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 atthe 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.
The first observations were made upon two of these points, two others being subsequently added. Professor Forbes also determined the velocity of two points on the Glacier du Géant, and found the ratio of motion, in the first instance, to be as 14 to 32. Subsequent measurements, however, showed the ratio to be as 14 to 18, the larger motion belonging to the station nearest to the centre of the glacier. These are the only measurements which I can find in his large work that establish the swifter motion of the centre of the glacier; and in these cases the velocity of the centre is compared with that ofone sideonly. In no instance that I am aware of, either in 1842 or subsequent years, did Professor Forbes extend his measurements quite across a glacier; and as regards completeness in this respect, no observations hitherto made can at all compare with those executed at the instance of Agassiz upon the glacier of the Aar.
In 1844 Professor Forbes made a series of interesting experiments on a portion of the Mer de Glace near l'Angle. He divided a length of 90 feet into 45 equal spaces, and fixed pins at the end of each. His theodolite was placed upon the ice, and in seventeen days he found that the ice 90 feet nearer the centre than the theodolite had moved 26 inches past the latter. These measurements were undertaken for a special object, and completely answered the end for which they were intended.
In 1846 Professor Forbes made another important observation. Fixing three stakes at the heights of 8, 54, and 143 feet above the bed of the glacier, he found that in five days they moved respectively 2.87, 4.18, and 4.66 feet. The stake nearest the bed moved most slowly,thus showing that the ice is retarded by friction. This result was subsequently verified by the measurements of M. Martins, and by my own.
If we add to the above an observation made during a short visit to the Aletsch glacier in 1844, which showed its lateral retardation, I believe we have before us the whole of the measurements executed by Professor Forbes, which show the analogy between the motion of a glacier and that of a viscous body.
MEASUREMENTS OF AGASSIZ.
Illustrative of the same point, we have the elaborate and extensive series of measurements executed by M. Wild under the direction of M. Agassiz upon the glacier of the Aar in 1842, 1843, 1844, and 1845, which exhibit on a grand scale, and in the most conclusive manner, the character of the motion of this glacier; and also show, on close examination, an analogy with fluid motion which neither M. Agassiz nor Professor Forbes suspected. The former philosopher publishes a section in his 'Système Glaciaire,' entitled 'Migrations of the Centre;' in which he shows that the middle of the glacier is not always the point of swiftest motion. The detection of this fact demonstrates the attention devoted by M. Agassiz to the discussion of his observations, but he gives no clue to the cause of the variation. On inspecting the shape of the valley through which the Aar glacier moves, I find that these "migrations" follow the law established in 1857 upon the Mer de Glace, and enunciated at page286.
To sum up this part of the question:—Theideaof semi-fluid motion belongs entirely to Rendu; theproofof the quicker central flow belongs in part to Rendu, but almost wholly to Agassiz and Forbes; the proof of the retardation of the bed belongs to Forbes alone; while the discovery of the locus of the point of maximum motion belongs, I suppose, to me.
The formal statement of this theory is given in the following words:—"A glacier is an imperfect fluid, or viscous body, which is urged down slopes of a certain inclination by the mutual pressure of its parts." The consistency of the glacier is illustrated by reference to treacle, honey, and tar, and the theory thus enunciated and exemplified is called the 'Viscous Theory.'
It has been the subject of much discussion, and great differences of opinion are still entertained regarding it. Able and sincere men take opposite sides; and the extraordinary number of Reviews which have appeared upon the subject during the last two years show the interest which the intellectual public of England take in the question. The chief differences of opinion turn upon the inquiry as to what Professor Forbes really meant when he propounded the viscous theory; some affirm one thing, some another, and, singularly enough, these differences continue, though the author of the theory has at various times published expositions of his views.
"FACTS AND PRINCIPLES."
The differences referred to arise from the circumstances that a sufficient distinction has not been observed betweenfactsandprinciples, and that the viscous theory has assumed various forms since its first promulgation. It has been stated to me that the theory of Professor Forbes is "the congeries of facts" which he has discovered. But it is quite evident that no recognition, however ample, of these facts would be altogether satisfactory to Professor Forbes himself. He claims recognition of histheory,[A]and no writerwith whom I am acquainted makes such frequent use of the term. What then can the viscous theory mean apart from the facts? I interpret it as furnishing the principle from which the facts follow as physical consequences—that the glacier moves as a river because the ice is viscous. In this sense only can Professor Forbes's views be called a theory; in any other, his experiments are mere illustrations of the facts of glacier motion, which do not carry us a hair's breadth towards their physical cause.
