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diff --git a/old/63803-0.txt b/old/63803-0.txt deleted file mode 100644 index d86f49b..0000000 --- a/old/63803-0.txt +++ /dev/null @@ -1,6100 +0,0 @@ -The Project Gutenberg EBook of The Forms of Water in Clouds and Rivers, -Ice and Glaciers, by John Tyndall - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: The Forms of Water in Clouds and Rivers, Ice and Glaciers - -Author: John Tyndall - -Release Date: November 18, 2020 [EBook #63803] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK FORMS OF WATER--CLOUDS, RIVERS, ICE *** - - - - -Produced by Tom Cosmas produced from files generously -provided on The Internet Archive. All resultant materials -are placed in the Public Domain. - - - - - - - - - - - -Transcriber Note - -Text emphasis denoted by _Italics_. - - - - -[Illustration: JOHN TYNDALL] - - - - - THE FORMS OF WATER IN CLOUDS AND RIVERS ICE AND GLACIERS - - - BY - - JOHN TYNDALL, LL.D., F. R. S. - - - - - _WITH TWENTY-FIVE ILLUSTRATIONS - DRAWN AND ENGRAVED UNDER THE DIRECTION - OF THE AUTHOR_ - - - NEW YORK - D. APPLETON AND COMPANY - 1899 - - - Copyright, 1872, - By D. APPLETON AND COMPANY. - - Electrotyped and Printed - at the Appleton Press, U. S. A. - - - - -AMERICAN PREFACE TO THE INTERNATIONAL SCIENTIFIC SERIES. - - -The rapid development of science in the present age, and the -increasing public interest in its results, make it desirable that the -most efficient measures should be adopted to elevate the character -of its popular literature. The tendency of careless and unscrupulous -book-makers to cater to public ignorance and love of the marvellous, -and to foist their crude productions upon those who are too little -instructed to judge of their real quality, has hitherto been so -strong as to cast discredit upon the idea of "popular science." It -is highly important to counteract this evil tendency by furnishing -the public with popular scientific books of a superior character. -The publication of the present volume is the first step in carrying -out a systematic enterprise of this kind. It initiates a series of -such works on a wide range of scientific subjects, to be prepared -by the leading thinkers of different countries, and known as the -"International Scientific Series." - -It is designed to consist of compendious scientific treatises, -representing the latest advances of thought upon subjects of general -interest, theoretical and practical, to all classes of readers. The -familiar phenomena of surrounding Nature, in their physical and -chemical aspects, the knowledge of which has recently undergone -marked extension or revision, will be considered in their latest -interpretations. Biology, or the general science of life, which has -lately come into prominence, will be explained in its leading and -most important principles. The subject of mind, which, under the -inductive method and on the basis of its physical accompaniments -and conditions, is giving rise to a new psychology, will be treated -with the fulness to which it is entitled. The laws of man's social -development, or the natural history of society, which are now being -studied by the scientific method, will also receive a due share of -attention. While the books of this series are to deal with a wide -diversity of topics, it will be a leading object of the enterprise -to present the bearings of inquiry upon the higher questions of the -time, and to throw the latest light of science upon the phenomena of -human nature and the economy of human life. - -As the first requisite of such a series of works is trustworthiness, -their preparation has been confided only to men of eminent ability, -and who are recognized authorities in their several departments. -As they are to address the non-scientific public, it is a further -requisite that they should be written in familiar and intelligible -language. It is not to be expected that the authors will all attain -to the same standard in this respect, but they are pledged to the -utmost simplicity of exposition that is possible consistently with -clear and accurate representation. - -As science is now the supreme interest of civilization, and concerns -alike the people of every country, and as, moreover, it affords a -common ground upon which men of all races, tongues, faiths, and -nationalities, may work together in harmony, it seemed fitting that -an undertaking of this kind should be of comprehensive scope and -stand upon an international basis. With the growing sentiment of -sympathy and brotherhood among the most widely-separated students of -Nature, and the extensive facilities of business intercourse that now -exist, there appeared no reason why an international combination of -authors and publishers should not be effected that would be equally -favourable to their own private interests and advantageous to the -public. To gain this end and guarantee to authors better remuneration -for their work, is a distinctive purpose of the present enterprise. -But there was this difficulty in the way of any such arrangement, -that, while the rights of foreign authors are guarded by all other -civilized governments, they are not protected by the government of -the United States. To escape this difficulty, and secure American -coöperation, the first thing needed was to obtain the consent of an -American publishing-house to grant voluntarily to foreign authors -the justice which our government denies them. It was agreed by -Messrs. Appleton that they would pay the foreign contributors to -this series the full rates of copyright that are usually allowed -to American authors. When this was done, engagements were made -with distinguished scientists of England, France, Germany, and the -United States, to prepare works for the series, and with Henry S. -King & Co., of London, Germer Baillière, of Paris, and Messieurs -Brockhaus, of Leipsic, to publish them. Negotiations are pending for -the reproduction of the series in other countries, but the present -arrangements secure to the authors the benefits of the four leading -markets of the world. - -It is a fact not without significance, that the proposal of this -enterprise was received with the most cordial favour by the eminent -scientific men who were solicited to aid in carrying it forward. Most -of them consented at once; but, while some were so heavily burdened -with work that they could enter into no immediate engagements, not -one of them declined to coöperate, and all promised to do so at the -earliest practicable opportunity. The feeling of the desirableness -of such an undertaking was strong and unanimous. The old dislike of -the cultivators of science to participate in the work of popular -teaching, seems very much to have passed away; and in England, -France, and Germany, alike it was freely acknowledged that _savants_ -have an imperative duty to discharge in relation to the work of -general scientific education. As remarked by Prof. Virchow, of -Berlin, "the destiny of science is the service of humanity." - -It was stipulated by the authors that they should have ample time -for the preparation of their books, and, as the arrangements were -recently made, only a few of the works are yet ready. Several, -however, are now in press, and will shortly appear. - -Those interested in the series are under many obligations to Prof. -Tyndall for his kindness in consenting to furnish its commencing -volume. Being prepared in a short time, amid great pressure both -of laboratory and literary work, it contains somewhat less matter -than may be expected in the ensuing volumes. It treats of subjects -upon which he is perhaps the highest living authority; and it is an -admirable example of that vivid, stirring, impressive style for which -its author is so distinguished. Prof. Tyndall is not only a master in -the "scientific use of the imagination," but in kindling the action -of that faculty in his readers. He writes in pictures, so as to make -them see what he sees. In this volume he addresses himself directly -to his juvenile friends, groups them around him, takes them with him -to his favourite mountains, and thus adds a dramatic element and the -effect of personal sympathy to familiar colloquial exposition. - -The "International Scientific Series" will form an elegant and -valuable library of popular science, fresh in treatment, attractive -in form, strong in character, moderate in price, and indispensable to -all who care for the acquisition of solid and serviceable knowledge; -and it is commended to American readers as a help in the important -work of sound public education. - - E. L. Y. - - New York, _September, 1872_. - - - - -AUTHOR'S PREFACE. - - -After an absence of twelve years, I visited the Mer de Glace last -June. It exhibited in a striking degree that excess of consumption -over supply which, if continued, would eventually reduce the Swiss -glaciers to the mere spectres of their former selves. When I first -saw the Mer de Glace its ice-cliffs towered over Les Mottets, and an -arm of the Arveiron, issuing from the cliffs, plunged as a powerful -cascade down the rocks. The ice has now shrunk far behind them. A -huge moraine, left behind by the retreating glacier, will mark, for -some time to come, its recent magnitude. The vault of the Arveiron -has dwindled considerably. The way up to the Chapeau lies on the -top of a lateral moraine, reached a few years ago by the surface -of the glacier, the present surface lying far below. The visible -and continual breaking away of the moraines, left thus stranded on -the mountain flank, explains the absence of ancient ridges on the -mountains where the slopes are steep. The ice-cascades of the Géant -has suffered much from the general waste. Its crevasses are still -wild, but the ice-cliffs and séracs of former days are but poorly -represented to-day. - -The great Aletsch and its neighbours exhibit similar evidences of -diminution. I found moreover this year that the two ancient moraines -mentioned in paragraph 364 are parts of the same great lateral -moraine which flanked the glacier for a long period, during which its -magnitude must have remained practically constant. The place occupied -by the ancient ice-river is rendered strikingly conspicuous by this -well-preserved boundary. - -During my residence at the Bel Alp this year, a catastrophe occurred -which renders, for the time being, the description of the Märgelin -See given in § 50 inappropriate. In company with two young friends I -had descended the glacier and passed through the gorge of the Massa. -On our return to the Bel Alp we found the domestics of the hotel -leaning out of the windows and looking excitedly towards the glacier. -From it proceeded a sound which resembled the roar of a cataract. -The servants remarked that the Märgelin See must have broken loose. -This was the case. For a time, however, the water flowed beneath the -glacier; but at a point about midway between the Bel Alp and the -Æggischhorn, it broke forth on the Æggischhorn side, and formed a -torrent between the glacier and the slope of the mountain. In some -places this river was more than sixty yards wide, at others it was -contracted to less than one-fifth of this width. Broken cascades of -great height were formed here and there by successive ledges of ice, -the torrent leaping with indescribable fury from ledge to ledge, and -sending a smoke of spray into the air. At one place the bottom of -the torrent was deep soft sand, which, after the water had passed, -could be seen to have been tortured into huge funnels by the whirling -eddies overhead. - -Soon after we reached the Bel Alp, on the occasion just referred to, -the front of the torrent appeared at the opposite side of the valley -carrying everything movable before it, and immediately afterwards -swept through the hollow that we had traversed a little earlier in -the day. When at the end of the glacier I was struck by the force and -volume of the Massa, and the grandeur of its vault, but I could not -then account for the huge blocks of ice which it incessantly carried -down. Doubtless the eruption above had been partial before the grand -rush set in. The Rhone was considerably swollen, crops were damaged -or ruined, and the driver of the diligence was sorely perplexed to -find himself in three feet of water, without any apparent reason, -on the public highway. Two or three days subsequently I learned -at the Æggischhorn that an engineer had been sent up to report on -the possibility of opening a channel, so as to prevent any future -accumulation of water in the Märgelin See. If this be done a useful -end will be gained, by the abolition, however, of one of the most -beautiful objects in Switzerland. - - J. Tyndall. - - _September, 1872._ - - - - -PREFACE TO THE FOURTH EDITION. - - -At a meeting of the Managers of the Royal Institution held on -December 12, 1825, "the Committee appointed to consider what lectures -should be delivered in the Institution in the next session," reported -"that they had consulted Mr. Faraday on the subject of engaging him -to take a part in the juvenile lectures proposed to be given during -the Christmas and Easter recesses, and they found his avocations were -such that it would be exceedingly inconvenient for him to engage in -such lectures." - -At a general monthly meeting of the members of the Royal Institution, -held on December 4, 1826, the Managers reported "that they had -engaged Mr. Wallis to deliver a course of lectures on Astronomy, -adapted to a juvenile auditory, during the Christmas vacation." - -In a report dated April 16, 1827, the Board of Visitors express -"their satisfaction at finding that the plan of juvenile courses of -lectures had been resorted to. They feel sure that the influence of -the Institution cannot be extended too far, and that the system of -instructing the younger portion of the community is one of the most -effective means which the Institution possesses for the diffusion of -science." - -Faraday's holding aloof was but temporary, for at Christmas 1827 we -find him giving a "Course of Six Elementary Lectures on Chemistry, -adapted to a Juvenile Auditory."[A] - -[A] There is no record to show that Mr. Wallis gave the Astronomical -lectures referred to, and our librarian believes that the _Christmas_ -courses were opened by Faraday. - -The Easter lectures were soon abandoned; but from the date here -referred to to the present time the Christmas lectures have been a -marked feature of the Royal Institution. - -In 1871 it fell to my lot to give one of these courses. I had been -frequently invited to write on Glaciers in encyclopædias, journals, -and magazines, but had always declined to do so. I had also abstained -from making them the subject of a course of lectures, wishing to take -no advantage of my position here, and indeed to avoid writing a line -or uttering a sentence on the subject for which I could not be held -personally responsible. In view of the discussions which the subject -had provoked, I thought this the fairest course. - -But, in 1871, the time (I imagined) had come when, without risk of -offence, I might tell our young people something about the labours -of those who had unravelled for their instruction the various -problems of the ice-world. My lamented friend and ever-helpful -counsellor, Dr. Bence Jones, thought the subject a good one, and -accordingly it was chosen. Strong in my sympathy with youth, and -remembering the damage done by defective exposition to my own young -mind, I sought, to the best of my ability, to confer upon these -lectures clearness, thoroughness, and life. - -Wishing, moreover, to render them of permanent value, I wrote out -copious Notes of the course, and had them distributed among the boys -and girls. In preparing these Notes I aimed at nothing less than -presenting to my youthful audience, in a concentrated but perfectly -digestible form, every essential point embraced in the literature of -the glaciers, and some things in addition, which, derived as they -were from my own recent researches, no book previously published on -the subject contained. - -But my theory of education agrees with that of Emerson, according -to which instruction is only half the battle, what he calls -_provocation_ being the other half. By this he means that power of -the teacher, through the force of his character and the vitality -of his thought, to bring out all the latent strength of his pupil, -and to invest with interest even the driest matters of detail. In -the present instance I was determined to shirk nothing essential, -however dry; and, to keep my mind alive to the requirements of my -pupil, I proposed a series of ideal ramblings, in which he should be -always at my side. Oddly enough, though I was here dealing with what -might be called the abstract idea of a boy, I realised his presence -so fully as to entertain for him, before our excursions ended, an -affection consciously warm and real. - -The "Notes" here referred to were at first intended for the use of my -audience alone. At the urgent request of a friend I slightly expanded -them, and converted them into the little book here presented to the -reader. - -The amount of attention bestowed upon the volume induces me to give -this brief history of its origin. - -A German critic, whom I have no reason to regard as specially -favourable to me or it, makes the following remark on the style of -the book: "This passion [for the mountains] tempts him frequently -to reveal more of his Alpine wanderings than is necessary for -his demonstrations. The reader, however, will not find this a -disagreeable interruption of the course of thought; for the book -thereby gains wonderfully in vividness." This, I would say, was the -express aim of the breaks referred to. I desired to keep my companion -fresh as well as instructed, and these interruptions were so many -breathing-places where the intellectual tension was purposely -relaxed and the mind of the pupil braced to fresh action. - -Of other criticisms, flattering and otherwise, I forbear to speak. -As regards some of them, indeed, it would be a reproach to that -manliness which I have sought to encourage in my pupil to return -blow for blow. If the reader be acquainted with them, this will let -him know how I regard them; and if he be not acquainted with them, -I would recommend him to ignore them, and to form his own judgment -of this book. No fair-minded person who reads it will dream that I, -in writing it, had a thought of acting otherwise than justly and -generously towards my predecessors, the last of whom, to the grief of -all who knew him, has recently passed away. - - John Tyndall. - - _April, 1874._ - - - - -CONTENTS. - - - Cloud-banner of the Aiguille du Dru _Frontispiece_ - - PAGE - - § 1, 2. Clouds, Rains, and Rivers 1, 6 - - 3. The Waves of Light 8 - - 4. The Waves of Heat which produce the Vapour of our - Atmosphere and melt our Glaciers 11 - - 5. Experiments to prove the foregoing statements 14 - - 6. Oceanic Distillation 19 - - 7. Tropical Rains 23 - - 8. Mountain Condensers 27 - - 9. Architecture of Snow 29 - - 10. Atomic Poles 32 - - 11. Architecture of Lake Ice 35 - - 12. The Source of the Arveiron. Ice Pinnacles, Towers, and - Chasms of the Glacier des Bois. Passage to the Montanvert 38 - - 13. The Mer de Glace and its Sources. Our First Climb to the - Cleft Station 43 - - 14. Ice-cascade and Snows of the Col du Géant 46 - - 15. Questioning the Glaciers 48 - - 16. Branches and Medial Moraines of the Mer de Glace from the - Cleft Station 51 - - 17. The Talèfre and the Jardin. Work among the Crevasses 52 - - 18. First Questions regarding Glacier Motion. Drifting of - Bodies buried in a Crevasse 54 - 19. The Motion of Glaciers. Measurements by Hugi and Agassiz. - Drifting of Huts on the Ice 69 - - 20. Precise Measurements of Agassiz and Forbes. Motion of a - Glacier proved to resemble the Motion of a River 60 - - 21. The Theodolite and its Use. Our own Measurements 62 - - 22. Motion of the Mer de Glace 66 - - 23. Unequal Motion of the two Sides of the Mer de Glace 70 - - 24. Suggestion of a new Likeness of Glacier Motion to River - Motion. Conjecture tested 72 - - 25. New Law of Glacier Motion 76 - - 26. Motion of Axis of Mer de Glace 78 - - 27. Motion of Tributary Glaciers 79 - - 28. Motion of Top and Bottom of Glacier 80 - - 29. Lateral Compression of a Glacier 81 - - 30. Longitudinal Compression of a Glacier 84 - - 31. Sliding and Flowing. Hard Ice and Soft Ice 86 - - 32. Winter on the Mer de Glace 88 - - 33. Winter Motion of the Mer de Glace 93 - - 34. Motion of the Grindelwald and Aletsch Glacier 93 - - 35. Motion of Morteratsch Glacier 95 - - 36. Birth of a Crevasse: Reflections 98 - - 37. Icicles 99 - - 38. The Bergschrund 102 - - 39. Transverse Crevasses 103 - - 40. Marginal Crevasses 105 - - 41. Longitudinal Crevasses 109 - - 42. Crevasses in relation to Curvature of Glacier 110 - - 43. Moraine-ridges, Glacier Tables, and Sand-Cones 112 - - 44. The Glacier Mills or Moulins 116 - - 45. The Changes of Volume of Water by Heat and Cold 118 - - 46. Consequences flowing from the foregoing Properties of - Water. Correction of Errors 122 - - 47. The Molecular Mechanism of Water-Congelation 125 - - 48. The Dirt Bands of the Mer de Glace 127 - - 49. Sea-ice and Icebergs 132 - - 50. The Æggischhorn, the Märgelin See and its Icebergs 136 - - 51. The Bel Alp 139 - - 52. The Riffelberg and Görner Glacier 140 - - 53. Ancient Glaciers of Switzerland 145 - - 54. Erratic Blocks 147 - - 55. Ancient Glaciers of England, Ireland, Scotland, and Wales 150 - - 56. The Glacier Epoch 152 - - 57. Glacial Theories 155 - - 58. Dilatation and Sliding Theories 155 - - 59. Plastic Theory 156 - - 60. Viscous Theory 161 - - 61. Regelation Theory 163 - - 62. Cause of Regelation 167 - - 63. Faraday's View of Regelation 171 - - 64. The Blue Veins of Glaciers 176 - - 65. Relation of Structure to Pressure 183 - - 66. Slate Cleavage and Glacier Lamination 187 - - 67. Conclusion 191 - -[Illustration: CLOUD-BANNER OF THE AIGUILLE DU DRU (par. 84 and 227).] - - - - -THE FORMS OF WATER - -IN CLOUDS AND RIVERS, ICE AND GLACIERS. - - -§ 1. _Clouds, Rains, and Rivers._ - -1. Every occurrence in Nature is preceded by other occurrences which -are its causes, and succeeded by others which are its effects. The -human mind is not satisfied with observing and studying any natural -occurrence alone, but takes pleasure in connecting every natural fact -with what has gone before it, and with what is to come after it. - -2. Thus, when we enter upon the study of rivers and glaciers, our -interest will be greatly augmented by taking into account not only -their actual appearances, but also their causes and effects. - -3. Let us trace a river to its source. Beginning where it empties -itself into the sea, and following it backwards, we find it from -time to time joined by tributaries which swell its waters. The river -of course becomes smaller as these tributaries are passed. It shrinks -first to a brook, then to a stream; this again divides itself into a -number of smaller streamlets, ending in mere threads of water. These -constitute the source of the river, and are usually found among hills. - -4. Thus the Severn has its source in the Welsh Mountains; the Thames -in the Cotswold Hills; the Danube in the hills of the Black Forest; -the Rhine and the Rhone in the Alps; the Ganges in the Himalaya -Mountains; the Euphrates near Mount Ararat; the Garonne in the -Pyrenees; the Elbe in the Giant Mountains of Bohemia; the Missouri in -the Rocky Mountains, and the Amazon in the Andes of Peru. - -5. But it is quite plain that we have not yet reached the real -beginning of the rivers. Whence do the earliest streams derive their -water? A brief residence among the mountains would prove to you that -they are fed by rains. In dry weather you would find the streams -feeble, sometimes indeed quite dried up. In wet weather you would see -them foaming torrents. In general these streams lose themselves as -little threads of water upon the hill sides; but sometimes you may -trace a river to a definite spring. The river Albula in Switzerland, -for instance, rushes at its origin in considerable volume from a -mountain side. But you very soon assure yourself that such springs -are also fed by rain, which has percolated through the rocks or soil, -and which, through some orifice that it has found or formed, comes to -the light of day. - -6. But we cannot end here. Whence comes the rain which forms the -mountain streams? Observation enables you to answer the question. -Rain does not come from a clear sky. It comes from clouds. But what -are clouds? Is there nothing you are acquainted with which they -resemble? You discover at once a likeness between them and the -condensed steam of a locomotive. At every puff of the engine a cloud -is projected into the air. Watch the cloud sharply: you notice that -it first forms at a little distance from the top of the funnel. Give -close attention and you will sometimes see a perfectly clear space -between the funnel and the cloud. Through that clear space the thing -which makes the cloud must pass. What, then, is this thing which -at one moment is transparent and invisible, and at the next moment -visible as a dense opaque cloud? - -7. It is the _steam_ or _vapour of water_ from the boiler. Within -the boiler this steam is transparent and invisible; but to keep it -in this invisible state a heat would be required as great as that -within the boiler. When the vapour mingles with the cold air above -the hot funnel it ceases to be vapour. Every bit of steam shrinks -when chilled, to a much more minute particle of water. The liquid -particles thus produced form a kind of _water-dust_ of exceeding -fineness, which floats in the air, and is called _a cloud_. - -8. Watch the cloud-banner from the funnel of a running locomotive; -you see it growing gradually less dense. It finally melts away -altogether, and if you continue your observations you will not fail -to notice that the speed of its disappearance depends upon the -character of the day. In humid weather the cloud hangs long and -lazily in the air; in dry weather it is rapidly licked up. What has -become of it? It has been reconverted into true invisible vapour. - -9. The _drier_ the air, and the _hotter_ the air, the greater is the -amount of cloud which can be thus dissolved in it. When the cloud -first forms, its quantity is far greater than the air is able to -maintain in an invisible state. But as the cloud mixes gradually -with a larger mass of air it is more and more dissolved, and finally -passes altogether from the condition of a finely-divided liquid into -that of transparent vapour or gas. - -10. Make the lid of a kettle air-tight, and permit the steam to issue -from the pipe; a cloud is precipitated in all respects similar to -that issuing from the funnel of the locomotive. - -11. Permit the steam as it issues from the pipe to pass through the -flame of a spirit-lamp, the cloud is instantly dissolved by the heat, -and is not again precipitated. With a special boiler and a special -nozzle the experiment may be made more striking, but not more -instructive, than with the kettle. - -12. Look to your bedroom windows when the weather is very cold -outside; they sometimes stream with water derived from the -condensation of the aqueous vapour from your own lungs. The windows -of railway carriages in winter show this condensation in a striking -manner. Pour cold water into a dry drinking-glass on a summer's day: -the outside surface of the glass becomes instantly dimmed by the -precipitation of moisture. On a warm day you notice no vapour in -front of your mouth, but on a cold day you form there a little cloud -derived from the condensation of the aqueous vapour from the lungs. - -13. You may notice in a ball-room that as long as the door and -windows are kept closed, and the room remains hot, the air remains -clear; but when the doors or windows are opened a dimness is visible, -caused by the precipitation to fog of the aqueous vapour of the -ball-room. If the weather be intensely cold the entrance of fresh -air may even cause _snow_ to fall. This has been observed in Russian -ball-rooms; and also in the subterranean stables at Erzeroom, when -the doors are opened and the cold morning air is permitted to enter. - -14. Even on the driest day this vapour is never absent from our -atmosphere. The vapour diffused through the air of this room may be -congealed to hoar-frost in your presence. This is done by filling -a vessel with a mixture of pounded ice and salt, which is colder -than the ice itself, and which, therefore, condenses and freezes the -aqueous vapour. The surface of the vessel is finally coated with a -frozen fur, so thick that it may be scraped away and formed into a -snow-ball. - -15. To produce the cloud, in the case of the locomotive and the -kettle, _heat_ is necessary. By heating the water we first convert -it into steam, and then by chilling the steam we convert it into -cloud. Is there any fire in nature which produces the clouds of our -atmosphere? There is: the fire of the sun. - -16. Thus, by tracing backward, without any break in the chain of -occurrences, our river from its end to its real beginnings, we come -at length to the sun. - - -§ 2. - -17. There are, however, rivers which have sources somewhat different -from those just mentioned. They do not begin by driblets on a hill -side, nor can they be traced to a spring. Go, for example, to the -mouth of the river Rhone, and trace it backwards to Lyons, where it -turns to the east. Bending round by Chambery, you come at length -to the Lake of Geneva, from which the river rushes, and which you -might be disposed to regard as the source of the Rhone. But go to -the head of the lake, and you find that the Rhone there enters it, -that the lake is in fact a kind of expansion of the river. Follow -this upwards; you find it joined by smaller rivers from the mountains -right and left. Pass these, and push your journey higher still. You -come at length to a huge mass of ice the--end of a glacier--which -fills the Rhone valley, and from the bottom of the glacier the river -rushes. In the glacier of the Rhone you thus find the source of the -river Rhone. - -18. But again we have not reached the real beginning of the river. -You soon convince yourself that this earliest water of the Rhone is -produced by the melting of the ice. You get upon the glacier and walk -upwards along it. After a time the ice disappears and you come upon -snow. If you are a competent mountaineer you may go to the very top -of this great snow-field, and if you cross the top and descend at the -other side you finally quit the snow, and get upon another glacier -called the Trift, from the end of which rushes a river smaller than -the Rhone. - -19. You soon learn that the mountain snow feeds the glacier. By some -means or other the snow is converted into ice. But whence comes the -snow? Like the rain, it comes from the clouds, which, as before, can -be traced to vapour raised by the sun. Without solar fire we could -have no atmospheric vapour, without vapour no clouds, without clouds -no snow, and without snow no glaciers. Curious then as the conclusion -may be, the cold ice of the Alps has its origin in the heat of the -sun. - - -§ 3. _The Waves of Light._ - -20. But what is the sun? We know its size and its weight. We also -know that it is a globe of fire far hotter than any fire upon earth. -But we now enter upon another enquiry. We have to learn definitely -what is the meaning of solar light and solar heat; in what way they -make themselves known to our senses; by what means they get from the -sun to the earth, and how, when there, they produce the clouds of our -atmosphere, and thus originate our rivers and our glaciers. - -21. If in a dark room you close your eyes and press the eyelid -with your finger-nail, a circle of light will be seen opposite to -the point pressed, while a sharp blow upon the eye produces the -impression of a flash of light. There is a nerve specially devoted to -the purposes of vision which comes from the brain to the back of the -eye, and there divide into fine filaments, which are woven together -to a kind of screen called the _retina_. The retina can be excited -in various ways so as to produce the consciousness of light; it may, -as we have seen, be excited by the rude mechanical action of a blow -imparted to the eye. - -22. There is no spontaneous creation of light by the healthy eye. -To excite vision the retina must be affected by something coming -from without. What is that something? In some way or other luminous -bodies have the power of affecting the retina--but _how?