I.
OBSERVATIONS ON THE MER DE GLACE.

The law established by Forbes and Agassiz, that the central portions of a glacier moved faster than the sides, was amply illustrated and confirmed by the deportment of lines of stakes placed across the Mer de Glace and its tributaries in 1857. The portions of the trunk glacier derived from these tributaries were easily traceable throughout the glacier by means of the moraines. Thus, for example, the portion of the trunk stream derived from the Glacier du Géant might be distinguished in a moment from the other portions by the absence of débris upon its surface. Attention was drawn by Prof. Forbes to the fact that the eastern side of the Mer de Glace in particular is ‘excessively crevassed;’ and he accounted for this crevassing by supposing that the Glacier du Géant moves most swiftly, and in its effort to drag its more sluggish companions along with it tears them asunder, thus producing the fissures and dislocation for which the eastern side of the glacier is remarkable. Too much weight must not be attached to this explanation. It was one of those suggestions which are perpetually thrown out by men of science during the course of an investigation, and the fulfilment or non-fulfilment of which cannot materially affect the merits of the investigator. Indeed, the merits of Forbes must be judged on far broader grounds. The qualities of mind and the physical culture invested in his ‘Travels in the Alps’ are such as to make it, in the estimation of the physical investigator at least, outweigh all other books upon the subject.

While thus acknowledging its merits, however, let a free and frank comparison of its statements with facts be instituted. To test whether the Glacier du Géant moved more quickly than its fellows, five different lines were set out across the Mer de Glace, in the vicinity of the Montanvert. In each case it was found that the point of swiftest motion did not lie upon the Glacier du Géant at all, but was displaced so as to bring it comparatively close to the eastern side of the glacier. But though the special opinion of Forbes just referred to here falls to the ground, the deviation of the point of swiftest motion from the centre of the glacier will probably, when its cause is pointed out, be regarded as of special importance to his theory.

At the place where these five lines were run across it the glacier turns its convex curvature to the eastern side of the valley, being concave towards the Montanvert. Let us then take a bolder analogy than even that suggested in the explanation of Forbes, where he compares the Glacier du Géant to a strong and swiftly flowing river. Let us enquire how a river, would behave in sweeping round a curve similar to that here existing. The point of swiftest motion would undoubtedly lie on that side of the centre of the stream towards which it turns its convex curvature. Can this be the case with the trunk of the Mer de Glace? If so, then we ought to have a shifting of the point of maximum motion towards the eastern side of the valley, when the curvature of the glacier so changes as to turn its convexity to the western side.

Now, such a change of flexure actually occurs opposite the passages called Les Ponts, and at this place the view just enunciated was tested. It was immediately ascertained that the point of swiftest motion here lay at a different side of the axis from that observed lower down. But to confer strict numerical accuracy upon the result, stakes were fixed at certain distances from the western side of the glacier, and others at equal distances from the eastern side. The velocities of these stakes were compared with each other, two by two, a stake on the western side being always compared with a second one which stood at the same distance from the eastern side. The results of this measurement are given in the following table, the numbers denoting inches:

It is here seen that in each case the western stake moved more swiftly than its eastern fellow stake; thus proving, beyond a doubt, that opposite the Ponts the western side of the Mer de Glace moves swiftest—a result precisely the reverse of that observed where the curvature of the valley was different.

But an additional test of the explanation is possible. Between the Ponts and the promontory of Trélaporte the glacier passes another point of contrary flexure, its convex curvature opposite to Trélaporte being turned towards the base of the Aiguille du Moine, on the eastern side. A series of stakes was placed across the glacier here; and the velocities of those placed at certain distances from the western side were compared, as before, with those of stakes placed at the same distances from the eastern side. The following table shows the result of these measurements; the numbers, as before, denote inches:

Here we find that in each case the eastern stake moved faster than its fellow. The point of maximum motion has therefore once more crossed the axis of the glacier.

Determining the point of maximum motion for a great number of transverse sections of the Mer de Glace, and uniting these points, we have what is called the locus of the point. The dotted line in the annexed figure represents the centre of the Mer de Glace; the hard line which crosses the axis of the glacier at the points A A is then the locus of the point of swiftest motion. It is a curve more deeply sinuous than the valley itself, and it crosses the central line of the valley at each point of contrary flexure. The position of towns upon the banks of rivers is usually on the convex side of the stream, where the rush of the water renders silting-up impossible; and the same law which regulated the flow of the Thames, and determined the position of the towns upon its banks, is at this moment operating with silent energy among the Alpine glaciers.

Another peculiarity of glacier motion is now to be noticed.

