume
this form so long as the snow remains solid; but the process to which I
have referred--the saturation of the lower portions of the snow by the
water produced by the melting of the superficial portions--enables the
air to form itself into globules, and to give the ice of the _neve_ its
peculiar character. Thus we see that, though the sun cannot get directly
at the deeper portions of the snow, by liquefying the upper layer he
charges it with heat, and makes it his messenger to the cold subjacent
mass.
The frost of the succeeding winter may, I think, or may not, according
to circumstances, penetrate through this layer, and solidify the water
which it still retains in its interstices. If the winter set in with
clear frosty weather, the penetration will probably take place; but if
heavy snow occur at the commencement of winter, thus throwing a
protective covering over the _neve_, freezing to any great depth may be
prevented. Mr. Huxley's idea seems to be quite within the range of
possibility, that water-cells may be transmitted from the origin of the
glacier to its end, retaining their contents always liquid.
[Sidenote: SNOW PRESSED TO ICE.]
It was formerly supposed, and is perhaps still supposed by many, that
the snow of the mountains is converted into the ice of the glacier by
the process of saturation and freezing just indicated. But the frozen
layer would not yet resemble glacier ice; it is only at the deeper
portions of the _neve_ that we find an approximation to the true ice of
the glacier. This brings us to the second great agent in the process of
glacification, namely, pressure. The ice of the _neve_ at 32 deg. may be
squeezed or crushed with extreme facility; and if the force be applied
slowly and with caution, the yielding of the mass may be made to
resemble the yielding of a plastic body. In the depths of the _neve_,
where each portion of the ice is surrounded by a resistant mass, rude
crushing is of course out of the question. The layers underneath yield
with extreme slowness to the pressure of the mass above them; they are
squeezed, but not rudely fractured; and even should rude fracture occur,
the ice, as shall subsequently be shown, possesses the power of
restoring its own continuity. Thus, then, the lower portions of the
_neve_ are removed by pressure more and more from the condition of snow,
the air-bubbles which give to the _neve_-ice its whiteness are more and
more expelled, and this process, continued throughout the entire
glacier, finally brings the ice to that state of magnificent
transparency which we find at the termination of the glacier of
Rosenlaui and elsewhere. This is all capable of experimental proof. The
Messrs. Schlagintweit compressed the snow of the _neve_ to compact ice;
and I have myself frequently obtained slabs of ice from snow in London.
COLOUR OF WATER AND ICE.
(6.)
The sun is continually sending forth waves of different lengths, all of
which travel with the same velocity through the ether. When these waves
enter a prism of glass they are retarded, but in different degrees. The
shorter waves suffer the greatest retardation, and in consequence of
this are most deflected from their straight course. It is this property
which enables us to separate one from the other in the solar spectrum,
and this separation proves that the waves are by no means inextricably
entangled with each other, but that they travel independently through
space.
In consequence of this independence, the same body may intercept one
system of waves while it allows another to pass: on this quality,
indeed, depend all the phenomena of colour. A red glass, for example, is
red because it is so constituted that it destroys the shorter waves
which produce the other colours, and transmits only the waves which
produce red. I may remark, however, that scarcely any glass is of a pure
colour; along with the predominant waves, some of the other waves are
permitted to pass. The colours of flowers are also very impure; in fact,
to get pure colours we must resort to a delicate prismatic analysis of
white light.
[Sidenote: LONG WAVES MOST ABSORBED.]
It has already been stated that a layer of water less than the twentieth
of an inch in thickness suffices to stop and destroy all waves of
radiant heat emanating from an obscure source. The longer waves of the
obscure heat cannot get through water, and I find that all transparent
compounds which contain _hydrogen_ are peculiarly hostile to the longer
undulations. It is, I think, the presence of this element in the
humours of the eye which prevents the extra red rays of the solar
spectrum from reaching the retina. It is interesting to observe that
while bisulphide of carbon, chloride of phosphorus, and other liquids
which contain no hydrogen, permit a large portion of the rays emanating
from an iron or copper ball, at a heat below redness, to pass through
them with facility, the same thickness of substances equally
transparent, but which contain hydrogen, such as ether, alcohol, water,
or the vitreous humour of the eye of an ox, completely intercepts these
obscure rays. The same is true of solid bodies; a very slight thickness
of those which contain hydrogen offers an impassable barrier to all rays
emanating from a non-luminous source.[A] But the heat thus intercepted
is by no means lost; its _radiant form_ merely is destroyed. Its waves
are shivered upon the particles of the body, but they impart warmth to
it, while the heat which retains its radiant form contributes in no way
to the warmth of the body through which it passes.
[Sidenote: FINAL COLOUR OF ICE AND WATER BLUE.]
Water then absorbs all the extra red rays of the sun, and if the layer
be thick enough it invades the red rays themselves. Thus the greater the
distance the solar beams travel through pure water the more are they
deprived of those components which lie at the red end of the spectrum.
The consequence is, that the light finally transmitted by the water, and
which gives to it its colour, is _blue_.
[Sidenote: EXPERIMENT.]
I find the following mode of examining the colour of water both
satisfactory and convenient:--A tin tube, fifteen feet long and three
inches in diameter, has its two ends stopped securely by pieces of
colourless plate glass. It is placed in a horizontal position, and pure
water is poured into it through a small lateral pipe, until the liquid
reaches half way up the glasses at the ends; the tube then holds a
semi-cylinder of water and a semi-cylinder of air. A white plate, or a
sheet of white paper, well illuminated, is then placed at a little
distance from one end of the tube, and is looked at through the tube.
Two semicircular spaces are then seen, one by the light which has passed
through the air, the other by the light which has passed through the
water; and their proximity furnishes a means of comparison, which is
absolutely necessary in experiments of this kind. It is always found
that, while the former semicircle remains white, the latter one is
vividly coloured.[B]
When the beam from an electric lamp is sent through this tube, and a
convex lens is placed at a suitable distance from its most distant end,
a magnified image of the coloured and uncoloured semicircles may be
projected upon a screen. Tested thus, I have sometimes found, after
rain, the ordinary pipe-water of the Royal Institution quite opaque;
while, under other circumstances, I have found the water of a clear
green. The pump-water of the Institution thus examined exhibits a rich
sherry colour, while distilled water is blue-green.