_ - -23. It was long supposed that from such bodies issued, with -inconceivable rapidity, an inconceivably fine matter, which flew -through space, passed through the pores supposed to exist in the -humours of the eye, reached the retina behind, and by their shock -against the retina, aroused the sensation of light. - -24. This theory, which was supported by the greatest men, among -others by Sir Isaac Newton, was found competent to explain a great -number of the phenomena of light, but it was not found competent -to explain _all_ the phenomena. As the skill and knowledge of -experimenters increased, large classes of facts were revealed which -could only be explained by assuming that light was produced, not by a -fine matter flying through space and hitting the retina, but by the -shock of minute waves against the retina. - -25. Dip your finger into a basin of water, and cause it to quiver -rapidly to and fro. From the point of disturbance issue small ripples -which are carried forward by the water, and which finally strike the -basin. Here, in the vibrating finger, you have a source of agitation; -in the water you have a vehicle through which the finger's motion -is transmitted, and you have finally the side of the basin which -receives the shock of the little waves. - -26. In like manner, according to the _wave theory_ of light, you -have a source of agitation in the vibrating atoms, or smallest -particles, of the luminous body; you have a vehicle of transmission -in a substance which is supposed to fill all space, and to be -diffused through the humours of the eye; and finally, you have the -retina, which receives the successive shocks of the waves. These -shocks are supposed to produce the sensation of light. - -27. We are here dealing for the most part with suppositions and -assumptions merely. We have never seen the atoms of a luminous body, -nor their motions. We have never seen the medium which transmits -their motions, nor the waves of that medium. How, then, do we come to -assume their existence? - -28. Before such an idea could have taken any real root in the human -mind, it must have been well disciplined and prepared by observations -and calculations of ordinary wave-motion. It was necessary to know -how both water-waves and sound-waves are formed and propagated. It -was above all things necessary to know how waves, passing through -the same medium, act upon each other. Thus disciplined, the mind was -prepared to detect any resemblance presenting itself between the -action of light and that of waves. Great classes of optical phenomena -accordingly appeared which could be accounted for in the most -complete and satisfactory manner by assuming them to be produced by -waves, and which could not be otherwise accounted for. It is because -of its competence to explain all the phenomena of light that the wave -theory now receives universal acceptance on the part of scientific -men. - -Let me use an illustration. We infer from the flint implements -recently found in such profusion all over England and in other -countries that they were produced by men, and also that the Pyramids -of Egypt were built by men, because, as far as our experience goes, -nothing but men could form such implements or build such Pyramids. -In like manner, we infer from the phenomena of light the agency of -waves, because, as far as our experience goes, no other agency could -produce the phenomena. - - -§ 4. _The Waves of Heat which produce the Vapour of our Atmosphere -and melt our Glaciers._ - -29. Thus, in a general way, I have given you the conception and -the grounds of the conception, which regards light as the product -of wave-motion; but we must go farther than this, and follow the -conception into some of its details. We have all seen the waves of -water, and we know they are of different sizes different in length -and different in height. When, therefore, you are told that the -atoms of the sun, and of almost all other luminous bodies, vibrate -at different rates, and produce waves of different sizes, your -experience of water-waves will enable you to form a tolerably clear -notion of what is meant. - -30. As observed above, we have never seen the light-waves, but we -judge of their presence, their position, and their magnitude, by -their effects. Their lengths have been thus determined, and found to -vary from about 1/30000th to 1/60000th of an inch. - -31. But besides those which produce light, the sun sends forth -incessantly a multitude of waves which produce no light. The largest -waves which the sun sends forth are of this non-luminous character, -though they possess the highest heating power. - -32. A common sunbeam contains waves of all kinds, but it is possible -to _sift_ or _filter_ the beam so as to intercept all its light, and -to allow its obscure heat to pass unimpeded. For substances have been -discovered which, while intensely opaque to the light-waves, are -almost perfectly transparent to the others. On the other hand, it is -possible, by the choice of proper substances, to intercept in a great -degree the pure heat-waves, and to allow the pure light-waves free -transmission. This last separation is, however, not so perfect as the -first. - -33. We shall learn presently how to detach the one class of waves -from the other class, and to prove that waves competent to light a -fire, fuse metal, or burn the hand like a hot solid, may exist in a -perfectly dark place. - -34. Supposing, then, that we withdraw, in the first instance the -large heat-waves, and allow the light-waves alone to pass. These -may be concentrated by suitable lenses and sent into water without -sensibly warming it. Let the light-waves now be withdrawn, and the -larger heat-waves concentrated in the same manner; they may be caused -to boil the water almost instantaneously. - -35. This is the point to which I wished to lead you, and which -without due preparation could not be understood. You now perceive the -important part played by these large darkness-waves, if I may use -the term, in the work of evaporation. When they plunge into seas, -lakes, and rivers, they are intercepted close to the surface, and -they heat the water at the surface, thus causing it to evaporate; -the light-waves at the same time entering to great depths without -sensibly heating the water through which they pass. Not only, -therefore, is it the sun's fire which produces evaporation, but a -particular constituent of that fire, the existence of which you -probably were not aware of. - -36. Further, it is these selfsame lightless waves which, falling upon -the glaciers of the Alps, melt the ice and produce all the rivers -flowing from the glaciers; for I shall prove to you presently that -the light-waves, even when concentrated to the uttermost, are unable -to melt the most delicate hoar-frost; much less would they be able to -produce the copious liquefaction observed upon the glaciers. - -37. These large lightless waves of the sun, as well as the -heat-waves issuing from non-luminous hot bodies, are frequently -called obscure or invisible heat. - -We have here an example of the manner in which phenomena, apparently -remote, are connected together in this wonderful system of things -that we call Nature. You cannot study a snow-flake profoundly without -being led back by it step by step to the constitution of the sun. It -is thus throughout Nature. All its parts are interdependent, and the -study of any one part completely would really involve the study of -all. - - -§ 5. _Experiments to prove the foregoing Statements._ - -38. Heat issuing from any source not visibly red cannot be -concentrated so as to produce the intense effects just referred to. -To produce these it is necessary to employ the obscure heat of a body -raised to the highest possible state of incandescence. The sun is -such a body, and its dark heat is therefore suitable for experiments -of this nature. - -39. But in the atmosphere of London, and for experiments such as -ours, the heat-waves emitted by coke raised to intense whiteness by a -current of electricity are much more manageable than the sun's waves. -The electric light has also the advantage that its dark radiation -embraces a larger proportion of the total radiation than the dark -heat of the sun. In fact, the force or energy, if I may use the term, -of the dark waves of the electric light is fully seven times that of -its light-waves. The electric light, therefore, shall be employed in -our experimental demonstrations. - -40. From this source a powerful beam is sent through the room, -revealing its track by the motes floating in the air of the room; for -were the motes entirely absent the beam would be unseen. It falls -upon a concave mirror (a glass one silvered behind will answer) and -is gathered up by the mirror into a cone of reflected rays; the -luminous apex of the cone, which is the _focus_ of the mirror, being -about fifteen inches distant from its reflecting surface. Let us mark -the focus accurately by a pointer. - -41. And now let us place in the path of the beam a substance -perfectly opaque to light. This substance is iodine dissolved in a -liquid called bisulphide of carbon. The light at the focus instantly -vanishes when the dark solution is introduced. But the solution is -intensely transparent to the dark waves, and a focus of such waves -remains in the air of the room after the light has been abolished. -You may feel the heat of these waves with your hand; you may let them -fall upon a thermometer, and thus prove their presence; or, best of -all, you may cause them to produce a current of electricity, which -deflects a large magnetic needle. The magnitude of the deflection is -a measure of the heat. - -42. Our object now is, by the use of a more powerful lamp, and -a better mirror (one silvered in front and with a shorter focal -distance), to intensify the action here rendered so sensible. As -before, the focus is rendered strikingly visible by the intense -illumination of the dust particles. We will first filter the beam so -as to intercept its dark waves, and then permit the purely luminous -waves to exert their utmost power on a small bundle of gun-cotton -placed at the focus. - -43. No effect whatever is produced. The gun-cotton might remain there -for a week without ignition. Let us now permit the unfiltered beam -to act upon the cotton. It is instantly dissipated in an explosive -flash. This experiment proves that the light-waves are incompetent to -explode the cotton, while the waves of the full beam are competent to -do so; hence we may conclude that the dark waves are the real agents -in the explosion. - -44. But this conclusion would be only probable; for it might be urged -that the _mixture_ of the dark waves and the light-waves is necessary -to produce the result. Let us then, by means of our opaque solution, -isolate our dark waves and converge them on the cotton. It explodes -as before. - -45. Hence it is the dark waves, and they only, that are concerned in -the ignition of the cotton. - -46. At the same dark focus sheets of platinum are raised to vivid -redness; zinc is burnt up; paper instantly blazes; magnesium wire is -ignited; charcoal within a receiver containing oxygen is set burning; -a diamond similarly placed is caused to glow like a star, being -afterward gradually dissipated. And all this while the _air_ at the -focus remains as cool as in any other part of the room. - -47. To obtain the light-waves we employ a clear solution of alum in -water; to obtain the dark waves we employ the solution of iodine -above referred to. But as before stated (32), the alum is not so -perfect a filter as the iodine; for it transmits a portion of the -obscure heat. - -48. Though the light-waves here prove their incompetence to ignite -gun-cotton, they are able to burn up black paper; or, indeed, to -explode the cotton when it is blackened. The white cotton does not -absorb the light, and without absorption we have no heating. The -blackened cotton absorbs, is heated, and explodes. - -49. Instead of a solution of alum, we will employ for our next -experiment a cell of pure water, through which the light passes -without sensible absorption. At the focus is placed a test-tube also -containing water, the full force of the light being concentrated upon -it. The water is not sensibly warmed by the concentrated waves. We -now remove the cell of water; no change is visible in the beam, but -the water contained in the test-tube now boils. - -50. The light-waves being thus proved ineffectual, and the full beam -effectual, we may infer that it is the dark waves that do the work of -heating. But we clench our inference by employing our opaque iodine -filter. Placing it on the path of the beam, the light is entirely -stopped, but the water boils exactly as it did when the full beam -fell upon it. - -51. The truth of the statement made in paragraph (34) is thus -demonstrated. - -52. And now with regard to the melting of ice. On the surface of -a flask containing a freezing mixture we obtain a thick fur of -hoar-frost. Sending the beam through a water-cell, its luminous waves -are concentrated upon the surface of the flask. Not a spicula of the -frost is dissolved. We now remove the water-cell, and in a moment a -patch of the frozen fur as large as half-a-crown is melted. Hence, -inasmuch as the full beam produces this effect, and the luminous part -of the beam does not produce it, we fix upon the dark portion the -melting of the frost. - -53. As before, we clench this inference by concentrating the dark -waves alone upon the flask. The frost is dissipated exactly as it was -by the full beam. - -54. These effects are rendered strikingly visible by darkening with -ink the freezing mixture within the flask. When the hoar-frost is -removed, the blackness of the surface from which it had been melted -comes out in strong contrast with the adjacent snowy whiteness. When -the flask itself, instead of the freezing mixture, is blackened, the -purely luminous waves being absorbed by the glass, warm it; the glass -reacts upon the frost, and melts it. Hence the wisdom of darkening, -instead of the flask itself, the mixture within the flask. - -55. This experiment proves to demonstration the statement in -paragraph (36): that it is the dark waves of the sun that melt the -mountain snow and ice, and originate all the rivers derived from -glaciers. - -There are writers who seem to regard science as an aggregate of -facts, and hence doubt its efficacy as an exercise of the reasoning -powers. But all that I have here taught you is the result of reason, -taking its stand, however, upon the sure basis of observation and -experiment. And this is the spirit in which our further studies are -to be pursued. - - -§ 6. _Oceanic Distillation._ - -56. The sun, you know, is never exactly overhead in England. But at -the equator, and within certain limits north and south of it, the sun -at certain periods of the year is directly overhead at noon. These -limits are called the Tropics of Cancer and of Capricorn. Upon the -belt comprised between these two circles the sun's rays fall with -their mightiest power; for here they shoot directly downward, and -heat both earth and sea more than when they strike slantingly. - -57. When the vertical sunbeams strike the land they heat it, and the -air in contact with the hot soil becomes heated in turn. But when -heated the air expands, and when it expands it becomes lighter. This -lighter air rises, like wood plunged into water, through the heavier -air overhead. - -58. When the sunbeams fall upon the sea the water is warmed, though -not so much as the land. The warmed water expands, becomes thereby -lighter, and therefore continues to float upon the top. This upper -layer of water warms to some extent the air in contact with it, -but it also sends up a quantity of aqueous vapour, which being far -lighter than air, helps the latter to rise. Thus both from the land -and from the sea we have ascending currents established by the action -of the sun. - -59. When they reach a certain elevation in the atmosphere, these -currents divide and flow, part towards the north and part towards -the south; while from the north and the south a flow of heavier and -colder air sets in to supply the place of the ascending warm air. - -60. Incessant circulation is thus established in the atmosphere. The -equatorial air and vapour flow above towards the north and south -poles, while the polar air flows below towards the equator. The two -currents of air thus established are called the upper and the lower -trade winds. - -61. But before the air returns from the poles great changes have -occurred. For the air as it quitted the equatorial regions was laden -with aqueous vapour, which could not subsist in the cold polar -regions. It is there precipitated, falling sometimes as rain, or -more commonly as snow. The land near the pole is covered with this -snow, which gives birth to vast glaciers in a manner hereafter to be -explained. - -62. It is necessary that you should have a perfectly clear view of -this process, for great mistakes have been made regarding the manner -in which glaciers are related to the heat of the sun. - -63. It was supposed that if the sun's heat were diminished, greater -glaciers than those now existing would be produced. But the lessening -of the sun's heat would infallibly diminish the quantity of aqueous -vapour, and thus cut off the glaciers at their source. A brief -illustration will complete your knowledge here. - -64. In the process of ordinary distillation, the liquid to be -distilled is heated and converted into vapour in one vessel, and -chilled and reconverted into liquid in another. What has just been -stated renders it plain that the earth and its atmosphere constitute -a vast distilling apparatus in which the equatorial ocean plays the -part of the boiler, and the chill regions of the poles the part of -the condenser. In this process of distillation _heat_ plays quite as -necessary a part as _cold_, and before Bishop Heber could speak of -"Greenland's icy mountains," the equatorial ocean had to be warmed by -the sun. We shall have more to say upon this question afterwards. - - -Illustrative Experiments. - -65. I have said that when heated, air expands. If you wish to verify -this for yourself, proceed thus. Take an empty flask, stop it by a -cork; pass through the cork a narrow glass tube. By heating the tube -in a spirit-lamp you can bend it downwards, so that when the flask is -standing upright the open end of the narrow tube may dip into water. -Now cause the flame of your spirit-lamp to play against the flask. -The flame heats the glass, the glass heats the air; the air expands, -is driven through the narrow tube, and issues in a storm of bubbles -from the water. - -66. Were the heated air unconfined, it would rise in the heavier -cold air. Allow a sunbeam or any other intense light to fall upon a -white wall or screen in a dark room. Bring a heated poker, a candle, -or a gas flame underneath the beam. An ascending current rises from -the heated body through the beam, and the action of the air upon the -light is such as to render the wreathing and waving of the current -strikingly visible upon the screen. When the air is hot enough, and -therefore light enough, if entrapped in a paper bag it carries the -bag upwards, and you have the Fire-balloon. - -67. Fold two sheets of paper into two cones and suspend them with -their closed points upwards from the end of a delicate balance. See -that the cones balance each other. Then place for a moment the flame -of a spirit-lamp beneath the open base of one of them; the hot air -ascends from the lamp and instantly tosses upwards the cone above it. - -68. Into an inverted glass shade introduce a little smoke. Let the -air come to rest, and then simply place your hand at the open mouth -of the shade. Mimic hurricanes are produced by the air warmed by the -hand, which are strikingly visible when the smoke is illuminated by a -strong light. - -69. The heating of the tropical air by the sun is _indirect_. The -solar beams have scarcely any power to heat the air through which -they pass; but they heat the land and ocean, and these communicate -their heat to the air in contact with them. The air and vapour start -upwards charged with the heat thus communicated. - - -§ 7. _Tropical Rains._ - -70. But long before the air and vapour from the equator reach the -poles, precipitation occurs. Wherever a humid warm wind mixes with a -cold dry one, rain falls. Indeed the heaviest rains occur at those -places where the sun is vertically overhead. We must enquire a little -more closely into their origin. - -71. Fill a bladder about two-thirds full of air at the sea level, -and take it to the summit of Mont Blanc. As you ascend, the bladder -becomes more and more distended; at the top of the mountain it is -fully distended, and has evidently to bear a pressure from within. -Returning to the sea level you find that the tightness disappears, -the bladder finally appearing as flaccid as at first. - -72. The reason is plain. At the sea level the air within the -bladder has to bear the pressure of the whole atmosphere, being -thereby squeezed into a comparatively small volume. In ascending -the mountain, you leave more and more of the atmosphere behind; the -pressure becomes less and less, and by its expansive force the air -within the bladder swells as the outside pressure is diminished. At -the top of the mountain the expansion is quite sufficient to render -the bladder tight, the pressure within being then actually greater -than the pressure without. By means of an air-pump we can show the -expansion of a balloon partly filled with air, when the external -pressure has been in part removed. - -73. But why do I dwell upon this? Simply to make plain to you that -the _unconfined air_, heated at the earth's surface, and ascending by -its lightness, must expand more and more the higher it rises in the -atmosphere. - -74. And now I have to introduce to you a new fact, towards the -statement of which I have been working for some time. It is this: -_The ascending air is chilled by its expansion._ Indeed this chilling -is one source of the coldness of the higher atmospheric regions. And -now fix your eye upon those mixed currents of air and aqueous vapour -which rise from the warm tropical ocean. They start with plenty of -heat to preserve the vapour as vapour; but as they rise they come -into regions already chilled, and they are still further chilled by -their own expansion. The consequence might be foreseen. The load of -vapour is in great part precipitated, dense clouds are formed, their -particles coalesce to rain-drops, which descend daily in gushes so -profuse that the word "torrential" is used to express the copiousness -of the rainfall. I could show you this chilling by expansion, and -also the consequent precipitation of clouds. - -75. Thus long before the air from the equator reaches the poles -its vapour is in great part removed from it, having redescended to -the earth as rain. Still a good quantity of the vapour is carried -forward, which yields hail, rain, and snow in northern and southern -lands. - - -Illustrative Experiments. - -76. I have said that the air is chilled during its expansion. Prove -this, if you like, thus. With a condensing syringe, you can force air -into an iron box furnished with a stopcock, to which the syringe is -screwed. Do so till the density of the air within the box is doubled -or trebled. Immediately after this condensation, both the box and -the air within it are warm, and can be proved to be so by a proper -thermometer. Simply turn the cock and allow the compressed air to -stream into the atmosphere. The current, if allowed to strike the -thermometer, visibly chills it; and with other instruments the chill -may be made more evident still. Even the hand feels the chill of the -expanding air. - -77. Throw a strong light, a concentrated sunbeam for example, across -the issuing current; if the compressed air be ordinary humid air, you -see the precipitation of a little cloud by the chill accompanying the -expansion. This cloud-formation may, however, be better illustrated -in the following way:-- - -78. In a darkened room send a strong beam of light through a glass -tube three feet long and three inches wide, stopped at its ends by -glass plates. Connect the tube by means of a stopcock with a vessel -of about one-fourth its capacity, from which the air has been removed -by an air-pump. The exhausted cylinder of the pump itself will answer -capitally. Fill the glass tube with humid air; then simply turn on -the stopcock which connects it with the exhausted vessel. Having -more room the air expands, cold accompanies the expansion, and, as a -consequence, a dense and brilliant cloud immediately fills the tube. -If the experiment be made for yourself alone you may see the cloud -in ordinary daylight; indeed, the brisk exhaustion of any receiver -filled with humid air is known to produce this condensation. - -79. Other vapours than that of water may be thus precipitated, -some of them yielding clouds of intense brilliancy, and displaying -iridescences, such as are sometimes, but not frequently, seen in the -clouds floating over the Alps. - -80. In science what is true for the small is true for the large. Thus -by combining the conditions observed on a large scale in nature we -obtain on a small scale the phenomena of atmospheric clouds. - - -§ 8. _Mountain Condensers._ - -81. To complete our view of the process of atmospheric precipitation -we must take into account the action of mountains. Imagine a -south-west wind blowing across the Atlantic towards Ireland. In -its passage it charges itself with aqueous vapour. In the south of -Ireland it encounters the mountains of Kerry: the highest of these -is Magillicuddy's Reeks, near Killarney. Now the lowest stratum of -this Atlantic wind is that which is most fully charged with vapour. -When it encounters the base of the Kerry mountains it is tilted up -and flows bodily over them. Its load of vapour is therefore carried -to a height, it expands on reaching the height, it is chilled in -consequence of the expansion, and comes down in copious showers -of rain. From this, in fact, arises the luxuriant vegetation of -Killarney; to this, indeed, the lakes owe their water supply. The -cold crests of the mountains also aid in the work of condensation. - -82. Note the consequence. There is a town called Cahirciveen to -the south-west of Magillicuddy's Reeks, at which observations of -the rainfall have been made, and a good distance farther to the -north-east, right in the course of the south-west wind, there is -another town, called Portarlington, at which observations of rainfall -have also been made. But before the wind reaches the latter station -it has passed over the mountains of Kerry and left a great portion of -its moisture behind it. What is the result? At Cahirciveen, as shown -by Dr. Lloyd, the rainfall amounts to 59 inches in a year, while at -Portarlington it is only 21 inches. - -83. Again, you may sometimes descend from the Alps when the fall of -rain and snow is heavy and incessant, into Italy, and find the sky -over the plains of Lombardy blue and cloudless, the wind at the same -time _blowing over the plain towards the Alps_. Below the wind is hot -enough to keep its vapour in a perfectly transparent state; but it -meets the mountains, is tilted up, expanded, and chilled. The cold of -the higher summits also helps the chill. The consequence is that the -vapour is precipitated as rain or snow, thus producing bad weather -upon the heights, while the plains below, flooded with the same air, -enjoy the aspect of the unclouded summer sun. Clouds blowing _from_ -the Alps are also sometimes dissolved over the plains of Lombardy. - -84. In connection with the formation of clouds by mountains, one -particularly instructive effect may be here noticed. You frequently -see a streamer of cloud many hundred yards in length drawn out from -an Alpine peak. Its steadiness appears perfect, though a strong -wind may be blowing at the same time over the mountain head. Why -is the cloud not blown away? It is blown away; its permanence is -only apparent. At one end it is incessantly dissolved, at the other -end it is incessantly renewed: supply and consumption being thus -equalized, the cloud appears as changeless as the mountain to which -it seems to cling. When the red sun of the evening shines upon these -cloud-streamers they resemble vast torches with their flames blown -through the air. - - -§ 9. _Architecture of Snow._ - -85. We now resemble persons who have climbed a difficult peak, -and thereby earned the enjoyment of a wide prospect. Having made -ourselves masters of the conditions necessary to the production of -mountain snow, we are able to take a comprehensive and intelligent -view of the phenomena of glaciers. - -86. A few words are still necessary as to the formation of snow. The -molecules and atoms of all substances, when allowed free play, build -themselves into definite and, for the most part, beautiful forms -called crystals. Iron, copper, gold, silver, lead, sulphur, when -melted and permitted to cool gradually, all show this crystallizing -power. The metal bismuth shows it in a particularly striking manner, -and when properly fused and solidified, self-built crystals of great -size and beauty are formed of this metal. - -87. If you dissolve saltpetre in water, and allow the solution to -evaporate slowly, you may obtain large crystals, for no portion of -the salt is converted into vapour. The water of our atmosphere is -fresh though it is derived from the salt sea. Sugar dissolved in -water, and permitted to evaporate, yields crystals of sugar-candy. -Alum readily crystallizes in the same way. Flints dissolved, as they -sometimes are in nature, and permitted to crystallize, yield the -prisms and pyramids of rock crystal. Chalk dissolved and crystallized -yields Iceland spar. The diamond is crystallized carbon. All our -precious stones, the ruby, sapphire, beryl, topaz, emerald, are all -examples of this crystallizing power. - -88. You have heard of the force of gravitation, and you know that -it consists of an attraction of every particle of matter for every -other particle. You know that planets and moons are held in their -orbits by this attraction. But gravitation is a very simple affair -compared to the force, or rather forces, of crystallization. For here -the ultimate particles of matter, inconceivably small as they are, -show themselves possessed of attractive and repellent poles, by the -mutual action of which the shape and structure of the crystal are -determined. In the solid condition the attracting poles are rigidly -locked together; but if sufficient heat be applied the bond of union -is dissolved, and in the state of fusion the poles are pushed so far -asunder as to be practically out of each other's range. The natural -tendency of the molecules to build themselves together is thus -neutralized. - -89. This is the case with water, which as a liquid is to all -appearance formless. When sufficiently cooled the molecules are -brought within the play of the crystallizing force, and they then -arrange themselves in forms of indescribable beauty. When snow -is produced in calm air, the icy particles build themselves into -beautiful stellar shapes, each star possessing six rays. There is no -deviation from this type, though in other respects the appearances -of the snow-stars are infinitely various. In the polar regions these -exquisite forms were observed by Dr. Scoresby, who gave numerous -drawings of them. I have observed them in mid-winter filling the air, -and loading the slopes of the Alps. But in England they are also to -be seen, and no words of mine could convey so vivid an impression of -their beauty as the annexed drawings of a few of them, executed at -Greenwich by Mr. Glaisher. - -90. It is worth pausing to think what wonderful work is going on in -the atmosphere during the formation and descent of every snow-shower: -what building power is brought into play! and how imperfect seem the -productions of human minds and hands when compared with those formed -by the blind forces of nature! - -91. But who ventures to call the forces of nature blind? In reality, -when we speak thus we are describing our own condition. The blindness -is ours; and what we really ought to say, and to confess, is that our -powers are absolutely unable to comprehend either the origin or the -end of the operations of nature. - -92. But while we thus acknowledge our limits, there is also reason -for wonder at the extent to which science has mastered the system -of nature. From age to age, and from generation to generation, fact -has been added to fact, and law to law, the true method and order -of the Universe being thereby more and more revealed. In doing this -science has encountered and overthrown various forms of superstition -and deceit, of credulity and imposture. But the world continually -produces weak persons and wicked persons; and as long as they -continue to exist side by side, as they do in this our day, very -debasing beliefs will also continue to infest the world. - - -§ 10. _Atomic Poles._ - -93. "What did I mean when, a few moments ago (88), I spoke of -attracting and repellent poles?" Let me try to answer this question. -You know that astronomers and geographers speak of the earth's poles, -and you have also heard of magnetic poles, the poles of a magnet -being the points at which the attraction and repulsion of the magnet -are as it were concentrated. - -[Illustration: SNOW CRYSTALS.] - -94. Every magnet possesses, two such poles; and if iron filings be -scattered over a magnet, each particle becomes also endowed with -two poles. Suppose such particles devoid of weight and floating in -our atmosphere, what must occur when they come near each other? -Manifestly the repellent poles will retreat from each other, while -the attractive poles will approach and finally lock themselves -together. And supposing the particles, instead of a single pair, to -possess several pairs of poles arranged at definite points over their -surfaces; you can then picture them, in obedience to their mutual -attractions and repulsions, building themselves together to form -masses of definite shape and structure. - -95. Imagine the molecules of water in calm cold air to be gifted -with poles of this description, which compel the particles to lay -themselves together in a definite order, and you have before your -mind's eye the unseen architecture which finally produces the -visible and beautiful crystals of the snow. Thus our first notions -and conceptions of poles are obtained from the sight of our eyes -in looking at the effects of magnetism; and we then transfer these -notions and conceptions to particles which no eye has ever seen. The -power by which we thus picture to ourselves effects beyond the range -of the senses is what philosophers call the Imagination, and in the -effort of the mind to seize upon the unseen architecture of crystals, -we have an example of the "scientific use" of this faculty. Without -imagination we might have _critical_ power, but not _creative_ power -in science. - - -§ 11. _Architecture of Lake Ice._ - -96. We have thus made ourselves acquainted with the beautiful -snow-flowers self-constructed by the molecules of water in calm cold -air. Do the molecules show this architectural power when ordinary -water is frozen? What, for example, is the structure of the ice over -which we skate in winter? Quite as wonderful as the flowers of the -snow. The observation is rare, if not new, but I have seen in water -slowly freezing six-rayed ice-stars formed, and floating free on the -surface. A six-rayed star, moreover, is typical of the construction -of all our lake ice. It is built up of such forms wonderfully -interlaced. - -97. Take a slab of lake ice and place it in the path of a -concentrated sunbeam. Watch the track of the beam through the ice. -Part of the beam is stopped, part of it goes through; the former -produces internal liquefaction, the latter has no effect whatever -upon the ice. But the liquefaction is not uniformly diffused. From -separate spots of the ice little shining points are seen to sparkle -forth. Every one of those points is surrounded by a beautiful liquid -flower with six petals. - -98. Ice and water are so optically alike that unless the light fall -properly upon these flowers you cannot see them. But what is the -central spot? A vacuum. Ice swims on water because, bulk for bulk, -it is lighter than water; so that when ice is melted it shrinks in -size. Can the liquid flowers then occupy the whole space of the ice -melted? Plainly no. A little empty space is formed with the flowers, -and this space, or rather its surface, shines in the sun with the -lustre of burnished silver. - -99. In all cases the flowers are formed parallel to the surface of -freezing. They are formed when the sun shines upon the ice of every -lake; sometimes in myriads, and so small as to require a magnifying -glass to see them. They are always attainable, but their beauty is -often marred by internal defects of the ice. Even one portion of the -same piece of ice may show them exquisitely, while the second portion -shows them imperfectly. - -100. Annexed is a very imperfect sketch of these beautiful figures. - -101. Here we have a reversal of the process of crystallization. The -searching solar beam is delicate enough to take the molecules down -without deranging the order of their architecture. Try the experiment -for yourself with a pocket-lens on a sunny day. You will not find the -flowers confused; they all lie parallel to the surface of freezing. -In this exquisite way every bit of the ice over which our skaters -glide in winter is put together. - -102. I said in (97) that a portion of the sunbeam was stopped by the -ice and liquefied it. What is this portion? The dark heat of the -sun. The great body of the light waves and even a portion of the dark -ones, pass through the ice without losing any of their heating power. -When properly concentrated on combustible bodies, even after having -passed through the ice, their burning power becomes manifest. - -[Illustration: LIQUID