VISCOUS THEORY;—WHAT IS IT?
What then is the meaning of viscosity or viscidity? I have heard it defined by men of high culture as "gluey tenacity;" and such tenacity they once supposed a glacier to possess. If we dip a spoon into treacle, honey, or tar, we can draw the substance out into filaments, and the same may be done with melted caoutchouc or lava. All these substances are viscous, and all of them have been chosen to illustrate the physical property in virtue of which a glacier moves. Viscosity then consists in the power of being drawn out when subjected to a force of tension, the substance, after stretching, being in a state of molecular equilibrium, or, in other words, devoid of that elasticity which would restore it to its original form. This certainly was the idea attached to Professor Forbes's words by some of his most strenuous supporters, and also by eminent men who have never taken part in any controversy on the subject. Mr. Darwin, for example, speaks of felspathic rocks being "stretched" while flowing slowly onwards in a pasty condition, in precisely the same manner as Professor Forbes believes that the ice of moving glaciers is stretched and fissured; and Professor Forbes himself quotes these words of Mr. Darwin as illustrative of his theory.[B]
The question now before us is,—Does a glacier exhibit that power of yielding to a force of tension which would entitle its ice to be regarded as a viscous substance?
THEORY TESTED.
With a view to the solution of this question Mr. Hirst took for me the inclinations of the Mer de Glace and all its tributaries in 1857; the effect of a change of inclination being always noted. I will select from those measurements a few which bear more specially upon the subject now under consideration, commencing with the Glacier des Bois, down which the ice moves in that state of wild dislocation already described. The inclination of the glacier above this cascade is 5° 10', and that of the cascade itself is 22° 20', the change of inclination being therefore 17° 10'.
Fig. 22. Inclinations of ice cascasde of the Glacier des Bois.
InFig. 22I have protracted the inclination of the cascade and of the glacier above it; the line A B representing the former and B C the latter. Now a stream of molten lava, of treacle, or tar, would, in virtue of its viscosity, be able to flow over the brow at B without breaking across; but this is not the case with the glacier; it is so smashed and riven in crossing this brow, that, to use the words of Professor Forbes himself, "it pours into the valley beneath in a cascade of icy fragments."
INCLINATIONS OF THE MER DE GLACE.
But this reasoning will appear much stronger when we revert to other slopes upon the Mer de Glace. For example, its inclination above l'Angle is 4°, and it afterwards descends a slope of 9° 25', the change of inclination being 5° 25'. If we protract these inclinations to scale, we havethe line A B,Fig. 23, representing the steeper slope, and B C that of the glacier above it. One would surely think that a viscous body could cross the brow B without transverse fracture, but this the glacier cannot do, and Professor Forbes himself pronounces this portion of the Mer de Glace impassable. Indeed it was the profound crevasses here formed which placed me in a difficulty already referred to. Higher up again, the glacier is broken on passing from a slope of 3° 10' to one of 5°. Such observations show how differently constituted a glacier is from a stream of lava in a "pasty condition," or of treacle, honey, tar, or melted caoutchouc, to all which it has been compared. In the next section I shall endeavour to explain the origin of the crevasses, and shall afterwards make a few additional remarks on the alleged viscosity of ice.
Fig. 23. Inclinations of Mer de Glace above l'Angle.
FOOTNOTES:[A]"Mr. Hopkins," writes Professor Forbes, "has done me the honour, in the memoirs before alluded to, to mention with approbation my observations and experiments on the subject of glaciers. He has been more sparing either in praise or criticism of the theory which I have founded upon them. Had Mr. Hopkins," &c.—Eighth Letter; 'Occ. Papers,' p. 66.[B]'Occ. Papers,' p. 92.
[A]"Mr. Hopkins," writes Professor Forbes, "has done me the honour, in the memoirs before alluded to, to mention with approbation my observations and experiments on the subject of glaciers. He has been more sparing either in praise or criticism of the theory which I have founded upon them. Had Mr. Hopkins," &c.—Eighth Letter; 'Occ. Papers,' p. 66.
