UC-NRLF 
 
 576 
 
 TV 
 
NOT 
 
 OF 
 
 PROFESSOR TYND 
 
 THE LIBRARY 
 
 OF 
 
 THE UNIVERSITY 
 OF CALIFORNIA 
 
 PRESENTED BY 
 
 PROF. CHARLES A. KOFOID AND 
 MRS. PRUDENCE W. KOFOID 
 
 ICE, WATER, VAPOUR, AND AIR. 
 

 
 HANDBOOK 
 
 OP 
 
 ICE AND GLACIEKS. 
 
NOTES 
 
 " ' ;'/. 1 
 
 A COURSE OF SIX LECTURES 
 
 (Adapted to a Juvenile Auditory}, 
 OX 
 
 ICE, WATEE, VAPOUB, AND AIR 
 
 BY 
 
 PEOFESSOR TYNDALL, LL.D., F.R.S. 
 
 l\ 
 
 CHRISTMAS, 1871-2. 
 
 ROYAL INSTITUTION OF GREAT BRITAIN, 
 ALBEMARLE STREET. 
 
 The Rights of Publication and Translation reserved. 
 
LONDON I PRINTED BY WILLIAM CLOWES AND SONS, STAMFORD STREET AND CHARING CROS3. 
 
EARTH 
 
 SCIENCES 
 
 LIBRARY 
 
 IN preparing these Notes, which I have tried to render clear and 
 compact, I have not forgotten that you are growing older and 
 wiser: otherwise I might have made them even shorter and 
 simpler than they are. 
 
 Probably but few of you, and these the very youngest, will find 
 the notes either long or difficult ; but even in their case all diffi- 
 culty will soon disappear. 
 
 Doubtless many of you when you become full-grown men, and 
 not I trust before, will try your strength upon the mountains. 
 I hope, when there, that you will never confound courage with 
 recklessness, or forget that on the Alps bravery and caution ought 
 always to be combined. 
 
 JOHN TYNDALL. 
 
 CHRISTMAS, 1871. 
 
CONTENTS. 
 
 PAGE 
 
 1,2. Clouds, Rains, and Rivers .. .. 1 
 
 3. The Waves of Light .. .. ;.. ., 4 
 
 4. The Waves of Darkness which produce the Vapour of our Atmosphere 
 
 and melt our Glaciers 5 
 
 5. Oceanic Distillation 7 
 
 6. Tropical Eains .; .V .. .-. .. .. 8 
 
 7. Mountain Condensers 9 
 
 8. Architecture of Snow 10 
 
 9. Architecture of Lake Ice 12 
 
 10. The Source of the Arveiron. Ice Pinnacles, Towers, and Chasms of the 
 
 Glacier des Bois. Passage to the Montanvert 14 
 
 11. The Mer de Glace and its Sources : Our First Climb to the Cleft Station 16 
 
 12. Ice-cascade and Snows of the Col du Geant 18 
 
 13. Questioning the Glaciers 19 
 
 14. Branches and Medial Moraines of the Mer de Glace from the Cleft 
 
 Station .. .. .. .. 20 
 
 15. The Talefre and the Jardin. Work among the Crevasses 22 
 
 16. First Questions regarding Glacier Motion. Drifting of Bodies buried in 
 
 a Crevasse 23 
 
 17. The Motion of Glaciers. Measurements by Hugi and Agassiz. Drifting 
 
 of Huts on the Ice 25 
 
 18. Precise Measurements of Agassiz and Forbes. Motion of a Glacier 
 
 proved to resemble the Motion of a Kiver 25 
 
 19. The Theodolite and its Use. Our own Measurements 27 
 
 20. Motion of the Mer de Glace 28 
 
 21. Unequal Motion of the two Sides of the Mer de Glace 31 
 
 22. Suggestion of a new likeness of Glacier Motion to River Motion. Con- 
 
 jecture tested 32 
 
 23. 24. New Law of Glacier Motion .- 34 
 
 25. Motion of Tributary Glaciers 35 
 
 26. Lateral Compression of a Glacier 36 
 
 27. Longitudinal Compression of a Glacier 37 
 
 28. Motion of Top and Bottom of Glacier 38 
 
 29. Sliding and Flowing. Hard Ice and Soft Ice 38 
 
 30. Winter on the Mer de Glace . . 39 
 
 
$ PAGE 
 
 31. Winter Motion of the Mer de Glace 42 
 
 32. Birth of a Crevasse : Reflections 42 
 
 33. Icicles ..." .. 43 
 
 34. Crevasses. The Bergschrund 44 
 
 35. Transverse Crevasses 45 
 
 36. Marginal Crevasses . . . . . . . . 46 
 
 37. Longitudinal Crevasses 48 
 
 38. Crevasses in relation to Curvature of Glacier 48 
 
 39. Moraine-ridges, Glacier Tables, and Sand Cones 49 
 
 40. The Glacier Mills or Moulins .. .. .. 52 
 
 41. Sea-ice and Icebergs 53 
 
 42. The Margelin See and its Icebergs -' 54 
 
 43. Motion of the Aletsch Glacier . .* .. 56 
 
 44. Ancient Glaciers of Switzerland 56 
 
 45. Erratic Blocks 57 
 
 46. Ancient Glaciers of England, Ireland, Scotland, and Wales 59 
 
 47. The Glacial Epoch .. .. 60 
 
 48. Glacier Theories .. .. ;, v .i.*Y- 61 
 
 49. Dilatation and Sliding Theories 61 
 
 50. Plastic Theory .. ;.;.. >-' r 62 
 
 51. Viscous Theory .,....*.. 64 
 
 52. Regelation Theory ; ., .. .. 65 
 
 53. Veined Structure of Glacier Ice 67 
 
NOTES 
 
 OF 
 
 A COUKSE OF SIX LECTUKES 
 
 ON 
 
 ICE, WATEE, VAPOUK, AND AIE. 
 
 DELIVERED ON DECEMBER 28, 30, 1871 ; JANUARY 2, 4, 6, 9, 1872. 
 
 1. 
 Clouds, Rains, and Ewers. 
 
 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 con' 
 stitute 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 ; while the Rhine and the Rhone have their sources in the 
 Alps. 
 
 5. But it is quite plain that we have not yet reached the real 
 beginning of the river. Whence do the hill streams derive their 
 
( 2 )' 
 
 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 some- 
 times 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 ques- 
 tion. Eain 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 fine- 
 ness, which floats in the air, and is called a cloud. 
 
 8. Watch the cloud-banner from the funnel of a running loco- 
 motive ; 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. 
 
( 3 ) 
 
 10. Even on the driest day this vapour is never absent from our 
 atmosphere. I will condense the vapour diffused through the air of 
 our lecture-room, and freeze it to hoar-frost in your presence. 
 
 11. To produce the cloud, in the case of the locomotive, 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 whicli produces the clouds of our atmosphere ? There is : 
 the fire of the sun. 
 
 12. 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. 
 
 13. 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 Ehone, and trace it backwards to Lyons 3 
 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 Khone. But go 
 to the head of the lake, and you find that the Ehone 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 Ehone valley, and from the bottom of the glacier the river 
 rushes. In the glacier of the Ehone you thus find the source of 
 the river Ehone. 
 