The blueness of the Grotto of Capri is due to the fact that the light
which enters it has previously traversed a great depth of clear water.
According to Bunsen's account, the _laugs_, or cisterns of hot water, in
Iceland must be extremely beautiful. The water contains silica in
solution, which, as the walls of the cistern arose, was deposited upon
them in fantastic incrustations. These, though white, when looked at
through the water appear of a lovely blue, which deepens in tint as the
vision plunges deeper into the liquid.
[Sidenote: ICE OPAQUE TO RADIANT HEAT.]
Ice is a crystal formed from this blue liquid, the colour of which it
retains. Ice is the most opaque of transparent solids to radiant heat,
as water is the most opaque of liquids. According to Melloni, a plate of
ice one twenty-fifth of an inch thick, which permits the rays of light
to pass without sensible absorption, cuts off 94 per cent. of the rays
of heat issuing from a powerful oil lamp, 99-1/2 per cent. of the rays
issuing from incandescent platinum, and the whole of the rays issuing
from an obscure source. The above numbers indicate how large a portion
of the rays emitted by our artificial sources of light is obscure.
When the rays of light pass through a sufficient thickness of ice the
longer waves are, as in the case of water, more and more absorbed, and
the final colour of the substance is therefore blue. But when the ice is
filled with minute air-bubbles, though we should loosely call it
_white_, it may exhibit, even in small pieces, a delicate blue tint.
This, I think, is due to the frequent interior reflection which takes
place at the surfaces of the air-cells; so that the light which reaches
the eye from the interior may, in consequence of its having been
reflected hither and thither, really have passed through a considerable
thickness of ice. The same remark, as we have already seen, applies to
the delicate colour of newly fallen snow.
FOOTNOTES:
[A] What is here stated regarding hydrogen is true of all the liquids
and solids which have hitherto been examined,--but whether any
exceptions occur, future experience must determine. It is only when in
combination that it exhibits this impermeability to the obscure rays.
[B] In my own experiments I have never yet been able to obtain a pure
blue, the nearest approach to it being a blue-green.
COLOURS OF THE SKY.
(7.)
[Sidenote: NEWTON'S HYPOTHESIS.]
In treating of the Colours of Thin Plates we found that a certain
thickness was necessary to produce blue, while a greater thickness was
necessary for red. With that wonderful power of generalization which
belonged to him, Newton thus applies this apparently remote fact to the
blue of the sky:--"The blue of the first order, though very faint and
little, may possibly be the colour of some substances, and particularly
the azure colour of the skies seems to be of this order. For all
vapours, when they begin to condense and coalesce into small parcels,
become first of that bigness whereby such an azure is reflected, before
they can constitute clouds of other colours. And so, this being the
first colour which vapours begin to reflect, it ought to be the colour
of the finest and most transparent skies, in which vapours are not
arrived at that grossness requisite to reflect other colours, as we find
it is by experience."
M. Clausius has written a most interesting paper, which he endeavours to
show that the minute particles of water which are supposed by Newton to
reflect the light, cannot be little globes entirely composed of water,
but bladders or hollow spheres; the vapour must be in what is generally
termed the _vesicular_ state. He was followed by M. Bruecke, whose
experiments prove that the suspended particles may be so small that the
reasoning of M. Clausius may not apply to them.
But why need we assume the existence of such particles at all?--why not
assume that the colour of the air is blue, and renders the light of the
sun blue, after the fashion of a blue glass or a solution of the
sulphate of copper? I have already referred to the great variation which
the colour of the firmament undergoes in the Alps, and have remarked
that this seems to indicate that the blue depends upon some variable
constituent of the atmosphere. Further, we find that the blue light of
the sky is _reflected_ light; and there must be something in the
atmosphere capable of producing this reflection; but this thing,
whatever it is, produces another effect which the blue glass or liquid
is unable to produce. These _transmit_ blue light, whereas, when the
solar beams have traversed a great length of air, as in the morning or
the evening, they are yellow, or orange, or even blood-red, according to
the state of the atmosphere:--the transmitted light and the reflected
light of the atmosphere are then totally different in colour.
[Sidenote: GOETHE'S HYPOTHESIS.]
Goethe, in his celebrated 'Farbenlehre,' gives a theory of the colour of
the sky, and has illustrated it by a series of striking facts. He
assumed two principles in the universe--Light and Darkness--and an
intermediate stage of Turbidity. When the darkness is seen through a
turbid medium on which the light falls, the medium appears blue; when
the light itself is viewed through such a medium, it is yellow, or
orange, or ruby-red. This he applies to the atmosphere, which sends us
blue light, or red, according as the darkness of infinite space, or the
bright surface of the sun, is regarded through it.
As a theory of colours Goethe's work is of no value, but the facts which
he has brought forward in illustration of the action of turbid media are
in the highest degree interesting. He refers to the blueness of distant
mountains, of smoke, of the lower part of the flame of a candle (which
if looked at with a white surface behind it completely disappears), of
soapy water, and of the precipitates of various resins in water. One of
his anecdotes in connexion with this subject is extremely curious and
instructive. The portrait of a very dignified theologian having suffered
from dirt, it was given to a painter to be cleaned. The clergyman was
drawn in a dress of black velvet, over which the painter, in the first
place, passed his sponge. To his astonishment the black velvet changed
to the colour of blue plush, and completely altered the aspect of its
wearer. Goethe was informed of the fact; the experiment was repeated in
his presence, and he at once solved it by reference to his theory. The
varnish of the picture when mixed with the water formed a turbid medium,
and the black coat seen through it appeared blue; when the water
evaporated the coat resumed its original aspect.
[Sidenote: SUSPENDED PARTICLES.]