[A]"Mr. Hopkins," writes Professor Forbes, "has done me the honour, in the memoirs before alluded to, to mention with approbation my observations and experiments on the subject of glaciers. He has been more sparing either in praise or criticism of the theory which I have founded upon them. Had Mr. Hopkins," &c.—Eighth Letter; 'Occ. Papers,' p. 66.
[B]'Occ. Papers,' p. 92.
[B]'Occ. Papers,' p. 92.
CREVASSES CAUSED BY THE MOTION.
Having made ourselves acquainted with the motion of the glacier, we are prepared to examine those rents, fissures, chasms, or, as they are most usually called,Crevasses, by which all glaciers are more or less intersected. They result from the motion of the glacier, and the laws of their formation are deduced immediately from those of the motion. The crevasses are sometimes very deep and numerous, and apparently without law or order in their distribution. They cut the ice into long ridges, and break these ridges transversely into prisms; these prisms gradually waste away, assuming, according to the accidents of their melting, the most fantastic forms. I have seen them like the mutilated statuary of an ancient temple, like the crescent moon, like huge birds with outstretched wings, like the claws of lobsters, and like antlered deer. Such fantastic sculpture is often to be found on the ice cascades, where the riven glacier has piled vast blocks on vaster pedestals, and presented them to the wasting action of sun and air. InFig. 24I have given a sketch of a mass of ice of this character, which stood in 1859 on the dislocated slope of the Glacier des Bois.
FANTASTIC ICE-MASSES.
Fig. 24. Fantastic Mass of ice.
It is usual for visitors to the Montanvert to descend to the glacier, and to be led by their guides to the edges of the crevasses, where, being firmly held, they look down into them; but those who have only made their acquaintance in this way know but little of their magnitude and beauty in the more disturbed portions of glaciers. As might be expected, they have been the graves of many a mountaineer; and the skeletons found upon the glacier prove that even the chamois itself, with its elastic muscles andadmirable sureness of foot, is not always safe among the crevasses. They are grandest in the higher ice-regions, where the snow hangs like a coping over their edges, and the water trickling from these into the gloom forms splendid icicles. The Görner Glacier, as we ascend it towards the old Weissthor, presents many fine examples of such crevasses; the ice being often torn in a most curious and irregular manner. You enter a porch, pillared by icicles, and look into a cavern in the very body of the glacier, encumbered with vast frozen bosses which are fringed all round by dependent icicles. At the peril of your life from slipping, or from the yielding of the stalactites, you may enter these caverns, and find yourself steeped in the blue illumination of the place. Their beauty is beyond description; but you cannot deliver yourself up, heart and soul, to its enjoyment. There is a strangeness about the place which repels you, and not without anxiety do you look from yourledge into the darkness below, through which the sound of subglacial water sometimes rises like the tolling of distant bells. You feel that, however the cold splendours of the place might suit a purely spiritual essence, they are not congenial to flesh and blood, and you gladly escape from its magnificence to the sunshine of the world above.
BIRTH OF A CREVASSE.
From their numbers it might be inferred that the formation of crevasses is a thing of frequent occurrence and easy to observe; but in reality it is very rarely observed. Simond was a man of considerable experience upon the ice, but the first crevasse he ever saw formed was during the setting out of one of our lines, when a narrow rent opened beneath his feet, and propagated itself through the ice with loud cracking for a distance of 50 or 60 yards. Crevasses always commence in this way as mere narrow cracks, which open very slowly afterwards. I will here describe the only case of crevasse-forming which has come under my direct observation.
On the 31st of July, 1857, Mr. Hirst and myself, having completed our day's work, were standing together upon the Glacier du Géant, when a loud dull sound, like that produced by a heavy blow, seemed to issue from the body of the ice underneath the spot on which we stood. This was succeeded by a series of sharp reports, which were heard sometimes above us, sometimes below us, sometimes apparently close under our feet, the intervals between the louder reports being filled by a low singing noise. We turned hither and thither as the direction of the sounds varied; for the glacier was evidently breaking beneath our feet, though we could discern no trace of rupture. For an hour the sounds continued without our being able to discover their source; this at length revealed itself by a rush of air-bubbles from one of the little pools upon the surface of the glacier, which was intersected bythe newly-formed crevasse. We then traced it for some distance up and down, but hardly at any place was it sufficiently wide to permit the blade of my penknife to enter it. M. Agassiz has given an animated description of the terror of his guides upon a similar occasion, and there was an element of awe in our own feelings as we heard the evening stillness of the glacier thus disturbed.