Before any observations had been made upon the subject, it was surmised by Prof. Forbes that the portions of a glacier near its bed were retarded by friction against the latter. This view was afterwards confirmed by his own observations, and by those of M. Martins. Nevertheless the state of our knowledge upon the subject rendered further confirmation of the fact highly desirable. A rare opportunity for testing the question was furnished in 1857 by an almost vertical precipice of ice, constituting the side of the Glacier du Géant, exposed near the Tacul. The precipice was about 140 feet in height. At the top and near the bottom stakes were fixed, and by hewing steps in the ice I succeeded in fixing a stake in the face of the precipice at a point about forty feet above the base.[31] After the lapse of a sufficient number of days, the progress of the three stakes was measured; reduced to the diurnal rate, the motion was as follows:

We thus see that the top stake moved with more than twice the velocity of the bottom one, while the velocity of the middle stake lies between the two. But it also appears that the augmentation of velocity upwards is not proportional to the distance from the bottom, but increases in a quicker ratio. At a height of 100 feet from the bottom, the velocity would undoubtedly be practically the same as at the surface. Measurements made upon an adjacent ice-cliff proved this. We thus see the perfect validity of the reason assigned by Forbes for the continued verticality of the walls of transverse crevasses. Indeed a comparison of the result with his anticipations and reasonings will prove alike their sagacity and their truth.

The most commanding view of the Mer de Glace and its tributaries is obtained from a point above the remarkable cleft in the mountain-range underneath the Aiguille de Charmoz, which is sure to attract the attention of an observer standing at the Montanvert. This point, marked G on the map of Forbes, I succeeded in attaining. A Tübingen Professor once visited the glaciers of Switzerland, and seeing these apparently rigid masses enclosed in sinuous valleys, went home and wrote a book, flatly denying the possibility of their motion. An inspection from the point now referred to would have doubtless confirmed him in his opinion; and indeed nothing can be more calculated to impress the mind with the magnitude of the forces brought into play than the squeezing of the three tributaries of the Mer de Glace through the neck of the valley at Trélaporte.

But let me state numerical results. Previous to its junction with its fellows, the Glacier du Géant measures 1,134 yards across. Before it is influenced by the thrust of the Talèfre, the Glacier de Léchaud has a width of 825 yards; while the width of the Talèfre branch across the base of the cascade, before it joins the Léchaud, is approximately 638 yards. The sum of these widths is 2,597 yards. At Trélaporte those three branches are forced through a gorge 893 yards wide, with a central velocity of 20 inches a day! The result is still more astonishing if we confine our attention to one of the tributaries—that of the Léchaud. This broad ice-river, which before its junction with the Talèfre has a width of 825 yards, at Trélaporte is squeezed to a driblet of less than 88 yards in width, that is to say, to about one-tenth of its previous horizontal transverse dimension.

Whence is the force derived which drives the glacier through the gorge? No doubt pressure from behind. Other facts also suggest that the Glacier du Géant is throughout its length in a state of forcible longitudinal compression. Taking a series of points along the axis of this glacier—if these points, during the descent of the glacier, preserved their distances asunder perfectly constant, there could be no longitudinal compression. The mechanical meaning of this term, as applied to a substance capable of yielding like ice, must be that the hinder points are incessantly advancing upon the forward ones. I was particularly anxious to test this view, which first occurred to me on à priori grounds. Three points, A, B, C, were therefore fixed upon the axis of the Glacier du Géant, A being the highest up the glacier. The distance between A and B was 545 yards, and that between B and C was 487 yards. The daily velocities of these three points, determined by the theodolite, were as follows:

The result completely corroborates the foregoing anticipation. The hinder points are incessantly advancing upon those in front, and that to an extent sufficient to shorten a segment of this glacier, measuring 1,000 yards in length, at the rate of 8 inches a day. Were this rate uniform at all seasons, the shortening would amount to 240 feet in a year. When we consider the compactness of this glacier, and the uniformity in the width of the valley which it fills, this result cannot fail to excite surprise; and the exhibition of force thus rendered manifest must be mainly instrumental in driving the glacier through the jaws of the granite vice at Trélaporte.

When the Glacier du Géant is observed from a sufficient distance, a remarkable system of seams of white ice appears to sweep across it, in the direction of the ‘dirt-bands.’ These seams are more resistant than the ordinary ice of the glacier, and sometimes protrude above the surface to a height of three or four feet. Their origin was for some time a difficulty, and it was at the base of the ice-cascade which descends from the basin of the Talèfre that the key to their solution first presented itself. It was well known that the ice of a glacier is not of homogeneous structure, but that the general more or less milky mass is traversed by blue veins of a more compact and transparent texture. In the upper portions of the Mer de Glace these veins sweep across the glacier in gentle curves, leaning forward—to which leaning forward Prof. Forbes gave the name of the ‘frontal dip.’ A case of ‘backward dip’ has never been described. But at the base of the ice-cascade referred to I had often noticed the veins exposed upon the walls of a longitudinal crevasse leaning backwards and forwards on both sides of a vertical line, like the joints of stones used to turn an arch.