With regard to the real explanation of these effects, it may be shown,
that, if a beam of white light be sent through a liquid which contains
extremely minute particles in a state of suspension, the short waves are
more copiously reflected by such particles than the long ones; blue, for
example, is more copiously reflected than red. This may be shown by
various fine precipitates, but the best is that of Bruecke. We know that
mastic and various resins are soluble in alcohol, and are precipitated
when the solution is poured into water: _Eau de Cologne_, for example,
produces a white precipitate when poured into water. If however this
precipitate be sufficiently diluted, it gives the liquid a bluish colour
by reflected light. Even when the precipitate is very thick and gross,
and floats upon the liquid like a kind of curd, its under portions often
exhibit a fine blue. To obtain particles of a proper size, Bruecke
recommends 1 gramme of colourless mastic to be dissolved in 87 grammes
of alcohol, and dropped into a beaker of water, which is kept in a state
of agitation. In this way a blue resembling that of the firmament may be
produced. It is best seen when a black cloth is placed behind the glass;
but in certain positions this blue liquid appears yellow; and these are
the positions when the _transmitted_ light reaches the eye. It is
evident that this change of colour must necessarily exist; for the blue
being partially withdrawn by more copious reflection, the transmitted
light must partake more or less of the character of the complementary
colour; though it does not follow that they should be exactly
complementary to each other.
[Sidenote: THE SUN THROUGH LONDON SMOKE.]
When a long tube is filled with clear water, the colour of the liquid,
as before stated, shows itself by transmitted light. The effect is very
interesting when a solution of mastic is permitted to drop into such a
tube, and the fine precipitate to diffuse itself in the water. The
blue-green of the liquid is first neutralized, and a yellow colour shows
itself; on adding more of the solution the colour passes from yellow to
orange, and from orange to blood-red. With a cell an inch and a half in
width, containing water, into which the solution of mastic is suffered
to drop, the same effect may be obtained. If the light of an electric
lamp be caused to form a clear sunlike disk upon a white screen, the
gradual change of this light by augmented precipitation into deep
glowing red, resembling the colour of the sun when seen through fine
London smoke, is exceedingly striking. Indeed the smoke acts, in some
measure, the part of our finely-suspended matter.
[Sidenote: MORNING AND EVENING RED.]
By such means it is possible to imitate the phenomena of the firmament;
we can produce its pure blue, and cause it to vary as in nature. The
milkiness which steals over the heavens, and enables us to distinguish
one cloudless day from another, can be produced with the greatest ease.
The yellow, orange, and red light of the morning and evening can also be
obtained: indeed the effects are so strikingly alike as to suggest a
common origin--that the colours of the sky are due to minute particles
diffused through the atmosphere. These particles are doubtless the
condensed vapour of water, and its variation in quality and amount
enables us to understand the variability of the firmamental blue, and of
the morning and the evening red. Professor Forbes, moreover, has made
the interesting observation that the steam of a locomotive, at a certain
stage of its condensation, is blue or red according as it is viewed by
reflected or transmitted light.
These considerations enable us to account for a number of facts of
common occurrence. Thin milk, when poured upon a black surface, appears
bluish. The milk is colourless; that is, its blueness is not due to
_absorption_, but to a _separation_ of the light by the particles
suspended in the liquid. The juices of various plants owe their blueness
to the same cause; but perhaps the most curious illustration is that
presented by a blue eye. Here we have no true colouring matter, no
proper absorption; but we look through a muddy medium at the black
choroid coat within the eye, and the medium appears blue.[A]
[Sidenote: COLOUR OF SWISS LAKES.]
Is it not probable that this action of finely-divided matter may have
some influence on the colour of some of the Swiss lakes--as that of
Geneva for example? This lake is simply an expansion of the river Rhone,
which rushes from the end of the Rhone glacier, as the Arveiron does
from the end of the Mer de Glace. Numerous other streams join the Rhone
right and left during its downward course; and these feeders, being
almost wholly derived from glaciers, join the Rhone charged with the
finer matter which these in their motion have ground from the rocks over
which they have passed. But the glaciers must grind the mass beneath
them to particles of all sizes, and I cannot help thinking that the
finest of them must remain suspended in the lake throughout its entire
length. Faraday has shown that a precipitate of gold may require months
to sink to the bottom of a bottle not more than five inches high, and
in all probability it would require _ages_ of calm subsidence to bring
_all_ the particles which the Lake of Geneva contains to its bottom. It
seems certainly worthy of examination whether such particles suspended
in the water contribute to the production of that magnificent blue which
has excited the admiration of all who have seen it under favourable
circumstances.
FOOTNOTES:
[A] Helmholtz, 'Das Sehen des Menschen.'
THE MORAINES.
(8.)
The surface of the glacier does not long retain the shining whiteness of
the snow from which it is derived. It is flanked by mountains which are
washed by rain, dislocated by frost, riven by lightning, traversed by
avalanches, and swept by storms. The lighter debris is scattered by the
winds far and wide over the glacier, sullying the purity of its surface.
Loose shingle rattles at intervals down the sides of the mountains, and
falls upon the ice where it touches the rocks. Large rocks are
continually let loose, which come jumping from ledge to ledge, the
cohesion of some being proof against the shocks which they experience;
while others, when they hit the rocks, burst like bomb-shells, and
shower their fragments upon the ice.
[Sidenote: LATERAL MORAINES.]
Thus the glacier is incessantly loaded along its borders with the ruins
of the mountains which limit it; and it is evident that the quantity of
rock and rubbish thus cast upon the glacier depends upon the character
of the adjacent mountains. Where the summits are bare and friable, we
may expect copious showers; where they are resistant, and particularly
where they are protected by a covering of ice and snow, the quantity
will be small. As the glacier moves downward, it carries with it the
load deposited upon it. Long ridges of debris thus flank the glacier,
and these ridges are called _lateral moraines_. Where two tributary
glaciers join to form a trunk-glacier, their adjacent lateral moraines
are laid side by side at the place of confluence, thus constituting a
ridge which runs along the middle of the trunk-glacier, and which is
called a _medial moraine_. The rocks and debris carried down by the
glacier are finally deposited at its lower extremity, forming there a
_terminal moraine_.
[Sidenote: MEDIAL AND TERMINAL MORAINES.]