MECHANICAL ORIGIN.
With regard to the mechanical origin of the crevasses the most vague and untenable notions had been entertained until Mr. Hopkins published his extremely valuable papers. To him, indeed, we are almost wholly indebted for our present knowledge of the subject, my own experiments upon this portion of the glacier-question being for the most part illustrations of the truth of his reasoning. To understand the fissures in their more complex aspects it is necessary that we should commence with their elements. I shall deal with the question in my own way, adhering, however, to the mechanical principles upon which Mr. Hopkins has based his exposition.
Fig. 25. Diagram explanatory of the mechanical origin of Crevasses.
Leta b,c d, be the bounding sides of a glacier moving in the direction of the arrow; letm,nbe two points upon the ice, one,m, close to the retarding side of the valley, and the other,n, at some distance from it. After a certain time, the pointmwill have moved downwards tom', but in consequence of the swifter movement of theparts at a distance from the sides,nwill have moved in the same time ton'. Thus the linem n, instead of being at right angles to the glacier, takes up the oblique positionm' n'; but to reach fromm'ton'the linem nwould have to stretch itself considerably; every other line that we can draw upon the ice parallel tom' n'is in a similar state of tension; or, in other words, the sides of the glacier are acted upon by an oblique pull towards the centre. Now, Mr. Hopkins has shown that the direction in which this oblique pull is strongest encloses an angle of 45° with the side of the glacier.
LINE OF GREATEST STRAIN.
Fig. 26. Diagram showing the line of Greatest Strain.
What is the consequence of this? Leta b,c d,Fig. 26, represent, as before, the sides of the glacier, moving in the direction of the arrow; let the shading lines enclose an angle of 45° with the sides.Alongthese lines the marginal ice suffers the greatest strain, and, consequentlyacrossthese lines and at right angles to them, the ice tends to break and to formmarginal crevasses. The lines,o p,o p, mark the direction of these crevasses; they are at right angles to the line of greatest strain, and hence also enclose an angle of 45° with the side of the valley,being obliquely pointed upwards.
MARGINAL AND TRANSVERSE CREVASSES.
This latter result is noteworthy; it follows from the mechanical data that the swifter motion of the centre tends to produce marginal crevasses which are inclined from the side of the glacier towards its source, and nottowards its lower extremity. But when we look down upon a glacier thus crevassed, the first impression is that the sides have been dragged down, and have left the central portions behind them; indeed, it was this very appearance that led M. de Charpentier and M. Agassiz into the error of supposing that the sides of a glacier moved more quickly than its middle portions; and it was also the delusive aspect of the crevasses which led Professor Forbes to infer the slower motion of the eastern side of the Mer de Glace.
The retardation of the ice is most evident near the sides; in most cases, the ice for a considerable distance right and left of the central line moves with a sensibly uniform velocity; there is no dragging of the particles asunder by a difference of motion, and, consequently, a compact centre is perfectly compatible with fissured sides. Nothing is more common than to see a glacier with its sides deeply cut, and its central portions compact; this, indeed, is always the case where the glacier moves down a bed of uniform inclination.
But supposing that the bed is not uniform—that the valley through which the glacier moves changes its inclination abruptly, so as to compel the ice to pass over a brow; the glacier is then circumstanced like a stick which we try to break by holding its two ends and pressing it against the knee. The brow, where the bed changes its inclination, represents the knee in the case of the stick, while the weight of the glacier itself is the force that tends to break it. It breaks; and fissures are formed across the glacier, which are hence calledtransverse crevasses.
GRINDELWALD GLACIER.