This fact was found to connect itself in the following way with the general state of the glacier. At the base of the ice-fall a succession of protuberances, with steep frontal slopes, followed each other, and were intersected by crevasses. Let the hand be placed flat upon the table, with the palm downwards; let the fingers be bent so as to render the space between the joints nearest the nails and the ends of the fingers nearly vertical. Let the second hand be now placed upon the back of the first, with its fingers bent as in the former case, and their ends resting upon the roots of the first fingers. The crumpling of the hands fairly represents the crumpling of the ice, and the spaces between the fingers represent the crevasses by which the protuberances are intersected. On the walls of these crevasses the change of dip of the veined structure above referred to was always observed, and at the base of each protuberance a vein of white ice was found firmly wedged into the mass of the glacier.

The next figure represents a series of these crumples with the veins of white ice i i i at their bases.

It was soon observed that the water which trickled down the protuberances, and gushed here and there from glacier orifices, collected at the bases of the crumples, and formed streams which cut for themselves deep channels in the ice. These streams seemed to be the exact matrices or moulds of the veins of white ice, and the latter were finally traced to the gorging up of the channels of glacial rivulets by winter snow. The same explanation applies to the system of bands upon the Glacier du Géant. I was enabled to trace the little arms of white ice which once were the tributaries of the streams, to see a trunk vein of the ice dividing into branches, and uniting again so as to enclose glacial islands. I finally traced them to the region of their formation, and by sketches of existing streams taken near the base of the séracs, and of bands of white ice taken lower down, a resemblance so striking was exhibited as to leave no doubt of their relationship. On the walls of some deep crevasses, moreover, which intersected the white ice-seams, I found that the latter penetrated the glacier only to a limited depth, having the appearance of a kind of glacial ‘trap’ intruded from above.

But how is the backward dip of the blue veins to be accounted for? Doubtless in the following way: At the base of the cascade the glacier is forcibly compressed by the thrust of the mass behind it; besides this, it changes its inclination suddenly and considerably; it is bent upwards, and the consequence of this bending is a system of wrinkles, such as those represented in the next figure. The interior of a bent umbrella-handle sometimes presents wrinkles which are the representatives, in little, of the protuberances upon the glacier. The coat-sleeve is an equally instructive illustration: when the arm is bent at the elbow the sleeve wrinkles, and as the places where these wrinkles occur in the cloth are determined, to some extent, by the previous creasing, so also the places where the wrinkles are formed upon the glacier are determined by the previous scarring of the ice during its descent down the cascade. The manner in which these crumples tend to scale off speaks strongly in favour of the explanation given. The following figure represents a type of numerous instances of scaling off. By means of a hydraulic press it is easy to produce a perfectly similar scaling in small masses of ice. One consequence of this crumpling of the glacier would be the backward and forward inclination of the veins as actually observed. The same appearance was noticed on the wrinkles of the Glacier du Géant. It was also proved, by measurements, that these wrinkles shorten as they descend.

In virtue of what quality, then, can ice be bent and squeezed, and have its form changed in the manner indicated in the foregoing observations? The only theory worthy of serious consideration at the present day is the celebrated Viscous Theory of glacial motion. Numerous appearances, as we have seen, favour the idea that ice is a viscous or ‘semi-fluid’ substance, and that it flows as such in the glaciers of the Alps. The aspect of many glaciers, as a whole—their power of closing up crevasses, and of reconstructing themselves after having been precipitated down glacial gorges—the obvious bendings and contortions of various portions of the ice, are all in harmony with the notion. The laminar structure of the glacier has also been regarded by eminent authorities as a crucial test in favour of the viscous theory, and affirmed to be impossible of explanation on any other hypothesis.

Nevertheless, this theory is so directly opposed to our ordinary experience of the nature of ice as to leave upon the mind a lingering doubt of its truth. Can we imitate the phenomena without invoking the explanation? We can. Moulds of various forms were hollowed out in boxwood, and pieces of ice were placed in these moulds and subjected to pressure. In this way spheres of ice were flattened into cakes, and cakes formed into transparent lenses. A straight bar of ice, six inches long, was passed through a series of moulds augmenting in curvature, and was finally bent into a semiring. A small block of ice was placed in a hemispherical cavity, and was pressed upon by a hemispherical protuberance, not large enough to fill the cavity; the ice yielded and filled the space between both, thus forming itself into a transparent cup. The specimens of ice here employed were so exceedingly brittle that a pricker driven into the ice was competent to split blocks of the substance eight cubic feet in volume, the surface of fracture being in all cases as clean and sharp as that of glass.