 14. But again we have not reached the real beginning of the river. 
 You soon convince yourself that this earliest water of the Ehone 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 Ehone. 
 
 15. 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, with- 
 out 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. 
 
 B 2 
 
3. 
 The Waves of Light. 
 
 16. 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 inquiry. 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. 
 
 17. 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 impres- 
 sion 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 divides 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. 
 
 18. Can the impression of real light be due to any action of this 
 kind ? In some way or other luminous bodies have the power of 
 affecting the retina but how ? 
 
 19. It was long supposed that from such bodies issued with incon- 
 ceivable 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. 
 
 20. 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 experi- 
 menters 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. 
 
 21. 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. 
 
 22. 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 sub- 
 
( 5 ) 
 
 stance which is supposed to fill all space, and to be diffused through 
 the humours of the eye ; and finally, you have the retina, which re- 
 ceives the successive shocks of the waves. These shocks are supposed 
 to produce the sensation of light. 
 
 23. 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 ? 
 
 24. 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. 
 
 25. Let me use an illustration. We infer from the flint imple- 
 ments recently found in such profusion all over England and in other 
 countries, that they were produced by men, because, as far as our 
 experience goes, nothing but man could form such implements. 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 Darkness which produce the Vapour of our Atmosphere 
 and melt our Glaciers. 
 
 26. 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 con- 
 ception into some of its details. We have all seen the waves of water, 
 a^nd we know that there are waves 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. 
 
 27. As observed above we have never seen the light-waves, but 
 we judge of their presence, their position, and their magnitude, by 
 
( 6 ) 
 
 their effects. Their lengths have been thus determined and found 
 to vary from about ^^th to ^-o-tfth of an inch. 
 
 28. But besides the waves 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. 
 These large waves possess the highest heating power. * 
 
 29. A common sunbeam contains waves of all kinds, but it is pos- 
 sible to sift or filter the beam so as to intercept all its light-yielding 
 waves, and at the same time to allow its purely heat-yielding waves 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. 
 
 30. I will show you how the one class of waves may be detached 
 from the other class, and that waves competent to light a fire, fuse metal, 
 or burn the hand like a hot solid, may exist in a perfectly dark place. 
 
 31. 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 through 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. 
 
 32. 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 heat- 
 ing 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. 
 
 33. Further, it is these self-same 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 that the light- 
 waves, even when concentrated to the uttermost, are hardly able to 
 melt the most delicate hoar-frost ; much less would they be able to 
 produce the copious liquefaction observed upon the glaciers. 
 
 34. We have here an example of the manner in which pheno- 
 mena, 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 by it 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. 
 
( 7 ) 
 
 5. 
 Oceanic Distillation. 
 
 35. But we must now be more precise. 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 downwards, and heat both earth and sea more than 
 when they strike slantingly. 
 
 36. When the vertical sunbeams strike the land they heat it, and 
 the air in contact with the hot soil becomes heated in turn. 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. 
 
 37. 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 qf the sun. 
 
 38. When they reach a certain elevation in the atmosphere, 
 these currents divide and flow, part towards the north and part to- 
 wards 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. 
 
 39. 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. 
 
 40. 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. 
 
 41. 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. 
 
 42. It was supposed that if the sun's heat were diminished greater 
 glaciers than those now existing would be produced. But the lessen- 
 ing of the sun's heat would infallibly diminish the quantity of aqueou* 
 
( 8 ) 
 
 vapour, and thus cut off the glaciers at their source. A brief illus- 
 tration will complete your knowledge here. 
 
 43. In the process of ordinary distillation, the liquid to be dis- 
 tilled 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 moun- 
 tains," the equatorial ocean had to be warmed by the sun. We shall 
 have more to say upon this question afterwards. 
 
 6. 
 Tropical Bains. 
 
 44. 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 
 inquire a little more closely into their origin. 
 
 45. 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 the tightness disappear, the bladder 
 finally appearing as flaccid as at first. 
 
 46. 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 I will show you 
 the expansion of a balloon partly filled with air, when the external 
 pressure has been in part removed. 
 
 47. 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. 
 
 48. 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. 
 
( 9 ) 
 
 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 will show you this chilling by 
 expansion, and also the consequent precipitation of clouds. 
 
 49. 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. 
 
 7. 
 Mountain Condensers. 
 
 50. To complete our view of the process of atmospheric precipita- 
 tion 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 Ire- 
 land it encounters the mountains of Kerry: the highest of these is 
 Magillicuddy's Eeeks, 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 conse- 
 quence of the expansion, and comes down in copious showers of rain. 
 From this, in fact, arises the luxuriant vegetation of the Killarney 
 lakes ; to this, indeed, they owe their water supply. The cold crests 
 of the mountains also aid in the work of condensation. 
 
 51. Note the consequence. There is a town called Cahirciveen 
 to the south-west of Magillicuddy's Eeeks, 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 the rain- 
 fall amounts to 59 inches in a year, while at Portarlington it is only 
 21 inches. 
 
 52. 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. 
 
 53. 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. 
 
 
 . .. 
 
 Architecture of Snow. 
 
 54. 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. 
 
 55. 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. 
 
 56. 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 practi- 
 

 Snow Crystals. 
 
( 12 ) 
 
 cally out of each other's range. The natural tendency of the mole- 
 cules to build themselves together is thus neutralized. 
 
 57. This is the case with water, which as a liquid is to all appear- 
 ance formless. When sufficiently cooled the molecules are brought 
 within the play of the crystallizing force, and they then arrange them- 
 selves 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 
 preceding drawings of a few of them, executed at Greenwich by Mr. 
 Glaisher. 
 
 58. 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 ! 
 
 59. 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 poor powers are absolutely unable to comprehend either 
 the origin or the end of the operations of nature. 
 
 60. 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 pro- 
 duces 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. 
 
 9. 
 Architecture of Lake Ice. 
 
 61. We have thus before us the beautiful snow-flowers self-con- 
 structed 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, 
 
( 13 ) 
 
 moreover, is typical of the construction of all our lake ice. It is 
 built up of such forms wonderfully interlaced. 
 
 62. Take a slab of lake ice and place it in the path of a con- 
 centrated 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 sepa- 
 rate 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. 
 
 63. 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. 
 
 64. Annexed is a very imperfect sketch of these beautiful figures. 
 
 Liquid Flowers in Lake Ice. 
 
 65. 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. 
 
 66. I said above that a portion of the sunbeam was stopped by 
 the ice and liquefied it. What is this portion ? The dark rays of the 
 sun. The great body of the light rays, 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, after having 
 passed through the ice, their burning power becomes manifest. 
 
 67. And the ice itself may be employed to concentrate them. 
 With an ice-lens in the polar regions Dr. Scoresby has often concen- 
 trated 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. 
 
 10. 
 
 The Source of the Arveiron. Ice Pinnacles, Towers, and Chasms 
 ' of the Glacier des Bois. Passage to the Montanvert. 
 