No glacier with which I am acquainted illustrates the mechanical laws just developed more clearly and fully than the Lower glacier of Grindelwald. Proceeding along the ordinary track beside the glacier, at about an hour'sdistance from the village the traveller reaches a point whence a view of the glacier is obtained from the heights above it. The marginal fissures are very cleanly cut, and point nearly in the direction already indicated; the glacier also changes its inclination several times along the distance within the observer's view. On crossing each brow the glacier is broken across, and a series of transverse crevasses is formed, which follow each other down the slope. At the bottom of the slope tension gives place to pressure, the walls of the crevasses are squeezed together, and the chasms closed up. They remain closed along the comparatively level space which stretches between the base of one slope and the brow of the next; but here the glacier is again transversely broken, and continues so until the base of the second slope is reached, where longitudinal pressure instead of longitudinal strain begins to act, and the fissures are closed as before. InFig. 27aI have given a sketchy section of a portion of the glacier, illustrating the formation of the crevasses at the top of a slope, and their subsequent obliteration at its base.
COMPRESSION AND TENSION.
Fig. 27a,b. Section and Plan of a portion of the Lower Grindelwald Glacier.
Another effect is here beautifully shown, namely, the union of the transverse and marginal crevasses to form continuous fissures which stretch quite across the glacier.Fig. 27bwill illustrate my meaning, though very imperfectly; it represents a plan of a portion of the Lower Grindelwald glacier, with both marginal and transverse fissures drawn upon it. I have placed it under the section so that each part of it may show in plan the portion of the glacier which is shown in section immediately above it. It shows how the marginal crevasses remain after the compression of the centre has obliterated the transverse ones; and how the latter join on to the former, so as to form continuous fissures, which sweep across the glacier in vast curves, with their convexities turned upwards.The illusion before referred to is here strengthened; the crevasses turn, so to say,againstthe direction of motion, instead of forming loops, with their convexities pointing downwards, and thus would impress a person unacquainted with the mechanical data with the idea that the glacier margins moved more quickly than the centre. The figures are intended to convey the idea merely; on the actual slopes of the glacier between twenty and thirty chasms may be counted: also the word "compression" ought to have been limited to the level portions of the sketch.
LONGITUDINAL CREVASSES.
Besides the two classes of fissures mentioned we often find others, which are neither marginal nor transverse. The terminal portions of many glaciers, for example, are in a state of compression; the snout of the glacier abuts against the ground, and having to bear the thrust of the mass behind it, if it have room to expand laterally, the icewill yield, andlongitudinal crevasseswill be formed. They are of very common occurrence, but the finest example of the kind is perhaps exhibited by the glacier of the Rhone. After escaping from the steep gorge which holds the cascade, this glacier encounters the bottom of a comparatively wide and level valley; the resistance to its forward motion is augmented, while its ability to expand laterally is increased; it has to bear a longitudinal thrust, and it splits at right angles to the pressure [strain?]. A series of fissures is thus formed, the central ones of which are truly longitudinal; but on each side of the central line the crevasses diverge, and exhibit a fan-like arrangement. This disposition of the fissures is beautifully seen from the summit of the Mayenwand on the Grimsel Pass.
Fig. 28. Diagram illustrating the crevassing of Convex Sides of glacier.
Here then we have the elements, so to speak, of glacier-crevassing, and through their separate or combined action the most fantastic cutting up of a glacier may be effected. And see how beautifully these simple principles enable us to account for the remarkable crevassing of the eastern side of the Mer de Glace. Leta b,c d, be the opposite sides of a portion of the glacier, near the Montanvert;c dbeing east, anda bwest, the glacier moving in the direction of the arrow; let the pointsm nrepresent the extremities of our line of stakes, and let us suppose an elastic string stretched across the glacier from one to the other. We have proved that the point of maximum motion here lies much nearer to the sidec dthan toa b. Letobe this point, and, seizing the string ato, let it be drawn in the direction of motion until it assumes the position,m,o',n. It is quite evident thato' nis in a state greater tension thano' m, and theice at the eastern side of the Mer de Glace is in a precisely similar mechanical condition. It suffers a greater strain than the ice at the opposite side of the valley, and hence is more fissured and broken. Thus we see that the crevassing of the eastern side of the glacier is a simple consequence of the quicker motion of that side, and does not, as hitherto supposed, demonstrate its slower motion. The reason why the eastern side of the glacier, as a whole, is much more fissured than the western side is, that there are two long segments which turn their convex curvature eastward, and only one segment of the glacier which turns its convexity westward.