These experiments, then, demonstrate a capacity on the part of small masses of ice which they have not been hitherto known to possess. They prove, to all appearance, that the substance is much more plastic than it was ever imagined to be. But the real germ from which these results have sprung is to be found in a lecture given at the Royal Institution in June 1850, and reported in the ‘Athenæum’ and ‘Literary Gazette’ for that year. Faraday then showed that when two pieces of ice, at a temperature of 32° Fahr., are placed in contact with each other, they freeze together, by the conversion of the film of moisture between them into ice. The case of a snowball is a familiar illustration of the principle. When the snow is below 32°, and therefore dry, it will not cohere, whereas when it is in a thawing condition it can be squeezed into a hard mass. During one of the hottest days of July 1857, when the thermometer was upwards of 100° Fahr. in the sun, and more than 80° in the shade, I observed a number of blocks of ice, which had been placed in a heap, frozen together at their places of contact; and I afterwards caused them to freeze together under water as hot as the hand could bear. Facts like these suggested the thought that if a piece of ice—a straight prism, for example—were placed in a bent mould and subjected to pressure it would break, but that the force would also bring its ruptured surfaces into contact, and thus the continuity of the mass might be re-established. Experiment, as we have seen, completely confirmed this surmise: the ice passed from a continuous straight bar to a continuous bent one, the transition being effected, not by a viscous movement of the particles, but through fracture and regelation.

Let the transition from curve to curve be only gradual enough, and we have the exact case of a transverse slice of a glacier.

All the phenomena of motion, on which the idea of viscosity has been based, are brought by such experiments as the above into harmony with the demonstrable properties of ice. In virtue of this property, the glacier accommodates itself to its bed while preserving its general continuity, crevasses are closed up, and the broken ice of a cascade, such as that of the Talèfre or the Rhone, is recompacted to a solid continuous mass.

The very essence of viscosity is the ability of yielding to a force of tension, the texture of the substance, after yielding, being in a state of equilibrium, so that it has no strain to recover from; and the substances chosen by Prof. Forbes as illustrative of the physical condition of a glacier possess this power of being drawn out in a very eminent degree. But it has been urged, and justly urged, that we ought not to conclude that viscosity is absent because hand specimens are brittle, any more than we ought to conclude that ice is not blue because small fragments of the substance do not exhibit this colour. To test the question of viscosity, then, we must appeal to the glacier itself. Let us do so.

An analogy between the motion of a glacier through a sinuous valley and of a river in a sinuous channel has been already pointed out. But the analogy fails in one important particular: the river, and much more so a mass of flowing treacle, honey, tar, or melted caoutchouc, sweeps round its curves without rupture of continuity. The viscous mass stretches, but the icy mass breaks, and the ‘excessive crevassing’ pointed out by Prof. Forbes himself is the consequence. The inclinations of the Mer de Glace and its three tributaries were, moreover, taken, and the association of transverse crevasses with the changes of inclination were accurately noted. Every traveller knows the utter dislocation and confusion produced by the descent of the Mer de Glace from the Chapeau downwards. A similar state of things exists in the ice-cascade of the Talèfre. Descending from the Jardin, as the ice approaches the fall, great transverse chasms are formed, which at length follow each other so speedily as to reduce the ice-masses between them to mere plates and wedges, along which the explorer has to creep cautiously. These plates and wedges are in some cases bent and crumpled by the lateral pressure, and some large pyramids are turned 90° round, so as to have their veins at right angles to the normal position. The ice afterwards descends the fall, the portions exposed to view being a fantastic assemblage of frozen boulders, pinnacles, and towers, some erect, some leaning, falling at intervals with a sound like thunder, and crushing the ice-crags on which they fall to powder. The descent of the ice through this fall has been referred to as a proof of its viscosity; but the description just given does not harmonise with our ideas of a viscous substance.

But the proof of the non-viscosity of the substance must be sought at places where the change of inclination is very small. Nearly opposite l’Angle there is a change from four to nine degrees, and the consequence is the production of transverse fissures which render the glacier here perfectly impassable. Further up the glacier transverse crevasses are produced by a change of inclination from three to five degrees. This change of inclination is protracted in fig. 5; the bend occurs at the point B; it is scarcely perceptible, and still the glacier is unable to pass over it without breaking across.

Again, the crevasses being due to a state of strain from which the ice relieves itself by breaking, the rate at which they widen may be taken as a measure of the amount of relief demanded by the ice. Both the suddenness of their formation and the slowness with which they widen are demonstrative of the non-viscosity of the ice. For were the substance capable of stretching, even at the small rate at which they widen, there would be no necessity for their formation.