It need hardly be stated that the number of medial moraines is only
limited by the number of branch glaciers. If a glacier have but two
branches, it will have only one medial moraine; if it have three
branches, it will have two medial moraines; if _n_ branches, it will
have _n_-1 medial moraines. The number of medial moraines, in short, is
always _one less_ than the number of branches. A glance at the annexed
figure will reveal the manner in which the lateral moraines of the Mer
de Glace unite to form medial ones. (See Fig. 19.)
[Illustration: MORAINES OF THE MER DE GLACE.
Fig. 19. _To face p. 264_.]
When a glacier diminishes in size it leaves its lateral moraines
stranded on the flanks of the valleys. Successive shrinkings may thus
occur, and _have_ occurred at intervals of centuries; and a succession
of old lateral moraines, such as many glacier-valleys exhibit, is the
consequence. The Mer de Glace, for example, has its old lateral
moraines, which run parallel with its present ones. The glacier may also
diminish _in length_ at distant intervals; the result being a succession
of more or less concentric terminal moraines. In front of the
Rhone-glacier we have six or seven such moraines, and the Mer de Glace
also possesses a series of them.
Let us now consider the effect produced by a block of stone upon the
surface of a glacier. The ice around it receives the direct rays of the
sun, and is acted on by the warm air; it is therefore constantly
melting. The stone also receives the solar beams, is warmed, and
transmits its heat, by conduction, to the ice beneath it. If the heat
thus transmitted to the ice through the stone be less than an equal
space of the surrounding ice receives, it is manifest that the ice
around the stone will waste more quickly than that beneath it, and the
consequence is, that, as the surface sinks, it leaves behind it a
pillar of ice, on which the block is elevated. If the stone be wide and
flat, it may rise to a considerable height, and in this position it
constitutes what is called a glacier-_table_. (See Fig. 6.)
[Sidenote: GLACIER TABLES ACCOUNTED FOR.]
Almost all glaciers present examples of such tables; but no glacier with
which I am acquainted exhibits them in greater number and perfection
than the Unteraar glacier, near the Grimsel. Vast masses of granite are
thus poised aloft on icy pedestals; but a limit is placed to their
exaltation by the following circumstance. The sun plays obliquely upon
the table all day; its southern extremity receives more heat than its
northern, and the consequence is, that it _dips_ towards the south.
Strictly speaking, the plane of the dip rotates a little during the day,
being a little inclined towards the east in the morning, north and south
a little after noon, and inclined towards the west in the evening; so
that, theoretically speaking, the block is a sun-dial, showing by its
position the hour of the day. This rotation is, however, too small to be
sensible, and hence _the dip of the stones upon a glacier sufficiently
exposed to the sunlight, enables us at any time to draw the meridian
line along its surface_. The inclination finally becomes so great that
the block slips off its pedestal, and begins to form another, while the
one which it originally occupied speedily disappears, under the
influence of sun and air. Fig. 20 represents a typical section of a
glacier-table, the sun's rays being supposed to fall in the direction of
the shading lines.
[Sidenote: TYPE "TABLE."]
[Illustration: Fig. 20. Typical section of a glacier Table.]
Stones of a certain size are always lifted in the way described. A
considerable portion of the heat which a large block receives is wasted
by radiation, and by communication to the air, so that the quantity
which reaches the ice beneath is trifling. Such a mass is, of course, a
protector of the ice beneath it. But if the stone be small, and dark in
colour, it absorbs the heat with avidity, communicates it quickly to
the ice with which it is in contact, and consequently sinks in the ice.
This is also the case with bits of dirt and the finer fragments of
debris; they sink in the glacier. Sometimes, however, a pretty thick
layer of sand is washed over the ice from the moraines, or from the
mountain-sides; and such sand-layers give birth to ice-cones, which grow
to peculiarly grand dimensions on the Lower Aar glacier. I say "grow,"
but the truth, of course, is, that the surrounding ice wastes, while the
portion underneath the sand is so protected that it remains as an
eminence behind. At first sight, these sand-covered cones appear huge
heaps of dirt, but on examination they are found to be cones of ice, and
that the dirt constitutes merely a superficial covering.
Turn we now to the moraines. Protecting, as they do, the ice from waste,
they rise, as might be expected, in vast ridges above the general
surface of the glacier. In some cases the surrounding mass has been so
wasted as to leave the spines of ice which support the moraines forty or
fifty feet above the general level of the glacier. I should think the
moraines of the Mer de Glace about the Tacul rise to this height. But
lower down, in the neighbourhood of the Echelets, these high ridges
disappear, and nought remains to mark the huge moraine but a strip of
dirt, and perhaps a slight longitudinal protuberance on the surface of
the glacier. How have the blocks vanished that once loaded the moraines
near the Tacul? They have been swallowed in the crevasses which
intersect the moraines lower down; and if we could examine the ice at
the Echelets we should find the engulfed rocks in the body of the
glacier.
[Sidenote: MORAINES ENGULFED AND DISGORGED.]
Cases occur, wherein moraines, after having been engulfed, and hidden
for a time, are again entirely disgorged by the glacier. Two moraines
run along the basin of the Talefre, one from the Jardin, the other from
an adjacent promontory, proceeding parallel to each other towards the
summit of the great ice-fall. Here the ice is riven, and profound chasms
are formed, in which the blocks and shingle of the moraines disappear.
Throughout the entire ice-fall the only trace of the moraines is a broad
dirt-streak, which the eye may follow along the centre of the fall, with
perhaps here and there a stone which has managed to rise from its frozen
sepulchre. But the ice wastes, and at the base of the fall large masses
of stone begin to reappear; these become more numerous as we descend;
the smaller debris also appears, and finally, at some distance below the
fall, the moraine is completely restored, and begins to exercise its
protecting influence; it rises upon its ridge of ice, and dominates as
before over the surface of the glacier.
[Sidenote: TRANSPARENCY OF ICE UNDER THE MORAINES.]