CREVASSING OF CONVEX SIDE.
The lower portion of the Rhone glacier sweeps round the side of the valley next the Furca, and turns throughout a convex curve to this side: the crevasses here are wide and frequent, while they are almost totally absent at the opposite side of the glacier. The lower Grindelwald glacier turns at one place a convex curve towards the Eiger, and is much more fissured at that side than at the opposite one; indeed, the fantastic ice-splinters, columns, and minarets, which are so finely exhibited upon this glacier, are mainly due to the deep crevassing of the convex side. Numerous other illustrations of the law might, I doubt not, be discovered, and it would be a pleasant and useful occupation to one who takes an interest in the subject, to determine, by strict measurements upon other glaciers, the locus of the point of maximum motion, and to observe the associated mechanical effects.
BERGSCHRUNDS.
The appearance of crevasses is often determined by circumstances more local and limited than those above indicated; a boss of rock, a protuberance on the side of the flanking mountain, anything, in short, which checks the motion of one part of the ice and permits an adjacent portion to be pushed away from it, produces crevasses. Some valleys are terminated by a kind of mountain-circuswith steep sides, against which the snow rises to a considerable height. As the mass is urged downwards, the lower portion of the snow-slope is often torn away from its higher portion, and a chasm is formed, which usually extends round the head of the valley. To such a crevasse the specific nameBergschrundis applied in the Bernese Alps; I have referred to one of them in the account of the"Passage of the Strahleck."
The phenomena described and accounted for in the last chapter have a direct bearing upon the question of viscosity. In virtue of the quicker central flow the lateral ice is subject to an oblique strain; but, instead of stretching, it breaks, and marginal crevasses are formed. We also see that a slight curvature in the valley, by throwing an additional strain upon one half of the glacier, produces an augmented crevassing of that side.
But it is known that a substance confessedly viscous may be broken by a sudden shock or strain. Professor Forbes justly observes that sealing-wax at moderate temperatures will mould itself (with time) to the most delicate inequalities of the surface on which it rests, but may at the same time be shivered to atoms by the blow of a hammer. Hence, in order to estimate the weight of the objection that a glacier breaks when subjected to strain, we must know the conditions under which the force is applied.
The Mer de Glace has been shown (p.287) to move through the neck of the valley at Trélaporte at the rate of twenty inches a day. Let the sides of this page represent the boundaries of the glacier at Trélaporte, and any one of its lines of print a transverse slice of ice. Supposing the lineto move down the page as the slice of ice moves down the valley, then the bending of the ice in twenty-four hours, shown on such a scale, would only be sufficient to push forward the centre in advance of the sides by a very small fraction of the width of the line of print. To such an extremely gradual strain the ice is unable to accommodate itself without fracture.
NUMERICAL TEST OF VISCOSITY.
Or, referring to actual numbers:—the stake No. 15 on our 5th line, page284, stood on the lateral moraine of the Mer de Glace; and between it and No. 14 a distance of 190 feet intervened. Leta b,Fig. 29, be the side of the glacier, moving in the direction of the arrow, and leta b c dbe a square upon the glacier with a side of 190 feet. The whole square moves with the ice, but the sideb dmoves quickest; the pointamoving 10 inches, whilebmoves 14.75 inches in 24 hours; the differential motion therefore amounts to an inch in five hours. Leta b' d' cbe the shape of the figure after five hours' motion; then the linea bwould be extended toa b'andc dtoc d'.
Fig. 29. Diagram illustrating test of viscosity.
The extension oftheselines does not however express themaximumstrain to which the ice is subjected. Mr. Hopkins has shown that this takes place along the linea d; in five hours then this line, if capable of stretching, would be stretched toa d'. From the data given every boy who has mastered the 47th Proposition of the First Book of Euclid can find the length both ofa danda d'; the former is 3224.4 inches, and the latter is 3225.1, the difference between them being seven-tenths of an inch.