 68. And now let us get to work. Through the village of Cha- 
 mouni, 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 you 
 come to what is called the source of the Arveiron a short hour's, 
 walk from Chamouni. Here, as in the case of the Ehone 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. 
 
 69. We must now be observant. Looking about us here, we find 
 in front of the ice curious heaps and ridges of debris, which are more 
 or less concentric. These are the terminal moraines of the glacier. 
 We shall examine them subsequently. 
 
 70. We now turn to the left, and ascend the slope beside the gla- 
 cier. 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 debris, like those lower down, but covered in part with trees, 
 and appearing to be literally as " old as the hills." It tells a won- 
 derful 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 moun- 
 tains at the opposite side. 
 
 71. Having passed the terminal portion of the glacier, which is 
 covered with rubbish, we find ourselves beside a very wonderful exhi- 
 bition 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 one of them. From deep chasms in the ice issues a delicate 
 shimmer of blue light. At times we hear a sound like that of thunder, 
 which arises either from the falling of a tower of ice, or from the 
 
( 16 ) 
 
 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. 
 
 Ice Figure on the Glacier des Bois. 
 
 72. 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. 
 
 73. 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. 
 
 74. 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. 
 
( 16 ) 
 
 75. 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. 
 
 11. 
 
 The Her de Glace and its Sources : Our First Climb to the 
 Cleft Station. 
 
 76. Here the view before us is very grand. We look across the 
 glacier at the beautiful pyramid of the Aiguille du Dru ; 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. 
 
 77. 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 Ponts. We cross them and reach I' Angle, where we quit the 
 land for the ice. We walk up the glacier, but before reaching the 
 promontory called Trelaporte, we take once more to the mountain 
 side ; for though this path 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. 
 
 78. Above Trelaporte 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 com- 
 manding and instructive view of the Mer de Glace and its sources. 
 
( 18 ) 
 
 79. 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. 
 
 80. 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. 
 
 81. 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. 
 Kegard the branch to the right, the Glacier du Geant. 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. 
 
 12. 
 
 Ice-cascade and Snows of the Col du Geant. 
 
 82. 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 Trelaporte, and walked right up the Glacier du 
 Geant. 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. 
 
 83. And with the aid of an axe to cut steps in the steeper ice- 
 walls and slopes we might 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 some- 
 times 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. 
 
84. 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 Geant. 
 
 85. 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 moun- 
 tain 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. 
 
 13. 
 Questioning the Glaciers. 
 
 86. 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, and in which we have left the deep traces of 
 our footsteps, 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 Geant the quantity of snow that falls annually exceeds the 
 quantity melted. 
 
 87. Had we come in the month of April or May we should have 
 found the glacier below the ice-fall also covered with snow, 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. 
 
 88. 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. 
 
 89. The French term Neve 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 Geant constitute the neve of the Glacier 
 du Geant, and in part, the neve of the Mer de Glace. 
 
 o 2 
 
( 20 ) 
 
 90. But if every year thus leaves a residue of snow upon the 
 plateau of the Col du Geant, it necessarily follows that the plateau 
 must get annually higher, provided the snow remains 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 ? 
 
 14 
 
 Branches and Medial Moraines of the Mer de Glace from the 
 Cleft Station. 
 
 91. Let us sink the question for a moment, and gather from the 
 scene before us knowledge which will help us to solve it. Look at 
 that ice-valley in front of us, stretching up between Mont Tacul and 
 the Aiguille de Lechaud, to the base of the great ridge called the 
 Grande Jorasse. This is called the Glacier de Lechaud. It receives 
 at its head the snows of the Jorasse and of Mont Mallet, and joins 
 the Glacier du Geant 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 debris ; you trace a similar 
 though smaller stripe from the junction of the Glacier du Geant with 
 the Glacier des Periades at the foot of the Aiguille Noire, which you 
 follow all along the Mer de Glace. 
 
 92. We also see another glacier, or a portion of it, to the left, 
 falling apparently in broken fragments through a narrow gorge, and 
 joining the Lechaud, and from their point of junction also a stripe of 
 debris runs downwards along the Mer de Glace. Beyond this again 
 we notice another stripe, 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. 
 
 93. These stripes are the Medial Moraines of the Mer de Glace. 
 We shall learn more about them immediately. 
 
 94. Annexed is a sketch-plan of the glacier and its moraines 
 taken from our present commanding position. 
 
 95. 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 
 
( 21 ) 
 
 our labour. You will find it thus through life ; without honest labour 
 there can be no deep joy. 
 
 Sketch-plan^ showing the Moraines a b c d e of the Her de Glace. 
 
( 22 ) 
 
 15. 
 The Talefre and the Jardin. Work among the Crevasses. 
 
 96. 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 debris 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. 
 
 97. We quit the ice at a place called the Couvercle, and wind 
 round this promontory, ascending all the time. We squeeze ourselves 
 through the Egralets, a kind of natural staircase in the rock, and soon 
 afterwards obtain a 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 Talefre, 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 fluted in front by the descending stones 
 and snow. 
 
 98. 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 por- 
 tion 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 Talefre ; 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. 
 
 99. 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 per- 
 fect 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 
 
( 23 ) 
 
 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. 
 
 100. And now for home, looking at the Tacul on our way. Here 
 we see the actual junction of the moraines of the Geant and the 
 Lechaud. They have brought down mighty stones, under one of 
 which I had a very hard bed on the night of the 24th of July, 1857. 
 Notice particularly a precipice of ice the naked side of the glacier 
 which is here displayed. We shall turn it to account subsequently. 
 
 16. 
 
 First Questions regarding Glacier Motion. Drifting of Bodies buried 
 in a Crevasse. 
 
 101. 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 moun- 
 tain 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. 
 
 102. But how are the medial moraines to be accounted for ? How 
 does the debris 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 examina- 
 tion. 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. 
 
 103. 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 sup- 
 posing that from the highest snow-fields of the Col du Geant, the 
 Lechaud, and the Talefre, to the extreme end of the Glacier des Bois, 
 the whole mass of frozen matter is moving downwards. 
 
 104. 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 VEchelle, because here a ladder is 
 usually placed to assist in crossing the chasms of the glacier. At the 
 Pierre TEchelle we should strike the ice, and passing under the 
 
( 24 ) 
 
 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, to the rocks of the Grands Mulcts, 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* of the Dome du Goute. 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. 
 
 105. 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. 
 
 106. Is this language correct ? A crevasse hardly to be dis- 
 tinguished 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 from the existing 
 crevasse. 
 
 107. The same observation proves to demonstration that it is the 
 ice near the bottom of the higher neve that becomes the surface-ice 
 of the glacier near its end. The uncompensated waste of the surface 
 below the snow-line brings the deeper portions of the ice more and 
 more to the light of day. 
 
 108. 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 some- 
 times from month to month ; and this could not be if the ice did not 
 move. Eocks 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 hills 
 or 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 
 
 * Called seracs from their resemblance in shape and colour to an inferior kind 
 of curdy cheese called by this name at Chamouni. 
 
