Chapter XIV: Part II: Chiefly Scientific (3)

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.

No. Vel. No. Vel. No. Vel. No. Vel. No. Vel.
West 17 15 16 17-1/4 15 22-1/4 13 23-1/4 12 23-1/4
East 3 12-1/2 4 15-1/4 5 15-1/2 7 18-1/4 9 19-1/2

[Sidenote: WESTERN HALF MOVES QUICKEST.]

The table explains itself. We see that while stake 17, which stands _west_ of the centre, moves 15 inches, stake 3, which stands an equal distance _east_ of the centre, moves only 12-1/2 inches. Comparing every pair of the other points, we find the same to hold good; the western stake moves in each case faster than the corresponding eastern one. Hence, _the entire western half of the Mer de Glace, at the place crossed by our fourth line, moves more quickly than the eastern half of the glacier_.

We next proceeded farther up, and tested the contrary curvature of the glacier, opposite to Trelaporte. The station chosen for this purpose was on a grassy platform of the promontory, whence, on the 28th of July, a row of stakes was fixed at right angles to the axis of the glacier. Their motions, measured on the 31st, gave the following results:--

Fifth Line.[B]--Daily Motion.

No. of stake. Inches.
West 1 moved 11-1/4
2 " 13-1/2
3 " 12-3/4
4 " 15
5 " 15-1/4
6 " 16
7 " 17-1/4
8 " 19-1/4
9 moved 19-3/4
10 " 19
11 " 19-1/2
12 " 17-1/2
13 " 16
14 " 14-3/4
15 " 10 East.

This line was set out and numbered from the Trelaporte side of the valley, and was also measured by Mr. Hirst, over boulders, ice-ridges, chasms, and moraines. The entire width of the glacier here was found to be 893 yards, or somewhat wider than it is at the Ponts. It will also be observed that its motion is somewhat slower.

An inspection of the notes of this line showed me that stakes 3 and 14, 4 and 12, 7 and 10, were "corresponding points;" the first of each pair standing as far from the western side, as the second stood from the eastern. In the following table these points and their velocities are arranged exactly as in the case of the fourth line.

Numbers and Velocities of the Corresponding Points on the Fifth Line.

No. Vel. No. Vel. No. Vel.
West 3 12-3/4 4 15 7 17-1/4
East 14 14-3/4 12 17-1/2 10 19

[Sidenote: EASTERN HALF MOVES QUICKEST.]

In each case we find that the stake on the eastern side moves more quickly than the corresponding one upon the western side: so that where the fifth line crosses the glacier _the eastern half of the Mer de Glace moves more quickly than the western half_. This is the reverse of the result obtained at our fourth line, but it agrees with that obtained on our first three lines, where the curvature of the valley is similar. The analogy between a river and a glacier moving through a sinuous valley is therefore complete.

Supposing the points of maximum motion to be determined for a great number of lines across the glacier, the line uniting all these points is what mathematicians would call the _locus_ of the point of maximum motion. At Trelaporte this line would lie east of the centre; at the Ponts it would lie west of the centre; hence, in passing from Trelaporte to the Ponts, it must cross the axis of the glacier. Again, at the Montanvert, it would lie east of the centre, and between the Ponts and the Montanvert the axis of the glacier would be crossed a second time. Supposing the dotted line in Fig. 21 to represent the middle line of the glacier, then the defined line would represent the locus of the point of maximum motion. _It is a curve more deeply sinuous than the valley itself, and it crosses the axis of the glacier at each point of contrary flexure._

[Sidenote: LOCUS OF POINT OF SWIFTEST MOTION.]

To complete our knowledge of the motion of the Mer de Glace, we afterwards determined the velocity of its two accessible tributaries--the Glacier du Geant, and the Glacier de Lechaud. On the 29th of July, a line of stakes was set out across the former, a little above the Tacul, and their motion was subsequently found to be as follows:

Sixth Line.--Daily Motion.

No. of stake. Inches.
1 moved 11
2 " 10
3 " 12
4 " 13
5 " 12
6 moved 12-3/4
7 " 10-1/2
8 " 10
9 " 9
10 " 5

The width of the glacier at this place we found to be 1134 yards, and its maximum velocity, as shown by the foregoing table, 13 inches a day.

On the 1st of August a line was set out across the Glacier de Lechaud, above its junction with the Talefre: it commenced beneath the block of stone known as the Pierre de Beranger. The displacements of the stakes, measured on the 3rd of August, gave the following results:--

Seventh Line.--Daily Motion.