This is the amount of yielding required from the ice in five hours, but it cannot grant this; the glacier breaks, and numerous marginal crevasses are formed. It must not be forgotten that the evidence here adduced merelyshows what ice cannot do; what itcando in the way of viscous yielding we do not know: there exists as yet no single experiment on great masses or small to show that ice possesses in any sensible degree that power of being drawn out which seems to be the very essence of viscosity.
I have already stated that the crevasses, on their first formation, are exceedingly narrow rents, which widen very slowly. The new crevasse observed by our guide required several days to attain a width of three inches; while that observed by Mr. Hirst and myself did not widen a single inch in three days. This, I believe, is the general character of the crevasses; they form suddenly and open slowly. Both facts are at variance with the idea that ice is viscous; for were this substance capable of stretching at the slow rate at which the fissures widen, there would be no necessity for their formation.
STRETCHING OF ICE NOT PROVED.
It cannot be too clearly and emphatically stated that theprovedfact of a glacier conforming to the law of semi-fluid motion is a thing totally different from theallegedfact of its being viscous. Nobody since its first enunciation disputed the former. I had no doubt of it when I repaired to the glaciers in 1856; and none of the eminent men who have discussed this question with Professor Forbes have thrown any doubt upon his measurements. It is the assertion that small pieces of ice are proved to be viscous[A]by the experiments made upon glaciers, and the consequent impression left upon the public mind—that ice possesses the "gluey tenacity" which the term viscous suggests—to which these observations are meant to apply.
FOOTNOTES:[A]"The viscosity, though it cannot be traced in the partsif very minuteneverthelessexiststhere, as unequivocally proved by experiments on the large scale."—Forbes in 'Phil. Mag.,' vol. x., p. 301.
[A]"The viscosity, though it cannot be traced in the partsif very minuteneverthelessexiststhere, as unequivocally proved by experiments on the large scale."—Forbes in 'Phil. Mag.,' vol. x., p. 301.
[A]"The viscosity, though it cannot be traced in the partsif very minuteneverthelessexiststhere, as unequivocally proved by experiments on the large scale."—Forbes in 'Phil. Mag.,' vol. x., p. 301.
CONNEXION OF NATURAL FORCES.
Great scientific principles, though usually announced by individuals, are often merely the distinct expression of thoughts and convictions which had long been entertained by all advanced investigators. Thus the more profound philosophic thinkers had long suspected a certain equivalence and connexion between the various forces of nature; experiment had shown the direct connexion and mutual convertibility of many of them, and the spiritual insight, which, in the case of the true experimenter, always surrounds and often precedes the work of his hands, revealed more or less plainly that natural forces either had a common root, or that they formed a circle, whose links were so connected that by starting from any one of them we could go through the circuit, and arrive at the point from which we set out. For the last eighteen years this subject has occupied the attention of some of the ablest natural philosophers, both in this country and on the Continent. The connexion, however, which has most occupied their minds is that betweenheatandwork; the absolute numerical equivalence of the two having, I believe, been first announced by a German physician named Mayer, and experimentally proved in this country by Mr. Joule.
MECHANICAL EQUIVALENT OF HEAT.
A lead bullet may be made hot enough to burn the hand, by striking it with a hammer, or by rubbing it against a board; a clever blacksmith can make a nail red-hot by hammering it; Count Rumford boiled water by the heat developed in the boring of cannon, and inferred from the experiment that heat was not what it was generallysupposed to be, an imponderable fluid, but a kind of motion generated by the friction. Now Mr. Joule's experiments enable us to state the exact amount of heat which a definite expenditure of mechanical force can originate. I sayoriginate, not drag from any hiding-place in which it had concealed itself, but actually bring into existence, so that the total amount of heat in the universe is thereby augmented. If a mass of iron fall from a tower 770 feet in height, we can state the precise amount of heat developed by its collision with the earth. Supposing all the heat thus generated to be concentrated in the iron itself, its temperature would thereby be raised nearly 10° Fahr. Gravity in this case has expended a certain amount of force in pulling the iron to the earth, and this force is themechanical equivalentof the heat generated. Furthermore, if we had a machine so perfect as to enable us to apply all the heat thus produced to the raising of a weight, we should be able, by it, to lift the mass of iron to the precise point from which it fell.