( 25 ) 
 
 act like ploughshares on the land in front of them, overturning with 
 slow but merciless force huts and chalets that stand in their way. 
 Facts like these hav v e 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. 
 
 17. 
 
 The Motion of Glaciers. Measurements by Hugi and Agassiz. Drifting 
 of Huts on the Ice. 
 
 109. 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 for ever 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. 
 
 110. 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. 
 
 111. On this great medial moraine in 1827 an intrepid and enthu- 
 siastic 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 2354 feet ; 
 and in 1841 M. Agassiz found it 4712 feet below its first position. 
 
 112. 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 Neufchatel, the hut was called 
 long afterwards the " Hotel des Neuchatelois." Two years subsequent 
 to its erection M. Agassiz found that the " hotel " had moved 486 feet 
 downwards. 
 
 18. 
 
 Precise Measurements of Agassiz and Forbes. Motion of a Glacier 
 proved to resemble the Motion of a River. 
 
 113. 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 
 
( 26 ) 
 
 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. 
 
 114. 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 the glacier, like 
 that of a river, moves more rapidly than the sides. 
 
 115. With the aid of trained engineers M. Agassiz followed up 
 these measurements in subsequent years. His researches are recorded 
 in a work entitled ' Systeme Glacier,' which is accompanied by a very 
 noble Atlas of the Glacier of the Unteraar, published in 1847. 
 
 116. These determinations were made by means of a theodolite, 
 of which I will give you some notion immediately. The same instru- 
 ment was employed the same year by the late Principal 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 (125). Professor 
 Forbes also triangulated the Mer de Glace, and laid down an ex- 
 cellent map of it. His first observations and his survey are recorded 
 in a celebrated book published in 1843, and entitled ' Travels in the 
 Alps.' 
 
 117. 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. 
 
( 27 ) 
 
 19. 
 The Theodolite and its Use. Our own Measurements. 
 
 118. 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 com- 
 mencement 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 pro- 
 perly planted. 
 
 119. 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 hairsbreadth the position occupied by the instrument to-day. 
 
 120. Look through the telescope ; you see it crossed by two fibres 
 of the finest spider's thread. In actual work we should 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 we sketch with its surroundings in a note 
 book, so that we shall immediately recognize 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. 
 
 121. A trained assistant is already upon the glacier. He erects his 
 staff and stands straight 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 tele- 
 scope probably strike the ice a little away from the staff of the assist- 
 ant ; 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. 
 
( 28 ) 
 
 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. 
 
 122. 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 mark at the opposite side of 
 the valley, being moved by a tangent screw until the point where the 
 spider's threads cross each other accurately covers the point at the 
 other side. 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 that 
 the stake has moved down. By our signals the assistant recovers the 
 point from which we started yesterday, and then determines the dis- 
 tance from this point to the stake. It is, say, 6 inches ; through this 
 distance, therefore, the stake has moved. 
 
 123. 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 was measured ; this enables us to calculate the accu- 
 rate daily motion of the point in question. The distances through 
 which all the other points have moved, are determined in precisely 
 the same way. 
 
 124. Thus we shall proceed to work, first making perfectly clear 
 to our minds what is to be done, and then loyally 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 obser- 
 vations together. The labour of measuring was undertaken for the 
 most part by Mr. Hirst. 
 
 20. 
 
 Motion of the Mer de Glace. 
 
 125. On the 14th of July, 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 cross hairs, 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. 
 
 126. 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 perpen- 
 
( 29 ) 
 
 clicular, or nearly so, to the axis or middle line of the glacier. We 
 send an instructed assistant to the middle ; he there lays down a long 
 staff or a rope in the direction of the axis, assuring himself by looking 
 up and down, that it is the true direction. With a second 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 suitable 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 here append an outline diagram of 
 the Mer de Glace, and of its tributaries. (See p. 30.) 
 
 127. Along the line just described ten stakes were set on the 17th 
 of July. 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 MER DE GLACE, JULY, 1857. 
 FIRST LINE : A A' UPON THE SKETCH. 
 
 Stake 1 2 3 4 5 7 9 10 
 
 Inches 12 17 23 26 25 26 27 33 
 
 128. 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 sure to excite wonder and delight. And not only does the 
 ice move bodily, but one part of it moves past another ; for the rate 
 of motion augments 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 had been already observed by Agassiz and Forbes ; 
 but come with me upon the glacier, and we shall find the germ of 
 something new. We cross the Glacier du Geant, which occupies more 
 than half the valley, and find that our line of stakes is not yet at an 
 end. The 10th stake stands a good way farther off, on the part of the 
 ice which comes from the Talefre. 
 
 129. 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 is least, the motion ought 
 to be greatest. This is clearly not the case ; for though nearer than 
 the 9th stake to the eastern or Chapeau side of the valley, the 10th 
 stake surpasses the 9th by 6 inches a day. 
 
 130. Here we have something to think of; but before we can think 
 with comfort we must be perfectly sure of our facts. The foregoing 
 line ran across the glacier a little below the Montanvert. We will run 
 another line of stakes across a little way above the hotel. On the 18th 
 of July 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 
 
Grande Jorasse. 
 
 Coldu 
 Geant. 
 
 Chapeau. 
 
 Outline Plan, showing the Measured Lines of the Mer de Glace and its Tributaries. 
 
( 31 ) 
 
 were found unfit for use. Hero are the distances passed over by the 
 remaining six-and-twenty in 24 hours. 
 
 SECOND LINE: BB' UPON THE SKETCH. 
 
 Stake 1 2 3 4 5 6 7 8 9 10 11 12 13 
 
 Inches 8 11 12 15 15 16 17 18 19 20 20 21 21 
 
 Stake 14 15 16 17 18 19 20 21 22 23 24 25 26 
 
 Inches 21 23 23 23 21 23 21 25 22 22 23 25 26 
 
 131. Look at these numbers. The first broad fact that they reveal 
 is the advance in the rate of motion from first to last. There are how- 
 ever small 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. 
 
 132. We note particularly that the 26th stake here, 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. 
 
 21. 
 Unequal Motion of the two Sides of the Mer de Glace. 
 
 133. But in neither the first line nor the second have we been 
 able to push our measurements quite across the glacier. Why ? In 
 attempting to do one thing we are often taught another, our defeats 
 becoming in this way means of instruction. We planted our theo- 
 dolite 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 higher than its sides, 
 and from our last two positions the centre cut off the view of the 
 ice near the opposite side of the glacier. The mountain slopes, in 
 fact, are warm in summer, and they melt the ice nearest them, thus 
 causing a fall from the centre to the sides. 
 
 134. Yonder on the heights at the other side 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; 
 
( 32 ) 
 
 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 fixed twelve 
 stakes on the 20th of July. On the 21st one of them had fallen ; but 
 the velocity of the remaining eleven measured in 24 hours were found 
 to be as follows : 
 
 THIED LINE: CO' UPON THE SKETCH. 
 
 Stake 123456789 10 11 
 
 Inches 20 23 29 30 34 28 25 25 25 18 9 
 
 135. Both the first stake and the eleventh in this series stood 
 near the side 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 goes to prove 
 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. 
 
 22. 
 