No. of stake. Inches.
1 moved 4-1/2
2 " 8-1/4
3 " 9-1/2
4 " 9
5 " 8-1/2
6 moved 7-1/2
7 " 6-1/4
8 " 8-1/2
9 " 7
10 " 5-1/2

The width of the Glacier de Lechaud at this place was found to be 825 yards; its maximum motion, as shown by the table, being 9-1/2 inches a day. This is the slowest rate which we observed upon either the Mer de Glace or its tributaries. The width of the Talefre-branch, as it descends the cascade, or, in other words, before it is influenced by the pressure of the Lechaud, was found approximately to be 638 yards.

[Sidenote: SQUEEZING AT TRELAPORTE.]

The widths of the tributaries were determined for the purpose of ascertaining the amount of lateral compression endured by the ice in its passage through the neck of the valley at Trelaporte. Adding all together we have--

Geant 1134 yards.
Lechaud 825 "
Talefre 638 "
Total 2597 yards.

These three branches, as shown by the actual measurement of our 5th line, are forced at Trelaporte through a channel 893 yards wide; the width of the trunk stream is a little better than one-third of that of its tributaries, and it passes through this gorge at a velocity of nearly 20 inches a day.

[Sidenote: THE LECHAUD A DRIBLET.]

Limiting our view to one of the tributaries only, the result is still more impressive. Previous to its junction with the Talefre, the Glacier de Lechaud stretches before the observer as a broad river of ice, measuring 825 yards across: at Trelaporte it is squeezed, in a frozen vice, between the Talefre on one side and the Geant on the other, to a driblet, measuring 85 yards in width, or about one-tenth of its former transverse dimension. It will of course be understood that it is the _form_ and not the _volume_ of the glacier that is affected to this enormous extent by the pressure.

Supposing no waste took place, the Glacier de Lechaud would force precisely the same amount of ice through the "narrows" at Trelaporte, in one day, as it sends past the Pierre de Beranger. At the latter place its velocity is about half of what it is at the former, but its width is more than nine times as great. Hence, if no waste took place, its _depth_, at Trelaporte, would be at _least_ 4-1/2 times its depth opposite the Pierre de Beranger. Superficial and subglacial melting greatly modify this result. Still I think it extremely probable that observations directed to this end would prove the comparative shallowness of the upper portions of the Glacier de Lechaud.

FOOTNOTES:

[A] Great care is necessary on the part of the man who measures the displacements. The staff ought to be placed along the original line, and the assistant ought to walk along it until the foot of a _perpendicular_ from the stake is attained. When several days' motion is to be measured, this precaution is absolutely necessary; the eye being liable to be grossly deceived in _guessing_ the direction of a perpendicular.

[B] The details of the measurement of the fourth and fifth lines are published in the 'Philosophical Transactions,' vol. cxlix., p. 261.

ICE-WALL AT THE TACUL.

VELOCITIES OF TOP AND BOTTOM.

(11.)

As regards the motion of the _surface_ of a glacier, two laws are to be borne in mind: 1st, that regarding the quicker movement of the centre; 2nd, that regarding the locus of the point of maximum motion. Our next care must be to compare the motion of the surface of a glacier with the motion of those parts which lie near its bed. Rendu first surmised that the bottom of the glacier was retarded by friction, and both Professor Forbes[A] and M. Martins[B] have confirmed the conjecture. Theirs are the only observations which we possess upon the subject; and I was particularly desirous to instruct myself upon this important head by measurements of my own.

[Sidenote: FIRST ATTEMPT AT MEASUREMENT.]

During the summer of 1857 the eastern side of the Glacier du Geant, near the Tacul, exposed a nearly vertical precipice of ice, measuring 140 feet from top to bottom. I requested Mr. Hirst to fix two stakes in the same vertical plane, one at the top of the precipice and one near the bottom. This he did upon the 3rd of August, and on the 5th I accompanied him to measure the progress of the stakes. On the summit of the precipice, and running along it, was the lateral moraine of the glacier. The day was warm and the ice liquefying rapidly, so that the boulders and debris, deprived incessantly of their support, came in frequent leaps and rushes down the precipice. Into this peril my guide was about to enter, to measure the displacement of the lower stake, while I was to watch, and call out the direction in which he was to run when a stone gave way. But I soon found that the initial motion was no sure index of the final motion. By striking the precipice, the stones were often deflected, and carried wide of their original direction. I therefore stopped the man, and sent him to the summit of the precipice to remove all the more dangerous blocks. This accomplished, he descended, and while I stood beside him, executed the required measurement. From the 3rd to the 5th of August the upper stake had moved twelve inches, and the lower one six.

Unfortunately some uncertainty attached itself to this result, due to the difficulty of fixing the lower stake. The guide's attention had been divided between his work and his safety, and he had to retreat more than a dozen times from the falling boulders and debris. I, on the other hand, was unwilling to accept an observation of such importance with a shade of doubt attached to it. Hence arose the desire to measure the motion myself. On the 11th of August I therefore reascended to the Tacul, and fixed a stake at the top of the precipice, and another at the bottom. While sitting on the old moraine looking at the two pickets, the importance of determining the motion of a point midway between the top and bottom forcibly occurred to me, but, on mentioning it to my guide, he promptly pronounced any attempt of the kind absurd.