The ice under the moraines and sand-cones is of a different appearance
from that of the surrounding glacier, and the principles we have laid
down enable us to explain the difference. The sun's rays, striking upon
the unprotected surface of the glacier, enter the ice to a considerable
depth; and the consequence is, that the ice near the surface of the
glacier is always disintegrated, being cut up with minute fissures and
cavities, filled with water and air, which, for reasons already
assigned, cause the glacier, when it is clean, to appear white and
opaque. The ice under the moraines, on the contrary, is usually dark and
transparent; I have sometimes seen it as black as pitch, the blackness
being a proof of its great transparency, which prevents the reflection
of light from its interior.
The ice under the moraines cannot be assailed in its depths by the solar
heat, because this heat becomes _obscure_ before it reaches the ice, and
as such it lacks the power of penetrating the substance. It is also
communicated in great part by way of contact instead of by radiation. A
thin film at the surface of the moraine-ice engages all the heat that
acts upon it, its deeper portions remaining intact and transparent.
GLACIER MOTION.
PRELIMINARY.
(9.)
[Sidenote: NEVE AND GLACIER.]
Though a glacier is really composed of two portions, one above and the
other below the snow-line, the term glacier is usually restricted to the
latter, while the French term _neve_ is applied to the former. It is
manifest that the snow which falls upon the glacier proper can
contribute nothing to its growth or permanence; for every summer is not
only competent to abolish the accumulations of the foregoing winter, but
to do a great deal more. During each summer indeed a considerable
quantity of the ice below the snow-line is reduced to water; so that, if
the waste were not in some way supplied, it is manifest that in a few
years the lower portion of the glacier must entirely disappear. The end
of the Mer de Glace, for example, could never year after year thrust
itself into the valley of Chamouni, were there not some agency by which
its manifest waste is made good. This agency is the motion of the
glacier.
To those unacquainted with the fact of their motion, but who have stood
upon these vast accumulations of ice, and noticed their apparent fixity
and rigidity, the assertion that a glacier moves must appear in the
highest degree startling and incredible. They would naturally share the
doubts of a certain professor of Tuebingen, who, after a visit to the
glaciers of Switzerland, went home and wrote a book flatly denying the
possibility of their motion. But reflection comes to the aid of sense,
and qualifies first impressions. We ask ourselves how is the permanence
of the glacier secured? How are the moraines to be accounted for?
Whence come the blocks which we often find at the terminus of a glacier,
and which we know belong to distant mountains? The necessity of motion
to produce these results becomes more and more apparent, until at length
we resort to actual experiment. We take two fixed points at opposite
sides of the glacier, so that a block of stone which rests upon the ice
may be in the straight line which unites the points; and we soon find
that the block quits the line, and is borne downwards by the glacier. We
may well realize the interest of the man who first engaged in this
experiment, and the pleasure which he felt on finding that the block
moved; for even now, after hundreds of observations on the motion of
glaciers have been made, the actual observance of this motion for the
first time is always accompanied by a thrill of delight. Such pleasure
the direct perception of natural truth always imparts. Like Antaeus we
touch our mother, and are refreshed by the contact.
[Sidenote: HUGI'S MEASUREMENTS.]
The fact of glacier-motion has been known for an indefinite time to the
inhabitants of the mountains; but the first who made quantitative
observations of the motion was Hugi. He found that from 1827 to 1830 his
cabin upon the glacier of the Aar had moved 100 metres, or about 110
yards, downwards; in 1836 it had moved 714 metres; and in 1841 M.
Agassiz found it at a distance of 1,428 metres from its first position.
This is equivalent in round numbers to an average velocity of 100 metres
a year. In 1840 M. Agassiz fixed the position of the rock known as the
Hotel des Neufchatelois; and on the 5th of September, 1841, he found
that it had moved 213 feet downward. Between this date and September,
1842, the rock moved 273 feet, thus accomplishing a distance of 486 feet
in two years.
But much uncertainty prevailed regarding the motion of the boulders, for
they sometimes rolled upon the glacier, and hence it was resolved to
use stakes of wood driven into the ice. In the month of July, 1841, M.
Escher de la Linth fixed a system of stakes, every two of which were
separated from each other by a distance of 100 metres, across the great
Aletsch glacier. A considerable number of other stakes were fixed
_along_ the glacier, the longitudinal separation being also 100 metres.
On the 8th of July the stakes stood at a depth of about three feet in
the ice. On the 16th of August he returned to the glacier. Almost all
the stakes had fallen, and no trace, even of the holes in which they had
been sunk, remained. M. Agassiz was equally unsuccessful on the glacier
of the Aar. It must therefore be borne in mind, that, previous to the
introduction of the facile modes of measurement which we now employ,
severe labour and frequent disappointment had taught observers the true
conditions of success.
After his defeat upon the Aletsch, M. Escher joined MM. Agassiz and
Desor on the Aar glacier, where, between the 31st of August and the 5th
of September, they fixed in concert the positions of a series of blocks
upon the ice, with the view of measuring their displacements the
following year.
[Sidenote: AGASSIZ'S MEASUREMENTS.]
Another observation of great importance was also commenced in 1841.
Warned by previous failures, M. Agassiz had iron boring-rods carried up
the glacier, with which he pierced the ice at six places to a depth of
ten feet, and at each place drove a wooden pile into the ice. These six
stations were in the same straight line across the glacier; three of
them standing upon the Finsteraar and three on the Lauteraar tributary.
About this time also M. Agassiz conceived the idea of having the
displacements measured the year following with precise instruments, and
also of having constructed, by a professional engineer, a map of the
entire glacier, on which all its visible "accidents" should be drawn
according to scale. This excellent work was afterwards executed by M.
Wild, now Professor of Geodesy and Topography in the Polytechnic School
of Zuerich, and it is published as a separate atlas in connexion with M.
Agassiz's 'Systeme Glaciaire.'
[Sidenote: PROF. J. D. FORBES INVITED.]
M. Agassiz is a naturalist, and he appears to have devoted but little
attention to the study of physics. At all events, the physical portions
of his writings appear to me to be very often defective. It was probably
his own consciousness of this deficiency that led him to invoke the
advice of Arago and others previous to setting out upon his excursions.