FLOWERS IN LAKE ICE.] - -103. And the ice itself may be employed to concentrate them. With an -ice-lens in the polar regions Dr. Scoresby has often concentrated the -sun's rays so as to make them burn wood, fire gunpowder, and melt -lead; thus proving that the heating power is retained by the rays, -even after they have passed through so cold a substance. - -104. By rendering the rays of the electric lamp parallel, and then -sending them through a lens of ice, we obtain all the effects which -Dr. Scoresby obtained with the rays of the sun. - - -§ 12. _The Source of the Arveiron. Ice Pinnacles, Towers, and Chasms -of the Glacier des Bois. Passage to the Montanvert._ - -105. Our preparatory studies are for the present ended, and thus -informed, let us approach the Alps. Through the village of Chamouni, -in Savoy, a river rushes which is called the Arve. Let us trace this -river backwards from Chamouni. At a little distance from the village -the river forks; one of its branches still continues to be called -the Arve, the other is the Arveiron. Following this latter we come -to what is called the "source of the Arveiron"--a short hour's walk -from Chamouni. Here, as in the case of the Rhone already referred -to, you are fronted by a huge mass of ice, the end of a glacier, and -from an arch in the ice the Arveiron issues. Do not trust the arch in -summer. Its roof falls at intervals with a startling crash, and would -infallibly crush any person on whom it might fall. - -106. We must now be observant. Looking about us here, we find in -front of the ice curious heaps and ridges of débris, which are more -or less concentric. These are the _terminal moraines_ of the glacier. -We shall examine them subsequently. - -107. We now turn to the left, and ascend the slope beside the -glacier. As we ascend we get a better view, and find that the ice -here fills a narrow valley. We come upon another singular ridge, -not of fresh débris, like those lower down, but covered in part -with trees, and appearing to be literally as "old as the hills." It -tells a wonderful tale. We soon satisfy ourselves that the ridge is -an ancient moraine, and at once conclude that the glacier, at some -former period of its existence, was vastly larger than it is now. -This old moraine stretches right across the main valley, and abuts -against the mountains at the opposite side. - -[Illustration: SOURCE OF THE ARVEIRON.] - -108. Having passed the terminal portion of the glacier, which is -covered with stones and rubbish, we find ourselves beside a very -wonderful exhibition of ice. The glacier descends a steep gorge, and -in doing so is riven and broken in the most extraordinary manner. -Here are towers, and pinnacles, and fantastic shapes wrought out by -the action of the weather, which put one in mind of rude sculpture. -Annexed is a sketch of an ice-pinnacle. From deep chasms in the -glacier issues a delicate shimmer of blue light. At times we hear a -sound like thunder, which arises either from the falling of a tower -of ice, or from the tumble of a huge stone into a chasm. The glacier -maintains this wild and chaotic character for some time; and the best -iceman would find himself defeated in any attempt to get along it. - -[Illustration: ICE-PINNACLE.] - -109. We reach a place called the Chapeau, where, if we wish, we can -have refreshment in a little mountain hut. We then pass the _Mauvais -Pas_, a precipitous rock, on the face of which steps are hewn, and -the unpractised traveller is assisted by a rope. We pursue our -journey, partly along the mountain side, and partly along a ridge of -singularly artificial aspect a _lateral moraine_. We at length face a -house perched upon an eminence at the opposite side of the glacier. -This is the auberge of the Montanvert, well known to all visitors to -this portion of the Alps. - -110. Here we cross the glacier. I should have told you that its lower -part, including the broken portion we have passed, is called the -Glacier des Bois; while the place that we are now about to cross is -the beginning of the Mer de Glace. You feel that this term is not -quite appropriate, for the glacier here is much more like a _river_ -of ice than a sea. The valley which it fills is about half a mile -wide. - -111. The ice may be riven where we enter upon it, but with the -necessary care there is no difficulty in crossing this portion -of the Mer de Glace. The clefts and chasms in the ice are called -_crevasses_; we shall make their acquaintance on a grander scale by -and by. - -[Illustration: THE MER DE GLACE, SHOWING MONT TACUL AND THE GRANDE -JORASSE, WITH OUR CLEFT ABOVE TRÉLEPORTE TO THE RIGHT.] - -112. Look up and down this side of the glacier. It is considerably -riven, but as we advance the crevasses will diminish, and we shall -find very few of them at the other side. Note this for future use. -The ice is at first dirty; but the dirt soon disappears, and you come -upon the clean crisp surface of the glacier. You have already noticed -that the clean ice is white, and that from a distance it resembles -snow rather than ice. This is caused by the breaking up of the -surface by the solar heat. When you pound transparent rock-salt into -powder it is as white as table-salt, and it is the minute fissuring -of the surface of the glacier by the sun's rays that causes it to -appear white. _Within_ the glacier the ice is transparent. After an -exhilarating passage we get upon the opposite lateral moraine, and -ascend the steep slope from it to the Montanvert Inn. - - -§ 13. _The Mer de Glace and its Sources. Our First Climb to the Cleft -Station._ - -113. Here the view before us is very grand. We look across the -glacier at the beautiful pyramid of the Aiguille du Dru (shown in our -frontispiece); and to the right at the Aiguille des Charmoz, with -its sharp pinnacles bent as if they were ductile. Looking straight -up the glacier the view is bounded by the great crests called La -Grande Jorasse, nearly 14,000 feet high. Our object now is to get -into the very heart of the mountains, and to pursue to its origin the -wonderful frozen river which we have just crossed. - -114. Starting from the Montanvert with, the glacier below us to our -left, we soon reach some rocks resembling the Mauvais Pas; they are -called _les Fonts_. We cross them and reach _l'Angle_, where we quit -the land for the ice. We walk up the glacier, but before reaching -the promontory called Trélaporte, we take once more to the mountain -side; for though the path here has been forsaken on account of its -danger, for the sake of knowledge we are prepared to incur danger -to a reasonable extent. A little glacier reposes on the slope to -our right. We may see a huge boulder or two poised on the end of -the glacier, and, if fortunate, also see the boulder liberated and -plunging violently down the slope. Presence of mind is all that is -necessary to render our safety certain; but travellers do not always -show presence of mind, and hence the path which formerly led over -this slope has been forsaken. The whole slope is cumbered by masses -of rock which this little glacier has sent down. These I wished you -to see; by and by they shall be fully accounted for. - -115. Above Trélaporte to the right you see a most singular cleft in -the rocks, in the middle of which stands an isolated pillar, hewn out -by the weather. Our next object is to get to the tower of rock to -the left of that cleft, for from that position we shall gain a most -commanding and instructive view of the Mer de Glace and its sources. - -116. The cleft referred to, with its pillar, may be seen to the -right of the preceding engraving of the Mer de Glace. Below the cleft -is also seen the little glacier just referred to. - -117. We may reach this cleft by a steep gully, visible from our -present position, and leading directly up to the cleft. But these -gullies, or couloirs, are very dangerous, being the pathways of -stones falling from the heights. We will therefore take the rocks -to the left of the gully, by close inspection ascertain their -assailable points, and there attack them. In the Alps as elsewhere -wonderful things may be done by looking steadfastly at difficulties, -and testing them wherever they appear assailable. We thus reach our -station, where the glory of the prospect, and the insight that we -gain as to the formation of the Mer de Glace, far more than repay us -for the labour of our ascent. - -118. For we see the glacier below us, stretching its frozen tongue -downwards past the Montanvert. And we now find this single glacier -branching out into three others, some of them wider than itself. -Regard the branch to the right, the Glacier du Géant. It stretches -smoothly up for a long distance, then becomes disturbed, and then -changes to a great frozen cascade, down which the ice appears to -tumble in wild confusion. Above the cascade you see an expanse of -shining snow, occupying an area of some square miles. - - -§ 14. _Ice-cascade and Snows of the Col du Géant._ - -119. Instead of climbing to the height where we now stand, we might -have continued our walk upon the Mer de Glace, turned round the -promontory of Trélaporte, and walked right up the Glacier du Géant. -We should have found ice under our feet up to the bottom of the -cascade. It is not so compact as the ice lower down, but you would -not think of refusing to call it ice. - -120. As we approach the fall, the smooth and unbroken character of -the glacier changes more and more. We encounter transverse ridges -succeeding each other with augmenting steepness. The ice becomes more -and more fissured and confused. We wind through tortuous ravines, -climb huge ice-mounds, and creep cautiously along crumbling crests, -with crevasses right and left. The confusion increases until further -advance along the centre of the glacier is impossible. - -121. But with the aid of an axe to cut steps in the steeper ice-walls -and slopes we might, by swerving to either side of the glacier, work -our way to the top of the cascade. If we ascended to the right, -we should have to take care of the ice avalanches which sometimes -thunder down the slopes; if to the left, we should have to take care -of the stones let loose from the Aiguille Noire. After we had cleared -the cascade, we should have to beware for a time of the crevasses, -which for some distance above the fall yawn terribly. But by caution -we could get round them, and sometimes cross them by bridges of snow. -Here the skill and knowledge to be acquired only by long practice -come into play; and here also the use of the Alpine rope suggests -itself. For not only are the 'snow bridges often frail, but whole -crevasses are sometimes covered, the unhappy traveller being first -made aware of their existence by the snow breaking under his feet. -Many lives have thus been lost, and some quite recently. - -122. Once upon the plateau above the ice-fall we find the surface -totally changed. Below the fall we walked upon ice; here we are -upon snow. After a gentle but long ascent we reach a depression of -the ridge which bounds the snow-field at the top, and now look over -Italy. We stand upon the famous Col du Géant. - -123. They were no idle scamperers on the mountains that made these -wild recesses first known; it was not the desire for health which -now brings some, or the desire for grandeur and beauty which brings -others, or the wish to be able to say that they have climbed a -mountain or crossed a col, which I fear brings a good many more; it -was a desire for _knowledge_ that brought the first explorers here, -and on this col the celebrated De Saussure lived for seventeen days, -making scientific observations. - - -§ 15. _Questioning the Glaciers._ - -124. I would now ask you to consider for a moment the facts which -such an excursion places in our possession. The snow through which -we have in idea trudged is the snow of last winter and spring. Had -we placed last August a proper mark upon the surface of the snow, we -should find it this August at a certain depth beneath the surface. A -good deal has been melted by the summer sun, but a good deal of it -remains, and it will continue until the snows of the coming winter -fall and cover it. This again will be in part preserved till next -August, a good deal of it remaining until it is covered by the snow -of the subsequent winter. We thus arrive at the certain conclusion -that on the plateau of the Col du Géant _the quantity of snow that -falls annually exceeds the quantity melted_. - -125. Had we come in the month of April or May, we should have found -the glacier below the ice-fall also covered with snow, which is now -entirely cleared away by the heat of summer. Nay, more, the ice there -is obviously melting, forming running brooks which cut channels in -the ice, and expand here and there into small blue-green lakes. Hence -you conclude with certainty that below the ice-fall _the quantity of -frozen material falling upon the glacier is less than the quantity -melted_. - -126. And this forces upon us another conclusion: between the glacier -below the ice-fall and the plateau above it there must exist a line -where the quantity of snow which falls _is exactly equal_ to the -quantity annually melted. This is the _snow-line_. On some glaciers -it is quite distinct, and it would be distinct here were the ice less -broken and confused than it actually is. - -127. The French term _névé_ is applied to the glacial region above -the snow-line, while the word _glacier_ is restricted to the ice -below it. Thus the snows of the Col du Géant constitute the névé of -the Glacier du Géant, and in part, the névé of the Mer de Glace. - -128. But if every year thus leaves a residue of snow upon the plateau -of the Col du Géant, it necessarily follows that the plateau must get -annually higher, _provided the snow remain upon it_. Equally certain -is the conclusion that the whole length of the glacier below the -cascade must sink gradually lower, _if the waste of annual melting -be not made good_. Supposing two feet of snow a year to remain upon -the Col, this would raise it to a height far surpassing that of Mont -Blanc in five thousand years. Such accumulation must take place if -the snow remain upon the Col; but the accumulation does _not_ take -place, hence the snow does not remain on the Col. The question then -is, whither does it go? - -[Illustration: SKETCH-PLAN, SHOWING THE MORAINES, _a_, _b_, _c_, _d_, -_e_, OF THE MER DE GLACE.] - - -§ 16. _Branches and Medial Moraines of the Mer de Glace from the -Cleft Station._ - -129. We shall grapple with this question immediately. Meanwhile -look at that ice-valley in front of us, stretching up between Mont -Tacul and the Aiguille de Léchaud, to the base of the great ridge -called the Grande Jorasse. This is called the Glacier de Léchaud. It -receives at its head the snows of the Jorasse and of Mont Mallet, -and joins the Glacier du Géant at the promontory of the Tacul. The -glaciers seem welded together where they join, but they continue -distinct. Between them you clearly trace a stripe of débris (_c_ on -the annexed sketch-plan); you trace a similar though smaller stripe -(_a_ on the sketch), from the junction of the Glacier du Géant with -the Glacier des Périades at the foot of the Aiguille Noire, which you -also follow along the Mer de Glace. - -130. We also see another glacier, or a portion of it, to the left, -falling apparently in broken fragments through a narrow gorge -(Cascade du Talèfre on the sketch) and joining the Léchaud, and from -their point of junction also a stripe of débris (_d_) runs downwards -along the Mer de Glace. Beyond this again we notice another stripe -(_e_), which seems to begin at the bottom of the ice-fall, rising -as it were from the body of the glacier. Beyond all of these we can -notice the lateral moraine of the Mer de Glace. - -131. These stripes are the _medial moraines_ of the Mer de Glace. We -shall learn more about them immediately. - -132. And now, having informed our minds by these observations, let -our eyes wander over the whole glorious scene, the splintered peaks -and the hacked and jagged crests, the far-stretching snow-fields, the -smaller glaciers which nestle on the heights, the deep blue heaven -and the sailing clouds. Is it not worth some labour to gain command -of such a scene? But the delight it imparts is heightened by the -fact that we did not come expressly to see it; we came to instruct -ourselves about the glacier, and this high enjoyment is an incident -of our labour. You will find it thus through life; without honest -labour there can be no deep joy. - - -§ 17. _The Talèfre and the Jardin. Work among the Crevasses._ - -133. And now let us descend to the Mer de Glace, for I want to take -you across the glacier to that broken ice-fall the origin of which we -have not yet seen. We aim at the farther side of the glacier, and to -reach it we must cross those dark stripes of débris which we observed -from the heights. Looked at from above, these moraines seemed flat, -but now we find them to be ridges of stones and rubbish, from twenty -to thirty feet high. - -134. We quit the ice at a place called the Couvercle, and wind round -this promontory, ascending all the time. We squeeze ourselves through -the _Égralets_, a kind of natural staircase in the rock, and soon -afterwards obtain a full view of the ice-fall, the origin of which we -wish to find. The ice upon the fall is much broken; we have pinnacles -and towers, some erect, some leaning, and some, if we are fortunate, -falling like those upon the Glacier des Bois; and we have chasms from -which issues a delicate blue light. With the ice-fall to our right -we continue to ascend, until at length we command a view of a huge -glacier basin, almost level, and on the middle of which stands a -solitary island, entirely surrounded by ice. We stand at the edge of -the _Glacier du Talèfre_, and connect it with the ice-fall we have -passed. The glacier is bounded by rocky ridges, hacked and torn at -the top into teeth and edges, and buttressed by snow fluted by the -descending stones. - -135. We cross the basin to the central island, and find grass and -flowers at the place where we enter upon it. This is the celebrated -_Jardin_, of which you have often heard. The upper part of the Jardin -is bare rock. Close at hand is one of the noblest peaks in this -portion of the Alps, the Aiguille Verte. It is between thirteen and -fourteen thousand feet high, and down its sides, after freshly-fallen -snow, avalanches incessantly thunder. From one of its projections a -streak of moraine starts down the Talèfre; from the Jardin also a -similar streak of moraine issues. Both continue side by side to the -top of the ice-fall, where they are engulphed in the chasms. But at -the bottom of the fall they reappear, as if newly emerging from the -body of the glacier, and afterwards they continue along the Mer de -Glace. - -136. Walk with me now alongside the moraine from the Jardin down -towards the ice-fall. For a time our work is easy, such fissures as -appear offering no impediment to our march. But the crevasses become -gradually wider and wilder, following each other at length so rapidly -as to leave merely walls of ice between them. Here perfect steadiness -of foot is necessary a slip would be death. We look towards the fall, -and observe the confusion of walls and blocks and chasms below us -increasing. At length prudence and reason cry "Halt!" We may swerve -to the right or to the left, and making our way along crests of ice, -with chasms on both hands, reach either the right lateral moraine or -the left lateral moraine of the glacier. - - -§ 18. _First Questions regarding Glacier Motion. Drifting of Bodies -buried in a Crevasse._ - -137. But what are these lateral moraines? As you and I go from day -to day along the glaciers, their origin is gradually made plain. We -see at intervals the stones and rubbish descending from the mountain -sides and arrested by the ice. All along the fringe of the glacier -the stones and rubbish fall, and it soon becomes evident that we -have here the source of the lateral moraines. - -138. But how are the medial moraines to be accounted for? How does -the débris range itself upon the glacier in stripes some hundreds -of yards from its edge, leaving the space between them and the edge -clear of rubbish? Some have supposed the stones to have rolled -over the glacier from the sides, but the supposition will not bear -examination. Call to mind now our reasoning regarding the excess of -snow which falls above the snow-line, and our subsequent question, -How is the snow disposed of. Can it be that the entire mass is moving -slowly downwards? If so, the lateral moraines would be carried along -by the ice on which they rest, and when two branch glaciers unite -they would lay their adjacent lateral moraines together to form a -medial moraine upon the trunk glacier. - -139. There is, in fact, no way that we can see of disposing of the -excess of snow above the snow-line; there is no way of making good -the constant waste of the ice below the snow-line; there is no way of -accounting for the medial moraines of the glacier, but by supposing -that from the highest snow-fields of the Col du Géant, the Léchaud, -and the Talèfre, to the extreme end of the Glacier des Bois, the -whole mass of frozen matter is moving downwards. - -140. If you were older, it would give me pleasure to take you up -Mont Blanc. Starting from Chamouni, we should first pass through -woods and pastures, then up the steep hill-face with the Glacier des -Bossons to our right, to a rock known as the _Pierre Pointue_; thence -to a higher rock called the _Pierre l'Échelle_, because here a ladder -is usually placed to assist in crossing the chasms of the glacier. -At the Pierre l'Échelle we should strike the ice, and passing under -the Aiguille du Midi, which towers to the left, and which sometimes -sweeps a portion of the track with stone avalanches, we should cross -the Glacier des Bossons; amid heaped-up mounds and broken towers of -ice; up steep slopes; over chasms so deep that their bottoms are hid -in darkness. - -141. We reach the rocks of the Grands Mulets, which form a kind -of barren islet in the icy sea; thence to the higher snow-fields, -crossing the _Petit Plateau_, which we should find cumbered by blocks -of ice. Looking to the right, we should see whence they came, for -rising here with threatening aspect high above us are the broken -ice-crags[B] of the Dôme du Goûté. The guides wish to pass this place -in silence, and it is just as well to humour them, however much you -may doubt the competence of the human voice to bring the ice-crags -down. From the Petit Plateau a steep snow-slope would carry us to the -Grand Plateau, and at day-dawn I know nothing in the whole Alps more -grand and solemn than this place. - -[B] Named _séracs_ from their resemblance in shape and colour to an -inferior kind of curdy cheese called by this name at Chamouni. - -142. One object of our ascent would be now attained; for here at the -head of the Grand Plateau, and at the foot of the final slope of Mont -Blanc, I should show you a great crevasse, into which three guides -were poured by an avalanche in the year 1820. - -[Illustration: CREVASSE ON GRAND PLATEAU.] - -143. Is this language correct? A crevasse hardly to be distinguished -from the present one undoubtedly existed here in 1820. But was it -the identical crevasse now existing? Is the ice riven here to-day -the same as that riven fifty-one years ago? By no means. How is this -proved? By the fact that more than forty years after their interment, -the remains of those three guides were found near the end of the -Glacier des Bossons, many miles below the existing crevasse. - -144. The same observation proves to demonstration that it is the ice -near the _bottom_ of the higher névé that becomes the _surface-ice_ -of the glacier near its end. The waste of the surface below the -snow-line brings the deeper portions of the ice more and more to the -light of day. - -145. There are numerous obvious indications of the existence of -glacier motion, though it is too slow to catch the eye at once. -The crevasses change within certain limits from year to year, and -sometimes from month to month; and this could not be if the ice did -not move. Rocks and stones also are observed, which have been plainly -torn from the mountain sides. Blocks seen to fall from particular -points are afterwards observed lower down. On the moraines rocks -are found of a totally different mineralogical character from those -composing the mountains right and left; and in all such cases strata -of the same character are found bordering the glacier higher up. -Hence the conclusion that the foreign boulders have been _floated_ -down by the ice. Further, the ends or "snouts" of many glaciers act -like ploughshares on the land in front of them, overturning with slow -but merciless energy huts and chalets that stand in their way. Facts -like these have been long known to the inhabitants of the High Alps, -who were thus made acquainted in a vague and general way with the -motion of the glaciers. - - -§ 19. _The Motion of Glaciers. Measurements by Hugi and Agassiz. -Drifting of Huts on the Ice._ - -146. But the growth of knowledge is from vagueness towards precision, -and exact determinations of the rate of glacier motion were soon -desired. With reference to such measurements one glacier in the -Bernese Oberland will remain forever memorable. From the little town -of Meyringen in Switzerland you proceed up the valley of Hasli, past -the celebrated waterfall of Handeck, where the river Aar plunges into -a chasm more than 200 feet deep. You approach the Grimsel Pass, but -instead of crossing it you turn to the right and follow the course of -the Aar upwards. Like the Rhone and the Arveiron, you find the Aar -issuing from a glacier. - -147. Get upon the ice, or rather upon the deep moraine shingle which -covers the ice, and walk upwards. It is hard walking, but after -some time you get clear of the rubbish, and on to a wide glacier -with a great medial moraine running along its back. This moraine is -formed by the junction of two branch glaciers, the Lauteraar and the -Finsteraar, which unite at a promontory called the Abschwung to form -the trunk glacier of the Unteraar. - -148. On this great medial moraine in 1827 an intrepid and -enthusiastic Swiss professor, Hugi, of Solothurm (French Soleure), -built a hut with a view to observations upon the glacier. His hut -moved, and he measured its motion. In the three years--from 1827 to -1830--it had moved 330 feet downwards. In 1836 it had moved 2,354 -feet; and in 1841 M. Agassiz found it 4,712 feet below its first -position. - -149. In 1840, M. Agassiz himself and some bold companions took -shelter under a great overhanging slab of rock on the same moraine, -to which they added side walls and other means of protection. And -because he and his comrades came from Neufchâtel, the hut was called -long afterwards the "Hôtel des Neuchâtelois." Two years subsequent -to its erection M. Agassiz found that the "hotel" had moved 486 feet -downwards. - - -§ 20. _Precise Measurements of Agassiz and Forbes. Motion of a -Glacier proved to resemble the Motion of a River._ - -150. We now approach an epoch in the scientific history of glaciers. -Had the first observers been practically acquainted with the -instruments of precision used in surveying, _accurate_ measurements -of the motion of glaciers would probably have been earlier executed. -We are now on the point of seeing such instruments introduced almost -simultaneously by M. Agassiz on the glacier of the Unteraar, and by -Professor Forbes on the Mer de Glace. Attempts had been made by M. -Escher de la Linth to determine the motion of a series of wooden -stakes driven into the Aletsch glacier, but the melting was so -rapid that the stakes soon fell. To remedy this, M. Agassiz in 1841 -undertook the great labour of carrying boring tools to his "hotel," -and piercing the Unteraar glacier at six different places to a depth -of ten feet, in a straight line across the glacier. Into the holes -six piles were so firmly driven that they remained in the glacier for -a year, and in 1842 the displacements of all six were determined. -They were found to be 160 feet, 225 feet, 269 feet, 245 feet, 210 -feet, and 125 feet, respectively. - -151. A great step is here gained. You notice that the middle numbers -are the largest. They correspond to the central portion of the -glacier. Hence, these measurements conclusively establish, not only -the fact of glacier motion, but that _the centre of a glacier, like -that of a river, moves more rapidly than the sides_. - -152. With the aid of trained engineers M. Agassiz followed up these -measurements in subsequent years. His researches are recorded in a -work entitled "Système glaciaire," which is accompanied by a very -noble Atlas of the Glacier of the Unteraar, published in 1847. - -153. These determinations were made by means of a theodolite, of -which I will give you some notion immediately. The same instrument -was employed the same year by the late Professor Forbes upon the -Mer de Glace. He established independently the greater central -motion. He showed, moreover, that it is not necessary to wait a -year, or even a week to determine the motion of a glacier; with a -correctly-adjusted theodolite he was able to determine the motion -of various points of the Mer de Glace from day to day. He affirmed, -and with truth, that the motion of the glacier might be determined -from hour to hour. We shall prove this farther on (162). Professor -Forbes also triangulated the Mer de Glace, and laid down an excellent -map of it. His first observations and his survey are recorded in a -celebrated book published in 1843, and entitled "Travels in the Alps." - -154. These observations were also followed up in subsequent years, -the results being recorded in a series of detached letters and essays -of great interest. These were subsequently collected in a volume -entitled "Occasional Papers on the Theory of Glaciers," published in -1859. The labours of Agassiz and Forbes are the two chief sources of -our knowledge of glacier phenomena. - - -§ 21. _The Theodolite and its Use. Our own Measurements._ - -155. My object thus far is attained. I have given you proofs of -glacier motion, and a historic account of its measurement. And -now we must try to add a little to the knowledge of glaciers by -our own labours on the ice. Resolution must not be wanting at -the commencement of our work, nor steadfast patience during its -prosecution. Look then at this theodolite; it consists mainly of a -telescope and a graduated circle, the telescope capable of motion -up and down, and the circle, carrying the telescope along with it, -capable of motion right and left. When desired to make the motion -exceedingly fine and minute, suitable screws, called tangent screws, -are employed. The instrument is supported by three legs, movable, but -firm when properly planted. - -156. Two spirit-levels are fixed at right angles to each other on -the circle just referred to. Practice enables one to take hold of -the legs of the instrument, and so to fix them that the circle shall -be nearly horizontal. By means of four levelling screws we render it -_accurately_ horizontal. Exactly under the centre of the instrument -is a small hook from which a plummet is suspended; the point of the -bob just touches a rock on which we make a mark; or if the earth be -soft underneath, we drive a stake into it exactly under the plummet. -By re-suspending the plummet at any future time we can find to a -hairbreadth the position occupied by the instrument to-day. - -157. Look through the telescope; you see it crossed by two fibres -of the finest spider's thread. In actual work we first direct the -telescope across the glacier, until the intersection of the two -fibres accurately covers some well-defined point of rock or tree at -the other side of the valley. This, our fixed standard, we sketch -with its surroundings in a note-book, so as to be able immediately -to recognise it on our return to this place. Imagine a straight line -drawn from the centre of the telescope to this point, and that this -line is permitted to drop straight down upon the glacier, every point -of it falling as a stone would fall; along such a line we have now to -fix a series of stakes. - -158. A trained assistant is already upon the glacier. He erects -his staff and stands behind it; the telescope is lowered without -swerving to the right or to the left; in mathematical language it -remains _in the same vertical plane_. The crossed fibres of the -telescope probably strike the ice a little away from the staff of the -assistant; by a wave of the arm he moves right or left; he may move -too much, so we wave him back again. After a trial or two he knows -whether he is near the proper point, and if so makes his motions -small. He soon exactly strikes the point covered by the intersection -of the fibres. A signal is made which tells him that he is right; he -pierces the ice with an auger and drives in a stake. He then goes -forward, and in precisely the same manner takes up another point. -After one or two stakes have been driven in, the assistant is able to -take up the other points very rapidly. Any requisite number of stakes -may thus be fixed in a straight line across the glacier. - -159. Next morning we measure the motion of all the stakes. The -theodolite is mounted in its former position and carefully levelled. -The telescope is directed first upon the standard point at the -opposite side of the valley, being moved by a tangent screw until the -intersection of the spider's threads accurately covers the point. -The telescope is then lowered to the first stake, beside which our -trained assistant is already standing. He is provided with a staff -with feet and inches marked on it. A glance shows us the stake has -moved down. By our signals the assistant recovers the point from -which we started yesterday, and then determines the distance from -this point to the stake. It is, say, 6 inches; through this distance, -therefore, the stake has moved. - -160. We are careful to note the hour and minute at which each stake -is driven in, and the hour and the minute when its distance from its -first position is measured; this enables us to calculate the accurate -_daily motion_ of the point in question. The distances through which -all the other points have moved are determined in precisely the same -way. - -161. Thus we shall proceed to work, first making clear to our minds -what is to be done, and then making sure that it shall be accurately -done. To give our work reality, I will here record the actual -measurements executed, and the actual thoughts suggested, on the -Mer de Glace in 1857. The only unreality that I would ask you to -allow, is that you and I are supposed to be making the observations -together. The labour of measuring was undertaken for the most part -by Mr. Hirst. - - -§ 22. _Motion of the Mer de Glace._ - -162. On July 14, then, we find ourselves at the end of the Glacier -des Bois, not far from the source of the Arveiron. We direct our -telescope across the glacier, and fix the intersection of its -spider's threads accurately upon the edge of a pinnacle of ice. We -leave the instrument untouched, looking through it from hour to -hour. The edge of ice moves slowly, but plainly, past