[Sidenote: STAKES FIXED AT TOP, BOTTOM, AND CENTRE.]

On scanning the place carefully, however, the value of the observation appeared to me to outweigh the amount of danger. I therefore took my axe, placed a stake and an auger against my breast, buttoned my coat upon them, and cut an oblique staircase up the wall of ice, until I reached a height of forty feet from the bottom. Here the position of the stake being determined by Mr. Hirst, who was at the theodolite, I pierced the ice with the auger, drove in the stake, and descended without injury. During the whole operation however my guide growled audibly.

On the following morning we commenced the ascent of Mont Blanc, a narrative of which is given in Part I. We calculated on an absence of three days, and estimated that the stakes which had just been fixed would be ready for measurement on our return; but we did not reach Chamouni until the afternoon of Friday, the 14th. Heavy clouds settled, during our descent, upon the summits behind us, and a thunder-peal from the Aiguilles soon heralded a fall of rain, which continued without intermission till the afternoon of the 16th, when the atmosphere cleared, and showed the mountains clothed to their girdles with snow. The Montanvert was thickly covered, and on our way to it we met the servants in charge of the cattle, which had been driven below the snow-line to obtain food.

[Sidenote: THROUGH GLOOM TO THE TACUL.]

On Monday morning, the 17th, a dense fog filled the valley of the Mer de Glace. I watched it anxiously. The stakes which we had set at the Tacul had been often in my thoughts, and I wished to make some effort to save the labour and peril incurred in setting them from being lost. I therefore set out, in one of the clear intervals, accompanied by my friend and Simond, determined to measure the motion of the stakes, if possible, or to fix them more firmly, if they still stood. As we passed, however, from l'Angle to the glacier, the fog became so dense and blinding that we halted. At my request Mr. Hirst returned to the Montanvert; and Simond, leaving the theodolite in the shelter of a rock, accompanied me through the obscurity to the Tacul. We found the topmost stake still stuck by its point in the ice; but the two others had disappeared, and we afterwards discovered their fragments in a snow-buttress, which reared itself against the base of the precipice. They had been hit by the falling stones, and crushed to pieces. Having thus learned the worst, we descended to the Montanvert amid drenching rain.

[Sidenote: DESCENT OF BOULDERS.]

On the morning of the 18th there was no cloud to be seen anywhere, and the sunlight glistened brightly on the surface of the ice. We ascended to the Tacul. The spontaneous falling of the stones appeared more frequent this morning than I had ever seen it. The sun shone with unmitigated power upon the ice, producing copious liquefaction. The rustle of falling debris was incessant, and at frequent intervals the boulders leaped down the precipice, and rattled with startling energy amid the rocks at its base. I sent Simond to the top to remove the looser stones; he soon appeared, and urged the moraine-shingle in showers down the precipice, upon a bevelled slope of which some blocks long continued to rest. They were out of the reach of the guide's baton, and he sought to dislodge them by sending other stones down upon them. Some of them soon gave way, drawing a train of smaller shingle after them; others required to be hit many times before they yielded, and others refused to be dislodged at all. I then cut my way up the precipice in the manner already described, fixed the stake, and descended as speedily as possible. We afterwards fixed the bottom stake, and on the 20th the displacements of all three were measured.[C] The spaces passed over by the respective stakes in 24 hours were found to be as follows:--

Inches.
Top stake 6.00
Middle stake 4.50
Bottom stake 2.56

[Sidenote: MOTION OF STAKES.]

The height of the precipice was 140.8 feet, but it sloped off at its upper portion. The height of the middle stake above the ground was 35 feet, and of the bottom one 4 feet. It is therefore proved by these measurements that the bottom of the ice-wall at the Tacul moves with less than half the velocity of the top; while the displacement of the intermediate stake shows how the velocity gradually increases from the bottom upwards.

FOOTNOTES:

[A] 'Edinb. Phil. Journ.,' Oct. 1846, p. 417.

[B] Agassiz, 'Systeme Glaciaire,' p. 522.

[C] On this latter occasion my guide volunteered to cut the steps for me up to the pickets; and I permitted him to do so. In fact, he was at least as anxious as myself to see the measurement carried out.

WINTER MOTION OF THE MER DE GLACE.

(12.)