It was also his desire "to see a philosopher so justly celebrated occupy
himself with the subject," which induced him to invite Prof. J. D.
Forbes of Edinburgh to be his guest upon the Aar glacier in 1841. On the
8th of August they met at the Grimsel Hospice, and for three weeks
afterwards they were engaged together daily upon the ice, sharing at
night the shelter of the same rude roof. It is in reference to this
visit that Prof. Forbes writes thus at page 38 of the 'Travels in the
Alps':--"Far from being ready to admit, as my sanguine companions wished
me to do in 1841, that the theory of glaciers was complete, and the
cause of their motion certain, after patiently hearing all they had to
say and reserving my opinion, I drew the conclusion that no theory which
I had then heard of could account for the few facts admitted on all
hands." In 1842 Prof. Forbes repaired, as early as the state of the snow
permitted, to the Mer de Glace; he worked there, in the first instance,
for a week, and afterwards crossed over to Courmayeur to witness a solar
eclipse. The result of his week's observations was immediately
communicated to Prof. Jameson, then editor of the 'Edinburgh New
Philosophical Journal.'
[Sidenote: CENTRE MOVES QUICKEST.]
In that letter he announces the fact, but gives no details of the
measurement, that "the central part of the glacier moves faster than the
edges in a very considerable proportion; quite contrary to the opinion
generally entertained." He also announced at the same time the
continuous hourly advance of the glacier. This letter bears the date,
"Courmayeur, Piedmont, 4th July," but it was not published until the
month of October following.
Meanwhile M. Agassiz, in company with M. Wild, returned to complete his
experiment upon the glacier of the Aar. On the 20th of July, 1842, the
displacements of the six piles which he had planted the year before were
determined by means of a theodolite. Of the three upon the Finsteraar
affluent, that nearest the side had moved 160 feet, the next 225 feet,
while that nearest to the centre had moved 269 feet. Of those on the
Lauteraar, that nearest the side had moved 125 feet, the next 210 feet,
and that nearest the centre 246 feet. These observations were perfectly
conclusive as to the quicker motion of the centre: they embrace a year's
motion; and the magnitude of the displacements, causing errors of
inches, which might seriously affect small displacements, to vanish,
justifies us in ranking this experiment with the most satisfactory of
the kind that have ever been made. The results were communicated to
Arago in a letter dated from the glacier of the Aar, on the 1st of
August, 1842; they were laid before the Academy of Sciences on the 29th
of August, 1842, and are published in the 'Comptes Rendus' of the same
date.
The facts, then, so far as I have been able to collect them, are as
follows:--M. Agassiz commenced his experiment about ten months before
Professor Forbes, and the results of his measurements, with quantities
stated, were communicated to the French Academy about two months prior
to the publication of the letter of Professor Forbes in the 'Edinburgh
Philosophical Journal.' But the latter communication, announcing in
general terms the fact of the speedier central motion, was dated from
Courmayeur twenty-seven days before the date of M. Agassiz's letter
from the glacier of the Aar.
[Sidenote: STATE OF THE QUESTION.]
The speedier motion of the central portion of a glacier has been justly
regarded as one of cardinal importance, and no other observation has
been the subject of such frequent reference; but the general impression
in England is that M. Agassiz had neither part nor lot in the
establishment of the above fact; and in no English work with which I am
acquainted can I find any reference to the above measurements. Relying
indeed upon such sources for my information, I remained ignorant of the
existence of the paper in the 'Comptes Rendus' until my attention was
directed to it by Professor Wheatstone. In the next following chapters I
shall have to state the results of some of my own measurements, and
shall afterwards devote a little time to the consideration of the cause
of glacier-motion. In treating a question on which so much has been
written, it is of course impossible, as it would be undesirable, to
avoid subjecting both my own views and those of others to a critical
examination. But in so doing I hope that no expression shall escape me
inconsistent with the courtesy which ought to be habitual among
philosophers or with the frank recognition of the just claims of my
predecessors.
MOTION OF THE MER DE GLACE.
(10.)
[Sidenote: MY FIRST OBSERVATION.]
On Tuesday, the 14th of July, 1857, I made my first observation on the
motion of the Mer de Glace. Accompanied by Mr. Hirst I selected on the
steep slope of the Glacier des Bois a straight pinnacle of ice, the
front edge of which was perfectly vertical. In coincidence with this
edge I fixed the vertical fibre of the theodolite, and permitted the
instrument to stand for three hours. On looking through it at the end of
this interval, the cross hairs were found projected against the white
side of the pyramid; the whole mass having moved several inches
downwards.
The instrument here mentioned, which had long been in use among
engineers and surveyors, was first applied to measure glacier-motion in
1842; by Prof. Forbes on the Mer de Glace, and by M. Agassiz on the
glacier of the Aar. The portion of the theodolite made use of is easily
understood. The instrument is furnished with a telescope capable of
turning up and down upon a pivot, without the slightest deviation right
or left; and also capable of turning right or left without the slightest
deviation up or down. Within the telescope two pieces of spider's
thread, so fine as to be scarcely visible to the naked eye, are drawn
across the tube and across each other. When we look through the
telescope we see these fibres, their point of intersection being exactly
in the centre of the tube; and the instrument is furnished with screws
by means of which this point can be fixed upon any desired object with
the utmost precision.
[Sidenote: MODE OF MEASUREMENT.]