Further, the marginal crevasses of a glacier are known to be a consequence of the swifter flow of its central portions, which throws the sides into a state of strain from which they relieve themselves by breaking. Now it is easy to calculate the amount of stretching demanded of the ice in order to accommodate itself to the speedier central flow. Take the case of a glacier half a mile wide. A straight transverse element, or slice, of such a glacier, is bent in twenty-four hours to a curve. The ends of the slice move a little, but the centre moves more: let us suppose the versed side of the curve formed by the slice in twenty-four hours to be a foot, which is a fair average. Having the chord of this arc, and its versed side, we can calculate its length. In the case of the Mer de Glace, which is about half a mile wide, the amount of stretching demanded would be about the eightieth of an inch in twenty-four hours. Surely, if the glacier possessed a property which could with any propriety be called viscosity, it ought to be able to respond to this moderate demand; but it is not able to do so: instead of stretching as a viscous body, in obedience to this slow strain, it breaks as an eminently fragile one, and marginal crevasses are the consequence. It may be urged that it is not fair to distribute the strain over the entire length of the curve: but reduce the distance as we may, a residue must remain, which is demonstrative of the non-viscosity of the ice.

To sum up, then, two classes of facts present themselves to the glacier investigator—one class in harmony with the idea of viscosity, and another as distinctly opposed to it. Where pressure comes into play we have the former; where tension comes into play we have the latter. Both classes of facts are reconciled by the assumption, or rather the experimental verity, that the fragility of ice and its power of regelation render it possible for it to change its form without prejudice to its continuity.

[Very interesting experiments upon the bending of ice have been recently made by Mr. Matthews and Mr. Froude. In these experiments the temperature of the ice, I believe, was some degrees below the freezing point: it would be important to repeat these experiments with ice at the temperature which it actually possesses in glaciers, namely, at 32°.—April 1871.]

II.
STRUCTURE AND PROPERTIES OF ICE.

Being desirous of examining how the interior of a mass of ice is affected by a beam of radiant heat sent through it, I availed myself of the sunny weather of September and October 1857. The sunbeams, condensed by a lens, were sent in various directions through slabs of ice. The path of every beam was observed to be instantly studded with lustrous spots, which increased in magnitude and number as the action continued. On examining the spots more closely, they were found to be flattened spheroids, and around each of them the ice was so liquefied as to form a beautiful flower-shaped figure possessing six petals. From this number there was no deviation. At first the edges of the liquid leaves were unindented; but a continuance of the action usually caused the edges to become serrated like those of ferns. When the ice was caused to move across the beam, or when the beam was caused to traverse different portions of the ice in succession, the sudden generation and crowding together of these liquid flowers, with their central spots shining with more than metallic brilliancy, was exceedingly beautiful.

In almost all cases the flowers were formed in planes parallel to the surface of freezing; it mattered not whether the beam traversed the ice parallel to this surface or perpendicular to it. Some apparent exceptions to this rule were found, which will form the subject of future investigation.

The general appearance of the shining spots at the centres of the flowers was that of the bubbles of air entrapped in the ice; to examine whether they contained air or not, portions of ice containing them were immersed in warm water. When the ice surrounding the cavities had completely melted, the latter instantly collapsed, and no trace of air rose to the surface of the water. A vacuum, therefore, had been formed at the centre of each spot, due, doubtless, to the well-known fact that the volume of water in each flower was less than that of the ice, by the melting of which the flower was produced.

The associated air-and-water cells, found in such numbers in the ice of glaciers, and also observed in lake ice, were next examined. Two hypotheses have been started to account for these cells. One attributes them to the absorption of the sun’s heat by the air of the bubbles, and the consequent melting of the ice which surrounds them. The other hypothesis supposes that the liquid in the cells never has been frozen, but has continued in the liquid condition from the névé or origin of the glacier downwards. Now if the water in the cells be due to the melting of the ice, the associated air must be rarefied, because the volume of the liquid is less than that of the ice which produced it; whereas if the air be simply that entrapped in the snow of the névé, it will not be thus rarefied. Here, then, we have a test as to whether the water-cells have been produced by the melting of the ice.

Portions of ice containing these compound cells were immersed in hot water, the ice around the cavities being thus gradually melted away. When a liquid connexion was established between the bubble and the atmosphere, the former collapsed to a smaller bubble. In many cases the residual bubble did not reach the hundredth part of the magnitude of the primitive one. There was no exception to this rule, and it proves that the water of these particular cavities, at all events, is really due to the melting of the adjacent ice.

But how was the ice surrounding the bubbles melted? The hypothesis that the melting is due to the absorption of the solar rays by the air of the bubbles is that of M. Agassiz, which has been reproduced and subscribed to by the Messrs. Schlagintweit, and accepted generally as the true one. Let us pursue it to its consequences.

Comparing equal weights of air and water, experiment proves that to raise a given weight of water one degree in temperature, as much heat would be needed as would raise the same weight of air four degrees.

Comparing equal volumes of air and water, the water is known to be 770 times heavier than the air; consequently, for a given volume of air to raise an equal volume of water one degree in temperature, it must part with 770 × 4 = 3080 degrees.

