FOOTNOTES:

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.

[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.

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 thissnow-linewith 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 fallsannuallyis melted, and no more. Below this line each year's snow is completelycleared away by the summer heat; above it a residual layer abides, which gradually augments in thickness from the snow-line upwards.

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.

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 ofthe Arveiron were once powdery snow upon the slopes of the Col du Géant. 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.

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 isdry, 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°, and afterwards to melt it. The water thus formed percolates through the colder mass underneath, and this I take to bethe 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 asround 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 thenévéits 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 thenévé, 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.

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 thenévéthat we find an approximation to the true ice ofthe glacier. This brings us to the second great agent in the process of glacification, namely, pressure. The ice of thenévéat 32° 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 thenévé, 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 thenévéare removed by pressure more and more from the condition of snow, the air-bubbles which give to thenévé-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 thenévéto compact ice; and I have myself frequently obtained slabs of ice from snow in London.

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.

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 containhydrogenare peculiarly hostile to the longer undulations. It is, I think, the presence ofthis 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; itsradiant formmerely 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.

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, isblue.

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. Itis 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, thelaugs, 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.

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, 991/2per 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 itwhite, 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.

[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.

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 thevesicularstate. He was followed by M. Brücke, 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 ablue 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 isreflectedlight; 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. Thesetransmitblue 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.

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 withthis 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.

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 Brücke. 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, Brücke 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 positionsthis blue liquid appears yellow; and these are the positions when thetransmittedlight 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.

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.

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 doubtlessthe 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 toabsorption, but to aseparationof 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]

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 abottle not more than five inches high, and in all probability it would requireagesof calm subsidence to bringallthe 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.'

[A]Helmholtz, 'Das Sehen des Menschen.'

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 débris 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.

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 débris thus flank the glacier, and these ridges are calledlateral 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, andwhich is called amedial moraine. The rocks and débris carried down by the glacier are finally deposited at its lower extremity, forming there aterminal moraine.

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; ifnbranches, it will haven-1 medial moraines. The number of medial moraines, in short, is alwaysone lessthan 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. (SeeFig. 19.)

MORAINES OF THE MER DE GLACE.Fig.19.

When a glacier diminishes in size it leaves its lateral moraines stranded on the flanks of the valleys. Successive shrinkings may thus occur, andhaveoccurred 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 diminishin lengthat 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. (SeeFig. 6.)

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 itdipstowards 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 hencethe 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. 20represents a typical section of a glacier-table, the sun's rays being supposed to fall in the direction of the shading lines.

TYPE "TABLE."

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, communicatesit 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 débris; 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 morainesof 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.

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 Talèfre, 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 débris 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.

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 thesurface 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 becomesobscurebefore 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.

NÉVÉ 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 termnévéis 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 Tübingen, 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 beaccounted 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 Antæus we touch our mother, and are refreshed by the contact.

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 mètres, or about 110 yards, downwards; in 1836 it had moved 714 mètres; and in 1841 M. Agassiz found it at a distance of 1,428 mètres from its first position. This is equivalent in round numbers to an average velocity of 100 mètres a year. In 1840 M. Agassiz fixed the position of the rock known as the Hôtel des Neufchâtelois; 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 mètres, across the great Aletsch glacier. A considerable number of other stakes were fixedalongthe glacier, the longitudinal separation being also 100 mètres. 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.

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 workwas afterwards executed by M. Wild, now Professor of Geodesy and Topography in the Polytechnic School of Zürich, and it is published as a separate atlas in connexion with M. Agassiz's 'Système Glaciaire.'

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