the fibres, -and at the end of three hours we assure ourselves that the motion -has amounted to several inches. While standing near the vault of the -Arveiron, and talking about going into it, its roof gives way, and -falls with the sound of thunder. It is not, therefore, without reason -that I warned you against entering these vaults in summer. - -163. We ascend to the Montanvert Inn, fix on it as a residence, and -then descend to the lateral moraine of the glacier a little below the -inn. Here we erect our theodolite, and mark its exact position by -a plummet. We must first make sure that our line is perpendicular, -or nearly so, to the axis or middle line of the glacier. Our -instructed assistant lays down a long staff in the direction of -the axis, assuring himself, by looking up and down, that it is the -true direction. With another staff in his hand, pointed towards -our theodolite, he shifts his position until the second staff is -perpendicular to the first. Here he gives us a signal. We direct our -telescope upon him, and then gradually raising its end in a vertical -plane we find, and note by sketching, a standard point at the other -side of the glacier. This point known, and our plummet mark known, we -can on any future day find our line. (To render the measurements more -intelligible, I append on the next page an outline diagram of the Mer -de Glace, and of its tributaries.) - -164. Along the line just described ten stakes were set on July 17, -1857. Their displacements were measured on the following day. Two of -them had fallen, but here are the distances passed over by the eight -remaining ones in twenty-four hours. - -DAILY MOTION OF THE MER DE GLACE. - -First Line: A A' upon the Sketch. - - East West - Stake 1 2 3 4 5 7 9 10 - Inches 12 17 23 26 25 26 27 33 - -165. You have already assured yourself by actual contact that the -body of the glacier is real ice, and you may have read that glaciers -move; but the actual observation of the motion of a body apparently -so rigid is strangely interesting. And not only does the ice move -bodily, but one part of it moves past another; the rate of motion -augmenting gradually from 12 inches a day at the side to 33 inches a -day at a distance from the side. This quicker movement of the central -ice of glaciers had been already observed by Agassiz and Forbes; we -verify their results, and now proceed to something new. Crossing the -Glacier du Géant, which occupies more than half the valley, we find -that our line of stakes is not yet at an end. The 10th stake stands -on the part of the ice which comes from the Talèfre. - -[Illustration: OUTLINE PLAN, SHOWING THE MEASURED LINES OF THE MER DE -GLACE AND ITS TRIBUTARIES.] - -166. Now the motion of the sides is slow, because of the friction of -the ice against its boundaries; but then one would think that midway -between the boundaries, where the friction of the sides is least, the -motion ought to be greatest. This is clearly not the case; for though -the 10th stake is nearer than the 9th to the eastern or _Chapeau_ -side of the valley, the 10th stake surpasses the 9th by 6 inches a -day. - -167. Here we have something to think of; but before a natural -philosopher can think with comfort he must be perfectly sure of his -facts. The foregoing line ran across the glacier a little below the -Montanvert. We will run another line across a little way above the -hotel. On July 18 we set out this line, and to multiply our chances -of discovery we place along it 31 stakes. On the subsequent day five -of these were found unfit for use; but here are the distances passed -over by the remaining six-and-twenty in 24 hours. - -Second Line: B B' upon the Sketch. - - West - Stake 2 3 4 5 6 7 8 9 10 11 12 13 - Inches 11 12 15 15 16 17 18 19 20 20 21 21 - Stake 15 16 17 18 19 20 21 22 23 24 25 26 - Inches 23 23 23 21 23 21 25 22 22 23 25 26 - East - -168. Look at these numbers. The first broad fact they reveal is the -advance in the rate of motion from first to last. There are however -some irregularities; from 23 inches at the 17th stake we fall to 21 -inches at the 18th; from 23 inches at the 19th we fall to 21 inches -at the 20th; from 25 inches at the 21st we fall to 22 inches at the -22nd and 23rd; but notwithstanding these small ups and downs, the -general advance of the rate of motion is manifest. Now there may have -been some slight displacement of the stakes by melting, sufficient to -account for these small deviations from uniformity in the increase -of the motion. But another solution is also possible. We shall -afterwards learn that the glacier is retarded not only by its sides -but by its bed; that the upper portions of the ice slide over the -lower ones. Now if the bed of the Mer de Glace should have eminences -here and there rising sufficiently near to the surface to retard the -motion of the surface, they might produce the small irregularities -noticed above. - -169. We note particularly, while upon the ice, that the 26th stake, -like the 10th stake in our last line, stands much nearer to the -eastern than to the western side of the glacier; the measurements, -therefore, offer a further proof that the centre of this portion of -the glacier is not the place of swiftest motion. - - -§ 23. _Unequal Motion of the two Sides of the Mer de Glace._ - -170. But in neither the first line nor the second were we able to -push our measurements quite across the glacier. Why? In attempting -to do one thing we are often taught another, and thus in science, if -we are only steadfast in our work, our very defeats are converted -into means of instruction. We at first planted our theodolite on the -lateral moraine of the Mer de Glace, expecting to be able to command -the glacier from side to side. But we are now undeceived; the centre -of the glacier proves to be higher than its sides, and from our last -two positions the view of the ice near the opposite side of the -glacier was intercepted by the elevation at the centre. The mountain -slopes, in fact, are warm in summer, and they melt the ice nearest to -them, thus causing a fall from the centre to the sides. - -171. But yonder on the heights at the other side of the glacier we -see a likely place for our theodolite. We cross the glacier and plant -our instrument in a position from which we sweep the glacier from -side to side. Our first line was below the Montanvert, our second -line above it; this third line is exactly opposite the Montanvert; -in fact, the mark on which we have fixed the fibre-cross of the -theodolite is a corner of one of the windows of the little inn. Along -this line we fix twelve stakes on July 20. On the 21st one of them -had fallen; but the velocities of the remaining eleven in 24 hours -were found to be as follows:-- - -Third Line: C C' upon the Sketch. - - East West - Stake 1 2 3 4 5 6 7 8 9 10 11 - Inches 20 23 29 30 34 28 25 25 25 18 9 - -172. Both the first stake and the eleventh in this series stood -near the sides of the glacier. On the eastern side the motion is -20 inches, while on the western side it is only 9. It rises on -the eastern side from 20 to 34 inches at the 5th stake, which we, -standing upon the glacier, can see to be much nearer to the eastern -than to the western side. _The united evidence of these three lines -places the fact beyond doubt, that opposite the Montanvert, and for -some distance above it and below it, the whole eastern side of the -glacier is moving more quickly than the western side._ - - -§ 24. _Suggestion of a new Likeness of Glacier Motion to River -Motion. Conjecture tested._ - -173. Here we have cause for reflection, and facts are comparatively -worthless if they do not provoke this exercise of the mind. It -is because facts of nature are not isolated but connected, that -science, to follow them, must also form a connected whole. The mind -of the natural philosopher must, as it were, be a web of _thought_ -corresponding in all its fibres with the web of _fact_ in nature. - -174. Let us, then, ascend to a point which commands a good view -of this portion of the Mer de Glace. The ice-river we see is not -straight but curved, and its curvature is _from_ the Montanvert; -that is to say, its convex side is east, and its concave side is -west (look to the sketch). You have already pondered the fact that a -glacier, _in some respects_, moves like a river. How would a river -move through a curved channel? This is known. Were the ice of the -Mer de Glace displaced by water, the point of swiftest motion at the -Montanvert would not be the centre, but a point east of the centre. -Can it be then that this "water-rock," as ice is sometimes called, -acts in this respect also like water? - -175. This is a thought suggested on the spot; it may or it may not -be true, but the means of testing it are at hand. Looking up the -glacier, we see that at _les Ponts_ it also bends, but that there -its convex curvature is towards the western side of the valley (look -again to the sketch). If our surmise be true, the point of swiftest -motion opposite _les Ponts_ ought to lie west of the axis of the -glacier. - -176. Let us test this conjecture. On July 25 we fix in a line across -this portion of the glacier seventeen stakes; every one of them has -remained firm, and on the 26th we find the motion for 24 hours to be -as follows:-- - -Fourth Line: D D' upon the Sketch. - - East West - Stake 1 2 3 4 5 8 7 8 9 10 11 12 13 14 15 - Inches 7 8 13 15 16 19 20 21 21 23 23 21 22 17 15 - -177. Inspected by the naked eye alone, the stakes 10 and 11, where -the glacier reaches its greatest motion, are seen to be considerably -to the west of the axis of the glacier. Thus far we have a perfect -verification of the _guess_ which prompted us to make these -measurements. You will here observe that the "guesses" of science are -not the work of chance, but of thoughtful pondering over antecedent -facts. The guess is the "induction" facts, to be ratified or exploded -by the test of subsequent experiment. - -178. And though even now we have exceedingly strong reason for -holding that the point of maximum velocity obeys the law of liquid -motion, the strength of our conclusion will be doubled if we can -show that the point shifts back to the eastern side of the axis at -another place of flexure. Fortunately such a place exists opposite -Trélaporte. Here the convex curvature of the valley turns again to -the east. Across this portion of the glacier a line was set out on -July 28, and from measurements on the 31st, the rate of motion per 24 -hours was determined. - -Fifth Line: E E' upon the Sketch. - - West East - Stake 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 - Inches 11 14 13 15 15 16 17 19 20 19 20 18 16 15 10 - -179. Here, again, the mere estimate of distances by the eye would -show us that the three stakes which moved fastest, viz. the 9th, -10th, and 11th, were all to the east of the middle line of the -glacier. The demonstration that the point of swiftest motion wanders -to and fro across the axis, as the flexure of the valley changes, is, -therefore,--shall I say complete? - -180. Not yet. For if surer means are open to us we must not rest -content with estimates by the eye. We have with us a surveying chain: -let us shake it out and measure these lines, noting the distance of -every stake from the side of the glacier. This is no easy work among -the crevasses, but I confide it confidently to Mr. Hirst and you. -We can afterwards compare a number of stakes on the eastern side -with the same number of stakes taken at the same distances from the -western side. For example, a pair of stakes, one ten yards from the -eastern side and the other ten yards from the western side; another -pair, one fifty yards from the eastern side and the other fifty yards -from the western side, and so on, can be compared together. For the -sake of easy reference, let us call the points thus compared in -pairs, _equivalent points_. - -181. There were five pairs of such points upon our fourth line, D D', -and here are their velocities: - - Eastern points; motion in inches 13 15 16 18 20 - Western " " " " 15 17 22 23 23 - -In every case here the stake at the western side moved more rapidly -than the equivalent stake at the eastern side. - -182. Applying the same analysis to our fifth line, E E', we have the -following series of velocities of three pairs of equivalent points:-- - - Eastern points; motion in inches 15 18 19 - Western " " " " 13 15 17 - -183. Here the three points on the eastern side move more rapidly than -the equivalent points on the western side. - -184. It is thus proved:-- - -1. _That opposite the Montanvert the eastern half of the Mer de Glace -moves more rapidly than the western half._ - -2. _That opposite_ les Fonts _the western half of the glacier moves -more rapidly than the eastern half._ - -3. _That opposite Trélaporte the eastern half of the glacier again -moves more rapidly than the western half._ - -4. _That these changes in the place of greatest motion are determined -by the flexures of the valley through which the Mer de Glace moves._ - - -§ 25. _New Law of Glacier Motion._ - -185. Let us express these facts in another way. Supposing the points -of swiftest motion for a very great number of lines crossing the -Mer de Glace to be determined; the line joining all those points -together is what mathematicians would call the _locus_ of the point -of swiftest motion. - -186. At Trélaporte this line would lie east of the centre; at the -_Ponts_ it would lie west of the centre; hence in passing from -Trélaporte to the _Ponts_ it would cross the centre. But at the -Montanvert it would again lie east of the centre; hence between the -_Ponts_ and the Montanvert the centre must be crossed a second time. -If there were further sinuosities upon the Mer de Glace there would -be further crossings of the axis of the glacier. - -187. The points on the axis which mark the transition from eastern to -western bending, and the reverse, may be called _points of contrary -flexure_. - -188. Now what is true of the Mer de Glace is true of all other -glaciers moving through sinuous valleys; so that the facts -established in the Mer de Glace may be expanded into the following -general law of glacier motion:-- - -_When a glacier moves through a sinuous valley, the locus of the -points of maximum motion does not coincide with the centre of the -glacier, but, on the contrary, always lies on the convex side of -the central line. The locus is therefore a curved line more deeply -sinuous than the valley itself, and crosses the axis of the glacier -at each point of contrary flexure._ - -189. The dotted line on the Outline Plan (page 68) represents the -locus of the point of maximum motion, the firm line marking the -centre of the glacier. - -190. Substituting the word _river_ for _glacier_, this law is -also true. The motion of the water is ruled by precisely the same -conditions as the motion of the ice. - -191. Let us now apply our law to the explanation of a difficulty. -Turning to the careful measurements executed by M. Agassiz on -the glacier of the Unteraar, we notice in the discussion of these -measurements a section of the "Système glaciaire" devoted to the -"Migrations of the Centre." It is here shown that the middle of the -Unteraar glacier is not always the point of swiftest motion. This -fact has hitherto remained without explanation; but a glance at the -Unteraar valley, or at the map of the valley, shows the enigma to be -an illustration of the law which we have just established on the Mer -de Glace. - - -§ 26. _Motion of Axis of Mer de Glace._ - -192. We have now measured the rate of motion of five different lines -across the trunk of the Mer de Glace. Do they all move alike? No. -Like a river, a glacier at different places moves at different rates. -Comparing together the points of maximum motion of all five lines, we -have this result: - -MOTION OF MER DE GLACE. - - At Trélaporte 20 inches a day. - At _les Ponts_ 23 " " - Above the Montanvert 26 " " - At the Montanvert 34 " " - Below the Montanvert 33[C] " " - -[C] This is probably under the mark. I think it likely that the -swiftest motion of this portion of the Mer de Glace in 1857 amounted -to a yard in twenty-four hours. - -193. There is thus an increase of rapidity as we descend the glacier -from Trélaporte to the Montanvert; the maximum, motion at the -Montanvert being fourteen inches a day greater than at Trélaporte. - - -§ 27. _Motion of Tributary Glaciers._ - -194. So much for the trunk glacier; let us now investigate the -branches, permitting, as we have hitherto done, reflection on known -facts to precede our attempts to discover unknown ones. - -195. As we stood upon our "cleft station," whence we had so capital -a view of the Mer de Glace, we were struck by the fact that some of -the tributaries of the glacier were wider than the glacier itself. -Supposing water to be substituted for the ice, how do you suppose -it would behave? You would doubtless conclude that the motion down -the broad and slightly-inclined valleys of the Géant and the Léchaud -would be comparatively slow, but that the water would force itself -with increased rapidity through the "narrows" of Trélaporte. Let us -test this notion as applied to the ice. - -196. Planting our theodolite in the shadow of Mont Tacul, and -choosing a suitable point at the opposite side of the Glacier du -Géant, we fix on July 29 a series of ten stakes across the glacier. -The motion of this line in twenty-four hours was as follows:-- - -MOTION OF GLACIER DU GÉANT. - -Sixth Line: H H' upon Sketch. - - Stake 1 2 3 4 5 6 7 8 9 10 - Inches 11 10 12 13 12 13 11 10 9 5 - -197. Our conjecture is fully verified. The maximum motion here is -seven inches a day less than that of the Mer de Glace at Trélaporte -(192). - -198. And now for the Léchaud branch. On August 1 we fix ten stakes -across this glacier above the point where it is joined by the -Talèfre. Measured on August 3, and reduced to twenty-four hours, the -motion was found to be-- - -MOTION OF GLACIER DE LÉCHAUD. - -Seventh Line: K K' upon Sketch. - - Stake 1 2 3 4 5 6 7 8 9 10 - Inches 5 8 10 9 9 8 6 9 7 6 - -199. Here our conjecture is still further verified, the rate of -motion being even less than that of the Glacier du Géant. - - -§ 28. _Motion of Top and Bottom of Glacier._ - -200. We have here the most ample and varied evidence that the sides -of a glacier, like those of a river, are retarded by friction against -its boundaries. But the likeness does not end here. The motion of a -river is retarded by the friction against its bed. Two observers, -viz. Prof. Forbes and M. Charles Martins, concur in showing the same -to be the case with a glacier. The observations of both have been -objected to; hence it is all the more incumbent on us to seek for -decisive evidence. - -201. At the Tacul (near the point _a_ upon the sketch plan, p. 83) a -wall of ice about 150 feet high has already attracted our attention. -Bending round to join the Léchaud the Glacier du Géant is here drawn -away from the mountain side, and exposes a fine section. We try to -measure it top, bottom, and middle, and are defeated twice over. We -try it a third time and succeed. A stake is fixed at the summit of -the ice-precipice, another at 4 feet from the bottom, and a third at -35 feet above the bottom. These lower stakes are fixed at some risk -of boulders falling upon us from above; but by skill and caution -we succeed in measuring the motions of all three. For 24 hours the -motions are:-- - - Top stake 6 inches. - Middle stake 4½ " - Bottom stake 2⅔ " - -202. The retarding influence of the bed of the glacier is reduced to -demonstration by these measurements. The bottom does not move with -half the velocity of the surface. - - -§ 29. _Lateral Compression of a Glacier._ - -203. Furnished with the knowledge which these labours and -measurements have given us, let us once more climb to our station -beside the Cleft under the Aiguille de Charmoz. At our first visit -we saw the medial moraines of the glacier, but we knew nothing about -their cause. We now know that they mark upon the trunk its tributary -glaciers. Cast your eye, then, first upon the Glacier du Géant; -realise its width in its own valley, and see how much it is narrowed -at Trélaporte. The broad ice-stream of the Léchaud is still more -surprising, being squeezed upon the Mer de Glace to a narrow white -band between its bounding moraines. The Talèfre undergoes similar -compression. Let us now descend, shake out our chain, measure, and -express in numbers the width of the tributaries, and the actual -amount of compression suffered at Trélaporte. - -204. We find the width of the Glacier du Géant to be 5,155 links, or -1,134 yards. - -205. The width of the Glacier de Léchaud we find to be 3,725 links, -or 825 yards. - -206. The width of the Talèfre we find to be 2,900 links, or 638 yards. - -207. The sum of the widths of the three branch glaciers is therefore -2,597 yards. - -208. At Trélaporte these three branches are forced through a gorge -893 yards wide, or one-third of their previous width, at the rate of -twenty inches a day. - -209. If we limit our view to the Glacier de Léchaud, the facts are -still more astonishing. Previous to its junction with the Talèfre, -this glacier has a width of 825 yards; in passing through the jaws of -the granite vice at Trélaporte, its width is reduced to eighty-eight -yards, or in round numbers to one-tenth of its previous width. (Look -to the sketch on the next page.) - -[Illustration: SKETCH-PLAN SHOWING THE MORAINES, _a_, _b_, _c_, _d_, -_e_, OF THE MER DE GLACE.] - -210. Are we to understand by this that the ice of the Léchaud is -squeezed to one-tenth of its former _volume_? By no means. It is -mainly a change of _form_, not of volume, that occurs at Trélaporte. -Previous to its compression, the glacier resembles a plate of ice -_lying flat_ upon its bed. After its compression, it resembles a -plate _fixed upon its edge_. The squeezing, doubtless, has deepened -the ice. - - -§ 30. _Longitudinal Compression of a Glacier._ - -211. The ice is forced through the gorge at Trélaporte by a pressure -from behind; in fact the Glacier du Géant, immediately above -Trélaporte, represents a piston or a plug which drives the ice -through the gorge. What effect must this pressure have upon the plug -itself? Reasoning alone renders it probable that the pressure will -shorten the plug; that the lower part of the Glacier du Géant will to -some extent yield to the pressure from behind. - -212. Let us test this notion. About three-quarters of a mile above -the Tacul, and on the mountain slope to the left as we ascend, we -observe a patch of verdure. Thither we climb; there we plant our -theodolite, and set out across the Glacier du Géant, a line, which we -will call line No. 1 (F F' upon sketch, p. 68). - -213. About a quarter of a mile lower down we find a practicable -couloir on the mountain side; we ascend it, reach a suitable -platform, plant our instrument, and set out a second line, No. 2 (G -G' upon sketch). We must hasten our work here, for along this couloir -stones are discharged from a small glacier which rests upon the slope -of Mont Tacul. - -214. Still lower down by another quarter of a mile, which brings us -near the Tacul, we set out a third line, No. 3 (H H' upon sketch), -across the glacier. - -215. The daily motion of the centres of these three lines is as -follows:-- - - Inches Distances asunder - No. 1 50·55 } - } 545 yards. - No. 2 15·43 } - } 487 " - No. 3 12·75 } - -216. The first line here moves five inches a day more than the -second; and the second nearly three inches a day more than the third. -The reasoning is therefore confirmed. The ice-plug, which is in round -numbers one thousand yards long, is shortened by the pressure exerted -on its front at the rate of about eight inches a day. - -217. A river descending the Valley du Géant would behave in -substantially the same fashion. It would have its motion on -approaching Trélaporte diminished, and it would pour through the -defile with a velocity greater than that of the water behind. - - -§ 31. _Sliding and Flowing. Hard Ice and Soft Ice._ - -218. We have thus far confined ourselves to the measurement and -discussion of glacier motion; but in our excursions we have noticed -many things besides. Here and there, where the ice has retreated -from the mountain side, we have seen the rocks fluted, scored, and -polished; thus proving that the ice had slidden over them and ground -them down. At the source of the Arveiron we noticed the water rushing -from beneath the glacier charged with fine matter. All glacier rivers -are similarly charged. The Rhone carries its load of matter into the -Lake of Geneva; the rush of the river is here arrested, the matter -subsides, and the Rhone quits the lake clear and blue. The Lake of -Geneva, and many other Swiss lakes, are in part filled up with this -matter, and will, in all probability, finally be obliterated by it. - -219. One portion of the motion of a glacier is due to this bodily -sliding of the mass over its bed. - -220. We have seen in our journeys over the glacier streams formed by -the melting of the ice, and escaping through cracks and _crevasses_ -to the bed of the glacier. The fine matter ground down is thus washed -away; the bed is kept lubricated, and the sliding of the ice rendered -more easy than it would otherwise be. - -221. As a skater also you know how much ice is weakened by a thaw. -Before it actually melts it becomes rotten and unsafe. Test such ice -with your penknife: you can dig the blade readily into it, or cut the -ice with ease. Try good sound ice in the same way: you find it much -more resistant. The one, indeed, resembles soft chalk; the other hard -stone. - -222. Now the Mer de Glace in summer is in this thawing condition. Its -ice is rendered soft and yielding by the sun; its motion is thereby -facilitated. We have seen that not only does the glacier slide over -its bed, but that the upper layers slide over the under ones, and -that the centre slides past the sides. The softer and more yielding -the ice is, the more free will be this motion, and the more readily -also will it be forced through a defile like Trélaporte. - -223. But in winter the thaw ceases; the quantity of water reaching -the bed of the glacier is diminished or entirely cut off. The -ice also, to a certain depth at least, is frozen hard. These -considerations would justify the opinion that in winter the glacier, -if it moves at all, must move more slowly than in summer. At all -events, the summer measurements give no clue to the winter motion. - -224. This point merits examination. I will not, however, ask you to -visit the Alps in mid-winter; but, if you allow me, I will be your -deputy to the mountains, and report to you faithfully the aspect of -the region and the behaviour of the ice. - - -§ 32. _Winter on the Mer de Glace._ - -225. The winter chosen is an inclement one. There is snow in London, -snow in Paris, snow in Geneva; snow near Chamouni so deep that the -road fences are entirely effaced. On Christmas night--nearly at -mid-night--1859, your deputy reaches Chamouni. - -226. The snow fell heavily on December 26; but on the 27th, during a -lull in the storm, we turn out. There are with me four good guides -and a porter. They tie planks to their feet to prevent them from -sinking in the snow; I neglect this precaution and sink often to the -waist. Four or five times during our ascent the slope cracks with an -explosive sound, and the snow threatens to come down in avalanches.[D] - -[D] Four years later, viz. in the spring of 1863, a mighty climber -and noble guide and companion of mine, named Johann Joseph Bennen, -was lost, through the cracking and subsequent slipping of snow on -such a slope. - -The freshly-fallen snow was in that particular condition which causes -its granules to adhere, and hence every flake falling on the trees -had been retained there. The laden pines presented beautiful and -often fantastic forms. - -227. After five hours and a half of arduous work the Montanvert was -attained. We unlocked the forsaken auberge, round which the snow -was reared in buttresses. I have already spoken of the complex play -of crystallising forces. The frost figures on the window-panes of -the auberge were wonderful: mimic shrubs and ferns wrought by the -building power while hampered by the adhesion between the glass and -the film in which it worked. The appearance of the glacier was very -impressive; all sounds were stilled. The cascades which in summer -fill the air with their music were silent, hanging from the ledges -of the rocks in fluted columns of ice. The surface of the glacier -was obviously higher than it had been in summer; suggesting the -thought that while the winter cold maintained the lower end of the -glacier jammed between its boundaries, the upper portions still moved -downwards and thickened the ice. The peak of the Aiguille du Dru -shook out a cloud-banner, the origin and nature of which have been -already explained (84). (See _Frontispiece_.) - -[Illustration: SNOW-LADEN PINE-TREE.] - -228. On the morning of the 28th this banner was strikingly large and -grand, and reddened by the light of the rising sun, it glowed like a -flame. Roses of cloud also clustered round the crests of the Grande -Jorasse and hung upon the pinnacles of Charmoz. Four men, well roped -together, descended to the glacier. I had trained one of them in -1857, and he was now to fix the stakes. The storm had so distributed -the snow as to leave alternate lengths of the glacier bare and -thickly covered. Where much snow lay great caution was required, for -hidden crevasses were underneath. The men sounded with their staffs -at every step. Once while looking at the party through my telescope -the leader suddenly disappeared; the roof of a crevasse had given -way beneath him; but the other three men promptly gathered round and -lifted him out of the fissure. The true line was soon picked up by -the theodolite; one by one the stakes were fixed until a series of -eleven of them stood across the glacier. - -229. To get higher up the valley was impracticable; the snow was -too deep, and the aspect of the weather too threatening; so the -theodolite was planted amid the pines a Little way below the -Montanvert, whence through a vista I could see across the glacier. -The men were wrapped at intervals by whirling snow-wreaths which -quite hid them, and we had to take advantage of the lulls in the -wind. Fitfully it came up the valley, darkening the air, catching the -snow upon the glacier, and tossing it throughout its entire length -into high and violently agitated clouds, separated from each other -by cloudless spaces corresponding to the naked portions of the ice. -In the midst of this turmoil the men continued to work. Bravely and -steadfastly stake after stake was set, until at length a series of -ten of them was fixed across the glacier. - -230. Many of the stakes were fixed in the snow. They were four feet -in length, and were driven in to a depth of about three feet. But -that night, while listening to the wild onset of the storm, I thought -it possible that the stakes and the snow which held them might be -carried bodily away before the morning. The wind, however, lulled. -We rose with the dawn, but the air was thick with descending snow. -It was all composed of those exquisite six-petaled flowers, or -six-rayed stars, which have been already figured and described (§ -9). The weather brightening, the theodolite was planted at the end -of the first line. The men descended, and, trained by their previous -experience, rapidly executed the measurements. The first line was -completed before 11 A. M. Again the snow began to fall, filling -all the air. Spangles innumerable were showered upon the heights. -Contrary to expectation, the men could be seen and directed through -the shower. - -231. To reach the position occupied by the theodolite at the end of -our second line, I had to wade breast-deep through snow which seemed -as dry and soft as flour. The toil of the men upon the glacier in -breaking through the snow was prodigious. But they did not flinch, -and after a time the leader stood behind the farthest stake, and -cried, _Nous avons fini_. I was surprised to hear him so distinctly, -for falling snow had been thought very deadening to sound. The work -was finished, and I struck my theodolite with a feeling of a general -who had won a small battle. - -232. We put the house in order, packed up, and shot by glissade down -the steep slopes of _La Filia_ to the vault of the Arveiron. We found -the river feeble, but not dried up. Many weeks must have elapsed -since any water had been sent down from the surface of the glacier. -But at the setting in of winter the fissures were in a great measure -charged with water; and the Arveiron of to-day was probably due to -the gradual _drainage_ of the glacier. There was now no danger of -entering the vault, for the ice seemed as firm as marble. In the -cavern we were bathed by blue light. The strange beauty of the place -suggested magic, and put me in mind of stories about fairy caves -which I had read when a boy. At the source of the Arveiron our winter -visit to the Mer de Glace ends; next morning your deputy was on his -way to London. - - -§ 33. _Winter Motion of the Mer de Glace._ - -233. Here are the measurements executed in the winter of 1859:-- - -Line No. I. - - Stake 1 2 3 4 5 6 7 8 9 10 11 - Inches 7 11 14 13 14 14 16 16 12 12 7 - -Line No. II. - - Stake 1 2 3 4 5 6 7 8 9 10 - Inches 8 10 14 16 16 16 18 17 15 14 - -234. Thus the winter motion of the Mer de Glace near the Montanvert -is, in round numbers, half the summer motion. - -235. As in summer, the eastern side of the glacier at this place -moved quicker than the western. - - -§ 34. Motion of the Grindelwald and Aletsch Glaciers. - -236. As regards the question of motion, to no other glacier have we -devoted ourselves with such thoroughness as to the Mer de Glace; we -are, however, able to add a few measurements of other celebrated -glaciers. Rear the village of Grindelwald in the Bernese Oberland, -there are two great ice-streams called respectively the Upper and the -Lower Grindelwald glaciers, the second of which is frequently visited -by travellers in the Alps. Across it on August 6, 1860, a series of -twelve stakes was fixed by Mr. Vaughan Hawkins and myself. Measured -on the 8th and reduced to its daily rate, the motion of these stakes -was as follows:-- - -MOTION OF LOWER GRINDELWALD GLACIER. - - Stake 1 2 3 4 5 6 7 8 9 10 11 12 - Inches 18 19 20 21 21 21 22 20 19 18 17 14 - -237. The theodolite was here planted a little below the footway -leading to the higher glacier region, and at about a mile above -the end of the glacier. The measurement was rendered difficult by -crevasses. - -238. The largest glacier in Switzerland is the Great Aletsch, to -which further reference shall subsequently be made. Across it on -August 14, 1860, a series of thirty-four stakes was planted by Mr. -Hawkins and me. Measured on the 16th and reduced to their daily rate, -the velocities were found to be as follows:-- - -MOTION OF GREAT ALETSCH GLACIER. - - East - Stake 1 2 3 4 5 6 7 8 9 10 11 12 - Inches 2 3 4 6 8 11 13 14 16 17 17 19 - Stake 13 14 15 16 17 18 19 20 21 22 23 - Inches 19 18 18 17 19 19 19 19 17 17 15 - Stake 24 25 26 27 28 29 30 31 32 33 34 - Inches 16 17 17 17 17 17 17 17 16 12 12 - West - -239. The maximum motion here is nineteen inches a day. Probably the -eastern side of the glacier is shallow, the retardation of the bed -making the motion of the eastern stakes inconsiderable. The width of -the glacier here is 9,030 links, or about a mile and a furlong. The -theodolite was planted high among the rocks on the western flank of -the mountain, about half a mile above the Märgelin See. - - -§ 35. _Motion of Morteratsch Glacier._ - -240. Far to the east of the Oberland and in that interesting part -of Switzerland known as the Ober Engadin, stands a noble group of -mountains, less in height than those of the Oberland, but still -of commanding elevation. The group derives its name from its most -dominant peak, the Piz Bernina. To reach the place we travel by -railway from Basel to Zürich, and from Zürich to Chur (French Coire), -whence we pass by diligence over either the Albula pass or the Julier -pass to the village of Pontresina. Here we are in the immediate -neighbourhood of the Bernina mountains. - -241. From Pontresina we may walk or drive along a good coach road -over the Bernina pass into Italy. At about an hour above the village -you would look from the road into the heart of the mountains, the -line of vision passing through a valley, in which is couched a -glacier of considerable size. Along its back you would trace a medial -moraine, and you could hardly fail to notice how the moraine, from a -mere narrow streak at first, widens gradually as it descends, until -finally it quite covers the lower end of the glacier. Nor is this an -effect of perspective; for were you to stand upon the mountain slopes -which nourish the glacier, you would see thence also the widening -of the streak of rubbish, though the perspective here would tend to -narrow the moraine as it retreats downwards. - -242. The ice-stream here referred to is the Morteratsch glacier, the -end of which is a short hour's walk from the village of Pontresina. -We have now to determine its rate of motion and to account for the -widening of its medial moraine. - -243. In the summer of 1864 Mr. Hirst and myself set out three lines -of stakes across the glacier. The first line crossed the ice high up; -the second a good distance lower down, and the third lower still. -Even the third line, however, was at a considerable distance above -the actual snout of the glacier. The daily motion of these three -lines was as follows:-- - -First Line. - - Stake 1 2 3 4 5 6 7 8 9 10 11 - Inches 8 12 13 13 14 13 12 12 10 7 5 - -Second Line. - - Stake 1 2 3 4 5 6 7 8 9 10 11 - Inches 1 4 6 8 10 11 11 11 11 11 11 - -Third Line. - - Stake 1 2 3 4 5 6 7 8 9 10 11 - Inches 1 2 4 5 6 6 7 7 5 5 4 - -244. Compare these lines together. You notice the velocity of the -first is greater than that of the second, and the velocity of the -second greater than that of the third. - -245. The lines were permitted to move down wards for 100 hours, -at the end of which time the spaces passed over by the points of -swiftest motion of the three lines were as follows: - -Maximum Motion in 100 Hours. - - First line 56 inches. - Second line 45 " - Third line 30 " - -246. Here then is a demonstration that the upper portions of the -Morteratsch glacier are advancing on the lower ones. _In 1871 the -motion of a point on the middle of the glacier near its snout was -found to be less than two inches a day!