 Suggestion of a new likeness of Glacier Motion to River Motion. 
 .Conjecture tested. 
 
 136. Here we have food for reflection, and facts are comparatively 
 \ worthless if they do not provoke this exercise of the mind. Let us 
 
 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. 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 here 
 would not be the centre^ bat 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 ? 
 
 137. 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 you see that towards les Ponts it also bends, but that there its 
 convex curvature is towards the western side of the valley. If our 
 surmise be true, the point of swiftest motion opposite les Ponts ought 
 to lie west of the axis of the glacier. Let us test this conjecture. On 
 the 25th of July 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 123 4 5 6 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 
 
( 33 ) 
 
 138. 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 verifi- 
 cation of the guess which prompted us to make these measurements. 
 
 139. But 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 east of the axis at another place of 
 flexure. Fortunately such a place exists opposite Trelaporte. Here 
 the convex curvature of the valley turns again to the east. Across 
 this portion of the glacier a line was set out on the 28th of July, 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 
 
 140. Here, when we descend to the glacier, the mere estimate of 
 distances by the eye shows us that the three stakes which moved 
 fastest, viz. the 9th, 10th, and llth, are 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 ? 
 
 141. 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 get it out and measure these lines, noting the distance 
 of every stake from the side of the glacier. We can then 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. 
 
 142. There were five pairs of such points upon our Fourth Line, 
 D D', and here are their velocities : 
 
 ***v Eftotom points ; motion in inches ..... 13 15 16 18 20 
 ..... 15 17 22 23 23 
 
 In every case here the stake at the eastern side moved more rapidly 
 than the equivalent stake at the western side. 
 
 143. 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 17 
 
 Western points ....... 13 15 19 
 
 D 
 
( 34 ) 
 
 144. Here the three points on the eastern side move more rapidly 
 than the equivalent points on the western side. 
 
 145. 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 Trelaporte 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. 
 
 23. 
 New Law of Glacier Motion. 
 
 146. Let us look at these facts and express them 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. 
 
 147. At Trelaporte this line would lie east of the centre ; at the 
 Ponts it would lie west of the centre ; hence in passing from Trela- 
 porte 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. 
 
 148. The points on the axis which mark the transition from eastern 
 to western bending, and the reverse, may be called points of contrary 
 flexure. * 
 
 149. 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 : 
 
 150. When a glacier moves through a sinuous valley the locus of the 
 point 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 therefore crosses the axis of the glacier at each point of contrary 
 flexure. 
 
 151. The dotted line on the Outline Plan (page 30) represents 
 the locus of the point of maximum motion, the firm line marking the 
 centre of the glacier. 
 
 152. 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. 
 
( 35 ) 
 
 24. 
 
 153. 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 MEE DE GLACE. 
 
 At Trelaporte 20 inches a day. 
 
 At les Fonts 
 
 Above the Montanvert 
 At the Montanvert . . 
 Below the Montanvert 
 
 26 
 34 
 33* 
 
 154. There is thus an increase of rapidity as we descend the gla- 
 cier from Trelaporte to the Montanvert ; the maximum motion at the 
 Montanvert being fourteen inches a day greater than at Trelaporte. 
 
 25. 
 Motion of Tributary Glaciers. 
 
 155. 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. As we stood 
 beside that cleft 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 ? I think you would 
 conclude that the motion down the broad and slightly-inclined valleys 
 of the Geant and the Lechaud would be comparatively slow, but that 
 the water would force itself with increased rapidity through the 
 " narrows " of Trelaporte. Let us test this notion. 
 
 156. Planting our theodolite in the shadow of Mont Tacul, and 
 choosing a suitable point at the opposite side of the Glacier du Geant, 
 we fix on the 29th of July a series of ten stakes across the glacier. 
 The motion of this line in 24 hours was as follows : 
 
 MOTION OF GLACIER DU 
 
 SIXTH LINE: HH' UPON SKETCH. 
 
 Stake ..... 1 2 3 4 5 6 7 8 ' 9 10 
 Inches ..... 11 10 12 13 12 13 11 10 9 5 
 
 157. Our conjecture is fully verified. The maximum motion here 
 is 7 inches a day less than that of the Mer de Glace at Trelaporte 
 (139). _ 
 
 * 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. 
 
 D 2 
 
( 86 ) . 
 
 158. And now for the Lechaud branch. On the 1st of August 
 we fix ten stakes across this glacier above the point where it is joined 
 by the Talefre. Measured on the 3rd of August, and reduced to 
 24 hours, the motion was found to be 
 
 MOTION OF GLACIEK DE LECHAUD. 
 
 SEVENTH LINE: KK' UPON SKETCH. 
 
 Stake 12 3 456789 10 
 
 Inches 58 10 998697 6 
 
 159. Here our conjecture is still further verified, the rate of 
 motion being even less than that of the Glacier du Geant. 
 
 26. 
 Lateral Compression of a Glacier. 
 
 160. With our minds 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 divide upon the 
 trunk the tributary glaciers from each other. Cast your eye then first 
 upon the Glacier du Geant ; realize its width in its own valley ; and 
 see how much narrower it is at Trelaporte. The aspect of the Lechaud 
 is still more surprising. The broad ice-stream is squeezed upon the 
 Mer de Glace to a narrow white band between its bounding moraines. 
 The Talefre undergoes similar compression. Let us descend, shake 
 out our chain, measure, and express in numbers the width of the tribu- 
 taries, and the actual amount of compression suffered at Trelaporte. 
 
 161. We find the width of the Glacier du Geant to be 5155 links, 
 or 1134 yards. 
 
 162. The width of the Glacier de Lechaud we find to be 3725 
 links, or 825 yards. 
 
 163. The width of the Talefre we find to be 2900 links, or 638 
 yards. 
 
 164. The sum of the widths of the three branch glaciers is there- 
 fore 2597 yards. 
 
 165. At Trelaporte these three branches are forced through a 
 gorge 893 yards wide, or one-third of their previous width, at the rate 
 of 20 inches a day. 
 
 166. If we limit our view to the Glacier de Lechaud, the facts are 
 still more astonishing. Previous to its junction with the Talefre, this 
 glacier has a width of 825 "yards; in passing through the jaws of the 
 granite vice at Trelaporte, its width is reduced to 88 yards, or in 
 round numbers to one-tenth of its previous width. 
 
 167. Are we to understand by this that the ice of the Lechaud is 
 squeezed to one-tenth of its former volume f By no means. It is 
 
( 37 ) 
 
 mainly a change of form, not of volume, that occurs at Trelaporte. 
 Previous to its compression, the glacier resembles a plate of ice lying 
 flat upon the bed of the glacier. After its compression it resembles a 
 plate fixed upon its edge. The squeezing, doubtless, has deepened the 
 ice. 
 
 27. 
 Longitudinal Compression of a Glacier. 
 
 168. The ice is forced through the gorge at Trelaporte by a 
 pressure from behind. The Glacier du Geant, immediately above Tre- 
 laporte represents a piston or a plug which drives the ice through the 
 
 >rge. What effect must this pressure- have upon the plug itself ? 
 ming alone renders it probable that the pressure will shorten the 
 plug ; that the lower part of the Glacier du Geant will to some extent 
 yield to the pressure from behind. 
 