The winter measurements were executed in the manner already described, on the 28th and 29th of December, 1859. The theodolite was placed on the mountain's side flanking the glacier, and a well-defined object was chosen at the opposite side of the valley, so that a straight line between this object and the theodolite was approximately perpendicular to the axis of the glacier. Fixing the telescope in the first instance with its cross hairs upon the object, its end was lowered until it struck the point upon the glacier at which a stake was to be fixed. Thanks to the intelligence of my assistants, after the fixing of the first stake they speedily took up the line at all other points, requiring very little correction to make their positions perfectly accurate. On the day following that on which the stakes were driven in, the theodolite was placed in the same position, and the distances to which the stakes had moved from their original positions were accurately determined. As already stated, the first line crossed the glacier about 80 yards above the Montanvert Hotel.

[Sidenote: HALF OF SUMMER MOTION.]

Line No. I.--Winter Motion in Twenty-four Hours.

No. of stake. Inches.
West 1 7-1/4
2 11
3 13-1/2
4 13
5 13-3/4
6 14-1/4
7 15-3/4
8 15-3/4
9 12-1/4
10 12
11 6-1/2 East.

[Sidenote: THE SAME LAW IN SUMMER AND WINTER.]

The maximum here is fifteen and three-quarters inches; the maximum summer motion of the same portion of the glacier is about thirty inches. These measurements also show that in winter, as well as in summer, the side of the glacier opposite to the Montanvert moves quicker than that adjacent to it. The stake which moved with the maximum velocity was beyond the moraine of La Noire. The second line crossed the glacier about 130 yards below the Montanvert.

Line No. II.--Winter Motion in Twenty-four Hours.

No. of stake. Inches.
1 7-3/4
2 9-1/2
3 13-3/4
4 16
5 16
6 15-3/4
7 17-1/2
8 16-1/2
9 14-1/2
10 14

The maximum here is an inch and three-quarters greater than that of line No. 1. The summer maximum at this portion of the glacier also exceeds that of the part intersected by line No. 1. The surface of the glacier between the two lines is in a state of tension which relieves itself by a system of transverse fissures, and thus permits of the quicker advance of the forward portion.

My desire, in making these measurements, was, in the first place, to raise the winter observations of the motion to the same degree of accuracy as that already possessed by the summer ones. Auguste Balmat had already made a series of winter observations on the Mer de Glace; but they were made in the way employed before the introduction of the theodolite by Agassiz and Forbes, and shared the unavoidable roughness of such a mode of measurement. They moreover gave us no information as to the motion of the different parts of the glacier along the same transverse line, and this, for reasons which will appear subsequently, was the point of chief interest to me.

CAUSE OF GLACIER-MOTION.

DE SAUSSURE'S THEORY.

(13.)

Perhaps the first attempt at forming a glacier-theory is that of Scheuchzer in 1705. He supposed the motion to be caused by the conversion of water into ice within the glacier; the known and almost irresistible expansion which takes place on freezing, furnishing the force which pushed the glacier downward. This idea was illustrated and developed with so much skill by M. de Charpentier, that his name has been associated with it; and it is commonly known as the Theory of Charpentier, or the Dilatation-Theory. M. Agassiz supported this theory for a time, but his own thermometric experiments show us that the body of the glacier is at a temperature of 32 deg. Fahr.; that consequently there is no interior magazine of cold to freeze the water with which the glacier is supposed to be incessantly saturated. So that these experiments alone, if no other grounds existed, would prove the insufficiency of the theory of dilatation. I may however add, that the arguments most frequently urged against this theory deal with an assumption, which I do not think its author ever intended to make.

[Sidenote: THE GLACIER SLIDES.]

Another early surmise was that of Altmann and Gruener (1760), both of whom conjectured that the glacier slid along its bed. This theory received distinct expression from De Saussure in 1799; and has since been associated with the name of that great alpine traveller, being usually called the 'Theory of Saussure,' and sometimes the 'Sliding Theory.' It is briefly stated in these words:--

"Almost every glacier reposes upon an inclined bed, and those of any considerable size have beneath them, even in winter, currents of water which flow between the ice and the bed which supports it. It may therefore be understood that these frozen masses, drawn down the slope on which they repose, disengaged by the water from all adhesion to the bottom, sometimes even raised by this water, must glide by little and little, and descend, following the inclinations of the valleys, or of the slopes which they cover. It is this slow but continual sliding of the ice on its inclined base which carries it into the lower valleys."[A]

[Sidenote: STRAINED INTERPRETATION.]