Now the quantity of heat necessary to melt a given weight of ice would raise the same weight of water 142.6 Fahr. degrees in temperature. Hence to produce, by the melting of ice, an amount of water equal to itself in bulk, a bubble of air must yield up 3080 × 142.6, or upwards of four hundred thousand degrees Fahrenheit.

This is the amount of heat which, according to the hypothesis of M. Agassiz and the Messrs. Schlagintweit, is absorbed by the bubble of air under the eyes of the observer. That is to say, the air is capable of absorbing an amount of heat which, had it not been communicated to the surrounding ice, would raise the bubble to a temperature 160 times that of fused cast iron. Did air possess this enormous power of absorption it would not be without inconvenience for the animal and vegetable life of our planet.

The fact is, that a bubble of air at the earth’s surface is unable, in the slightest appreciable degree, to absorb the sun’s rays; for those rays before they reach the earth have been perfectly sifted by their passage through the atmosphere. I made the following experiment illustrative of this point: The rays from an electric lamp were condensed by a lens, and the concentrated beam sent through the bulb of a differential thermometer. The heat of the beam was intense; still not the slightest effect was produced upon the thermometer. In fact, all the rays that air could absorb had been absorbed before the thermometer was reached, while the rays that glass could absorb had been absorbed by the lens. The heat consequently passed through the thin glass envelope of the thermometer, and the air within it, without imparting the slightest sensible heat to either.

The liquid bubbles observed in lake ice, and those which occur in the deeper portions of glacier ice, are produced by heat which has been conducted through the substance without melting it. Regarding heat as a mode of motion, it seems natural to infer, that inasmuch as within the mass each molecule is controlled in its motion by the surrounding molecules, the liberty of liquidity must be attained by the molecules at the surface of ice before the molecules in the interior can attain this liberty. But if a cavity exist in the interior, the molecules surrounding that cavity are in a condition similar to those at the surface; and they may be liberated by an amount of motion which has been transmitted through the ice without prejudice to its solidity. The conception is helped when we call to mind the transmission of motion through a series of elastic balls, by which the last ball of the series is detached, while the others do not suffer visible separation. It may indeed be proved, by actual experiment, that the interior portion of a mass of ice can be liquefied by an amount of heat which has been conducted through the exterior portions without melting them.

Now precisely the converse of this takes place when two pieces of ice, at 32° Fahr., with moist surfaces, are brought into contact. Superficial portions are by this act transferred to the centre where a temperature of 32° is not quite sufficient to produce liquefaction. The motion of liquidity which the surfaces possessed before contact is now checked, and the pieces of ice freeze together. This appears to furnish a satisfactory explanation of all the cases of this nature which have hitherto been observed.

The particles of a crushed mass of ice at 32°, or a ball of moist snow, may, it is now well known, be squeezed into slabs or cups of ice. That moisture is necessary here, and that the same agent is necessary in the conversion of snow into glacier ice, was proved by the following experiment. A ball of ice was cooled in a bath of solid carbonic acid and ether, and thus rendered perfectly dry. Placed in a suitable mould, and subjected to hydraulic pressure, the ball was crushed; but the crushed fragments remained as white and opaque as those of crushed glass. The particles, while thus dry, could not be squeezed so as to form pellucid ice, which is so easily obtained when the compressed mass is at a temperature of 32° Fahr.

III.
STRUCTURE OF GLACIERS.

If a transparent colourless solid be reduced to powder, the powder is white. Thus rock crystal, rock salt, and glass in powder are all white. A glass jar, partially filled with a solution of carbonate of soda, with a little gum added to give it tenacity, presents, on the addition of a little tartaric acid, the appearance of a tall white column of foam. In all these cases, the whiteness and the opacity are due to the intimate and irregular admixture of a solid or a liquid with air; in like manner the whiteness of snow is due to the mixture of air and transparent particles of ice.

The snow falls upon mountain eminences, and, above the snow-line, each year leaves a residue; the substance thus collects in layers, forming masses of great depth. The lower portions are squeezed by the pressure of those above them, and a gradual approach to ice is the consequence. The air being gradually expelled, the transparency of the substance augments in proportion.

But even after the snow has been squeezed to hard ice in the upper glacier region, it always contains a large amount of the air originally entrapped in the snow. The air is distributed through the solid in the form of bubbles, which give the ice a milky appearance. At the lower extremity of a glacier the ice, as everybody knows, is blue and transparent. The transition from one state to the other is not, in all cases, a gradual change which takes place uniformly throughout the entire mass. The white ice, on the contrary, of the middle glacier region is usually striped by veins of a more transparent character, the air which gives to the ice its whiteness having been, by some means or other, wholly or partially ejected from the veins. These veins sometimes give the ice of many glaciers a beautiful laminated appearance; vast portions, indeed, of various glaciers consist of this laminated ice.