 169. 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 Geant, a line, which we 
 will call line No. 1 (F F' upon sketch, p. 30). 
 
 170. 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. 
 
 171. 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. 
 
 172. The daily motion of the centres of these three lines is as 
 follows : 
 
 LONGITUDINAL COMPRESSION OF GLACIER DU GEANT. 
 
 Inches. Distances asunder. 
 
 No.l .. .. 20-55J I 545yardg 
 
 No. 2 .. .. 15'43| 
 
 No. 3 .. .. 12-75J ' ' 487 " 
 
 173. The first line here moves 5 inches a day more than the second ; 
 and the second nearly 3 inches a day more than the third. Our 
 reasoning is therefore confirmed. Our ice-plug, which is in round 
 numbers 1000 yards long, is shortened by the pressure exerted on its 
 front at the rate of about 8 inches a day. 
 
 174. A river descending the Valley du Geant would behave in sub- 
 stantially the same fashion. It would have its motion on approaching 
 Trelaporte diminished, and it would pour through the defile with a 
 velocity greater than that of the water behind. 
 
( 38 ) 
 
 28. 
 Motion of Top and Bottom of Glacier. 
 
 175. 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. 
 
 176. At the Tacul a wall of ice about 150 feet high has already 
 attracted our attention (100). Bending round to join the Lechaud the 
 Glacier du Geant is here drawn away from the mountain side, expos- 
 ing this 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 the risk of boulders falling upon us from 
 above ; but we succeed in measuring the motions of all three. For 
 24 hours the motions are : 
 
 Top stake 6 inches. 
 
 Middle stake .. .. 4J 
 Bottom stake . . . . 2| 
 
 177. 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. 
 Sliding and Flowing. Hard Ice and Soft Ice. 
 
 178. 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. 
 
 179. One portion of the motion of a glacier is due to this bodily 
 sliding of the mass over its bed. We have seen in our journeys 
 streams formed by the melting of the ice, and escaping through 
 cracks . and crevasses to the bed of the glacier. The fine matter 
 
( 39 ) 
 
 ground down is thus washed away ; the bed is kept lubricated, and 
 the sliding of the ice rendered more easy than it would other- 
 wise be. 
 
 180. 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. 
 
 181. Now the Her de Glace in summer is in this thawing con- 
 dition. 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 yield- 
 ing the ice is, the more free will be this motion, and the more readily 
 also will it be forced through a defile like Trelaporte. 
 
 182. But in winter the thaw ceases ; the quantity of water reach- 
 ing the bed of the glacier is diminished or 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 mea- 
 surements give no clue to the winter motion. 
 
 183. I will not 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. 
 
 30. 
 Winter on the Her de Glace. 
 
 184. 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 midnight 1859, your deputy reaches Chamouni. 
 
 185. The snow fell heavily on the 26th of December; but on the 
 27th, during a lull in the storm, we turn out. There are with me 
 four good men. They tie planks to their feet to prevent them from 
 sinking in the mass ; I neglect this precaution and sink often to the 
 waist. Four or five times during an ascent the snow-slope cracks 
 with an explosive sound, and the snow threatens to come down in 
 avalanches. Four years later, in the spring of 1863, a mighty climber 
 and noble guide, named Johann Joseph Bennen, was lost, through the 
 cracking and subsequent slipping of snow on such a slope. The snow 
 falling during our ascent was in that particular condition when the 
 granules adhere, and hence every flake falling on the trees was 
 retained. The laden pines presented beautiful and often fantastic 
 
forms. Annexed is a sketch of one of them. After five hours and 
 a half of arduous work the Montanvert was attained. 
 
 Snow-laden Pine-tree. 
 
 186. We unlocked the forsaken auberge, round which the snow 
 was reared in buttresses. I have already spoken of the complex 
 play of crystallizing force. 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 to the glass of 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 down- 
 wards 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 (53). 
 
 187. 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 dis- 
 tributed 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. 
 
 188. To get higher up the valley was impracticable; the snow 
 was too deep, and the aspect of the weather too threatening; so I 
 planted the theodolite amid the pines a little way below the Mon- 
 tanvert, 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 bare portions of the glacier. In the 
 midst of this turmoil the men continued to work. Bravely and stead- 
 fastly stake after stake was set, until at length a series of ten of them 
 were fixed across the glacier. 
 
 189. 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 storm I thought it possible that the 
 stakes and the snow which held them might be carried bodily away 
 before the morning. The storm, however, lulled. We rose with the 
 dawn, but the air was thick with descending snow. It was all com- 
 posed of those exquisite six-petaled flowers, or six-rayed stars, which 
 have been already figured and described. 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 innu- 
 merable were showered upon the heights. Contrary to my expecta- 
 tion, I was able to see the men and direct them through the 
 shower. 
 
 190. 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 in breaking 
 through the glacier snow was prodigious. But they did not flinch, 
 and after a time the leader stood behind the farthest stake, and cried, 
 Nous avowfini. 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 the feeling of a general who 
 had won a small battle. 
 
( 42 ) 
 
 191. 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. Here our winter visit to the Mer 
 de Glace ends ; next morning your deputy was on his way to London. 
 
 31. 
 Winter Motion of the Mer de Glace. 
 
 192. Here are the measurements executed in the winter of 
 1859 : 
 
 LINE No. I. 
 
 Stake 1 2 34 5 6 7 8 9 10 11 
 
 Inches 7 11 14 13 14 14 16 16 12 12 7 
 
 LINE No. II. 
 
 Stake 12 345 678910 
 
 Inches 8 10 14 16 16 16 18 17 15 14 
 
 193. Thus the winter motion of the Mer de Glace near the Mon- 
 tanvert is, in round numbers, half the summer motion. 
 
 194. As in summer, the eastern side of the glacier at this place 
 moved quicker than the western. 
 
 32. 
 Birth of a Crevasse : Reflections. 
 
 195. Preserving the notion that we are working together, we will 
 now enter upon a new field of inquiry. We have wrapped up our 
 chain, and are turning homewards after a hard day's work upon the 
 Glacier du Geant, when under our feet, as if coming from the body 
 of the glacier, an explosion is heard. Somewhat startled, we look 
 inquiringly over the ice. The sound is repeated ; several shots being 
 fired in quick succession. They seem sometimes to our right, some- 
 times to our left, giving the impression that the glacier was breaking 
 all round us. Still nothing is to be seen. 
 
 196. We closely scan the ice, aud after an hour's strict search we 
 discover the cause of the reports. They announce the birth of a cre- 
 vasse. Through a pool upon the glacier we notice air bubbles ascend- 
 ing, and find the bottom of the pool crossed by a narrow crack, from 
 which the bubbles issue. Bight and left from this pool we trace the 
 
( 43 ) 
 
 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 thick 
 knife-blade. 
 