_ - -247. What, then, is the consequence of this swifter march of -the upper glacier? Obviously to squeeze this medial moraine -longitudinally, and to cause it to spread out laterally. We have here -distinctly revealed the cause of the widening of the medial moraine. - -248. It has been a question much discussed, whether a glacier is -competent to scoop out or deepen a valley through which it moves, and -this very Morteratsch glacier has been cited to prove that such is -not the case. Observers went to the snout of the glacier, and finding -it sensibly quiescent, they concluded that no scooping occurred. But -those who contended for the power of glaciers to excavate valleys -never stated, or meant to state, that it was the snout of the -glacier which did the work. In the Morteratsch glacier the work of -excavation, which certainly goes on to a greater or less extent, -must be far more effectual high up the valley than at the end of the -glacier. - - -§ 36. _Birth of a Crevasse: Reflections._ - -249. Preserving the notion that we are working together, we will -now enter upon a new field of enquiry. We have wrapped up our -chain, and are turning homewards after a hard day's work upon the -Glacier du Géant, when under our feet, as if coming from the body -of the glacier, an explosion is heard. Somewhat startled, we look -enquiringly over the ice. The sound is repeated, several shots -being fired in quick succession. They seem sometimes to our right, -sometimes to our left, giving the impression that the glacier is -breaking all round us. Still nothing is to be seen. - -250. We closely scan the ice, and after an hour's strict search -we discover the cause of the reports. They announce the birth of -a crevasse. Through a pool upon the glacier we notice air bubbles -ascending, and find the bottom of the pool crossed by a narrow crack, -from which the bubbles issue. Eight and left from this pool we trace -the young fissure through long distances. It is sometimes almost -too feeble to be seen, and at no place is it wide enough to admit a -knife-blade. - -251. It is difficult to believe that the formidable fissures among -which you and I have so often trodden with awe, could commence in -this small way. Such, however, is the case. The great and gaping -chasms on and above the ice-falls of the Géant and the Talèfre -begin as narrow cracks, which open gradually to crevasses. We are -thus taught in an instructive and impressive way that appearances -suggestive of very violent action may really be produced by processes -so slow as to require refined observations to detect them. In the -production of natural phenomena two things always come into play, the -_intensity_ of the acting force, and the _time_ during which it acts. -Make the intensity great, and the time small, and you have sudden -convulsion; but precisely the same apparent effect may be produced -by making the intensity small, and the time great. This truth is -strikingly illustrated by the Alpine ice-falls and crevasses; -and many geological phenomena, which at first sight suggest -violent convulsion, may be really produced in the selfsame almost -imperceptible way. - - -§ 37. _Icicles._ - -252. The crevasses are grandest on the higher névés, where they -sometimes appear as long yawning fissures, and sometimes as chasms -of irregular outline. A delicate blue light shimmers from them, but -this is gradually lost in the darkness of their profounder portions. -Over the edges of the chasms, and mostly over the southern edges, -hangs a coping of snow, and from this depend like stalactites rows -of transparent icicles, 10, 20, 30 feet long. These pendent spears -constitute one of the most beautiful features of the higher crevasses. - -253. How are they produced? Evidently by the thawing of the snow. But -why, when once thawed, should the water freeze again to solid spears? -You have seen icicles pendent from a house-eave, which have been -manifestly produced by the thawing of the snow upon the roof. If we -understand these, we shall also understand the vaster stalactites of -the Alpine crevasses. - -254. Gathering up such knowledge as we possess, and reflecting upon -it patiently, let us found upon it, if we can, a theory of icicles. - -255. First, then, you are to know that the _air_ of our atmosphere -is hardly heated at all by the rays of the sun, whether visible or -invisible. The air is highly transparent to all kinds of rays, and -it is only the scanty fraction to which it is _not_ transparent that -expend their force in warming it. - -256. Not so, however, with the snow on which the sunbeams fall. It -absorbs the solar heat, and on a sunny day you may see the summits of -the high Alps glistening with the water of liquefaction. The _air_ -above and around the mountains may at the same time be many degrees -below the freezing point in temperature. - -257. You have only to pass from sunshine into shade to prove this. A -single step suffices to carry you from a place where the thermometer -stands high to one where it stands low; the change being due, not -to any difference in the temperature of the _air_, but simply to the -withdrawal of the thermometer from the direct action of the solar -rays. Nay, without shifting the thermometer at all, by interposing a -suitable screen, which cuts off the sun's rays, the coldness of the -air may be demonstrated. - -258. Look now to the snow upon your house roof. The sun plays upon -it, and melts it; the water trickles to the eave and then drops down. -If the eave face the sun the water remains water; but if the eave -do not face the sun, the drop, before it quits its parent snow, _is -already in shadow_. Now the shaded space, as we have learnt, may be -below the freezing temperature. If so the drop, instead of falling, -congeals, and the rudiment of an icicle is formed. Other drops and -driblets succeed, which trickle over the rudiment, congeal upon it in -part and _thicken_ it at the root. But a portion of the water reaches -the free end of the icicle, hangs from it, and is there congealed -before it escapes. The icicle is thus _lengthened_. In the Alps, -where the liquefaction is copious and the cold of the shaded crevasse -intense, the icicles, though produced in the same way, naturally grow -to a greater size. The drainage of the snow after the sun's power is -withdrawn also produces icicles. - -259. It is interesting and important that you should be able to -explain the formation of an icicle; but it is far more important -that you should realise the way in which the various threads of what -we call Nature are woven together. You cannot fully understand an -icicle without first knowing that solar beams powerful enough to -fuse the snows and blister the human skin, nay, it might be added, -powerful enough, when concentrated, to burn up the human body itself, -may pass through the air, and still leave it at an icy temperature. - - -§ 38. _The Bergschrund._ - -260. Having cleared away this difficulty, let us turn once more to -the crevasses, taking them in the order of their formation. First -then above the névé we have the final Alpine peaks and crests, -against which the snow is often reared as a steep buttress. We have -already learned that both névés and glaciers are moving slowly -downwards; but it usually happens that the attachment of the highest -portion of the buttress to the rocks is great enough to enable it to -hold on while the lower portion breaks away. A very characteristic -crevasse is thus formed, called in the German-speaking portion of the -Alps a _Bergschrund_. It often surrounds a peak like a fosse, as if -to defend it against the assaults of climbers. - -261. Look more closely into its formation. Imagine the snow as yet -unbroken. Its higher portions cling to the rocks, and move downwards -with extreme slowness. But its lower portions, whether from their -greater depth and weight, or their less perfect attachment, are -compelled to move more quickly. _A pull_ is therefore exerted, -tending to separate the lower from the upper snow. For a time this -pull is resisted by the cohesion of the névé; but this at length -gives way, and a crack is formed exactly across the line in which -the pull is exerted. In other words, _a crevasse is formed at right -angles to the line of tension_. - - -§ 39. _Transverse Crevasses._ - -262. Both on the névé and on the glacier the origin of the crevasses -is the same. Through some cause or other the ice is thrown into a -state of strain, and as it cannot _stretch_ it _breaks_ across the -line of tension. Take for example, the ice-fall of the Géant, or -of the Talèfre, above which you know the crevasses yawn terribly. -Imagine the névé and the glacier entirely peeled away, so as to -expose the surface over which they move. From the Col du Géant we -should see this surface falling gently to the place now occupied by -the brow of the cascade. Here the surface would fall steeply down to -the bed of the present Glacier du Géant, where the slope would become -gentle once more. - -263. Think of the névé moving over such a surface. It descends from -the Col till it reaches the brow just referred to. It crosses the -brow, and must bend down to keep upon its bed. Realise clearly what -must occur. The surface of the névé is evidently thrown into a -state of strain; it breaks and forms a crevasse. Each fresh portion -of the névé as it passes the brow is similarly broken, and thus a -succession of crevasses is sent down the fall. Between every two -chasms is a great transverse ridge. Through local strains upon the -fall those ridges are also frequently broken across, towers of -ice--_séracs_--being the result. Down the fall both ridges and séracs -are borne, the dislocation being augmented during the descent. - -264. What must occur at the foot of the fall? Here the slope suddenly -lessens in steepness. It is plain that the crevasses must not only -cease to open here, but that they must in whole or in part close -up. At the summit of the fall, the bending was such as to make the -surface convex; at the bottom of the fall the bending renders the -surface concave. In the one case we have _strain_, in the other -_pressure_. In the one case, therefore, we have the _opening_, and -in the other the _closing_ of crevasses. This reasoning corresponds -exactly with the facts of observation. - -265. Lay bare your arm and stretch it straight. Make two ink dots -half an inch or an inch apart, exactly opposite the elbow. Bend your -arm, the dots approach each other, and are finally brought together. -Let the two dots represent the two sides of a crevasse at the bottom -of an ice-fall; the bending of the arm resembles the bending of the -ice, and the closing up of the dots resembles the closing of the -fissures. - -266. The same remarks apply to various portions of the Mer de Glace. -At certain places the inclination changes from a gentler to a steeper -slope, and on crossing the brow between both the glacier breaks its -back. _Transverse crevasses_ are thus formed. There is such a change -of inclination opposite to the Angle, and a still greater but similar -change at the head of the Glacier des Bois. The consequence is that -the Mer de Glace at the former point is impassable, and at the latter -the rending and dislocation are such as we have seen and described. -Below the Angle, and at the bottom of the Glacier des Bois, the -steepness relaxes, the crevasses heal up, and the glacier becomes -once more continuous and compact. - - -§ 40. _Marginal Crevasses._ - -267. Supposing, then, that we had no changes of inclination, should -we have no crevasses? We should certainly have less of them, but they -would not wholly disappear. For other circumstances exist to throw -the ice into a state of strain, and to determine its fracture. The -principal of these is the more rapid movement of the centre of the -glacier. - -268. Helped by the labours of an eminent man, now dead, the late Mr. -Wm. Hopkins, of Cambridge, let us master the explanation of this -point together. But the pleasure of mastering it would be enhanced -if we could see beforehand the perplexing and delusive appearances -accounted for by the explanation. Could my wishes be followed out, I -would at this point of our researches carry you off with me to Basel, -thence to Thun, thence to Interlaken, thence to Grindelwald, where -you would find yourself in the actual presence of the Wetterhorn and -the Eiger, with all the greatest peaks of the Bernese Oberland, the -Finsteraarhorn, the Schreckhorn, the Monch, the Jungfrau, at hand. -At Grindelwald, as we have already learnt, there are two well-known -glaciers--the Ober Grindelwald and the Unter Grindelwald glaciers--on -the latter of which our observations should commence. - -269. Dropping down from the village to the bottom of the valley, we -should breast the opposite mountain, and with the great limestone -precipices of the Wetterhorn to our left, we should get upon a path -which commands a view of the glacier. Here we should see beautiful -examples of the opening of crevasses at the summit of a brow, and -their closing at the bottom. But the chief point of interest would -be the crevasses formed at the _side_ of this glacier--the _marginal -crevasses_, as they may be called. - -270. We should find the side copiously fissured, even at those places -where the centre is compact; and we should particularly notice that -the fissures would neither run in the direction of the glacier, nor -straight across it, but that they would be _oblique_ to it, enclosing -an angle of about 45 degrees with the sides. Starting from the side -of the glacier the crevasses would be seen to point _upwards_; that -is to say, the ends of the fissures abutting against the bounding -mountain would appear to be _dragged down_. Were you less instructed -than you now are, I might lay a wager that the aspect of these -fissures would cause you to conclude that the centre of the glacier -is left behind by the quicker motion of the sides. - -271. This indeed was the conclusion drawn by M. Agassiz from this -very appearance, before he had measured the motion of the sides and -centre of the glacier of the Unteraar. Intimately versed with the -treatment of mechanical problems, Mr. Hopkins immediately deduced -the obliquity of the lateral crevasses from the quicker flow of the -centre. Standing beside the glacier with pencil and note-book in -hand, I would at once make the matter clear to you thus. - -[Illustration] - -272. Let A C, in the annexed figure, be one side of the glacier, and -B D the other; and let the direction of motion be that indicated -by the arrow. Let S T be a transverse slice of the glacier, taken -straight across it, say to-day. A few days or weeks hence this -slice will have been carried down, and because the centre moves more -quickly than the sides it will not remain straight, but will bend -into the form S' T'. - -273. Supposing T _i_ to be a small square of the original slice near -the side of the glacier. In its new position the square will be -distorted to the lozenge-shaped figure T' _i'_. Fix your attention -upon the diagonal T _i_ of the square; in the lowest position this -diagonal, _if the ice could stretch_, would be lengthened to T' _i'_. -But the ice does not stretch; it breaks, and we have a crevasse -formed at right angles to T' _i'_. The mere inspection of the diagram -will assure you that the crevasse will point obliquely _upwards_. - -274. Along the whole side of the glacier the quicker movement of the -centre produces a similar state of strain; and the consequence is -that the sides are copiously cut by those oblique crevasses, even at -places where the centre is free from them. - -275. It is curious to see at other places the transverse fissures -of the centre uniting with those at the sides, so as to form great -curved crevasses which stretch across the glacier from side to -side. The convexity of the curve is turned _upwards_, as mechanical -principles declare it ought to be. (See sketch on opposite page.) -But if you were ignorant of those principles, you would never infer -from the aspect of these curves the quicker motion of the centre. In -landslips, and in the motion of partially indurated mud, you may -sometimes notice appearances similar to those exhibited by the ice. - -[Illustration: SKETCH OF CURVED CREVASSES: THE GLACIER MOVES FROM -LEFT TO RIGHT.] - - -§ 41. _Longitudinal Crevasses._ - -276. We have thus unravelled the origin of both transverse and -marginal crevasses. But where a glacier issues from a steep and -narrow defile upon a comparatively level plain which allows it room -to expand laterally, its motion is in part arrested, and the level -portion has to bear the thrust of the steeper portions behind. Here -the line of thrust is in the direction of the glacier, while the -direction at right angles to this is one of tension. Across this -latter the glacier breaks, and _longitudinal crevasses_ are formed. - -277. Examples of this kind of crevasse are furnished by the lower -part of the Glacier of the Rhone, when looked down upon from the -Grimsel Pass, or from any commanding point on the flanking mountains. - - -§ 42. _Crevasses in relation to Curvature of Glacier._ - -278. One point in addition remains to be discussed, and your present -knowledge will enable you to master it in a moment. You remember at -an early period of OUT researches that we crossed the Mer de Glace -from the Chapeau side to the Montanvert side. I then desired you to -notice that the Chapeau side of the glacier was more fissured than -either the centre or the Montanvert side (75). Why should this be so? -Knowing as we now do that the Chapeau side of the glacier moves more -quickly than the other; that the point of maximum motion does not -lie on the centre but far east of it, we are prepared to answer this -question in a perfectly satisfactory manner. - -279. Let A B and C D, in the diagram opposite, represent the two -curved sides of the Mer de Glace at the Montanvert, and let _m n_ be -a straight line across the glacier. Let _o_ be the point of maximum -motion. The mechanical state of the two sides of the glacier may be -thus made plain. Supposing the line _m n_ to be a straight elastic -string with its ends fixed; let it be grasped firmly at the point -o by the finger and thumb, and drawn to _o'_, keeping the distance -between _o'_ and the side C D constant. Here the length, _n o_ of -the string would have stretched to _n o'_, and the length _m o_ to -_m o'_ and you see plainly that the stretching of the short line, in -comparison with its length, is greater than that of the long line in -comparison with its length. In other words, the strain upon _n o'_ is -greater than that upon _m o'_; so that if one of them were to break -under the strain, it would be the short one. - -[Illustration: _Montanvert_] - -280. These two lines represent the conditions of strain upon the -two sides of the glacier. The sides are held back, and the centre -tries to move on, a strain being thus set up between the centre and -sides. But the displacement of the point of maximum motion through -the curvature of the valley makes the strain upon the eastern ice -greater than that upon the western. The eastern side of the glacier -is therefore more crevassed than the western. - -281. Here indeed resides the difficulty of getting along the eastern -side of the Mer de Glace: a difficulty which was one reason for our -crossing the glacier opposite to the Montanvert. There are two convex -sweeps on the eastern side to one on the western side, hence on the -whole the eastern side of the Mer de Glace is most riven. - - -§ 43. _Moraine-ridges, Glacier Tables, and Sand-Cones._ - -282. When you and I first crossed the Mer de Glace from Trélaporte -to the Couvercle, we found that the stripes of rocks and rubbish -which constituted the medial moraines were ridges raised above the -general level of the glacier to a height at some places of twenty or -thirty feet. On examining these ridges we found the rubbish to be -superficial, and that it rested upon a great spine of ice which ran -along the back of the glacier. By what means has this ridge of ice -been raised? - -283. Most boys have read the story of Dr. Franklin's placing bits of -cloth of various colours upon snow on a sunny day. The bits of cloth -sank in the snow, the dark ones most. - -284. Consider this experiment. The sun's rays first of all fall -upon the upper surface of the cloth and warm it. The heat is then -conducted through the cloth to the under surface, and the under -surface passes it on to the snow, which is finally liquefied by the -heat. It is quite manifest that the quantity of snow melted will -altogether depend upon the amount of heat sent from the upper to the -under surface of the cloth. - -285. Now cloth is what is called a bad conductor. It does not permit -heat to travel freely through it. But where it has merely to pass -through the thickness of a single bit of cloth, a good quantity of -the heat gets through. But if you double or treble or quintuple the -thickness of the cloth; or, what is easier, if you put several pieces -one upon the other, you come at length to a point where no sensible -amount of heat could get through from the upper to the under surface. - -286. What must occur if such a thick piece, or such a series of -pieces of cloth, were placed upon snow on which a strong sun is -falling? The snow round the cloth is melted, but that underneath the -cloth is protected. If the action continue long enough the inevitable -result will be, that the level of the snow all round the cloth will -sink, and the cloth will be left behind perched upon an eminence of -snow. - -287. If you understand this, you have already mastered the cause of -the moraine-ridges. They are not produced by any swelling of the ice -upwards. But the ice underneath the rocks and rubbish being protected -from the sun, the glacier right and left melts away and leaves a -ridge behind. - -288. Various other appearances upon the glacier are accounted for -in the same way. Here upon the Mer de Glace we have flat slabs of -rock sometimes lifted up on pillars of ice. These are the so-called -_Glacier Tables_. They are produced, not by the growth of a stalk of -ice out of the glacier, but by the melting of the glacier all round -the ice protected by the stone. Here is a sketch of one of the Tables -of the Mer de Glace. - -[Illustration] - -289. Notice moreover that a glacier table is hardly ever set square -upon its pillar. It generally leans to one side, and repeated -observation teaches you that it so leans as to enable you always to -draw the north and south line upon the glacier. For the sun being -south of the zenith at noon pours its rays against the southern end -of the table, while the northern end remains in shadow. The southern -end, therefore, being most warmed does not protect the ice underneath -it so effectually as the northern end. The table becomes inclined, -and ends by sliding bodily off its pedestal. - -290. In the figure opposite we have what maybe called an ideal -Table. The oblique lines represent the direction of the sunbeams, -and the consequent tilting of the table here shown resembles that -observed upon the glaciers. - -291. A pebble will not rise thus: like Franklin's single bit of -cloth, a dark-coloured pebble sinks in the ice. A spot of black mould -will not rest upon the surface, but will sink; and various parts of -the Glacier du Géant are honeycombed by the sinking of such spots of -dirt into the ice. - -[Illustration] - -292. But when the dirt is of a thickness sufficient to protect the -ice the case is different. Sand is often washed away by a stream from -the mountains, or from the moraines, and strewn over certain spaces -of the glacier. A most curious action follows: the sanded surface -rises, the part on which the sand lies thickest rising highest. -Little peaks and eminences jut forth, and when the distribution -of the sand is favourable, and the action sufficiently prolonged, -you have little mountains formed, sometimes singly, and sometimes -grouped so as to mimic the Alps themselves. The _Sand-Cones_ of the -Mer de Glace are not striking; but on the Görner, the Aletsch, the -Morteratsch, and other glaciers, they form singly and in groups, -reaching sometimes a height of ten or twenty feet. - - -§ 44. _The Glacier Mills or Moulins._ - -293. You and I have learned by long experience the character of the -Mer de Glace. We have marched over it daily, with a definite object -in view, but we have not closed our eyes to other objects. It is -from side glimpses of things which are not at the moment occupying -our attention that fresh subjects of enquiry arise in scientific -investigation. - -294. Thus in marching over the ice near Trélaporte we were often -struck by a sound resembling low rumbling thunder. We subsequently -sought out the origin of this sound, and found it. - -295. A large area of this portion of the glacier is unbroken. -Driblets of water have room to form rills; rills to unite and form -streams; streams to combine to form rushing brooks, which sometimes -cut deep channels in the ice. Sooner or later these streams reach -a strained portion of the glacier, where a crack is formed across -the stream. A way is thus opened for the water to the bottom of the -glacier. By long action the stream hollows out a shaft, the crack -thus becoming the starting-point of a funnel of unseen depth, into -which the water leaps with the sound of thunder. - -296. This funnel and its cataract form a glacier Mill or _Moulin_. - -297. Let me grasp your hand firmly while you stand upon the edge of -this shaft and look into it. The hole, with its pure blue shimmer, -is beautiful, but it is terrible. Incautious persons have fallen -into these shafts, a second or two of bewilderment being followed by -sudden death. But caution upon the glaciers and mountains ought, by -habit, to be made a second nature to explorers like you and me. - -298. The crack into which the stream first descended to form the -moulin, moves down with the glacier. A succeeding portion of the ice -reaches the place where the breaking strain is exerted. A new crack -is then formed above the moulin, which is thenceforth forsaken by the -stream, and moves downward as an empty shaft. Here upon the Mer de -Glace, in advance of the _Grand Moulin_, we see no less than six of -these forsaken holes. Some of them we sound to a depth of 90 feet. - -299. But you and I both wish to determine, if possible, the entire -depth of the Mer de Glace. The Grand Moulin offers a chance of doing -this which we must not neglect. Our first effort to sound the moulin -fails through the breaking of our cord by the impetuous plunge of the -water. A lump of grease in the hollow of a weight enables a mariner -to judge of a sea bottom. We employ such a weight, but cannot reach -the bed of the glacier. A depth of 163 feet is the utmost reached by -our plummet. - -300. From July 28 to August 8 we have watched the progress of the -Grand Moulin. On the former date the position of the Moulin was -fixed. On the 31st it had moved down 50 inches; a little more than -a day afterwards it had moved 74 inches. On August 8 it had moved -198 inches, which gives an average of about 18 inches in twenty-four -hours. No doubt next summer upon the Mer de Glace a Grand Moulin will -be found thundering near Trélaporte; but like the crevasse of the -Grand Plateau, already referred to (§ 16), it will not be our Moulin. -This, or rather the ice which it penetrated, is now probably more -than a mile lower down than it was in 1857. - - -§ 45. _The Changes of Volume of Water by Heat and Cold._ - -301. We have noticed upon the glacier shafts and pits filled with -water of the most delicate blue. In some cases these have been the -shafts of extinct moulins closed at the bottom. A theory has been -advanced to account for them, which, though it may be untenable, -opens out considerations regarding the properties of water that -ought to be familiar to enquirers like you and me. - -302. In our dissection of lake ice by a beam of heat (§ 11) we -noticed little vacuous spots at the centres of the liquid flowers -formed by the beam. These spots we referred to the fact that when ice -is melted the water produced is less in volume than the ice, and that -hence the water of the flower was not able to occupy the whole space -covered by the flower. - -303. Let us more fully illustrate this subject. Stop a small flask -water-tight with a cork, and through the cork introduce a narrow -glass tube also water-tight. It is easy to fill the flask with water -so that the liquid shall stand at a certain height in the glass tube. - -304. Let us now warm the flask with the flame of a spirit-lamp. On -first applying the flame you notice a momentary sinking of the liquid -in the glass tube. This is due to the momentary expansion of the -flask by heat; it becomes suddenly larger when the flame is first -applied. - -305. But the expansion of the water soon overtakes that of the flask -and surpasses it. We immediately see the rise of the liquid column -in the glass tube, exactly as mercury rises in the tube of a warmed -thermometer. - -306. Our glass tube is ten inches long, and at starting the water -stood in it at a height of five inches. We will apply the spirit-lamp -flame until the water rises quite to the top of the tube and trickles -over. This experiment suffices to show the expansion of the water by -heat. - -307. We now take a common finger-glass and put into it a little -pounded ice and salt. On this we place the flask, and then build -round it the freezing mixture. The liquid column retreats down the -tube, proving the contraction of the liquid by cold. We allow the -shrinking to continue for some minutes, noticing that the downward -retreat of the liquid becomes gradually slower, and that it finally -ceases altogether. - -308. Keep your eye upon the liquid column; it remains quiescent for -a fraction of a minute, and then moves once more. But its motion is -now _upwards_ instead of downwards. _The freezing mixture now acts -exactly like the flame._ - -309. It would not be difficult to pass a thermometer through the cork -into the flask, and it would tell us the exact temperature at which -the liquid ceased to contract and began to expand. At that moment we -should find the temperature of the liquid a shade over 39° Fahr. - -310. At this temperature, then, water attains _its maximum density_. - -311. Seven degrees below this temperature, or at 32° Fahr., the -liquid begins to turn into solid crystals of ice, which you know -swims upon water because it is bulkier for a given weight. In fact, -this halt of the approaching molecules at the temperature of 39°, -is but the preparation for the subsequent act of crystallisation, -in which the expansion by cold culminates. Up to the point of -solidification the increase of volume is slow and gradual; while in -the act of solidification it is sudden, and of overwhelming strength. - -312. By this force of expansion the Florentine Academicians long -ago burst a sphere of copper nearly three quarters of an inch in -thickness. By the same force the celebrated astronomer Huyghens burst -in 1667 iron cannons a finger breadth thick. Such experiments have -been frequently made since. Major Williams during a severe Quebec -winter filled a mortar with water, and closed it by driving into -its muzzle a plug of wood. Exposed to a temperature 50° Fahr. below -the freezing point of water, the metal resisted the strain, but the -plug gave way, being projected to a distance of 400 feet. At Warsaw -howitzer shells have been thus exploded; and you and I have shivered -thick bombshells to fragments, by placing them for half an hour in a -freezing mixture. - -313. The theory of the shafts and pits referred to at the beginning -of this section is this: The water at the surface of the shaft is -warmed by the sun, say to a temperature of 39° Fahr. The water at -the bottom, in contact with the ice, must be at 32° or near it. The -heavier water is therefore at the top; it will descend to the bottom, -melt the ice there, and thus deepen the