The theory of the veins which perhaps first presents itself to the mind, and which is still entertained by many intelligent Alpine explorers, is that the veining of the middle glaciers is simply a continuation of the bedding of the névé; that not only do the annual snow-falls produce beds of great thickness, but every successive fall tends to produce a layer of less thickness, which layers, or the surfaces separating them, ultimately appear as the blue veins. This theory demands respectful consideration: on the exposed sections of the névé the lines of stratification are very manifest, exhibiting in many cases appearances strongly resembling that of the veined structure. Indeed, it was with a view to examine this subject more closely that I withheld my observations on the structure of the Mer de Glace in 1857, and betook myself once more to the mountains during the summer of 1858. My desire at that time was to settle once for all the rival claims of the only two theories which then deserved serious attention—namely, those of pressure and of stratification.

In pursuance of this idea, I first visited the Lower glacier of Grindelwald, one of the most accessible, and at the same time most instructive, in the entire range of the Alps. Ascending the branch of this glacier which descends from the Schreckhorn, the Strahleck, and the Finsteraarhorn, I came to the base of an ice-fall which forbade further advance. Quitting the glacier here, I ascended the side of the flanking mountain, so as to reach a point from which the fall, and the glacier below it, are distinctly visible; and from this position I observed the gradual development and perfecting of the structure at the base of the fall. On the middle of the fall itself no trace of the structure was manifest; but where the glacier changed its inclination at the bottom, being bent upwards so as throw its surface into a state of intense longitudinal compression, the blue veins first made their appearance. The base of the fall was a true structure mill, where the transverse veins were manufactured, being afterwards sent forward, giving a character to portions of the glacier which had no share in their formation.

I afterwards examined the fall from the opposite side of the valley, and corroborated the observations. It is difficult, in words, to convey the force of the evidence which this glacier presents to the observer who sees it; it seems in fact like a grand laboratory experiment made by Nature herself with especial reference to the point in question. The squeezing of the mass, its yielding to the force brought to bear upon it, its wrinkling and scaling off, and the appearance of the veins at the exact point where the pressure begins to manifest itself, leave no doubt on the mind that pressure and structure stand to each other in the relation of cause and effect, and that the stratification could have nothing to do with the phenomenon.

I subsequently crossed the Strahleck, descended the glaciers of the Aar, crossed the Grimsel, and examined the glacier of the Rhone. This glacier has also its grand ice-fall. In company with Prof. Ramsay, I climbed in 1858 the precipices flanking the fall at the Grimsel side. What has been stated regarding the Grindelwald ice-fall is true of that of the Rhone; the base of the cascade is the manufactory of the structure; and, as all the ice has to pass through this mill, the entire mass of the glacier from the base of the fall downwards is beautifully laminated.

Descending the valley of the Rhone to Viesch, I went thence to the Æggischhorn, and remained for eight days in the vicinity of the Great Aletsch glacier—the noblest ice-stream of the Alps. A highly intelligent explorer had adduced certain phenomena of this glacier as an evidence against the pressure theory of the veined structure; and I did not think myself justified in quitting the place until I had perfectly satisfied myself that the Aletsch not only presented no phenomena at variance with the pressure theory, but exhibited some which seemed fatal to the theory of the stratification.

I subsequently proceeded to Zermatt, and spent ten days on the Riffelberg, exploring the entire system of glaciers between Monte Rosa and the Mont Cervin. These glaciers exhibit, perhaps in a more striking manner than any others in the Alps, the yielding of glacier ice when subjected to intense pressure. The great western glacier of Monte Rosa, the Schwartze glacier, the Trifti glacier, and the glaciers of St. Theodule, are first spread out as wide and extensive névés over the breasts of the mountains. They move down, and are finally forced into the valley containing the trunk, or Görner glacier. Here they are squeezed to narrow strips, which gradually dwindle in width until they form driblets not more than a few yards across. From the Görner Grat, or from the summit of the Riffelhorn, these parallel strips of glacier, each separated from its neighbour by a medial moraine, present a most striking and instructive appearance.

The structure of these glaciers was carefully examined, and in all cases as I travelled from regions where the pressure was feeble to others where it was intense, the ice changed from a state almost, if not entirely, structureless, to one in which the veining was exhibited in great perfection. Each glacier, for example, where it met the opposing mass in the trunk valley, and was pressed against the latter by the thrust from behind, exhibited a beautifully developed structure.

Proofs have been already adduced that the Glacier du Géant is in a state of longitudinal compression; it has also been shown that the seams of white ice which intersect this glacier are due to the filling up of the channels of glacier streams by snow, and the subsequent compression of the substance. Here, then, we have a vast ice-press which furnishes us with a test of the pressure theory. Both in 1857 and 1858 I found many of these seams of white ice intersected by blue veins of the finest and most distinct character, their general direction being at right angles to the direction of pressure.