De Saussure devoted but little time to the subject of glacier-motion; and the absence of completeness in the statement of his views, arising no doubt from this cause, has given subsequent writers occasion to affix what I cannot help thinking a strained interpretation to the sliding theory. It is alleged that he regarded a glacier as a perfectly rigid body; that he considered it to be "a mass of ice of small depth, and considerable but uniform breadth, sliding down a uniform valley, or pouring from a narrow valley into a wider one."[B] The introduction "of the smallest flexibility or plasticity" is moreover emphatically denied to him.[C]

It is by no means probable that the great author of the 'Voyages' would have subscribed to this "rigid" annotation. His theory, be it remembered, is to some extent _true_: the glacier moves over its bed in the manner supposed, and the rocks of Britain bear to this day the traces of these mighty sliders. De Saussure probably contented himself with a general statement of what he believed to be the substantial cause of the motion. He visited the Jardin, and saw the tributaries of the Mer de Glace turning round corners, welding themselves together, and afterwards moving through a sinuous trunk-valley; and it is scarcely credible that in the presence of such facts he would have denied all flexibility to the glacier.

The statement that he regarded a glacier to be a mass of ice of uniform width, is moreover plainly inconsistent with the following description of the glacier of Mont Dolent: "Its most elevated plateau is a great circus, surrounded by high cliffs of granite, of pyramidal forms; thence the glacier descends through a gorge, in which _it is narrowed_; but after having passed the gorge, it _enlarges again_, spreading out like a fan. Thus it has on the whole the form of a sheaf tied in the middle and dilated at its two extremities."[D]

[Sidenote: GLACIER OF MONT DOLENT.]

Curiously enough this very glacier, and these very words, are selected by M. Rendu as illustrative of the plasticity of glaciers. "Nothing," he says, "shows better the extent to which a glacier moulds itself to its locality than the form of the glacier of Mont Dolent in the Valley of Ferret;" and he adds, in connexion with the same passage, these remarkable words:--"There is a multitude of facts which would seem to necessitate the belief that the substance of glaciers enjoys a kind of ductility which permits it to mould itself to the locality which it occupies, to grow thin, to swell, and to narrow itself like a soft paste."[E]

FOOTNOTES:

[A] 'Voyages,' Sec. 535.

[B] James D. Forbes, 'Occasional Papers on the Theory of Glaciers,' 1859, p. 100.

[C] "I adhere to the definition as excluding the introduction of the smallest flexibility or plasticity." 'Occ. Pap.,' p. 96.

[D] 'Voyages,' tome ii. p. 290.

[E] In connexion with this brief sketch of the 'Sliding Theory,' it ought to be stated, that Mr. Hopkins has proved experimentally, that ice may descend an incline at a sensibly uniform rate, and that the velocity is augmented by increasing the weight. In this remarkable experiment the motion was due to the slow disintegration of the lower surface of the ice. See 'Phil. Mag.,' 1845, vol. 26.

RENDU'S THEORY.

(14.)

[Sidenote: RENDU'S CHARACTER.]

M. Rendu, Bishop of Annecy, to whose writings I have just referred, died last autumn.[A] He was a man of great repute in his diocese, and we owe to him one of the most remarkable essays upon glaciers that have ever appeared. His knowledge was extensive, his reasoning close and accurate, and his faculty of observation extraordinary. With these were associated that intuitive power, that presentiment concerning things as yet untouched by experiment, which belong only to the higher class of minds. Throughout his essay a constant effort after quantitative accuracy reveals itself. He collects observations, makes experiments, and tries to obtain numerical results; always taking care, however, so to state his premises and qualify his conclusions that nobody shall be led to ascribe to his numbers a greater accuracy than they merit. It is impossible to read his work, and not feel that he was a man of essentially truthful mind, and that science missed an ornament when he was appropriated by the Church.

The essay above referred to is printed in the tenth volume of the Memoirs of the Royal Academy of Sciences of Savoy, published in 1841, and is entitled, '_Theorie des Glaciers de la Savoie, par M. le Chanoine Rendu, Chevalier du Merite Civil et Secretaire perpetuel_.' The paper had been written for nearly two years, and might have remained unprinted, had not another publication on the same subject called it forth.

I will place a few of the leading points of this remarkable production before the reader; commencing with a generalization which is highly suggestive of the character of the author's mind.

[Sidenote: "THEORIE DES GLACIERS DE LA SAVOIE."]

He reflects on the accumulation of the mountain-snows, each year adding fifty-eight inches of ice to a glacier. This would make Mont Blanc four hundred feet higher in a century, and four thousand feet higher in a thousand years. "It is evident," he says, "that nothing like this occurs in nature." The escape of the ice then leads him to make some general remarks on what he calls the "law of circulation." "The conserving will of the Creator has employed for the permanence of His work the great law of _circulation_, which, strictly examined, is found to reproduce itself in all parts of nature. The waters circulate from the ocean to the air, from the air to the earth, and from the earth to the ocean.... The elements of organic substances circulate, passing from the solid to the liquid or aeriform condition, and thence again to the state of solidity or of organisation. That universal agent which we designate by the names of fire, light, electricity, and magnetism, has probably also a _circulation_ as wide as the universe." The italics here are Rendu's own. This was published in 1841, but written, we are informed, nearly two years before. In 1842 Mr. Grove wrote thus:--"Light, heat, magnetism, motion, and chemical affinity, are all convertible material affections." More recently Helmholtz, speaking of the "circuit" formed by "heat, light, electricity, magnetism, and chemical affinity," writes thus:--"Starting from each of these different manifestations of natural forces, we can set every other in action." I quote these passages because they refer to the same agents as those named by M. Rendu, and to which he ascribes "_circulation_." Can it be doubted that this Savoyard priest had a premonition of the Conservation of Force? I do not want to lay more stress than it deserves upon a conjecture of this kind; but its harmony with an essay remarkable for its originality gives it a significance which, if isolated, it might not possess.