shaft. - -314. The circulation here referred to undoubtedly goes on, and -some curious effects are due to it; but not, I think, the one here -ascribed to it. The _deepening_ of a shaft implies a quicker melting -of its bottom than of the surface of the glacier. It is not easy to -see how the fact of the solar heat being first absorbed by water, -and then conveyed by it to the bottom of the shaft, should make the -melting of the bottom more rapid than that of the ice which receives -the direct impact of the solar rays. The surface of the glacier must -sink _at least_ as rapidly as the bottom of the pit, so that the -circulation, though actually existing, cannot produce the effect -ascribed to it. - - -§ 46. _Consequences flowing from the foregoing Properties of Water. -Correction of Errors._ - -315. I was not much above your age when the property of water ceasing -to contract by cold at a temperature of 39° Fahr. was made known to -me, and I still remember the impression it made upon me. For I was -asked to consider what would occur in case this solitary exception to -an otherwise universal law ceased to exist. - -316. I was asked to reflect upon the condition of a lake stored with -fish and offering its surface to very cold air. It was made clear -to me that the water on being first chilled would shrink in volume -and become heavier, that it would therefore sink and have its place -supplied by the warmer and lighter water from the deeper portions of -the lake. - -317. It was pointed out to me that without the law referred to -this process of circulation would go on until the whole water of -the lake had been lowered to the freezing temperature. Congelation -would then begin, and would continue as long as any water remained -to be solidified. One consequence of this would be to destroy every -living thing contained in the lake. Other calamities were added, -all of which were said to be prevented by the perfectly exceptional -arrangement, that after a certain time the _colder_ water becomes the -_lighter_, floats on the surface of the lake, is there congealed, -thus throwing a protecting roof over the life below. - -318. Count Rumford, one of the most solid of scientific men, writes -in the following strain about this question:--"It does not appear to -me that there is anything which human sagacity can fathom, within the -wide-extended bounds of the visible creation, which affords a more -striking or more palpable proof of the wisdom of the Creator, and -of the special care He has taken in the general arrangement of the -universe, to preserve animal life, than this wonderful contrivance. - -319. "Let me beg the attention of my readers while I endeavour to -investigate this most interesting subject; and let me at the same -time bespeak his candour and indulgence. I feel the danger to which a -mortal exposes himself who has the temerity to explain the designs -of Infinite Wisdom. The enterprise is adventurous, but it surely -cannot be improper. - -320. "Had not Providence interfered on this occasion in a manner -which may well be considered as _miraculous_, all the fresh water -within the polar circle must inevitably have been frozen to a very -great depth in winter, and every plant and tree destroyed." - -321. Through many pages of his book Count Rumford continues in this -strain to expound the ways and intentions of the Almighty, and he -does not hesitate to apply very harsh words to those who cannot -share his notions. He calls them hardened and degraded. We are here -warned of the fact, which is too often forgotten, that the pleasure -or comfort of a belief, or the warmth or exaltation of feeling which -it produces, is no guarantee of its truth. For the whole of Count -Rumford's delight and enthusiasm in connexion with this subject, and -the whole of his ire against those who did not share his opinions, -were founded upon an erroneous notion. - -322. Water is _not_ a solitary exception to an otherwise general law. -There are other molecules than those of this liquid which require -more room in the solid crystalline condition than in the adjacent -molten condition. Iron is a case in point. Solid iron floats upon -molten iron exactly as ice floats upon water. Bismuth is a still -more impressive case, and we could shiver a bomb as certainly by the -solidification of bismuth as by that of water. There is no fish, to -be taken care of here, still the "contrivance" is the same. - -323. I am reluctant to mention them in the same breath with Count -Rumford, but I am told that in our own day there are people who -profess to find the comforts of a religion in a superstition lower -than any that has hitherto degraded the civilized human mind. So that -the _happiness_ of a faith and the _truth_ of a faith are two totally -different things. - -324. Life and the conditions of life are in necessary harmony. This -is a truism, for without the suitable conditions life could not -exist. But both life and its conditions set forth the operations of -inscrutable Power. We know not its origin; we know not its end. And -the presumption, if not the degradation, rests with those who place -upon the throne of the universe a magnified image of themselves, and -make its doings a mere colossal imitation of their own. - - -§ 47. _The Molecular Mechanism of Water-Congelation._ - -325. But let us return to our science. How are we to picture this act -of expansion on the part of freezing water? By what operation do the -molecules demand with such irresistible emphasis more room in the -solid than in the adjacent liquid condition? In all cases of this -kind we must derive our conceptions from the world of the senses, and -transfer them afterwards to a world transcending the range of the -senses. - -326. You have not forgotten our conversation regarding "atomic -poles" (§ 10), and how the notion of polar force came to be applied -to crystals. With this fresh in your memory, you will have no great -difficulty in understanding how expansion of volume may accompany the -act of crystallisation. - -327. I place a number of magnets before you. They, as matter, are -affected by gravity, and, if perfectly free, they would move towards -each other in obedience to the attraction of gravity. - -328. But they are not only matter, but _magnetic_ matter. They not -only act upon each other by the simple force of gravity, but by the -polar force of magnetism. Imagine them placed at a distance from each -other, and perfectly free to move. Gravity first makes itself felt -and draws them together. For a time the magnetic force issuing from -the poles is insensible; but when a certain nearness is attained, the -polar force comes into play. The mutually attracting points close up, -the mutually repellent points retreat, and it is easy to see that -this action may produce an arrangement of the magnets which requires -more room. Suppose them surrounded by a box which exactly encloses -them at the moment the polar force first comes into play. It is easy -to see that in arranging themselves subsequently the repelled corners -and ends of the magnets may be caused to press against the sides of -the box, and even to burst it, if the forces be sufficiently strong. - -329. Here then we have a conception which may be applied to the -molecules of water. They, like the magnets, are acted upon by two -distinct forces. For a time while the liquid is being cooled they -approach each other, in obedience to their general attraction for -each other. But at a certain point new forces, some attractive, -some repulsive, _emanating from special points_ of the molecules, -come into play. The attracted points close up, the repelled points -retreat. Thus the molecules turn and rearrange themselves, demanding, -as they do so, more space, and overcoming all ordinary resistance by -the energy of their demand. This, in general terms, is an explanation -of the expansion of water in solidifying: it would be easy to -construct an apparatus for its illustration. - - -§ 48. _The Dirt Bands of the Mer de Glace._ - -330. Pass from bright sunshine into a moderately lighted room; for a -time all appears so dark that the objects in the room are not to be -clearly distinguished. Hit violently by the waves of light (§ 3) the -optic nerve is numbed, and requires time to recover its sensitiveness. - -331. It is for this reason that I choose the present hour for a -special observation on the Mer de Glace. The sun has sunk behind the -ridge of Charmoz, and the surface of the glacier is in sober shade. -The main portion of our day's work is finished, but we have still -sufficient energy to climb the slopes adjacent to the Montanvert to -a height of a thousand feet or thereabouts above the ice. - -[Illustration] - -332. We now look fairly down upon the glacier, and see it less -foreshortened than from the Montanvert. We notice the diet -overspreading its eastern side, due to the crowding together of -its medial moraines. We see the comparatively clean surface of the -Glacier du Géant; but we notice upon this surface an appearance -which we have not hitherto seen. It is crossed by a series of grey -bent bands, which follow each other in succession, from Trélaporte -downwards. We count eighteen of these from our present position. (See -sketch, page 128.) - -[Illustration] - -333. These are the _Dirt Bands_ of the Mer de Glace; they were first -observed by Professor Forbes in 1842. - -[Illustration] - -334. They extend down the glacier further than we can see; and if we -cross the valley of Chamouni, and climb the mountains at the opposite -side, to a point near the little auberge, called La Flégère, we shall -command a view of the end of the glacier and observe the completion -of the series of bands. We notice that they are confined throughout -to the portion of the glacier derived from the Col du Géant. (See -sketch, page 129.) - -335. We must trace them to their source. You know how noble and -complete a view is obtained of the glacier and Col du Géant from the -Cleft Station above Trélaporte. Thither we must once more climb; -and thence we can see the succession of bands stretching downwards -to the Montanvert, and upwards to the base of the ice-cascade upon -the Glacier du Géant. The cascade is evidently concerned in their -formation. (See sketch opposite.) - -336. And how? Simply enough. The glacier, as we know, is broken -transversely at the summit of the ice-fall, and descends the -declivity in a series of great transverse ridges. At the base of the -fall, the chasms are closed, but the ridges in part remain forming -protuberances, which run like vast wrinkles across the glacier. These -protuberances are more and more bent because of the quicker motion -of the centre, and the depressions between them form receptacles for -the fine mud and débris washed by the little rills from the adjacent -slopes. - -337. The protuberances sink gradually through the wasting action of -the sun, so that long before Trélaporte is reached they have wholly -disappeared. Not so the dirt of which they were the collectors: it -continues to occupy, in transverse bands, the flat surface of the -glacier. At Trélaporte, moreover, where the valley becomes narrow, -the bands are much sharpened, obtaining there the character which -they afterwards preserve throughout the Mer de Glace. Other glaciers -with cascades also exhibit similar bands. - - -§ 49. _Sea Ice and Icebergs._ - -338. We are now equipped intellectually for a campaign into another -territory. Water becomes heavier and more difficult to freeze when -salt is dissolved in it. Sea water is therefore heavier than fresh, -and the Greenland Ocean requires to freeze it a temperature 3½ -degrees lower than fresh water. When concentrated till its specific -gravity reaches 1.1045, sea water requires for its congelation a -temperature 18⅓ degrees lower than the ordinary freezing-point.[E] - -[E] Scoresby. - -339. But even when the water is saturated with salt, the -crystallising force studiously rejects the salt, and devotes itself -to the congelation of the water alone. Hence the ice of sea water, -when melted, produces fresh water. The only saline particles existing -in such ice are those entangled 'mechanically in its pores. They have -no part or lot in the structure of the crystal. - -340. This _exclusiveness_, if I may use the term, of the water -molecules; this entire rejection of all foreign elements from the -edifices which they build, is enforced to a surprising degree. -Sulphuric acid has so strong an affinity for water that it is one -of the most powerful agents known to the chemist for the removal of -humidity from air. Still, as shown by Faraday, when a mixture of -sulphuric acid and water is frozen, the crystal formed is perfectly -sweet and free from acidity. The water alone has lent itself to the -crystallising force. - -341. Every winter in the Arctic regions the sea freezes, roofing -itself with ice of enormous thickness and vast extent. By the summer -heat, and the tossing of the waves, this is broken up; the fragments -are drifted by winds and borne by currents. They clash, they crush -each other, they pile themselves into heaps, thus constituting the -chief danger encountered by mariners in the polar seas. - -342. But among the drifting masses of flat sea-ice, vaster masses -sail, which spring from a totally different source. These are the -_Icebergs_ of the Arctic seas. They rise sometimes to an elevation of -hundreds of feet above the water, while the weight of ice submerged -is about seven times that seen above. - -343. The first observers of striking natural phenomena generally -allow wonder and imagination more than their due place. But to -exclude all error arising from this cause, I will refer to the -journal of a cool and intrepid Arctic navigator, Sir Leopold -McClintock. He describes an iceberg 250 feet high, which was aground -in 500 feet of water. This would make the entire height of the berg -750 feet, not an unusual altitude for the greater icebergs. - -344. From Baffin's Bay these mighty masses come sailing down through -Davis' Straits into the broad Atlantic. A vast amount of heat is -demanded for the simple liquefaction of ice (§ 48); and the melting -of icebergs is on this account so slow, that when large they -sometimes maintain themselves till they have been drifted 2000 miles -from their place of birth. - -345. What is their origin? The Arctic glaciers. From the mountains -in the interior the indurated snows slide into the valleys and fill -them with ice. The glaciers thus formed move like the Swiss ones, -incessantly downward. But the Arctic glaciers reach the sea, enter -it, often ploughing up its bottom into submarine moraines. Undermined -by the lapping of the waves, and unable to resist the strain imposed -by their own weight, they break across, and discharge vast masses -into the ocean. Some of these run aground on the adjacent shores, -and often maintain themselves for years. Others escape southward, to -be finally dissolved in the warm waters of the Atlantic. The first -engraving on the opposite page is copied from a photograph taken by -Mr. Bradford during a recent expedition to the Northern seas. The -second represents a mass of ice upon the Glacier des Bossons. Their -likeness suggests their common origin. - -[Illustration] - -[Illustration] - - -§ 50. _The Æggischhorn, the Märgelin See and its Icebergs._ - -346. I am, however, unwilling that you should quit Switzerland -without seeing such icebergs as it can show, and indeed there are -other still nobler glaciers than the Mer de Glace with which you -ought to be acquainted. In tracing the Rhone to its source, you have -already ascended the valley of the Rhone. Let us visit it again -together; halt at the little town of Viesch, and go from it straight -up to the excellent hostelry on the slope of the Æggischhorn. This -we shall make our head-quarters while we explore that monarch of -European ice-streams,--the great Aletsch glacier. - -347. Including the longest of its branches, this noble ice-river is -about twenty miles long, while at the middle of its trunk it measures -nearly a mile and a quarter from side to side. The grandest mountains -of the Bernese Oberland, the Jungfrau, the Monch, the Trugberg, the -Aletschhorn, the Breithorn, the Gletscherhorn, and many another noble -peak and ridge, are the collectors of its névés. From three great -valleys formed in the heart of the mountains these névés are poured, -uniting together to form the trunk of the Aletsch at a place named -by a witty mountaineer, the "Place de la Concorde of Nature." If the -phrase be meant to convey the ideas of tranquil grandeur, beauty of -form, and purity of hue, it is well bestowed. - -348. Our hotel is not upon the peak of the Æggischhorn, but a brisk -morning walk soon places us upon the top. Thence we see the glacier -like a broad river stretching upwards to the roots of the Jungfrau, -and downwards past the Bel Alp towards its end. Prolonging the vision -downwards, we strike the noblest mountain group in all the Alps,--the -Dom and its attendant peaks, the Matterhorn and the Weisshorn. The -scene indeed is one of impressive grandeur, a multitude of peaks and -crests here unnamed contributing to its glory. - -349. But low down to our right, and surrounded by the sheltering -mountains, is an object the beauty of which startles those who are -unprepared for it. Yonder we see the naked side of the glacier, -exposing glistening ice-cliffs sixty or seventy feet high. It would -seem as if the Aletsch here were engaged in the vain attempt to -thrust an arm through a lateral valley. It once did so; but the arm -is now incessantly broken off close to the body of the glacier, a -great space formerly covered by the ice being occupied by its water -of liquefaction. A lake of the loveliest blue is thus formed, which -reaches quite to the base of the ice-cliffs, saps them, as the Arctic -waves sap the Greenland glaciers, and receives from them the broken -masses which it has undermined. As we look down upon the lake, small -icebergs sail over the tranquil surface, each resembling a snowy swan -accompanied by its shadow. - -350. This is the beautiful little lake of Märgelin, or, as the Swiss -here call it, the Märgelin See. You see that splash, and immediately -afterwards hear the sound of the plunging ice. The glacier has broken -before our eyes, and dropped an iceberg into the lake. All over the -lake the water is set in commotion, thus illustrating on a small -scale the swamping waves produced by the descent of vast islands of -ice from the Arctic glaciers. Look to the end of the lake. It is -cumbered with the remnants of icebergs now aground, which have been -in part wafted thither by the wind, but in part slowly borne by the -water which moves gently in this direction. - -351. Imagine us below upon the margin of the lake, as I happened to -be on one occasion. There is one large and lonely iceberg about the -middle. Suddenly a sound like that of a cataract is heard; we look -towards the iceberg and see water teeming from its sides. Whence -comes the water? the berg has become top-heavy through the melting -underneath; it is in the act of performing a somersault, and in -rolling over carries with it a vast quantity of water, which rushes -like a waterfall down its sides. And notice that the iceberg, which -a moment ago was snowy-white, now exhibits the delicate blue colour -characteristic of compact ice. It will soon, however, be rendered -white again by the action of the sun. The vaster icebergs of the -Northern seas sometimes roll over in the same fashion. A week may be -spent with delight and profit at the Æggischhorn. - - -§ 51. _The Bel Alp._ - -352. From the Æggischhorn I might lead you along the mountain ridge -by the Betten See, the fish of which we have already tasted, to the -Rieder Alp, and thence across the Aletsch to the Bel Alp. This is a -fine mountain ramble, but you and I prefer making the glacier our -highway downwards. Easy at some places, it is by no means child's -play at others to unravel its crevasses. But the steady constancy and -close observation which we have hitherto found availing in difficult -places do not forsake us here. We clear the fissures; and, after four -hours of exhilarating work, we find ourselves upon the slope leading -up to the Bel Alp hotel. - -353. This is one of the finest halting-places in the Alps. Stretching -before us up to the Æggischhorn and Märgelin See is the long last -reach of the Aletsch, with its great medial moraine running along its -back. At hand is the wild gorge of the Massa, in which the snout of -the glacier lies couched like the head of a serpent. The beautiful -system of the Oberaletsch glaciers is within easy reach. Above us -is a peak called the Sparrenhorn, accessible to the most moderate -climber, and on the summit of which little more than an hour's -exertion will place you and me. Below us now is the Oberaletsch -glacier, exhibiting the most perfect of medial moraines. Near us -is the great mass of the Aletschhorn, clasped by its névés, and -culminating in brown rock. It is supported by other peaks almost as -noble as itself. The Nesthorn is at hand; while sweeping round to the -west we strike the glorious triad already referred to, the Weisshorn, -the Matterhorn, and the Dom. Take one glance at the crevasses of the -glacier immediately below us. It tumbles at its end down a steep -incline, and is greatly riven. But the crevasses open before the -steep part is reached, and you notice the coalescence of marginal and -transverse crevasses, producing a system of curved fissures with the -convexities of the curves pointing upwards. The mechanical reason of -this is now known to you. The glacier-tables are also numerous and -fine. I should like to linger with you here for a week, exploring the -existing glaciers, and tracing out the evidences of others that have -passed away. - - -§ 52. _The Riffelberg and Görner Glacier._ - -354. And though our measurements and observations on the Mer de Glace -are more or less representative of all that can be made or solved -elsewhere, I am unwilling to leave you unacquainted with the great -system of glaciers which stream from the northern slopes of Monte -Rosa and the adjacent mountains. From the Bel Alp we can descend to -Brieg, and thence drive to Visp; but you and I prefer the breezy -heights, so we sweep round the promontory of the Nessel, until we -stand over the Rhone valley, in front of Visp. From this village an -hour's walking carries us to Stalden, where the valley divides into -two branches: the one leading through Saas over the Monte Moro, and -the other through St. Nicholas to Zermatt. The latter is our route. - -355. We reach Zermatt, but do not halt here. On the mountain ridge, -4,000 feet above the valley, we discern the Riffelberg hotel. This -we reach. Right in front of us is the pinnacle of the Matterhorn, -upon the top of which it must appear incredible to you that a human -foot could ever tread. Constancy and skill, however, accomplished -this, but in the first instance at a terrible price. In the little -churchyard of Zermatt we have seen the graves of two of the greatest -mountaineers that Savoy and England have produced: and who, with two -gallant young companions, fell from the Matterhorn in 1865. - -356. At the Riffelberg we are within an hour's walk of the famous -Görner Grat, which commands so grand a view of the glaciers of Monte -Rosa. But yonder huge knob of perfectly bare rock, which is called -the Riffelhorn, must be our station. What the Cleft Station is to -the Mer de Glace, the Riffelhorn is to the Görner glacier and its -tributaries. From its lower side the rock, easy as it may seem, is -inaccessible. Here, indeed, in 1865, a fifth good man met his end, -and he also lies beside his fellow countrymen in the churchyard of -Zermatt. Passing a little tarn, or lake, called the Riffel See, we -assail the Riffelhorn on its upper side. It is capital rock-practice -to reach the summit; and from it we command a most extraordinary -scene. - -357. The huge and many-peaked mass of Monte Rosa faces us, and we -scan its snows from bottom to top. To the right is the mighty ridge -of the Lyskamm, also laden with snow; and between both lies the -Western Glacier of Monte Rosa. This glacier meets another from the -vast snow-fields of the Cima di Jazzi; they join to form the Görner -glacier, and from their place of junction stretches the customary -medial moraine. On this side of the Lyskamm rise two beautifully -snowy eminences, the Twins Castor and Pollux; then come the brown -crags of the Breithorn, then the Little Matterhorn, and then the -broad snow-field of the Théodule, out of which springs the Great -Matterhorn, and which you and I will cross subsequently into Italy. - -358. The valleys and depressions between these mountains are filled -with glaciers. Down the flanks of the Twin Castor comes the Glacier -des Jumeaux, from Pollux comes the Schwartze glacier, from the -Breithorn the Trifti glacier, then come the Little Matterhorn glacier -and the Théodule glacier, each, as it welds itself to the trunk, -carrying with it its medial moraine. We can count nine such moraines -from our present position. And to a still more surprising degree -than on the Mer de Glace, we notice the power of the ice to yield to -pressure; the broad névés being squeezed on the trunk of the Görner -into white stripes, which become ever narrower between the bounding -moraines, and finally disappear under their own shingle. - -[Illustration: THE GÖRNER GLACIER, WITH MONTE ROSA IN THE DISTANCE, -AND THE RIFFELHORN TO THE LEFT.] - -359. On the two main tributaries we also notice moraines which seem -in each case to rise from the body of the glacier, appearing in the -middle of the ice without any apparent origin higher up. These at -their sources, are sub-glacial moraines, which have been rubbed away -from rocky promontories entirely covered with ice. They lie hidden -for a time in the body of the glacier, and appear at the surface -where the ice above them has been melted away by the sun. - -360. This is the place to mention a notion long entertained by the -inhabitants of the high Alps, that glaciers possess the power of -thrusting out all impurities from them. On the Mer de Glace you and -I have noticed large patches of clay and black mud which evidently -came from the body of the glacier, and we can therefore understand -how natural was this notion of extrusion to people unaccustomed to -close observation. But the power of the glacier in this respect is -in reality the power of the sun, which fuses the ice above concealed -impurities, and, like the bodies of the guides on the Glacier des -Bossons (143), brings them to the light of day. - -361. On no other glacier will you find more objects of interest than -on the Görner. Sand-cones, glacier-tables, deep ice-gorges cut by -streams and bridged fantastically by boulders, moulins, sometimes -arched ice-caverns of extraordinary size and beauty. On the lower -part of the glacier we notice the partial disappearance of the medial -moraine in the crevasses, and its reappearance at the foot of the -incline. For many years this glacier was steadily advancing on the -meadow in front of it, ploughing up the soil and overturning the -chalets in its way. It now shares in the general retreat exhibited -during the last fifteen years among the glaciers of the Alps. As -usual, a river, the Visp, rushes from a vault at the extremity of the -Görner glacier. - - -§ 53. _Ancient Glaciers of Switzerland._ - -362. You have not lost the memory of the old Moraine, which -interested us so much in our first ascent from the source of the -Arveiron; for it opened our minds to the fact that at one period of -its history the Mer de Glace attained far greater dimensions than it -now exhibits. Our experience since that time has enabled us to pursue -these evidences of ice action to an extent of which we had then no -notion. - -363. Close to the existing glacier, for example, we have repeatedly -seen the mountain side laid bare by the retreat of the ice. This is -especially conspicuous just now, because for the last fifteen or -sixteen years the glaciers of the Alps have been steadily shrinking; -so that it is no uncommon thing to see the marginal rocks laid bare -for a height of fifty, sixty, eighty, or even one hundred feet above -the present glacier. On the rocks thus exposed we see the evident -marks of the sliding; and our eyes and minds have been so educated in -the observation of these appearances that we are now able to detect, -with certainty, icemarks, or moraines, ancient or modern, wherever -they appear. - -364. But the elevations at which we have found such evidence might -well shake belief in the conclusions to which they point. Beside -the Massa Gorge, at 1,000 feet above the present Aletsch, we found -a great old moraine. Descending the meadows between the Bel Alp -and Flatten, we found another, now clothed with grass, and bearing -a village on its back. But I wish to carry you to a region which -exhibits these evidences on a still grander and more impressive -scale. We have already taken a brief flight to the valley of -Hasli and the Glacier of the Aar. Let us make that glacier our -starting-point. Walking from it downwards towards the Grimsel, we -pass everywhere over rocks singularly rounded, and fluted, and -scarred. These appearances are manifestly the work of the glacier in -recent times. But we approach the Grimsel, and at the turning of the -valley stand before the precipitous granite flank of the mountain. -The traces of the ancient ice are here as plain as they are amazing. -The rocks are so hard that not only the fluting and polishing, but -even the fine scratches which date back unnamable thousands of years -are as evident as if they had been made yesterday. We may trace these -evidences to a height of two thousand feet above the present valley -bed. It is indubitable that an ice-river of this astounding depth -once flowed through the vale of Hasli. - -365. Yonder is the summit of the Siedelhorn; and if we gain it, the -Unteraar glacier will lie like a map below us. From this commanding -point we plainly see marked upon the mountain sides the height to -which the ancient ice extended. The ice-ground part of the mountains -is clearly distinguished from the splintered crests which in those -distant days rose above the surface of the glacier, and which must -have then appeared as island peaks and crests in the midst of an -ocean of ice. - -366. We now scamper down the Siedelhorn, get once more into the -valley of Hasli, along which we follow for more than twenty miles -the traces of the ice. Fluted precipices, polished slabs, and -beautifully-rounded granite domes. Right and left upon the mountain -flanks, at great elevations, the evidences appear. We follow the -footsteps of the glacier to the Lake of Brientz; and if we prolonged -our enquiries, we should learn that all the lake beds of this region, -at the time now referred to, bore the burden of immense masses of ice. - -367. Instead of the vale of Hasli, we might take the valley of the -Rhone. The traces of a mighty glacier, which formerly filled it, -may be followed all the way to Martigny, which is 60 miles distant -from the present ice. At Martigny the Rhone glacier was reinforced -by another from Mont Blanc, and the welded masses moved onward, -planing the mountains right and left, to the Lake of Geneva, the -basin of which they entirely filled. Other evidences prove that the -glacier did not end here, but pushed across the low country until it -encountered the limestone barrier of the Jura Mountains. - - -§ 54. _Erratic Blocks._ - -368. What are these other evidences? We have seen mighty rocks poised -on the moraines of the Mer de Glace, and we now know that, unless -they are split and shattered by the frost, these rocks will, at -some distant day, be landed bodily by the Glacier des Bois in the -valley of Chamouni. You have already learned that these boulders -often reveal the mineralogical nature of the mountains among which -the glacier has passed; that specimens are thus brought down of a -character totally different from the rocks among which they are -finally landed; this is strikingly the case with the _erratic blocks_ -stranded along the Jura. - -369. For the Jura itself, as already stated, is limestone; there is -no trace of native granite to be found amongst these hills. Still -along the breast of the mountain above the town of Neufchâtel, and at -about 800 feet above the lake of Neufchâtel, we find stranded a belt -of granite boulders from Mont Blanc. And when we clear the soil away -from the adjacent mountain side, we find upon the limestone rocks the -scarrings of the ancient glacier which brought the boulders here. - -370. The most famous of these rocks, called the Pierre à Bôt, -measures 50 feet in length, 40 in height, and 20 in width. -Multiplying these three numbers together, we obtain 40,000 cubic feet -as the volume of the boulder. - -371. But this is small compared with some of the rocks which -constitute the freight of even recent glaciers. Let us visit another -of them. We have already been to Stalden, where the valley divides -into two branches, the right branch running to St. Nicholas and -Zermatt, and the left one to Saas and the Monte Moro. Three hours -above Saas we come upon the end of the Allelein glacier, not filling -the main valley, but thrown athwart it so as to stop its drainage -like a dam. Above this ice-dam we have the Mattmark Lake, and at the -head of the lake a small inn well known to travellers over the Monte -Moro. - -372. Close to this inn is the greatest boulder that we have ever -seen. It measures 240,000 cubic feet. Looking across the valley we -notice a glacier with its present end half a mile from the boulder. -The stone, I believe, is serpentine, and were you and I to explore -the Schwartzberg glacier to its upper fastnesses, we should find -among them the birthplace of this gigantic stone. Four-and-twenty -years ago, when the glacier reached the place now occupied by the -boulder, it landed there its mighty freight, and then retreated. -There is a second ice-borne rock at hand which would be considered -vast were it not dwarfed by the aspect of its huger neighbour. - -373. Evidence of this kind might be multiplied to any extent. In -fact, at this moment, distinguished men, like Professor Favre of -Geneva, are determining from the distribution of the erratic blocks -the extent of the ancient glaciers of Switzerland. It was, however, -an engineer named Venetz that first brought these evidences to light, -and announced to an incredulous world the vast extension of the -ancient ice. M. Agassiz afterwards developed and wonderfully expanded -the discovery. Perhaps the most interesting observation regarding -ancient glaciers is that of Dr. Hooker, who, during a recent visit -to Palestine, found the celebrated Cedars of Lebanon growing upon -ancient moraines. - - -§ 55. _Ancient Glaciers of England, Ireland, Scotland, and Wales._ - -374. At the time the ice attained this extraordinary development in -the Alps, many other portions of Europe, where no glaciers now exist, -were covered with them. In the Highlands of Scotland, among the -mountains of England, Ireland, and Wales, the ancient glaciers have -written their story as plainly as in the Alps themselves. I should -like to wander with you through Borrodale in Cumberland, or through -the valleys near Bethgellert