But the notions of M. Agassiz as to the turning up of the strata so as to expose their edges at the surface, and the acute remarks and arguments of Mr. John Ball on the same subject, might still cast a doubt upon the pressure theory, by suggesting a possible, though extremely improbable, explanation of the structure in accordance with the theory of stratification.

Hence my strong desire to discover some crucial phenomenon which should set this question for ever at rest, and leave no room for doubt, even on the minds of those who never saw a glacier. On Wednesday, August 18, I was fortunate enough to make this discovery upon the Furgge glacier.

This ice-field spreads out as an almost level plain at the base of Mont Cervin. The strata pile themselves one above the other without disturbance, and hence with great regularity. The ice at length reaches a brow, over which it is precipitated, forming in its descent four great terraces, and shutting up the lower valley as a cul de sac. When I reached this place huge blocks of ice stood, like rocking stones, upon the topmost ledge, and numbers, which had fallen, had been caught by the other ledges, and occupied very threatening positions: the base of the fall was cumbered with crushed ice, and large boulders of the substance had been cast a considerable way down the glacier.

On the faces of the terraces horizontal lines of stratification were shown in the most perfect manner. Here and there the exertion of a powerful lateral squeeze was manifest, causing the beds to crumple, and producing numerous faults. Examining the fall from a distance through an opera-glass, I thought I could discover lines of veining running through the strata, at a high angle, exactly as the planes of cleavage often run at a high angle to the bedding of slate rocks. The surface of the ice was, however, weathered; and I was unwilling to accept an observation upon such a cardinal point with a shade of doubt attached to it. Leaving my field-glass with my guide, who was to give me warning should the blocks overhead give way, I advanced to the wall of ice, and at several places cut away with my axe the weathered superficial portions. Underneath I found the true veined structure, running nearly at right angles to the planes of stratification.

I afterwards climbed the glacier to the right, and, as I ascended, still better illustrations of the coexistence of the structure and the strata than those observed upon the terraces exhibited themselves. The ice was greatly dislocated, and on the faces of the crevasses the beds were distinctly shown, with the veins crossing them. The idea that the veins could be due to the turning up of the strata is plainly irreconcileable with these observations.

The same year I visited the Mer de Glace and its tributaries, and found the pressure key applicable to their phenomena also. The transverse structure of the Glacier du Géant is formed at the base of the séracs; that of the Talèfre branch of the Mer de Glace at the base of the Talèfre ice-fall, where the change of inclination and the thrust from behind produce the requisite longitudinal compression. I have already had occasion to remark upon the peculiar dipping of the structure, and the scaling-off of the protuberances, which are effects of the same cause. These phenomena are exhibited at the base of all the ice-cascades.

The principal kinds of structure may be divided into three; as follows:

1st, Marginal structure, developed by pressure due to the swifter motion of the centre of the glacier.

2nd, Longitudinal structure, due to mutual pressure of two tributary glaciers; the structure here is parallel to the medial moraine which divides the tributaries.

3rd, Transverse structure, produced by pressure due to the change of inclination, and to the longitudinal thrust endured by the glacier at the base of an ice-fall.

The lamination of a glacier is a peculiarly interesting case of cleavage. It is produced in the same manner as the lamination of slate rock, which is known, through the distortion of its fossils, to have suffered great pressure at right angles to the planes of cleavage.

IV.
HELMHOLTZ ON ICE AND GLACIERS.

Switzerland has attractions for the scientific philosophers of Germany, and around the Titlis, Bunsen, Helmholtz, Kirchhoff, and Wiedemann are not unfamiliar names. Nor have their visits to the Alps been unproductive of results. Some time ago I was favoured by Professor Helmholtz with the First Part of his ‘Popular Scientific Lectures.’ It contains four of them—the first, ‘On the Relation of the Natural Sciences to Science in general;’ the second, ‘On Goethe’s Labours in Natural Science;’ the third, ‘On the Physiological Origin of Musical Harmony;’ and the fourth, ‘On Ice and Glaciers.’ The lectures are in German, and it is much to be desired that some competent person should undertake their translation into English.[32]

I turned with natural interest to the last-mentioned discourse, to see how my notions and experiments on the formation and motion of glaciers were regarded by so eminent a man. I will here endeavour to give a summary of the scientific portion of the lecture.

Professor Helmholtz refers the cold of the upper regions of the atmosphere to the causes generally assigned; but he adds a remark important at the present moment, when the origin of the hot wind called Föhn in Switzerland is the subject of so much discussion. This wind, as Helmholtz justly observes, may not only be a cold wind upon the mountain-summits, but a wet one, and it may deposit its moisture there. A wind thus dried upon the heights, and warmed by its subsequent fall into the valleys, would possess the heat and dryness of the Föhn. These qualities are, therefore, no proof that the origin of the Föhnwind is Sahara.