[Sidenote: GLACIERS RIGHTLY DIVIDED.]

With regard to the glaciers, Rendu commences by dividing them into two kinds, or rather the selfsame glacier into two parts, one of which he calls the "_glacier reservoir_," the other the "_glacier d'ecoulement_,"--two terms highly suggestive of the physical relationship of the _neve_ and the glacier proper. He feeds the reservoirs from three sources, the principal one of which is the snow, to which he adds the rain, and the vapours which are condensed upon the heights without passing into the state of either rain or snow. The conversion of the snow into ice he supposes to be effected by four different causes, the most efficacious of which is _pressure_.[B] It is needless to remark that this quite agrees with the views now generally entertained.

In page 60 of the volume referred to there is a passage which shows that the "veined structure" of the glacier had not escaped him, though it would seem that he ascribed it to stratification. "When," he writes, "we perceive the profile of a glacier on the walls of a crevasse, we see different layers distinct in colour, but more particularly in density; some seem to have the hardness, as they have the greenish colour, of glass; others preserve the whiteness and porosity of the snow." There is also a very close resemblance between his views of the influence of "time and cohesion" and those of Prof. Forbes. "We may conclude," he writes, "that _time_, favouring the action of _affinity_, and the pressure of the layers one upon the other, causes the little crystals of which snow is composed to approach each other, bring them into contact, and convert them into ice."[C] Regelation also appears to have attracted his notice.[D] "When we fill an ice-house," he writes, "we break the ice into very small fragments; afterwards we wet it with water 8 or 10 degrees above zero (Cent.) in temperature; but, notwithstanding this, the whole is converted into a compact mass of ice." He moreover maintains, in almost the same language as Prof. Forbes,[E] the opinion, that ice has always an inner temperature lower than zero (Cent.). He believed this to be a property "inherent to ice." "Never," he says, "can a calorific ray pass the first surface of ice to raise the temperature of the interior."[F]

[Sidenote: OBSERVATIONS AND HYPOTHESES.]

He notices the direction of the glacier as influencing the wasting of its ridges by the sun's heat; ascribing to it the effect to which I have referred in explaining the wave-like forms upon the surface of the Mer de Glace. His explanation of the Moulins, too, though insufficient, assigns a true cause, and is an excellent specimen of physical reasoning.

With regard to the diminution of the _glaciers reservoirs_, or, in other words, to the manner in which the ice disappears, notwithstanding the continual additions made to it, we have the following remarkable passage:--"In seeking the cause of the diminution of glaciers, it has occurred to my mind that the ice, notwithstanding its hardness and its rigidity, can only support a given pressure without breaking or being squeezed out. According to this supposition, whenever the pressure exceeds that force, there will be rupture of the ice, and a flow in consequence. Let us take, at the summit of Mont Blanc, a column of ice reposing on a horizontal base. The ice which forms the first layer of that column is compressed by the weight of all the layers above it; but if the solidity of the said first layer can only support a weight equal to 100, when the weight exceeds this amount there will be rupture and spreading out of the ice of the base. Now, something very similar occurs in the immense crust of ice which covers the summits of Mont Blanc. This crust appears to augment at the upper surface and to diminish by the sides. To assure oneself that the movement is due to the force of pressure, it would be necessary to make a series of experiments upon the solidity of ice, such as have not yet been attempted."[G] I may remark that such experiments substantially verify M. Rendu's notion.

But it is his observations and reasoning upon the _glaciers d'ecoulement_ that chiefly interest us. The passages in his writings where he insists upon the power of the glaciers to mould themselves to their localities, and compares them to a soft paste, to lava at once ductile and liquid, are well known from the frequent and flattering references of Professor Forbes; but there are others of much greater importance, which have hitherto remained unknown in this country. Regarding the motion of the Mer de Glace, Rendu writes as follows:--

[Sidenote: MEASUREMENT OF MOTION.]

[Sidenote: THE SIDES OF THE GLACIER RETARDED.]