in Wales. Under all the beauty of the -present scenery we should discover the memorials of a time when the -whole region was locked in the embrace of ice. Professor Ramsay is -especially distinguished by his writings on the ancient glaciers of -Wales. - -375. We have made the acquaintance of the Reeks of Magillicuddy as -the great condensers of Atlantic vapour. At the time now referred to, -this moisture did not fall as soft and fructifying rain, but as snow, -which formed the nutriment of great glaciers. A chain of lakes now -constitutes the chief attraction of Killarney, the Lower, the Middle, -and the Upper Lake. Let us suppose ourselves rowing towards the head -of the Upper Lake with the Purple Mountain to our left. Remembering -our travels in the Alps, you would infallibly call my attention to -the planing of the rocks, and declare the action to be unmistakably -that of glaciers. With our attention thus sharpened, we land at -the head of the lake, and walk up the Black Valley to the base of -Magillicuddy's Reeks. Your conclusion would be, that this valley -tells a tale as wonderful as that of Hasli. - -376. We reach our boat and row homewards along the Upper Lake. Its -islands now possess a new interest for us. Some of them are bare, -others are covered wholly or in part with luxuriant vegetation; but -both the naked and clothed islands are glaciated. The weathering of -ages has not altered their forms: there are the Cannon Rock, the -Giant's Coffin, the Man of War, all sculptured as if the chisel had -passed over them in our own lifetime. These lakes, now fringed with -tender woodland beauty, were all occupied by the ancient ice. It has -disappeared, and seeds from other regions have been wafted thither -to sow the trees, the shrubs, the ferns, and the grasses which now -beautify Killarney. Man himself, they say, has made his appearance in -the world since that time of ice; but of the real period and manner -of man's introduction little is professed to be known since, to make -them square with science, new meanings have been found for the -beautiful myths and stories of the Bible. - -377. It is the nature and tendency of the human mind to look backward -and forward; to endeavour to restore the past and predict the future. -Thus endowed, from data patiently and painfully won, we recover in -idea a state of things which existed thousands, it may be millions, -of years before the history of the human race began. - - -§ 56. _The Glacial Epoch._ - -378. This period of ice-extension has been named the _Glacial Epoch_. -In accounting for it great minds have fallen into grave errors, as we -shall presently see. - -379. The substance on which we have thus far been working exists in -three different states: as a solid in ice; as a liquid in water; as -a gas in vapour. To cause it to pass from one of these states to the -next following one, _heat_ is necessary. - -380. Dig a hole in the ice of the Mer de Glace in summer, and place -a thermometer in the hole; it will stand at 32° Fahr. Dip your -thermometer into one of the glacier streams; it will still mark 32°. -_The water is therefore as cold as ice._ - -381. Hence the whole of the heat poured by the sun upon the glacier, -and which has been absorbed by the glacier, is expended in simply -liquefying the ice, and not in rendering either ice or water a single -degree warmer. - -382. Expose water to a fire; it becomes hotter for a time. It boils, -and from that moment it ceases to get hotter. After it has begun to -boil, all the heat communicated by the fire is carried away by the -steam, _though the steam itself is not the least fraction of a degree -hotter than the water_. - -383. In fact, simply to liquefy ice a large quantity of heat -is necessary, and to vaporize water a still larger quantity is -necessary. And inasmuch as this heat does not render the water warmer -than the ice, nor the steam warmer than the water, it was at one time -supposed to be _hidden_ in the water and in the steam. And it was -therefore called _latent_ heat. - -384. Let us ask how much heat must the sun expend in order to convert -a pound weight of the tropical ocean into vapour? This problem has -been accurately solved by experiment. It would require in round -numbers 1,000 times the amount of heat necessary to raise one pound -of water one degree in temperature. - -385. But the quantity of heat which would raise the temperature of a -pound of water one degree would raise the temperature of a pound of -iron _ten_ degrees. This has been also proved by experiment. Hence -to convert one pound of the tropical ocean into vapour the sun must -expend 10,000 times as much heat as would raise one pound of iron one -degree in temperature. - -386. This quantity of heat would raise the temperature of 5 lbs. of -iron 2,000 degrees, which is the fusing point of cast iron; at this -temperature the metal would not only be _white hot_, but would be -passing into the molten condition. - -387. Consider the conclusions at which we have now arrived. For every -pound of tropical vapour, or for every pound of Alpine ice produced -by the congelation of that vapour, an amount of heat has been -expended by the sun sufficient to raise 5 lbs. of cast iron to its -melting-point. - -388. It would not be difficult to calculate approximately the weight -of the Mer de Glace and its tributaries--to say, for example, that -they contained so many millions of millions of tons of ice and snow. -Let the place of the ice be taken by a mass of white-hot iron of -quintuple the weight; with such a picture before your mind you get -some notion of the enormous amount of heat paid out by the sun to -produce the present glacier. - -389. You must think over this, until it is as clear as sunshine. -For you must never henceforth fall into the error already referred -to, and which has entangled so many. So natural was the association -of ice and cold, that even celebrated men assumed that all that is -needed to produce a great extension of our glaciers is a diminution -of the sun's temperature. Had they gone through the foregoing -reflections and calculations, they would probably have demanded more -heat instead of less for the production of a "glacial epoch." What -they really needed were _condensers_ sufficiently powerful to congeal -the vapour generated by the heat of the sun. - - -§ 57. _Glacier Theories._ - -390. You have not forgotten, and hardly ever can forget, our climbs -to the Cleft Station. Thoughts were then suggested which we have not -yet discussed. We saw the branch glaciers coming down from their -névés, welding themselves together, pushing through Trélaporte, and -afterwards moving through the sinuous valley of the Mer de Glace. -These appearances alone, without taking into account subsequent -observations, were sufficient to suggest the idea that glacier -ice, however hard and brittle it may appear, is really a viscous -substance, resembling treacle, or honey, or tar, or lava. - - -§ 58. _Dilatation and Sliding Theories._ - -391. Still this was not the notion expressed by the majority of -writers upon glaciers. Scheuchzer of Zürich, a great naturalist, -visited the glaciers in 1705, and propounded a theory of their -motion. Water, he knew, expands in freezing, and the force of -expansion is so great, that thick bombshells filled with water, and -permitted to freeze, are, as we know (312), shattered to pieces by -the ice within. Scheuchzer supposed that the water in the fissures of -the glaciers, freezing there and expanding with resistless force, was -the power which urged the glacier downwards. He added to this theory -other notions of a less scientific kind. - -392. Many years subsequently, De Charpentier of Bex renewed and -developed this theory with such ability and completeness, that it was -long known as Charpentier's Theory of Dilatation. M. Agassiz for a -time espoused this theory, and it was also more or less distinctly -held by other writers. The glacier, in fact, was considered to be a -magazine of cold, capable of freezing all water percolating through -it. The theory was abandoned when this notion of glacier cold was -proved by M. Agassiz to be untenable. - -393. In 1760, Altmann and Grüner propounded the view that glaciers -moved by sliding over their beds. Nearly forty years subsequently, -this notion was revived by De Saussure, and it has therefore been -called "De Saussure's Theory," or the "Sliding Theory" of glacier -motion. - -394. There was, however, but little reason to connect the name of -De Saussure with this or any other theory of glaciers. Incessantly -occupied in observations of another kind, this celebrated man devoted -very little time or thought to the question of glacier motion. What -he has written upon the subject reads less like the elaboration of a -theory than the expression of an opinion. - - -§ 59. _Plastic Theory._ - -395. By none of these writers is the property of viscosity or -plasticity ascribed to glacier ice; the appearances of many glaciers -are, however, so suggestive of this idea that we may be sure it -would have found more frequent expression, were it not in such -apparent contradiction with our every-day experience of ice. - -396. Still the idea found its advocates. In a little book, published -in 1773, and entitled "Picturesque Journey to the Glaciers of Savoy," -Bordier of Geneva wrote thus:--"It is now time to look at all these -objects with the eyes of reason; to study, in the first place, the -position and the progression of glaciers, and to seek the solution of -their principal phenomena. At the first aspect of the ice-mountains -an observation presents itself, which appears sufficient to explain -all. It is that the entire mass of ice is connected together, and -presses from above downwards after the manner of fluids. Let us then -regard the ice, not as a mass entirely rigid and immobile, but as a -heap of coagulated matter, or as softened wax, flexible and ductile -to a certain point."[F] Here probably for the first time the quality -of plasticity is ascribed to the ice of glaciers. - -[F] I am indebted to my distinguished friend Prof. Studer of Berne -for directing my attention to Bordier's book, and to my friends at -the British Museum for the great trouble they have taken to find it -for me. - -397. To us, familiar with the aspect of the glaciers, it must seem -strange that this idea once expressed did not at once receive -recognition and development. But in those early days explorers were -few, and the "Picturesque Journey" probably but little known, so -that the notion of plasticity lay dormant for more than half a -century. But Bordier was at length succeeded by a man of far greater -scientific grasp and insight than himself. This was Rendu, a Catholic -priest and canon when he wrote, and afterwards Bishop of Annecy. In -1841 Rendu laid before the Royal Academy of Sciences of Savoy his -"Theory of the Glaciers of Savoy," a contribution for ever memorable -in relation to this subject.[G] - -[G] "Memoirs of the Academy," vol. x. - -398. Rendu seized the idea of glacier plasticity with great power and -clearness, and followed it resolutely to its consequences. It is not -known that he had ever seen the work of Bordier; probably not, as he -never mentions it. Let me quote for you some of Rendu's expressions, -which, however, fail to give an adequate idea of his insight and -precision of thought:--"Between the Mer de Glace and a river there -is a resemblance so complete that it is impossible to find in the -glacier a circumstance which does not exist in the river. In currents -of water the motion is not uniform either throughout their width or -throughout their depth. The friction of the bottom and of the sides, -with the action of local hindrances, causes the motion to vary, and -only towards the middle of the surface do we obtain the full motion." - -399. This reads like a prediction of what has since been established -by measurement. Looking at the glacier of Mont Dolent, which -resembles a sheaf in form, wide at both ends and narrow in the -middle, and reflecting that the upper wide part had become narrow, -and the narrow middle part again wide, Rendu observes, "There is a -multitude of facts which seem to necessitate the belief that glacier -ice enjoys a kind of ductility which enables it to mould itself to -its locality, to thin out, to swell, and to contract as if it were a -soft paste." - -400. To fully test his conclusions, Rendu required the accurate -measurement of glacier motion. Had he added to his other endowments -the practical skill of a land-surveyor, he would now be regarded as -the prince of glacialists. As it was he was obliged to be content -with imperfect measurements. In one of his excursions he examined the -guides regarding the successive positions of a vast rock which he -found upon the ice close to the side of the glacier. The mean of five -years gave him a motion for this block of 40 feet a year. - -401. Another block, the transport of which he subsequently measured -more accurately, gave him a velocity of 400 feet a year. Note his -explanation of this discrepancy:--"The enormous difference of these -two observations arises from the fact that one block stood near the -centre of the glacier, which moves most rapidly, while the other -stood near the side, where the ice is held back by friction." So -clear and definite were Rendu's ideas of the plastic motion of -glaciers, that had the question of curvature occurred to him, I -entertain no doubt that he would have enunciated beforehand the -shifting of the point of maximum motion from side to side across the -axis of the glacier (§ 25). - -402. It is right that you should know that scientific men do not -always agree in their estimates of the comparative value of facts -and ideas; and it is especially right that you should know that your -present tutor attaches a very high value to ideas when they spring -from the profound and persistent pondering of superior minds, and -are not, as is too often the case, thrown out without the warrant -of either deep thought or natural capacity. It is because I believe -Rendu's labours fulfil this condition, that I ascribe to them so -high a value. But when you become older and better informed, you may -differ from me; and I write these words lest you should too readily -accept my opinion of Rendu. Judge me, if you care to do so, when your -knowledge is matured. I certainly shall not fear your verdict. - -403. But, much as I prize the prompting idea, and thoroughly as -I believe that often in it the force of genius mainly lies, it -would, in my opinion, be an error of omission of the gravest kind, -and which, if habitual, would ensure the ultimate decay of natural -knowledge, to neglect verifying our ideas, and giving them outward -reality and substance when the means of doing so are at hand. In -science thought, as far as possible, ought to be wedded to fact. -This was attempted by Rendu, and in part accomplished by Agassiz and -Forbes. - - -§ 60. _Viscous Theory._ - -404. Here indeed the merits of the distinguished glacialist last -named rise conspicuously to view. From the able and earnest advocacy -of Professor Forbes, the public knowledge of this doctrine of glacial -plasticity is almost wholly derived. He gave the doctrine a more -distinctive form; he first applied the term viscous to glacier ice, -and sought to found upon precise measurements a "Viscous Theory" of -glacier motion. - -405. I am here obliged to state facts in their historic sequence. -Professor Forbes when he began his investigations was acquainted with -the labours of Rendu. In his earliest work upon the Alps he refers -to those labours in terms of flattering recognition. But though as -a matter of fact Rendu's ideas were there to prompt him, it would -be too much to say that he needed their inspiration. Had Rendu -not preceded him, he might none the less have grasped the idea of -viscosity, executing his measurements and applying his knowledge to -maintain it. Be that as it may, the appearance of Professor Forbes on -the Unteraar glacier in 1841, and on the Mer de Glace in 1842, and -his labours then and subsequently, have given him a name not to be -forgotten in the scientific history of glaciers. - -406. The theory advocated by Professor Forbes was enunciated by -himself in these words:--"A glacier is an imperfect fluid, or viscous -body, which is urged down slopes of certain inclination by the -natural pressure of its parts." In 1773 Bordier wrote thus:--"As the -glaciers always advance upon the plain, and never disappear, it is -absolutely essential that new ice shall perpetually take the place -of that which is melted: it must therefore be pressed forward from -above. One can hardly refuse then to accept the astonishing truth, -that this vast extent of hard and solid ice moves as a single piece -downwards." In the passage already quoted he speaks of the ice being -pressed as a fluid from above. These constitute, I believe, Bordier's -contributions to this subject. The quotations show his sagacity at an -early date; but, in point of completeness, his views are not to be -compared with those of Rendu and Forbes. - -407. I must not omit to state here that though the idea of viscosity -has not been espoused by M. Agassiz, his measurements, and maps of -measurements, on the Unteraar glacier have been recently cited as the -most clear and conclusive illustrations of a quality which, at all -events, closely resembles viscosity. - -408. But why, with proofs before him more copious and characteristic -than those of any other observer, does M. Agassiz hesitate to accept -the idea of viscosity as applied to ice? Doubtless because he -believes the notion to be contradicted by our every-day experience of -the substance. - -409. Take a mass of ice ten or even fifteen cubic feet in volume; -draw a saw across it to a depth of half an inch or an inch; and -strike a pointed pricker, not thicker than a very small round file, -into the groove; the substance will split from top to bottom with a -clean crystalline fracture. How is this brittleness to be reconciled -with the notion of viscosity? - -410. We have, moreover, been upon the glacier and have witnessed the -birth of crevasses. We have seen them beginning as narrow cracks -suddenly formed, days being required to open them a single inch. In -many glaciers fissures may be traced narrow and profound for hundreds -of yards through the ice. What does this prove? Did the ice possess -even a very small modicum of that power of stretching, which is -characteristic of a viscous substance, such crevasses could not be -formed. - -411. Still it is undoubted that the glacier moves like a viscous -body. The centre flows past the sides, the top flows over the -bottom, and the motion through a curved valley corresponds to fluid -motion. Mr. Mathews, Mr. Froude, and above all Signor Bianconi, -have, moreover, recently made experiments on ice which strikingly -illustrate the flexibility of the substance. These experiments merit, -and will doubtless receive, full attention at a future time. - - -§ 61. _Regelation Theory._ - -412. I will now describe to you an attempt that has been made of -late years to reconcile the brittleness of ice with, its motion in -glaciers. It is founded on the observation, made by Mr. Faraday in -1850, that when two pieces of thawing ice are placed together they -freeze together at the place of contact. - -413. This fact may not surprise you; still it surprised Mr. Faraday -and others, and men of very great distinction in science have -differed in their interpretation of the fact. The difficulty is to -explain where, or how, in ice already thawing the cold is to be -found requisite to freeze the film of water between the two touching -surfaces. - -414. The word _Regelation_ was proposed by Dr. Hooker to express the -freezing together of two pieces of thawing ice observed by Faraday; -and the memoir in which the term was first used was published by Mr. -Huxley and Mr. Tyndall in the Philosophical Transactions for 1857. - -415. The _fact_ of regelation, and its application irrespective of -the _cause_ of regelation, may be thus illustrated:--Saw two slabs -from a block of ice, and bring their flat surfaces into contact; they -immediately freeze together. Two plates of ice, laid one upon the -other, with flannel round them overnight, are sometimes so firmly -frozen in the morning that they will rather break elsewhere than -along their surface of junction. If you enter one of the dripping -ice-caves of Switzerland, you have only to press for a moment a slab -of ice against the roof of the cave to cause it to freeze there and -stick to the roof. - -416. Place a number of fragments of ice in a basin of water, and -cause them to touch each other; they freeze together where they -touch. You can form a chain of such fragments; and then, by taking -hold of one end of the chain, you can draw the whole series after it. -Chains of icebergs are sometimes formed in this way in the Arctic -seas. - -417. Consider what follows from these observations. Snow consists of -small particles of ice. Now if by pressure we squeeze out the air -entangled in thawing snow, and bring the little ice-granules into -close contact, they may be expected to freeze together; and if the -expulsion of the air be complete, the squeezed snow may be expected -to assume the appearance of compact ice. - -418. We arrive at this conclusion by reasoning; let us now test it by -experiment, employing a suitable hydraulic press, and a mould to hold -the snow. In exact accordance with our expectation, we convert by -pressure the snow into ice.[H] - -[H] A similar experiment was made by the Messrs. Schlagintweit prior -to the discovery which explains it, and which therefore remained -unsolved. - -419. Place a compact mass of ice in a proper mould, and subject -it to pressure. It breaks in pieces: squeeze the pieces forcibly -together; they re-unite by regelation, and a compact piece of ice, -totally different in shape from the first one, is taken from the -press. To produce this effect the ice must be in a thawing condition. -When its temperature is much below the melting point it is crushed -by pressure, not into a pellucid mass of another shape, but into a -white powder. - -420. By means of suitable moulds you may in this way change the shape -of ice to any extent, turning out spheres, and cups, and rings, and -twisted ropes of the substance; the change of form in these cases -being effected through rude fracture and regelation. - -421. By applying the pressure carefully, rude fracture may be -avoided, and the ice compelled slowly to change its form as if it -were a plastic body. - -422. Now our first experiment illustrates the consolidation of the -snows of the higher Alpine regions. The deeper layers of the névé -have to bear the weight of all above them, and are thereby converted -into more or less perfect ice. And our last experiment illustrates -the changes of form observed upon the glacier, where, by the slow and -constant application of pressure, the ice gradually moulds itself to -the valley, which it fills. - -423. In glaciers, however, we have also ample illustrations of -rude fracture and regelation. The opening and closing of crevasses -illustrate this. The glacier is broken on the cascades and mended -at their bases. When two branch glaciers lay their sides together, -the regelation is so firm that they begin immediately to flow in -the trunk glacier as a single stream. The medial moraine gives no -indication by its slowness of motion that it is derived from the -sluggish ice of the sides of the branch glaciers. - -424. The gist of the Regelation Theory is that the ice of glaciers -changes its form and preserves its continuity under _pressure_ which -keeps its particles together. But when subjected to _tension_, sooner -than stretch it _breaks_, and behaves no longer as a viscous body. - - -§ 62. _Cause of Regelation._ - -425. Here the fact of regelation is applied to explain the plasticity -of glacier ice, no attempt being made to assign the cause of -regelation itself. They are two entirely distinct questions. But a -little time will be well spent in looking more closely into the cause -of regelation. You may feel some surprise that eminent men should -devote their attention to so small a point, but we must not forget -that in nature nothing is small. Laws and principles interest the -scientific student most, and these may be as well illustrated by -small things as by large ones. - -426. The question of regelation immediately connects itself with that -of "latent heat," already referred to (383), but which we must now -subject to further examination. To melt ice, as already stated, a -large amount of heat is necessary, and in the case of the glaciers -this heat is furnished by the sun. Neither the ice so melted nor -the water which results from its liquefaction can fall below 32° -Fahrenheit. The freezing point of water and the melting point of ice -touch each other, as it were, at this temperature. A hair's-breadth -lower water freezes; a hair's-breadth higher ice melts. - -427. But if the ice could be caused to melt without this supply of -solar heat, a temperature lower than that of ordinary thawing ice -would result. When snow and salt, or pounded ice and salt, are mixed -together, the salt causes the ice to melt, and in this way a cold of -20 or 30 degrees below the freezing point may be produced. Here, in -fact, the ice consumes _its own warmth_ in the work of liquefaction. -Such a mixture of ice and salt is called "a freezing mixture." - -428. And if by any other means ice at the temperature of 32° -Fahrenheit could be liquefied without access of heat from without, -the water produced would be colder than the ice. Now Professor James -Thomson has proved that ice may be liquefied by mere _pressure_, and -his brother, Sir William Thomson, has also shown that water under -pressure requires a lower temperature to freeze it than when the -pressure is removed. Professor Mousson subsequently liquefied large -masses of ice by a hydraulic press; and by a beautiful experiment -Professor Helmholtz has proved that water in a vessel from which -the air has been removed, and which is therefore relieved from the -pressure of the atmosphere, freezes and forms ice-crystals when -surrounded by melting ice. All these facts are summed up in the -brief statement _that the freezing point of water is lowered by -pressure_.[I] - -[I] Professor James Thomson and Professor Clausius proved this -independently and almost contemporaneously. - -429. For our own instruction we may produce the liquefaction of ice -by pressure in the following way:--You remember the beautiful flowers -obtained when a sunbeam is sent through lake ice (§ 11), and you have -not forgotten that the flowers always form parallel to the surface of -freezing. Let us cut a prism, or small column of ice with the planes -of freezing running across it at right angles; we place that prism -between two slabs of wood, and bring carefully to bear upon it the -squeezing force of a small hydraulic press. - -430. It is well to converge by means of a concave mirror a good light -upon the ice, and to view it through a magnifying lens. You already -see the result. Hazy surfaces are formed in the very body of the -ice, which gradually expand as the pressure is slowly augmented. -Here and there you notice something resembling crystallisation; -fern-shaped figures run with considerable rapidity through the ice, -and when you look carefully at their points and edges you find them -in visible motion. These hazy surfaces are spaces of liquefaction, -and the motion you see is that of the ice falling to water under the -pressure. That water is colder than the ice was before the pressure -was applied, and if the pressure be relieved, not only does the -liquefaction cease, but the water re-freezes. The cold produced by -its liquefaction under pressure is sufficient to re-congeal it when -the pressure is removed. - -431. If instead of diffusing the pressure over surfaces of -considerable extent, we concentrate it on a small surface, the -liquefaction will of course be more rapid, and this is what Mr. -Bottomley has recently done in an experiment of singular beauty and -interest. Let us support on blocks of wood the two ends of a bar of -ice 10 inches long, 4 inches deep, and 3 wide, and let us loop over -its middle a copper wire one-twentieth, or even one-tenth, of an -inch in thickness. Connecting the two ends of the wire together, and -suspending from it a weight of 12 or 14 pounds, the whole pressure -of this weight is concentrated on the ice which supports the wire. -What is the consequence? The ice underneath the wire liquefies; the -water of liquefaction escapes round the wire, but the moment it is -relieved from the pressure it freezes, and round about the wire, even -before it has entered the ice, you have a frozen casing. The wire -continues to sink in the ice; the water incessantly escapes, freezing -as it does so behind the wire. In half an hour the weight falls; the -wire has gone clean through the ice. You can plainly see where it -has passed, but the two severed pieces of ice are so firmly frozen -together that they will break elsewhere as soon as along the surface -of regelation. - -432. Another beautiful experiment bearing upon this point has -recently been made by M. Boussingault. He filled a hollow steel -cylinder with water and chilled it. In passing to ice, water, as you -know, expands (§ 45); in fact, room for expansion is a necessary -condition of solidification. But in the present case the strong -steel resisted the expansion, the water in consequence remaining -liquid at a temperature of more than 30° Fahr. below the ordinary -freezing point. A bullet within the cylinder rattled about at this -temperature, showing that the water was still liquid. On opening the -tap the liquid, relieved of the pressure, was instantly converted -into ice. - -433. It is only substances which _expand_ on solidifying that behave -in this manner. The metal bismuth, as we know, is an example similar -to water; while lead, wax, or sulphur, all of which contract on -solidifying, have their point of fusion _heightened_ by pressure. - -434. And now you are prepared to understand Professor James Thomson's -theory of regelation. When two pieces of ice are pressed together -liquefaction, he contends, results. The water spreads out around the -points of pressure, and when released re-freezes, thus forming a kind -of cement between the pieces of ice. - - -§ 63. _Faraday's View of Regelation._ - -435. Faraday's view of regelation is not so easily expressed, still -I will try to give you some notion of it, dealing in the first place -with admitted facts. Water, even in open vessels, may be lowered many -degrees below its freezing temperature, and still remain liquid; -it may also be raised to a temperature far higher than its boiling -point, and still resist boiling. This is due to the mutual cohesion -of the water particles, which resists the change of the liquid -either into the solid or the vaporous condition. - -436. But if into the over-chilled water you throw a particle of ice, -the cohesion is ruptured, and congelation immediately sets in. And if -into the superheated water you introduce a bubble of air or of steam, -cohesion is likewise ruptured, and ebullition immediately commences. - -437. Faraday concluded that _in the interior_ of any body, whether -solid or liquid, where every particle is grasped so to speak by the -surrounding particles, and grasps them in turn, the bond of cohesion -is so strong as to require a higher temperature to change the state -of aggregation than is necessary _at the surface_. At the surface -of a piece of ice, for example, the molecules are free on one side -from the control of other molecules; and they therefore yield to -heat more readily than in the interior. The bubble of air or steam -in overheated water also frees the molecules on one side; hence the -ebullition consequent upon its introduction. Practically speaking, -then, the point of liquefaction of the interior ice is higher than -that of the superficial ice. Faraday also refers to the special -solidifying power which bodies exert upon their own molecules. -Camphor in a glass bottle fills the bottle with an atmosphere of -camphor. In such an atmosphere large crystals of the substance may -grow by the incessant deposition of camphor molecules upon camphor, -at a temperature too high to permit of the slightest deposit _upon -the adjacent glass_. A similar remark applies to sulphur, phosphorus, -and the metals in a state of fusion. They are deposited upon solid -portions of their own substance at temperatures not low enough to -cause them to solidify against other substances. - -438. Water furnishes an eminent example of this special solidifying -power. It may be cooled ten degrees and more below its freezing -point without freezing. But this is not possible if the smallest -fragment of ice be floating in the water. It then freezes accurately -at 32° Fahr., depositing itself, however, not upon the sides of the -containing vessel, but _upon the ice_. Faraday observed in a freezing -apparatus thin crystals of ice growing in ice-cold water to a length -of six, eight, or ten inches, at a temperature incompetent to produce -their deposition upon the sides of the containing vessel. - -439. And now we are prepared for Faraday's view of regelation. -When the surfaces of two pieces of ice, covered with a film of -the water of liquefaction, are brought together, the covering -film is transferred from the surface to the centre of the ice, -where the point of liquefaction, as before shown, is higher than -at the surface. The special solidifying power of ice upon water -is now brought into play _on both sides of the film_. Under these -circumstances, Faraday held that the film would congeal, and freeze -the two surfaces together. - -440. The lowering of the freezing point by pressure amounts to -no more than one-seventieth of a degree Fahrenheit for a whole -atmosphere. Considering the infinitesimal fraction of this pressure -which is brought into play in some cases of regelation, Faraday -thought its effect insensible. He suspended pieces of ice, and -brought them into contact without sensible pressure, still they -froze together. Professor James Thomson, however, considered that -even the capillary attraction exerted between two such masses would -be sufficient to produce regelation. You may make the following -experiments, in further illustration of this subject:-- - -441. Place a small piece of ice in water, and press it underneath the -surface by a second piece. The submerged piece may be so small as to -render the pressure infinitesimal; still it will freeze to the under -surface of the superior piece. - -442. Place two pieces of ice in a basin of warm water, and allow them -to come together; they freeze together when they touch. The parts -surrounding the place of contact melt away, but the pieces continue -for a time united by a narrow bridge of ice. The bridge finally -melts, and the pieces for a moment are separated. But capillary -attraction immediately draws them together, and regelation sets in -once more. A new bridge is formed, which in its turn is dissolved, -the separated pieces again closing up. A kind of pulsation is thus -established between the two pieces of ice. They touch, they freeze, -a bridge is formed and melted; and thus