But the heat cannot lift the weight and still continue heat; this is the peculiarity of the modern view of the matter. The heat is consumed, used up, it is no longer heat; but instead of it we have a certain amount of gravitating force stored up, which is ready to act again, and to regenerate the heat when the weight is let loose. In fact, when the falling weight is stopped by the earth, the motion of its mass is converted into a motion of its molecules; when the weight is lifted by heat, molecular motion is converted into ordinary mechanical motion, but for every portion of either of them brought into existence an equivalent portion of the other must be consumed.
What is true for masses is also true for atoms. As the earth and the piece of iron mutually attract each other, and produce heat by their collision, so the carbon of a burning candle and the oxygen of the surrounding airmutually attract each other; they rush together, and on collision the arrested motion becomes heat. In the former case we have the conversion of gravity into heat, in the latter the conversion of chemical affinity into heat; but in each case the process consists in the generation of motion by attraction, and the subsequent change of that motion into motion of another kind. Mechanically considered, the attraction of the atoms and its results is precisely the same as the attraction of the earth and weight anditsresults.
HEAT PRODUCED IF THE EARTH STRUCK THE SUN.
But what is true for an atom is also true for a planet or a sun. Supposing our earth to be brought to rest in her orbit by a sudden shock, we are able to state the exact amount of heat which would be thereby generated. The consequence of the earth's being thus brought to rest would be that it would fall into the sun, and the amount of heat which would be generated by this second collision is also calculable. Helmholtz has calculated that in the former case the heat generated would be equal to that produced by the combustion of fourteen earths of solid coal, and in the latter case the amount would be 400 times greater.
SHIFTING OF ATOMS.
Whenever a weight is lifted by a steam-engine in opposition to the force of gravity an amount of heat is consumed equivalent to the work done; and whenever the molecules of a body are shifted in opposition to their mutual attractions work is also performed, and an equivalent amount of heat is consumed. Indeed the amount of work done in the shifting of the molecules of a body by heat, when expressed in ordinary mechanical work, is perfectly enormous. The lifting of a heavy weight to the height of 1000 feet may be as nothing compared with the shifting of the atoms of a body by an amount so small that our finest means of measurement hardly enable us to determine it. Different bodies give heat different degrees of trouble, if I may use the term, in shifting their atoms and putting them in new places.Iron gives more trouble than lead; and water gives far more trouble than either. The heat expended in this molecular work is lost as heat; it does not show itself as temperature. Suppose the heat produced by the combustion of an ounce of candle to be concentrated in a pound of iron, a certain portion of that heat would go to perform the molecular work to which I have referred, and the remainder would be expended in raising the temperature of the body; and if the same amount of heat were communicated to a pound of iron and to a pound of lead, the balance in favour of temperature would be greater in the latter case than in the former, because the heat would have less molecular work to do; the lead would become more heated than the iron. To raise a pound of iron a certain number of degrees in temperature would, in fact, require more than three times the absolute quantity of heat which would be required to raise a pound of lead the same number of degrees. Conversely, if we place the pound of iron and the pound of lead, heated to the same temperature, into ice, we shall find that the quantity of ice melted by the iron will be more than three times that melted by the lead. In fact, the greater amount of molecular work invested in the iron now comes into play, the atoms again obey their own powerful forces, and an amount of heat corresponding to the energy of these forces is generated.
This molecular work is that which has usually been calledspecific heat, orcapacity for heat. According to thematerialisticview of heat, bodies are figured as sponges, and heat as a kind of fluid absorbed by them, different bodies possessing different powers of absorption. According to thedynamicview, as already explained, heat is regarded as a motion, and capacity for heat indicates the quantity of that motion consumed in internal changes.
The greatest of these changes occurs when a body passes from one state of aggregation to another, from the solidto the liquid, or from the liquid to the aëriform state; and the quantity of heat required for such changes is often enormous. To convert a pound of ice at 32° Fahr. into waterat the same temperaturewould require an amount of heat competent, if applied as mechanical force, to lift the same pound of ice to a height of 110,000 feet; it would raise a ton of ice nearly 50 feet, or it would lift between 49 and 50 tons to a height of one foot above the earth's surface. To convert a pound of water at 212° into a pound of steam at the same temperature would require an amount of heat which would perform nearly seven times the amount of mechanical work just mentioned.