"I sought to appreciate the quantity of its motion; but I could only collect rather vague data. I questioned my guides regarding the position of an enormous rock at the edge of the glacier, but still upon the ice, and consequently partaking of its motion. The guides showed me the place where it stood the preceding year, and where it had stood two, three, four, and five years previously; they showed me the place where it would be found in a year, in two years, &c.; _so certain are they of the regularity of the motion_. Their reports, however, did not always agree precisely with each other, and their indications of time and distance lack the precision without which we proceed obscurely in the physical sciences. In reducing these different indications to a mean, I found the total advance of the glacier to be about 40 feet a year. During my last journey I obtained more certain data, which I have stated in the preceding chapter. _The enormous difference between the two results arises from the fact that the latter observations were made at the centre of the glacier_, WHICH MOVES MORE RAPIDLY, _while the former were made at the side, where the ice_ IS RETAINED BY THE FRICTION AGAINST ITS ROCKY WALLS."[H]

An opinion, founded on a grave misapprehension which Rendu enables us to correct, is now prevalent in this country, not only among the general public, but also among those of the first rank in science. The nature of the mistake will be immediately apparent. At page 128 of the 'Travels in the Alps' its distinguished author gives a sketch of the state of our knowledge of glacier-motion previous to the commencement of his inquiries. He cites Ebel, Hugi, Agassiz, Bakewell, De la Beche, Shirwell, Rendu, and places them in open contradiction to each other. Rendu, he says, gives the motion of the Mer de Glace to be "242 feet per annum; 442 feet per annum; a foot a day; 400 feet per annum, and 40 feet per annum, or _one-tenth_ of the last!" ... and he adds, "I was not therefore wrong in supposing that the actual progress of a glacier was yet a new problem when I commenced my observations on the Mer de Glace in 1842."[I]

In the 'North British Review' for August, 1859, a writer equally celebrated for the brilliancy of his discoveries and the vigour of his pen, collected the data furnished by the above paragraph into a table, which he introduced to his readers in the following words:--"It is to Professor Forbes alone that we owe the first and most correct researches respecting the motion of glaciers; and in proof of this, we have only to give the following list of observations which had been previously made.

Observers. Name of glacier. Annual rate of motion.

Ebel Chamouni 14 feet
Ebel Grindelwald 25 "
Hugi Aar 240 "
Agassiz Aar 200 "
Bakewell Mer de Glace 540 "
De la Beche Mer de Glace 600 "
Shirwell Mer de Glace 300 "
M. Rendu Mer de Glace 365 "
Saussure's Ladder Mer de Glace 375 "

... Such was the state of our knowledge when Professor Forbes undertook the investigation of the subject."

I am persuaded that the writer of this article will be the first to applaud any attempt to remove an error which, advanced on his great authority, must necessarily be widely disseminated. The numbers in the above table certainly differ widely, and it is perhaps natural to conclude that such discordant results can be of no value; but the fact really is that _every one of them may be perfectly correct_. This fact, though overlooked by Professor Forbes, was clearly seen by Rendu, who pointed out with perfect distinctness the sources from which the discrepancies were derived.

[Sidenote: DISCREPANCIES EXPLAINED.]

"It is easy," he says, "to comprehend that it is impossible to obtain a general measure,--that there ought to be one for each particular glacier. The nature of the slope, the number of changes to which it is subjected, the depth of the ice, the width of the couloir, the form of its sides, and a thousand other circumstances, must produce variations in the velocity of the glacier, and these circumstances cannot be everywhere absolutely the same. Much more, it is not easy to obtain this velocity for a single glacier, and for this reason. In those portions where the inclination is steep, the layer of ice is thin, and its velocity is great; in those where the slope is almost nothing, the glacier swells and accumulates; the mass in motion being double, triple, &c., the motion is only the half, the third, &c.

[Sidenote: LIQUID MOTION ASCRIBED TO GLACIER.]

"But this is not all," adds M. Rendu: "_Between the Mer de Glace and a river, there is a resemblance so complete that it is impossible to find in the latter a circumstance which does not exist in the former._ In currents of water the motion is not uniform, neither throughout their width nor throughout their depth; _the friction of the bottom, that of the sides_, the action of obstacles, cause the motion to vary, _and only towards the middle of the surface is this entire...._"[J]

In 1845 Professor Forbes appears to have come to the same conclusion as M. Rendu; for after it had been proved that the centre of the Aar glacier moved quicker than the side in the ratio of fourteen to one, he accepted the result in these words:--"The movement of the centre of the glacier is to that of a point five metres from the edge as FOURTEEN to ONE: such is the effect of plasticity!"[K] Indeed, if the differences exhibited in the table were a proof of error, the observations of Professor Forbes himself would fare very ill. The measurements of glacier-motion made with his own hands vary from less than 42 feet a year to 848 feet a year, the minimum being less than _one-twentieth_ of the maximum; and if we include the observations made by Balmat, the fidelity of which has been certified by Professor Forbes, the minimum is only _one-thirty-seventh_ of the maximum.