the rhythmic action continues -until the ice disappears. - -443. According to Professor James Thomson's theory, pressure is -necessary to liquefy the ice. The heat necessary for liquefaction -must be drawn from the ice itself, and the cold water must escape -from the pressure to be re-frozen. Now in the foregoing experiments -the cold water, instead of being allowed to freeze, _issues into -the warm water_, still the floating fragments regelate in a moment. -The touching surfaces may, moreover, be convex; they may be reduced -practically to _points_, clasped all round by the warm water, which -indeed rapidly dissolve them as they approach each other; still they -freeze immediately when they touch. - -444. You may learn from this discussion that in scientific matters, -as in all others, there is room for differences of opinion. The frame -of mind to be cultivated here is a suspension of judgment as long -as the meaning remains in doubt. It may be that Faraday's action -and Thomson's action come both into play. I cannot do better than -finish these remarks by quoting Faraday's own concluding words, -which show how in his mind scientific conviction dwelt apart from -dogmatism:--"No doubt," he says, "nice experiments will enable us -hereafter to criticise such results as these, and separating the true -from the untrue will establish the correct theory of regelation." - - -§ 64. _The Blue Veins of Glaciers._ - -445. We now approach the end, one important question only remaining -to be discussed. Hitherto we have kept it back, for a wide -acquaintance with the glaciers was necessary to its solution. We had -also to make ourselves familiar by actual experiment with the power -of ice, softened by thaw, to yield to pressure, and to liquefy under -such pressure. - -446. Snow is white. But if you examine its individual particles you -would call them _transparent_, not white. The whiteness arises from -the mixture of the ice particles with small spaces of air. In the -case of all transparent bodies whiteness results from such a mixture. -The clearest glass or crystal when crushed becomes a white powder. -The foam of champagne is white through the intimate admixture of a -transparent liquid with transparent carbonic acid gas. The whitest -paper, moreover, is composed of fibres which are individually -transparent. - -447. It is not, however, the air or the gas, but the _optical -severance_ of the particles, giving rise to a multitude of reflexions -of the white solar light at their surfaces, that produces the -whiteness. - -448. The whiteness of the surface of a clean glacier (112), and of -the icebergs of the Märgelin See (357), has been already referred to -a similar cause. The surface is broken into innumerable fissures by -the solar heat, the reflexion of solar light from the sides of the -little fissures producing the observed appearance. - -449. In like manner if you freeze water in a test-tube by plunging it -into a freezing mixture, the ice produced is white. For the most part -also the ice formed in freezing machines is white. Examine such, ice, -and you will find it filled with small air-bubbles. When the freezing -is extremely slow the crystallising force pushes the air effectually -aside, and the resulting ice is transparent; when the freezing is -rapid, the air is entangled before it can escape, and the ice is -_translucent_. But even in the case of quick freezing Mr. Faraday -obtained transparent ice by skilfully removing the air-bubbles as -fast as they appeared with a feather. - -450. In the case of lake ice the freezing is not uniform, but -intermittent. It is sometimes slow, sometimes rapid. When slow the -air dissolved in the water is effectually squeezed out and forms a -layer of bubbles on the under-surface of the ice. An act of sudden -freezing entangles the air, and hence we find lake ice usually -composed of layers alternately clear, and filled with bubbles. Such -layers render it easy to detect the planes of freezing in lake ice. - -451. And now for the bearing of these facts. Under the fall of the -Géant, at the base of the Talèfre cascade, and lower down the Mer -de Glace; in the higher regions of the Grindelwald, the Aar, the -Aletsch and the Görner glaciers, the ice does not possess the -transparency which it exhibits near the ends of the glaciers. It -is white, or whitish. Why? Examination shows it to be filled with -small air-bubbles; and these, as we now learn, are the cause of its -whiteness. - -452. They are the residue of the air originally entangled in the -snow, and connected, as before stated, with the whiteness of the -snow. During the descent of the glacier, the bubbles are gradually -expelled by the enormous pressures brought to bear upon the ice. -Not only is the expulsion caused by the mechanical yielding of the -soft thawing ice, but the liquefaction of the substance at places of -violent pressure, opening, as it does, fissures for the escape of the -air, must play an important part in the consolidation of the glacier. - -453. The expulsion of the bubbles is, however, not uniform; for -neither ice nor any other substance offers an absolutely uniform -resistance to pressure. At the base of every cascade that we have -visited, and on the walls of the crevasses there formed, we have -noticed innumerable blue streaks drawn through the white translucent -ice, and giving the whole mass the appearance of lamination. These -blue veins turned out upon examination to be spaces from which the -air-bubbles had been almost wholly expelled, translucency being thus -converted into transparency. - -454. This is the _veined_ or _ribboned structure_ of glaciers, -regarding the origin of which diverse opinions are now entertained. - -455. It is now our duty to take up the problem, and to solve it if we -can. On the névés of the Col du Géant, and other glaciers, we have -found great cracks, and faults, and _Bergschrunds_, exposing deep -sections of the névé; and on these sections we have found marked the -edges of half-consolidated strata evidently produced by successive -falls of snow. The névé is stratified because its supply of material -from the atmosphere is intermittent, and when we first observed -the blue veins we were disposed to regard them as due to this -stratification. - -456. But observation and reflexion soon dispelled this notion. Indeed -it could hardly stand in the presence of the single fact that at the -bases of the ice-falls the veins are always _vertical_, or nearly so. -We saw no way of explaining how the horizontal strata of the névé -could be so tilted up at the base of the fall as to be set on edge. -Nor is the aspect of the veins that of stratification. - -457. On the central portions of the cascades, moreover, there are -no signs of the veins. At the bases they first appear, reaching in -each case their maximum development a little below the base. As you -and I stood upon the heights above the Zäsenberg and scrutinised the -cascade of the Strahleck branch of the Grindelwald glacier, we could -not doubt that the base of the fall was the birthplace of the veins. -We called this portion of the glacier a "Structure Mill," intimating -that here, and not on the névé, the veined structure was manufactured. - -[Illustration: SECTION OF ICEFALL, AND GLACIER BELOW IT, SHOWING -ORIGIN OF VEINED STRUCTURE.] - -458. This, however, is, at bottom, the language of strong _opinion_ -merely, not that of _demonstration_; and in science opinion ought to -content us only so long as positive proof is unattainable. The love -of repose must not prevent us from seeking this proof. There is no -sterner conscience than the scientific conscience, and it demands, -in every possible case, the substitution for private conviction of -demonstration which shall be conclusive to all. - -459. Let us, for example, be shown a case in which the stratification -of the névé is prolonged into the glacier; let us see the planes of -bedding and the planes of lamination existing side by side, and still -indubitably distinct. Such an observation would effectually exclude -stratification from the problem of the veined structure, and through -the removal of this tempting source of error, we should be rendered -more free to pursue the truth. - -460. We sought for this conclusive test upon the Mer de Glace, -but did not find it. We sought it on the Grindelwald, and the Aar -glaciers,[J] with an equal want of success. On the Aletsch glacier, -for the first time, we observed the apparent coexistence of bedding -and structure, the one _cutting_ the other upon the walls of the same -crevasse. Still the case was not sufficiently pronounced to produce -entire conviction, and we visited the Görner glacier with the view of -following up our quest. - -[J] M. Agassiz, however, reports a case of the kind upon the glacier -of the Aar. - -[Illustration: STRUCTURE AND BEDDING ON ALETSCH GLACIER.] - -461. Here day after day added to the conviction that the bedding and -the structure were two different things. Still day after day passed -without revealing to us the final proof. Surely we have not let our -own ease stand in the way of its attainment, and if we retire baffled -we shall do so with the consciousness of having done our best. -Yonder, however, at the base of the Matterhorn, is the Furgge glacier -that we have not yet explored. Upon it our final attempt must be made. - -462. We get upon the glacier near its end, and ascend it. We are soon -fronted by a barrier composed of three successive walls of névé, the -one rising above the other, and each retreating behind the other. The -bottom of each wall is separated from the top of the succeeding one -by a ledge, on which threatening masses of broken névé now rest. We -stand amid blocks and rubbish which have been evidently discharged -from these ledges, on which other masses, ready apparently to tumble, -are now poised. - -463. On the vertical walls of this barrier we see, marked with the -utmost plainness, the horizontal lines of stratification, while -something exceedingly like the veined structure appears to cross the -lines of bedding at nearly a right angle. The vertical surface is, -however weathered, and the lines of structure, if they be such, are -indistinct. The problem now is to remove the surface, and expose the -ice underneath. It is one of the many cases that have come before -us, where the value of an observation is to be balanced against the -danger which it involves. - -464. We do nothing rashly; but scanning the ledges and selecting a -point of attack, we conclude that the danger is not too great to -be incurred. We advance to the wall, remove the surface, and are -rewarded by the discovery underneath it of the true blue veins. They, -moreover, are vertical, while the bedding is horizontal. Bruce, as -you know, was defeated in many a battle, but he persisted and won at -last. Here, upon the Furgge glacier, you also have fought and won -your little Bannockburn. - -[Illustration: STRUCTURE AND BEDDING ON FURGGE GLACIER.] - -465. But let us not use the language of victory too soon. The -stratification theory has been removed out of the field of -explanation, but nothing has as yet been offered in its place. - - -§ 65. _Relation of Structure to Pressure._ - -466. This veined structure was first described by the distinguished -Swiss naturalist, Guyot, now a resident in the United States. From -the Grimsel Pass I have already pointed out to you the Gries glacier -overspreading the mountains at the opposite side of the valley of the -Rhone. It was on this glacier that M. Guyot made his observation. - -467. "I saw," he said, "under my feet the surface of the entire -glacier covered with regular furrows, from one to two inches wide, -hollowed out in a half-snowy mass, and separated by protruding plates -of harder and more transparent ice. It was evident that the glacier -here was composed of two kinds of ice, one that of the furrows, -snowy and more easily melted; the other of the plates, more perfect, -crystalline, glassy, and resistant; and that the unequal resistance -which the two kinds of ice presented to the atmosphere was the cause -of the ridges. - -468. "After having followed them for several hundred yards, I reached -a crevasse twenty or thirty feet wide, which, as it cut the plates -and furrows at right angles, exposed the interior of the glacier to -a depth of thirty or forty feet, and gave a beautiful transverse -section of the structure. As far as my eyes could reach, I saw -the mass of the glacier composed of layers of snowy ice, each two -of which were separated by one of the hard plates of which I have -spoken, the whole forming a regularly laminated mass, which resembled -certain calcareous slates." - -469. I have not failed to point out to you upon all the glaciers -that we have visited the little superficial furrows here described; -and you have, moreover, noticed that in the furrows mainly is lodged -the finer dirt which is scattered over the glacier. They suggest -the passage of a rake over the ice. And whenever these furrows were -interrupted by a crevasse, the veined structure invariably revealed -itself upon the walls of the fissure. The surface grooving is indeed -an infallible indication of the interior lamination of the ice. - -470. We have tracked the structure through the various parts of the -glaciers at which its appearance was most distinct; and we have -paid particular attention to the condition of the ice at these -places. The very fact of its cutting the crevasses at right angles -is significant. We know the mechanical origin of the crevasses; that -they are cracks formed at right angles to lines of tension. But since -the crevasses are also perpendicular to the planes of structure, -these planes must be parallel to the lines of tension. - -471. On the glaciers, however, tension rarely occurs alone. At the -sides of the glacier, for example, where marginal crevasses are -formed, the tension is always accompanied by pressure; the one -force acting at right angles to the other. Here, therefore, the -veined structure, which is parallel to the lines of tension, _is -perpendicular to the lines of pressure_. - -472. That this is so will be evident to you in a moment. Let the -adjacent figure represent the channel of the glacier moving in the -direction of the arrow. Suppose three circles to be marked upon the -ice, one at the centre and the two others at the sides. In a glacier -of uniform inclination all these circles would move downward, the -central one only remaining a circle. By the retardation of the sides -the marginal circles would be drawn out to ovals. The two circles -would be _elongated_ in one direction, and _compressed_ in another. -Across the long diameter, which is the direction of strain, we have -the marginal crevasses; across the short diameter _m n_, which is the -direction of pressure, we have the _marginal veined structure_. - -[Illustration] - -473. This association of pressure and structure is invariable. At -the bases of the cascades, where the inclination of the bed of the -glacier suddenly changes, the pressure in many cases suffices not -only to close the crevasses but to violently squeeze the ice. At -such places the structure always appears, sweeping quite across -the glacier. When two branch glaciers unite, their mutual thrust -intensifies the pre-existing marginal structure of the branches, -and developes new planes of lamination. Under the medial moraines, -therefore, we have usually a good development of the structure. It -is finely displayed, for example, under the great medial moraine of -the glacier of the Aar. - -474. Upon this glacier, indeed, the blue veins were observed -independently three years after M. Guyot had first described them. I -say independently, because M. Guyot's description, though written in -1838, remained imprinted, and was unknown in 1841 to the observers on -the Aar. These were M. Agassiz and Professor Forbes. To the question -of structure Professor Forbes subsequently devoted much attention, -and it was mainly his observations and reasonings that gave it the -important position now assigned to it in the phenomena of glaciers. - -475. Thus without quitting the glaciers themselves, we establish -the connexion between pressure and structure. Is there anything in -our previous scientific experience with which these facts may be -connected? The new knowledge of nature must always strike its roots -into the old, and spring from it as an organic growth. - - -§ 66. _Slate Cleavage and Glacier Lamination_. - -476. M. Guyot threw out an exceedingly sagacious hint, when he -compared the veined structure to the cleavage of slate rocks. We must -learn something of this cleavage, for it really furnishes the key to -the problem which now occupies us. Let us go then to the quarries -of Bangor or Cumberland, and observe the quarrymen in their sheds -splitting the rocks. With a sharp point struck skilfully into the -edge of the slate, they cause it to divide into thin plates, fit for -roofing or ciphering, as the case may be. The surfaces along which -the rock cleaves are called its _planes of cleavage_. - -477. All through the quarry you notice the direction of these planes -to be perfectly constant. How is this laminated structure to be -accounted for? - -478. You might be disposed to consider that cleavage is a case of -stratification or bedding; for it is true that in various parts of -England there are rocks which can be cloven into thin flags along the -planes of bedding. But when we examine these slate rocks we verify -the observation, first I believe made by the eminent and venerable -Professor Sedgwick, that the planes of bedding usually run across the -planes of cleavage. - -479. We have here, as you observe, a case exactly similar to that of -glacier lamination, which we were at first disposed to regard as due -to stratification. We afterwards, however, found planes of lamination -crossing the layers of the névé, exactly as the planes of cleavage -cross the beds of slate rocks. - -480. But the analogy extends further. Slate cleavage continued to -be a puzzle to geologists till the late Mr. Daniel Sharpe made the -discovery that shells and other fossils and bodies found in slate -rocks are invariably flattened out in the planes of cleavage. - -481. Turn into any well-arranged museum--for example, into the School -of Mines in Jermyn Street, and observe the evidence there collected. -Look particularly to the fossil trilobites taken from the slate rock. -They are in some cases squeezed to one third of their primitive -thickness. Numerous other specimens show in the most striking manner -the flattening out of shells. - -482. To the evidence adduced by Mr. Sharpe, Mr. Sorby added other -powerful evidence, founded upon the microscopic examination of slate -rock. Taking both into account, the conclusion is irresistible that -such rocks have suffered enormous pressure at right angles to the -planes of cleavage, exactly as the glacier has demonstrably suffered -great pressure at right angles to its planes of lamination. - -483. The association of pressure and cleavage is thus demonstrated; -but the question arises, do they stand to each other in the relation -of cause and effect? The only way of replying to this question is to -combine artificially the conditions of nature, and see whether we -cannot produce her results. - -484. The substance of slate rocks was once a plastic mud, in which -fossils were embedded. Let us imitate the action of pressure upon -such mud by employing, instead of it, softened white wax. Placing a -ball of the wax between two glass plates, wetted to prevent it from -sticking, we apply pressure and flatten out the wax. - -485. The flattened mass is at first too soft to cleave sharply; but -you can see, by tearing, that it is laminated. Let us chill it with -ice. We find afterwards that no slate rock ever exhibited so fine a -cleavage. The laminæ, it need hardly be said, are perpendicular to -the pressure. - -486. One cause of this lamination is that the wax is an aggregate -of granules the surfaces of which are places of weak cohesion; and -that by the pressure these granules are squeezed flat, thus producing -planes of weakness at right angles to the pressure. - -487. But the main cause of the cleavage I take to be the lateral -sliding of the particles of wax over each other. Old attachments -are thereby severed, which the new ones fail to make good. Thus the -tangential sliding produces lamination, as the rails near a station -are caused to exfoliate by the gliding of the wheel. - -488. Instead of wax we may take the slate itself, grind it to fine -powder, add water, and thus reproduce the pristine mud. By the proper -compression of such mud, in one direction, the cleavage is restored. - -489. Call now to mind the evidences we have had of the power of -thawing ice to yield to pressure. Recollect the shortening of the -Glacier du Géant, and the squeezing of the Glacier de Léchaud, at -Trélaporte. Such a substance, slowly acted upon by pressure, will -yield laterally. Its particles will slide over each other, the -severed attachments being immediately made good by regelation. It -will not yield uniformly, but along special planes. It will also -liquefy, not uniformly, but along special surfaces. Both the sliding -and the liquefaction will take place principally at right angles to -the pressure, and glacier lamination is the result. - -490. As long as it is sound the laminated glacier ice resists -cleavage. Regelation, as I have said, makes the severed attachments -good. But when such ice is exposed to the weather the structure is -revealed, and the ice can then be cloven into tablets a square foot, -or even a square yard in area. - - -§ 67. _Conclusion_. - -491. Here, my friend, our labours close. It has been a true pleasure -to me to have you at my side so long. In the sweat of our brows we -have often reached the heights where our work lay, but you have been -steadfast and industrious throughout, using in all possible cases -your own muscles instead of relying upon mine. Here and there I have -stretched an arm and helped you to a ledge, but the work of climbing -has been almost exclusively your own. It is thus that I should like -to teach you all things; showing you the way to profitable exertion, -but leaving the exertion to you more anxious to bring out your -manliness in the presence of difficulty than to make your way smooth -by toning difficulties down. - -492. Steadfast, prudent, without terror, though not at all times -without awe, I have found you on rock and ice, and you have shown -the still rarer quality of steadfastness in intellectual effort. As -here set forth, our task seems plain enough, but you and I know how -often we have had to wrangle resolutely with the facts to bring out -their meaning. The work, however, is now done, and you are master of -a fragment of that sure and certain knowledge which is founded on the -faithful study of nature. Is it not worth the price paid for it? Or -rather, was not the paying of the price the healthful, if sometimes -hard, exercise of mind and body, upon alp and glacier--a portion of -our delight? - -493. Here then we part. And should we not meet again, the memory of -these days will still unite us. Give me your hand. Good bye. - - - - -INDEX. - - - A - - Accurate measurements of the motions of glaciers, by Agassiz and - Forbes, 60-62. - Æggischhorn, view from the, 137. - Agassiz's measurements, 60; - conclusions, 107; - discovery by, 150; - observations made by, 187. - Aiguille du Dru, pyramid of, 43; - cloud-banner of, 90. - ---- des Charmoz, 43; - clouds about, 90. - ---- Noire, 51. - ---- Verte, height of, 53 - ---- du Midi, stone avalanches of, 56. - Air, its expansion, 24; - a chilling process, 25; - experiments illustrating, 25, 26. - Aletsch glacier, 136; - length of, 136; - arm of, 137. - Alpine ice, origin in the sun's heat, 7. - Ancient glaciers of England, Ireland, Scotland, and Wales, 150, 151. - Architecture of snow, 29-34, 91; - of lake-ice, 35, 169. - Arveiron, vault of, 66; - description and cause of, 92. - - B - - Bel Alp, description of the, 139, 140. - Bergschrund, formation of the, 102, 103, 179. - Blue veins of glaciers, 176; - whiteness of snow, 176; - whiteness of ice, 177; - freezing of lake-ice, 177; - explanation of cause of whiteness of glaciers, 178; - translucency converted into transparency, 178; - vertical veins, 179; - structure and bedding on glaciers, 181, 182; - stratification theory, 183; - observations, 187. - Bodies of guides found on Glacier des Bossons, 57, 144. - Boulders, size of, 148, 149. - - C - - Changes of volume resulting from heat and cold, 118-122; - illustrations of, 119, 120; - consequences from, 122; - opinions of Count Rumford, 12, 124. - Chapeau, refreshment at, 41. - Cleavage and glacier lamination, 187; - analogy between, 188, 189; - planes of, 188; - observations of Prof. Sedgwick, 188; - discovery by Daniel Sharp, 188; - additional evidence of Mr. Sorby, 189; - association of pressure and cleavage established, 189: - relation of cause and effect, 189; - artificial conditions of Nature combined, 189, 190. - Clouds, their formation, 3-6; - in tropical regions, 25; - illustration of the formation of, 26. - Col du Géant, snows and ice-cascade of the, 46, 47; - snow-fall on the plateau of, 48, 49; - cracks on, 179. - Conclusion, 191, 192. - Condensers needed, 154. - Conditions necessary for the production of natural phenomena, 99. - Conscience, scientific, 180. - Crevasses, 41; - work among the, 52; - widening of, 54; - drifting of bodies buried in, 57; - birth of, 98; - features of, 100; - characteristic, 102; - transverse, 103; - stalactites of Alpine, 100; - marginal, 105-107; - longitudinal, 109; - curvature of glacier related to number of, 110-112. - Crystallization of metals, 29; - of sugar, 30; - of saltpetre, 30; - of alum, 30; - of chalk, 30; - of carbon, 30; - reversal of the process of, 36. - - D - - De Saussure's theory of glacier-motion, 156. - Dilatation theory, 155, 156. - Dirt-bands of Mer de Glace, observed by Prof. Forbes, 130; - description and explanation of, 131, 132. - Distillation, oceanic, 18; ordinary, 21. - Dôme du Goûté, broken crags of, 50. - Drifting of huts on ice, 59, 60. - Dr. William Hopkins's conclusions regarding the obliquity of the - lateral crevasses, 107. - - E - - Égralets, passage through the, 53. - Electric light, dark waves of, 14. - Equivalent points, comparison of velocities of, 75, 76, 107. - Erratic blocks, 147-150. - Evaporation, caused by the heat-rays of the sun, 13. - Expansion of water, 121, 155. - Experiments to show the heat-power of the dark rays, 14-19; - illustrative, 22, 23, 25, 20; - Dr. Franklin's experiment, 112. - Extract from Bordier's book, 157, 162. - - F - - Faraday's theory regarding regelation, 171; - special solidifying power exerted by substances upon their own - molecules, 172, 173; - opinion of Prof. James Thomson, 174, 175; - experiments illustrating the subject, 174; - quotation from Faraday, 175. - Fog, its formation in ball-rooms, 5. - Forces, of crystallization, 30; - of gravitation, 30; - of Nature, 31; - attractive and repulsive, 127. - Freezing mixture, 120, 168. - - G - - Glacier, the source of the Rhone, 7; - fed by mountain-snow, 7, 21; - melted by the sun's dark rays, 13; - terminal moraines of, 38; - questions regarding motions of, 54-58; - action of the ends of, 58; - motion at top and bottom of, 80, 81; - lateral compression of, 81, 82; - longitudinal compression of, 84, 85; - slow movement in winter of, 87; - motion of Grindelwald, 94; - motion of Great Aletsch, 94; - motion of Morteratsch, 95; - crevasses at the side of, 106; - action, 146, 147; - ice, 155; - veined or ribboned structure of, 178; - blue veins of, 170; - tables, 113, 114; - mills, 116-118; - theory of Scheuchzer regarding, 155; - ancient, 145-147; - impurities thrust out by, 144; - whiteness of a clean, 43, 170; - measurements by Hugi and Agassiz of, 59, 60; - epoch, 152-167. - Glacier des Bois, description of, 38-43. - ---- des Bossons, 56; - mass of ice upon, 134. - ---- of Aletsch, sand-cones of, 116. - ---- des Périades, 51. - ---- du Talèfre, boundary of, 53; - width of, 92; - chasms of the, 98. - ---- de Léchaud, motion of, 80; - width of, 82; - compression of, 190. - ---- du Géant, _névé_ of the, 49; - motion of, 79; - width of, 82; - cracks above the ice-falls of, 119; - honey-combing of, 115; - shortening of, 190. - ---- Unteraar, movement of, 61; - appearance of Prof. Forbes on, 161; - measurements by Agassiz on, 162. - ---- Görner, sand-cones of, 116; - description of, 140-144; - moraines of, 143; - advance and retreat of, 144; - objects of interest on, 144; - structure and bedding on, 181. - Grand Plateau, crevasse on, 57, 118. - Greenland's icy mountains, 21. - Growth of knowledge, 59. - Guesses in science, 74. - - H - - Harmony of life and its conditions, 125. - Heat, waves of, 12; - invisible, 14; - office of the invisible waves of, 36; - absorption of solar, 100; - demanded for the liquefaction of ice, 131; - latent, 153. - Hoar-frost, 5, 6; - not melted by light-waves, 13, 18. - Hôtel des Neuchâtelois, movements of, at Riffelberg, 141. - - I - - Ice, structure of, 35, 36, 119; - towers of, 104; - sea, 132-134; - retreat of, 145; - development in the Alps of, 150; - freezing of pieces of, 164; - liquefaction by pressure of, 108, 160; - translucent, 177; - difference between hard and soft, 87. - Icebergs, of arctic seas, 133; - described by Sir Leopold McClintock, 133, 134; - drifting of, 134; - origin of, 134; - of Switzerland, 136-138; - colors of, 138; - formation of chains of, 165. - Ice-lens, concentration of sun's rays by means of, 37. - Ice-river through the vale of Hasli, 146. - Icicles, 99; - how produced, 100; - a theory of, 100-102 - Imagination, scientific use of, 34. - Impurities thrust out by glaciers, 144. - Infinite Wisdom, designs of, 124. - - J - - Jardin, description of, 53. - Jungfrau, 136, 137. - - K - - Killarney, luxuriant vegetation of, 27, 151. - - L - - La Grande Jorasse, crests of, 43; - roses of cloud about, 90. - Lake of Geneva, an expansion of the river Rhone, 6. - Latent heat, 153, 167. - Lateral moraines, origin of, 54. - Light, wave-theory of, 10, 11; - inference from the phenomena of, 11; - length of wave of, 12. - Likeness of glacier-motion to river-motion, 72-76. - Liquefaction of ice, 168, 169; - experiment by Mr. Bottomley, 170; - experiment by Mr. Boussingault, 170, 171. - Locus of the point of swiftest motion, 76. - Longitudinal crevasses, how formed, 109; - examples of, 110. - - M - - Magillicuddy's Reeks, 27, 150, 151. - Magnet, poles of, 32; - repelling corners and ends of, 126. - Märgelin See, 138; icebergs in the, 138. - Marginal crevasses, explanation of, 106-109. - Mauvais Pas, 41. - Measurements of glaciers by Hugi and Agassiz, 59, 60. - Medial moraines, how accounted for, 55. - Mer de Glace, 41; - its sources, 43-45; - view of the, 44; - branching of the, 45; - medial moraines of the, 51, 52; - triangulation of, 62; - motion of, 66, 70; - daily motion of, 67; - unequal motion of the two sides of, 70-72; - motion of axes of, 78; - summer condition of, 87; - winter on, 88-92; - dirt-bands of, 127; - dimensions of, 145; - winter motion of, 93; - greater number of crevasses on eastern side of, 112; - glacier tables of, 113; - grand moulin of, 117, 118; - approximate weight of, 154. - Molecules, of water, 31, 34; - expansion of, 125; - forces acting upon, 127; - exclusiveness of water, 132, 133. - Montanvert, auberge of the, 41, 43; - appearance in winter, 89. - Moraine, lateral, 41; - medial moraines of the Mer de Glace, 51, 112; - cedars of Lebanon growing on ancient, 150; - explanation of the cause of ridges on, 112, 113. - Morteratsch glacier, cause of the widening of the medial - moraine of, 97; - motion of, 95, 96; - sand-cones of, 116. - Motion, of Mer de Glace, 93; - of Grindelwald, 94; - of Great Aletsch glacier, 94; - of Morteratsch glacier, 95; - sand-cones of, 116. - Moulins, description of, 116; - dangers from, 117; - sounding of, 118. - Mountain condensers, 27, 150. - - N - - _Névé_, explanation of term, 49; - stratification of the, 179. - - O - - Obliquity of the lateral crevasses, 167; - illustration, 107, 108. - - P - - Petit Plateau, 56. - Piz Bernina, route to, 95. - Place de la Concorde of Nature, 136. - Plastic theory, 156; - advocated by Bordier, 157; - advocated by Rendu, 158. - Poles, atomic, 32, 126; - attractive and repellent, 30. - Pontresina, village of, 95. - Precious stones, examples of crystallizing power, 30. - Precipitation, 23, 25; - atmospheric, 27. - Promontory of Trélaporte, 44; - of Tacul, 51. - Proofs of glacier-motions, 58. - Pyramid of Aiguille du Dru, 43; - of Aiguille des Charmoz, 43. - - Q - - Quotation from Rendu, 158. - - R - - Rain, its source, 3; - tropical, 23, 25. - Rainfall, observations on amount of, 28. - Regulation, theory, 163-167; - observations made by Mr. Faraday, 164; - formation of chain of icebergs by, 165; - cause of, 167; - Faraday's view of, 171-176. - Relation of structure to pressure, 183; - veined structure described by Guyot, 183-185; - illustration, 186; - connection established, 187. - Retina, how excited, 8; - theory of Sir Isaac Newton, 9. - Riffelberg Hotel, location of, 144. - Rivers, their sources, 1, 7, 19. - - S - - Sand-cones, 116. - Scientific tacts, connection of, 72. - Siedelhorn, view from the summit of, 146. - Snow, its conversion into ice, 156; - consolidation in Alpine regions, 156; - line, 49; - its formation in Russian ball-rooms, 5; - in subterranean stables, 5, 14; - in polar regions, 21; - its architecture, 29-32, 34, 91; - absorbs solar heat, 100. - Solidifying power of camphor and metals, 17. - Source of the Severn, 2; - the Thames, 2; - the Danube, 2; - the Rhine, 2; - the Rhone, 2; - the Gauges, 2; - the Euphrates, 2; - the Garonne, 2; - the Elbe, 2; - the Missouri, 2; - the Amazon, 2; - Albula, 2; - the Arveiron, 38; - the Aar, 50. - Stalactites of Alpine crevasses, 100. - Steam, its condensation, 3, 4. - Sunbeams, office of, 10-21. - Sun, its heat the source of Alpine ice, 7; - vibratory motion of the atoms of the, 11; - position of, 19; - indirect heat, of the, 28. - Switzerland, ancient glaciers of, 145-147. - - T - - Theodolite, description of, 63; - use of, 64, 65. - Theory, of Dilatation, 155; - developed by De Charpentier, 156; - sliding, 156; - plastic, 156-160; - viscous, 161-103; - regulation, 163-167; - of glacial epoch, 155-167. - Trade-winds, 20. - Transverse crevasses, formation of, 104, 105. - Trélaporte, promontory of, 44; - motion of the water through the narrows of, 79. - - U - - Universe, order of the, 32. - - V - - Valley, of Hasli, 146, 147; - Black, 151. - Vapor, in the atmosphere, 5. - Viscous theory, advocated by Prof. Forbes, 161; - rejected by M. Agassiz, 162. - - W - - Water, changes of volume of, 118-122; - maximum density of, 120; - effects of expansion of, 121; - not a solitary exception to general law, 124; - molecular expansion of, 125-127; - temperature necessary to freeze the sea, 132; - special solidifying power of, 173; - freezing-point of, 168. - Waves, of light, 8-11, 127; - length of, 12; - of heat, 12. - Whiteness of a clean glacier, 43, 176; - of Märgelin See, 138, 176; - of snow, 176; - of ice formed in freezing mixtures, 177. - - -THE END. - - - * * * * * - - -Transcriber Note - -On page 61 (153), Principal was changed to Professor. Minor typos were -corrected. 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