 197. 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 Geant and the Talefre begin 
 as narrow cracks, which open gradually to crevasses. We are thus 
 taught in an instructive and impressive way that appearances sug- 
 gestive of very violent action may really be produced by processes so 
 slow as to require refined observations to detect them. In the pro- 
 duction 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 convul- 
 sion ; 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 self-same almost imperceptible way. 
 
 33. 
 Icicles. 
 
 198. Having thus informed ourselves regarding the origin of the 
 crevasses, we will now examine them and classify them. They are 
 grandest on the higher neves, 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. 
 
 199. 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. 
 
 200. Gathering up such knowledge as we possess, and reflecting 
 upon it patiently, let us found on it, if we can, a theory of icicles. 
 
 201. 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. 
 
( 44 ) 
 
 202. 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. 
 
 203. 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 with- 
 drawal 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 cold of the air may be 
 demonstrated. 
 
 204. 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 thus 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. Here, I think, we have the 
 secret of our Alpine stalactites. In the Alps, however, 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 
 vaster size. 
 
 205. 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 realize 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. 
 
 34. 
 Crevasses. The Bergschrund. 
 
 206. Having cleared away this difficulty, let us turn once more to 
 our crevasses, taking them in the order of their formation. First 
 then above the neve 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 neves 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 Berg- 
 schrund. It often surrounds a peak like a fosse, as if to defend it against 
 the assaults of climbers. 
 
 207. 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 com- 
 pelled 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 neve ; 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 tight angles to the line 
 of tension. 
 
 35. 
 Transverse Crevasses. 
 
 208. Both on the neve 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 ten- 
 sion. Take, for example, the ice-fall of the Geant, or of the Talefre, 
 above which you know the crevasses yawn terribly. Imagine the 
 neve and the glacier entirely peeled away, so as to expose the surface 
 over which they move. From the Col du Geant 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 Geant, where the slope would become gentle once 
 more. 
 
 209. Think of the neve 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. Realize clearly 
 what must occur. The surface of the neve is evidently thrown into a 
 state of strain ; it breaks and forms a crevasse. Each fresh portion of 
 the neve as it passes the brow is similarly broken, and thus a succes- 
 sion 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 seracs 'being 
 the result. Down the fall both ridges and seracs are borne, the 
 dislocation being augmented during the descent. 
 
 210. 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 pres- 
 sure. 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. 
 
 211. 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 closure of the fissures is a consequence of this 
 bending. 
 
 212. 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 -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 in 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 steep- 
 ness relaxes, the crevasses heal up, and the glacier becomes once more 
 continuous and compact. 
 
 36. 
 Marginal Crevasses. 
 
 213. 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. % 
 
 214. 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 en- 
 hanced if we could see beforehand the perplexing and delusive appear- 
 ances accounted for by the explanation. Could I follow out my wishes, 
 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 there are two well-known glaciers the Ober Grindel- 
 wald and the Unter Grindelwald glaciers on the latter of which our 
 observations should commence. 
 
 215. 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 
 
( 47 ) 
 
 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. 
 
 216. 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. 
 
 217. 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 my pencil and note-book in 
 my hand, I will at once make the matter clear to you. 
 
 ASS' C 
 
 T' 
 
 D 
 
 218. Let A C, in the annexed figure, be one side of the glacier, 
 and BD the other. The line AB being higher up than CD: the 
 direction of motion would then be 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'. 
 
 219. 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 dis- 
 torted to the lozenge-shaped figure T' i'. Fix your attention upon the 
 diagonal T i of the square ; in the lower 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. 
 
( 48 ) 
 
 220. 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 side is copiously fissured even at places where the centre 
 is free from crevasses. 
 
 221. 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 con- 
 vexity of the curve is turned upwards, as mechanical principles declare 
 it ought to be. But if you were ignorant of those principles you 
 would never infer from the aspect of these curved crevasses 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. 
 
 37. 
 Longitudinal Crevasses. 
 
 222. 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. 
 
 223. 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 moun- 
 tains. 
 
 38. 
 
 Crevasses in relation to Curvature of Glacier. 
 
 224. One point in addition remains to be discussed, and your pre- 
 sent knowledge will enable you to master it in a moment. You 
 remember at an early period of our 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. 
 
 225. Let A B and C D, in the annexed diagram, 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, I think, 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 no', and the 
 
 length m o to m o', and you see n 
 
 plainly that the stretching of the 
 
 short line, in comparison with its c - 
 
 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 mo' ; so that if one 
 
 of them were to break under the A Montanvert B 
 
 strain it would be the short one. 
 
 226. 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 then set up between the centre and sides. But 
 the displacement of the point of maximum motion through the curva- 
 ture 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. 
 
 227. Here indeed resides the difficulty of getting along the eastern 
 side of the Mer de Glace, a difficulty which was one reason for our cross- 
 ing 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. 
 
 39. 
 
 Moraine-ridges, Glacier Tables, and SandnGones. 
 
 228. When you and I first crossed the Mer de Glace from Trela- 
 porte 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 ? 
 
 229. 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. 
 
 230. 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-sur- 
 face passes it on to the snow which is liquefied by it. It is quite mani- 
 fest 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. 
 
( 50 ) 
 
 231. Now cloth is what is called a bad conductor of heat. It does 
 not permit heat to travel freely through it. But where the heat 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. 
 
 232. 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 fall- 
 ing ? 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. 
 
 233. If you understand this you have already mastered the cause 
 of the moraine-ridges. They are produced by no swelling of the ice 
 upwards. But the ice underneath the rocks and rubbish being pro- 
 tected from the sun, the glacier right and left melts away and leaves 
 a ridge behind. 
 
 234. 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. Annexed is a sketch of one of the Tables 
 of the Mer de Glace. 
 
 235. Notice that a glacier table is hardly ever set square upon its 
 stalk. It generally leans to one side, and repeated observation 
 
( 51 ) 
 
 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 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. 
 
 236. Here is what may be 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. 
 
 237. 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 Geant are honeycombed by the sinking of such spots of 
 dirt into the ice. 
 
 238. 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 moun- 
 tains 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 Gorner, the Aletsch, the Morteratsch, and other 
 glaciers, they, reach sometimes a height of ten or twenty feet. 
 
 E 2 
 
( 52 ) 
 
 40. 
 The Glacier Mills or Moulins. 
 
 239. 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 occupy- 
 ing our attention that fresh subjects of inquiry arise in scientific 
 investigation. 
 
 240. Thus in marching over the ice near Trelaporte we were often 
 struck by a sound resembling low rumbling thunder. We subse- 
 quently sought out the origin of this sound and found it. 
 
 241. A large area of this portion of the glacier is unbroken. 
 Driblets of water have room here 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. 
 
 242. This funnel and its cataract form a glacier Mill or Moulin. 
 
 243. 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 fol- 
 lowed by sudden death. But caution upon the glaciers and moun- 
 tains ought, by habit, to be made a second nature to explorers like 
 you and me. 
 
 244. 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. 
 
 245. 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 bottom of a weight 
 enables a mariner to judge of a sea bottom. We employ such a 
 weight, but cannot reach the bottom of the glacier. A depth of 163 
 feet is the utmost reached by our plummet. 
 