[Sidenote: NORTH BRITISH REVIEW.]

There is another point connected with Rendu's theory which needs clearing up:--"The idea," writes the eminent reviewer, "that a glacier is a semifluid body is no doubt startling, especially to those who have seen the apparently rigid ice of which it is composed. M. Rendu himself shrank from the idea, and did not scruple to say that 'the rigidity of a mass of ice was in direct opposition to it;' and we think that Professor Forbes himself must have stood aghast when his fancy first associated the notion of imperfect fluidity with the solid or even the fissured ice of the glacier, and when he saw in his mind's eye the glaciers of the Alps flowing like a river along their rugged bed. A truth like this was above the comprehension and beyond the sympathy of the age; and it required a moral power of no common intensity to submit it to the ordeal of a shallow philosophy, and the sneers of a presumptuous criticism."

These are strong words; but the fact is that, so far from "shrinking" from the idea, Rendu affirmed, with a clearness and an emphasis which have not been exceeded since, that all the phenomena of a river were reproduced upon the Mer de Glace; its deeps, its shallows, its widenings, its narrowings, its rapids, its places of slow motion, and the quicker flow of its centre than of its sides. He did not shrink from accepting a difference between the central and lateral motion amounting to a ratio of ten to one--a ratio so large that Professor Forbes at one time regarded the acceptance of it as a simple absurdity. In this he was perhaps justified; for his own first observations, which, however valuable, were hasty and incomplete, gave him a maximum ratio of about one and a half to one, while the ratio in some cases was nearly one of _equality_. The observations of Agassiz however show that the ratio, instead of being ten to one, may be _infinity_ to one; for the lateral ice may be so held back by a local obstacle that in the course of a year it shall make no sensible advance at all.

[Sidenote: THE ICE AND THE GLACIER.]

From one thing only did M. Rendu shrink; and it is _the_ thing regarding which we are still disunited. He shrank from stating the physical quality of the ice in virtue of which a glacier moved like a river. He demands experiments upon snow and ice to elucidate this subject. The very observations which Professor Forbes regards as proofs are those of which we require the physical explanation. It is not the viscous flow, if you please to call it such, of the glacier as a whole that here concerns us; but it is the quality of the _ice_ in virtue of which this kind of motion is accomplished. Professor Forbes sees this difference clearly enough: he speaks of "fissured ice" being "flexible" in hand specimens; he compares the glacier to a mixture of ice and sand; and finally, in a more matured paper, falls back for an explanation upon the observations of Agassiz regarding the capillaries of the glacier.[L]

FOOTNOTES:

[A] Expressions such as "last summer," "last autumn," "recently," will be taken throughout in the sense which they had in the early half of 1860, when this book was first published.--L. C. T.

[B] 'Memoir,' p. 77.

[C] P. 75.

[D] P. 71.

[E] 'Philosophical Magazine,' 1859.

[F] 'Memoir,' p. 69.

[G] Page 80.

[H] Page 95.

[I] At page 38 of the 'Travels' the following passage also occurs:--"I believe that I may safely affirm that not one observation of the rate of motion of a glacier, either on the average or at any particular season of the year, existed when I commenced my experiments in 1842."

[J] 'Theorie,' p. 96.

[K] 'Occ. Pap.,' p. 74.

[L] In all that has been written upon glaciers in this country the above passages from the writings of Rendu are unquoted; and many who mingled very warmly in the discussions of the subject were, until quite recently, ignorant of their existence. I was long in this condition myself, for I never supposed that passages which bear so directly upon a point so much discussed, and of such cardinal import, could have been overlooked; or that the task of calling attention to them should devolve upon myself nearly twenty years after their publication. Now that they are discovered, I conceive no difference of opinion can exist as to the propriety of placing them in their true position.

(15.)

The measurements of Agassiz and Forbes completely verify the anticipations of Rendu; but no writer with whom I am acquainted has added anything essential to the Bishop's statements as to the identity of glacier and liquid motion. He laid down the conditions of the problem with perfect clearness, and, as regards the distribution of merit, the point to be decided is the relative importance of his idea, and of the measurements which were subsequently made.

[Sidenote: OBSERVATIONS OF FORBES.]

The observations on which Professor Forbes based the analogy between a glacier and a river are the following:--In 1842 he fixed four marks upon the Mer de Glace a little below the Montanvert, the first of which was 100 yards distant from the side of the glacier, while the last was at the centre "or a little beyond it." The relative velocity of these four points was found to be

1.000 1.332 1.356 1.367.