In setting a straight row of stakes across the glacier, our mode of
proceeding was in all cases this:--The theodolite was placed on the
mountain-side flanking the glacier, quite clear of the ice; and having
determined the direction of a line perpendicular to the axis of the
glacier, a well-defined object was sought at the opposite side of the
valley as close as possible to this direction; the object being, in some
cases, the sharp edge of a cliff; in others, a projecting corner of
rock; and, in others, a well-defined mark on the face of the rock. This
object and those around it were carefully sketched, so that on returning
to the place it could be instantly recognized. On commencing a line the
point of intersection of the two spiders' threads within the telescope
was first fixed accurately upon the point thus chosen, and an assistant
carrying a straight baton was sent upon the ice. By rough signalling he
first stood near the place where the first stake was to be driven in;
and the object end of the telescope was then lowered until he came
within the field of view. He held his staff upright upon the ice, and,
in obedience to signals, moved upwards or downwards until the point of
intersection of the spiders-threads exactly hit the bottom of the baton;
a concerted signal was then made, the ice was pierced with an auger to a
depth of about sixteen inches, and a stake about two feet long was
firmly driven into it. The assistant then advanced for some distance
across the glacier; the end of the telescope was now gently raised until
he and his upright staff again appeared in the field of view. He then
moved as before until the bottom of his staff was struck by the point of
intersection, and here a second stake was fixed in the ice. In this way
the process was continued until the line of stakes was completed.
Before quitting the station, a plummet was suspended from a hook
directly underneath the centre of the theodolite, and the place where
the point touched the ground was distinctly marked. To measure the
motion of the line of stakes, we returned to the place a day or two
afterwards, and by means of the plummet were able to make the theodolite
occupy the exact position which it occupied when the line was set out.
The telescope being directed upon the point at the opposite side of the
valley, and gradually lowered, it was found that no single stake along
the line preserved its first position: they had all shifted downwards.
The assistant was sent to the first stake; the point which it had first
occupied was again determined, and its present distance from that point
accurately measured. The same thing was done in the case of each stake,
and thus the displacement of the whole row of stakes was ascertained.[A]
The time at which the stake was fixed, and at which its displacement was
measured, being carefully noted, a simple calculation determined _the
daily motion_ of the stake.
[Sidenote: THE FIRST LINE.]
Thus, on the 17th of July, 1857, we set out our first line across the
Mer de Glace, at some distance below the Montanvert; on the day
following we measured the progress of the stakes. The observed
displacements are set down in the following table:--
First Line.--Daily Motion.
No. of stake. Inches.
West 1 moved 12-1/4
2 " 16-3/4
3 " 22-1/2
4 " ...
5 " 24-1/2
6 moved ...
7 " 26-1/4
8 " ...
9 " 28-3/4
10 " 35-1/2 East.
[Sidenote: THE CENTRE-POINT NOT THE QUICKEST.]
The theodolite in this case stood on the Montanvert side of the valley,
and the stakes are numbered from this side. We see that the motion
gradually augments from the 1st stake onward--the 1st stake being held
back by the friction of the ice against the flanking mountain-side. The
stakes 4, 6, and 8 have no motion attached to them, as an accident
rendered the measurement of their displacements uncertain. But one
remarkable fact is exhibited by this line; the 7th stake stood upon the
_middle_ of the glacier, and we see that its motion is by no means the
quickest; it is exceeded in this respect by the stakes 9 and 10.
The portion of the glacier on which the 10th stake stood was very much
cut up by crevasses, and, while the assistant was boring it with his
auger, the ice beneath him was observed, through the telescope, to slide
suddenly forward for about 4 inches. The other stakes retained their
positions, so that the movement was purely local. Deducting the 4 inches
thus irregularly obtained, we should have a daily motion of 31-1/2
inches for stake No. 10. The place was watched for some time, but the
slipping was not repeated; and a second measurement on the succeeding
day made the motion of the 10th stake 32 inches, whilst that of the
centre of the glacier was only 27.
Here, then, was a fact which needed explanation; but, before attempting
this, I resolved, by repeated measurements in the same locality, to
place the existence of the fact beyond doubt. We therefore ascended to a
point upon the old and now motionless moraine, a little above the
Montanvert Hotel; and choosing, as before, a well-defined object at the
opposite side of the valley, we set between it and the theodolite a row
of twenty stakes across the glacier. Their motions, measured on a
subsequent day, and reduced to their daily rate, gave the results set
down in the following table:--
Second Line.--Daily Motion.
No. of stake. Inches.
West 1 moved 7-1/2
2 " 10-3/4
3 " 12-1/4
4 " 14-1/2
5 " 16
6 " 16-3/4
7 " 17-1/2
8 " 19
9 " 19-1/2
10 " 21
11 moved 21
12 " 22-1/2
13 " 21
14 " 22-1/2
15 " 20-1/2
16 " 21-3/4
17 " 22-1/4
18 " 25-1/4
19 " ...
20 " 25-3/4 East.
[Sidenote: CORROBORATIVE MEASUREMENTS.]
As regards the retardation of the side, we observe here the same fact as
that revealed by our first line--the motion gradually augments from the
first stake to the last. The stake No. 20 stood upon the dirty portion
of the ice, which was derived from the Talefre tributary of the Mer de
Glace, and far beyond the middle of the glacier. These measurements,
therefore, corroborate that made lower down, as regards the
non-coincidence of the point of swiftest motion with the centre of the
glacier.
But it will be observed that the measurements do not show any
retardation of the ice at the eastern extremity of the line of
stakes--the motion goes on augmenting from the first stake to the last.
The reason of this is, that in neither of the cases recorded were we
able to get the line quite across the glacier; the crevasses and broken
ice-ridges, which intercepted the vision, compelled us to halt before we
came sufficiently close to the eastern side to make its retardation
sensible. But on the 20th of July my friend Hirst sought out an elevated
station on the Chapeau, or eastern side of the valley, whence he could
command a view from side to side over all the humps and inequalities of
the ice, the fixed point at the opposite side, upon which the telescope
was directed, being the corner of a window of the Montanvert Hotel.
Along this line were placed twelve stakes, the daily motions of which
were found to be as follows:--
Third Line.--Daily Motion.
No. of stake. Inches.
East 1 moved 19-1/2
2 " 22-3/4
3 " 28-3/4
4 " 30-1/4
5 " 33-3/4
6 " 28-1/4
7 moved 24-1/2
8 " 25
9 " 25
10 " 18
11 " ...
12 " 8-1/2 West.
The numbering of the stakes along this line commenced from the
Chapeau-side of the glacier, and the retardation of that side is now
manifest enough; the motion gradually augmenting from 19-1/2 to 33-1/2
inches. But, comparing the velocity of the two extreme stakes, we find
that the retardation of stake 12 is much greater than that of stake 1.
Stake 5, moreover, which moved with the _maximum_ velocity, was not upon
the centre of the glacier, but much nearer to the eastern than to the
western side.
[Sidenote: A NEW PECULIARITY OF GLACIER MOTION.]
It was thus placed beyond doubt that the point of maximum motion of the
Mer de Glace, at the place referred to, is not the centre of the
glacier. But, to make assurance doubly sure, I examined the comparative
motion along three other lines, and found in all the same undeviating
result.
This result is not only unexpected, but is quite at variance with the
opinions hitherto held regarding the motion of the Mer de Glace. The
reader knows that the trunk-stream is composed of three great
tributaries from the Geant, the Lechaud, and the Talefre. The Glacier du
Geant fills more than half of the trunk-valley, and the junction between
it and its neighbours is plainly marked by the dirt upon the surface of
the latter. In fact four medial moraines are crowded together on the
eastern side of the glacier, and before reaching the Montanvert they
have strewn their debris quite over the adjacent ice. A distinct limit
is thus formed between the clean Glacier du Geant and the other dirty
tributaries of the trunk-stream.
Now the eastern side of the Mer de Glace is observed on the whole to be
much more fiercely torn than the western side, and this excessive
crevassing has been referred to _the swifter motion of the Glacier du
Geant_. It has been thought that, like a powerful river, this glacier
drags its more sluggish neighbours after it, and thus tears them in the
manner observed. But the measurement of the foregoing three lines shows
that this cannot be the true cause of the crevassing. In each case the
stakes which moved quickest _lay upon the dirty portion of the
trunk-stream_, far to the east of the line of junction of the Glacier du
Geant, which in fact moved slowest of all.
[Sidenote: LAW OF MOTION SOUGHT.]
The general view of the glacier, and of the shape of the valley which it
filled, suggested to me that the analogy with a river might perhaps make
itself good beyond the limits hitherto contemplated. The valley was not
straight, but sinuous. At the Montanvert the convex side of the glacier
was turned eastward; at some distance higher up, near the passages
called _Les Ponts_, it was turned westward; and higher up again it was
turned once more, for a long stretch, eastward. Thus between Trelaporte
and the Ponts we had what is called a point of contrary flexure, and
between the Ponts and the Montanvert a second point of the same kind.
[Sidenote: CONJECTURE REGARDING CHANGE OF FLEXURE.]
Supposing a river, instead of the glacier, to sweep through this valley;
_its_ point of maximum motion would not always remain central, but would
deviate towards that side of the valley to which the river turned its
convex boundary. Indeed the positions of towns along the banks of a
navigable river are mainly determined by this circumstance. They are,
in most cases, situate on the convex sides of the bends, where the rush
of the water prevents silting up. Can it be then that the ice exhibits a
similar deportment? that the same principle which regulates the
distribution of people along the banks of the Thames is also acting with
silent energy amid the glaciers of the Alps? If this be the case, the
position of the point of maximum motion ought, of course, to shift with
the bending of the glacier. Opposite the Ponts, for example, the point
ought to be on the Glacier du Geant, and westward of the centre of the
trunk-stream; while, higher up, we ought to have another change to the
eastern side, in accordance with the change of flexure.
On the 25th of July a line was set out across the glacier, one of its
fixed termini being a mark upon the first of the three Ponts. The motion
of this line, measured on a subsequent day, and reduced to its daily
rate, was found to be as follows:--
Fourth Line.--Daily Motion.
No. of stake. Inches.
East 1 moved 6-1/2
2 " 8
3 " 12-1/2
4 " 15-1/4
5 " 15-1/2
6 " 18-3/4
7 " 18-1/4
8 " 18-3/4
9 " 19-1/2
10 moved 21
11 " 20-1/2
12 " 23-1/4
13 " 23-1/4
14 " 21
15 " 22-1/4
16 " 17-1/4
17 " 15 West.
This line, like the third, was set out and numbered from the eastern
side of the glacier, the theodolite occupying a position on the heights
of the Echelets. A moment's inspection of the table reveals a fact
different from that observed on the third line; _there_ the most
easterly stake moved with more than twice the velocity of the most
westerly one; _here_, on the contrary, the most westerly stake moves
with more than twice the velocity of the most easterly one.
To enable me to compare the motion of the eastern and western halves of
the glacier with greater strictness, my able and laborious companion
undertook the task of measuring with a surveyor's chain the line just
referred to; noting the pickets which had been fixed along the line, and
the other remarkable objects which it intersected. The difficulty of
thus directing a chain over crevasses and ridges can hardly be
appreciated except by those who have tried it. Nevertheless, the task
was accomplished, and the width of the Mer de Glace, at this portion of
its course, was found to be 863 yards, or almost exactly half a mile.
Referring to the last table, it will be seen that the two stakes
numbered 12 and 13 moved with a common velocity of 23-1/4 inches per
day, and that their motion is swifter than that of any of the others.
The point of swiftest motion may be taken midway between them, and this
point was found by measurement to lie 233 yards _west_ of the dirt which
marked the junction of the Glacier du Geant with its fellow tributaries:
whereas, in the former cases, it lay a considerable distance _east_ of
this limit. Its distance from the eastern side of the glacier was 601
yards, and from the western side 262 yards, being 170 yards west of the
centre of the glacier.
[Sidenote: CONJECTURE TESTED.]
But the measurements enabled me to take the stakes in pairs, and to
compare the velocity of a number of them which stood at certain
distances from the eastern side of the valley, with an equal number
which stood at the same distances from the western side. By thus
arranging the points two by two, I was able to compare the motion of the
entire body of the ice at the one side of the central line with that of
the ice at the other side. Stake 17 stood about as far from the western
side of the glacier as stake 3 did from its eastern side; 16 occupied
the same relation to 4; 15, to 5; 13, to 7; and 12, to 9.
Calling each pair of points which thus stand at equal distances from the
opposite sides _corresponding points_, the following little table
exhibits their comparative motions:--
Numbers and Velocities of Corresponding Points on the Fourth Line.