 246. From the 28th of July to the 8th of August the progress of 
 
( 53 ) 
 
 the Grand Moulin has been watched. 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 the 
 8th of August it had moved 198 inches, which amounts to about 
 18 inches in twenty-four hours. No doubt next summer upon the 
 Mer de Glace a Grand Moulin will be found thundering near Trela- 
 porte ; but like the crevasse of the Grand Plateau already referred to, 
 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. 
 
 41. 
 Sea- ice and Icebergs. 
 
 247. We are now equipped intellectually for a campaign into 
 another territory. Water becomes heavier when salt is dissolved in 
 it, and more difficult to freeze. Sea water is therefore heavier than 
 fresh, and the Greenland Ocean requires to freeze it a temperature 
 3^ degrees lower than fresh water. When concentrated till its specific 
 gravity reaches 1*1045, sea water requires for its congelation a tem- 
 perature 18^ degrees lower than the ordinary freezing-point.* 
 
 248. But even when the water is saturated with salt, the crystal- 
 lizing force studiously rejects the salt, and devotes itself to the conge- 
 lation 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. 
 
 249. This exclusiveness, if I may use the term, of the water mole- 
 cules ; 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 crystallizing force. 
 
 250. 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. 
 
 251. But among the drifting masses of flat sea-ice, vaster masses 
 sail, which spring from a totally different source. These are the Ice- 
 bergs 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. 
 
 * Scoresby. 
 
( 54 ) 
 
 252. 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. 
 
 253. 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 ; and the melting 
 of icebergs is on this account so slow, that when large they some- 
 times maintain themselves till they have been drifted 2000 miles 
 from their place of birth. 
 
 254. What is their origin ? The Arctic glaciers. From the moun- 
 tains in the interior the indurated snows slide into the valleys and 
 charge 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 south- 
 
 . ward to be finally dissolved in the warm waters of the Atlantic. I 
 shall have occasion to show you pictures of some extraordinary ice- 
 bergs photographed during a recent expedition by Mr. Bradford to 
 the Northern seas. 
 
 42. 
 The Mdrgelin See and its Icebergs. 
 
 255. I am, however, unwilling that you should quit Switzerland 
 without seeing such icebergs as it can show, and indeed there are 
 other nobler glaciers than the Mer de Glace with which you ought to 
 be acquainted. In tracing the Khone to its source, you have already 
 ascended the valley of the Khone. 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 ^Eggischhorn. This we shall 
 make our head-quarters while we explore that monarch of European 
 ice-streams,- the great Aletsch glacier. 
 
 256. Including the longest of its branches, this noble ice-river is 
 about 20 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 neves. From three great 
 valleys formed in the heart of the mountains these neves are poured, 
 uniting together to form the trunk of the Aletsch at a place named 
 
( 55 ) 
 
 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. 
 
 257. Our hotel is not upon the peak of the ^Eggischhorn, 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. Pro- 
 longing 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. 
 
 258. 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 60 or 70 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 now occupied by its water of lique- 
 faction. 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. 
 
 259. This is the beautiful little lake of Margelin, or, as the Swiss 
 here call it, the Margelin 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. 
 
 260. 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, how- 
 ever, be rendered white again by the action of the sun. The vaster 
 icebergs of the northern seas sometimes roll over in the same fashion. 
 
( 56 ) 
 
 [43. 
 Motion of the Aletsch Glacier. 
 
 261. While here we will dwell for a moment on the motion of the 
 great Aletsch glacier, as measured by Mr. Vaughan Hawkins and 
 myself in 1860. The theodolite was planted high among the rocks 
 on the western flank of the mountain, about half a mile above the 
 Margelin See. Here, on the 14th of August, thirty-four stakes were 
 fixed in a straight line across the glacier. The displacement of these 
 stakes was measured on the 16th. Reduced to its daily rate the 
 motion was as follows : 
 
 MOTION OF GKEAT ALETSCH GLACIEB. 
 
 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 16 12 12 
 
 West. 
 
 262. The maximum motion here is 19 inches a day. Probably 
 the eastern side of the glacier here is shallow, the retardation of the 
 bed making the motion of the eastern stakes inconsiderable. 
 
 263. The width of the glacier here was 9030 links, or about a 
 mile and a furlong. 
 
 44. 
 Ancient Glaciers of Switzerland. 
 
 264. You have not lost the memory of the old Moraine, which 
 interested us so much in our first ascent from the source of the Arvei- 
 ron ; 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. 
 
 265. Close to the existing glacier, for example, we have repeat- 
 edly 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 50, 60, 80, or even 100 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, ice- 
 marks, ancient or modern, wherever they appear. 
 
 266. But the elevations at which we have found such markings 
 
( 57 ) 
 
 might well shake belief in the conclusions to which they point. 
 I will carry you to a region which exhibits them on a grander and 
 more impressive scale than the Mer de Glace. 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 down- 
 wards 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 yes- 
 terday. We may trace these evidences to a height of fully 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. 
 
 267. Yonder is the summit of the Siedelhorn ; and if we gain it, 
 the Unteraar, glacier will be like a map below us. From this com- 
 manding 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. 
 
 268. We now scamper down the Siedelhorn, get once more into 
 the valley of Hasli, along which we follow for a score of 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 also appear. We follow the footsteps 
 of the glacier to the Lake of Brientz, and if we prolonged our inquiries 
 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. 
 
 269. Instead of the vale of Hasli we might take the valley of the 
 Ehone. 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 bed 
 of which they entirely filled. Other evidences prove that the glacier 
 did not end here, but pushed across the low country until it encoun- 
 tered the limestone barrier of the Jura mountains. 
 
 45. 
 Erratic Blocks. 
 
 270. What are these other evidences? We have seen mighty 
 rocks poised on the moraines of the Mer de Glace, and we now know 
 
( 58 ) 
 
 that unless they are split and shattered by the frost these rocks will, at 
 some distant day, be landed 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. 
 
 271. 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 Neufchatel, and at 
 about 800 feet above the lake of Neufchatel, 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 the scarrings of the 
 ancient glacier which landed the boulders here. 
 
 272. The most famous of these rocks, called the Pierre a Bot, 
 measures 50 feet in length, 40 in height, and 20 in width. Multiply- 
 ing these three numbers together, we obtain 40,000 cubic feet as the 
 volume of the boulder. 
 
 273. But even this is small compared to some of the rocks which 
 constitute the freight of even recent glaciers. Let us visit another 
 of them. From the town of Visp in the valley of the Rhone we walk 
 southwards to Stalden. Here the valley divides into two branches, 
 the right branch running to St. Nicolas 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. 
 
 274. Close to this inn is the greatest glacial 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-forty 
 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 boulder at hand which would be considered vast were 
 it not dwarfed by the aspect of its huger neighbour. 
 
 275. 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 
 
( 59 ) 
 
 ancient ice. M. Agassiz afterwards developed and wonderfully ex- 
 panded the discovery. 
 
 46. 
 Ancient Glaciers of England, Ireland, Scotland, and Wales. 
 
 276. 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. Dr. Hooker 
 found the cedars of Lebanon growing on ancient moraines. 
 
 277. We have made the acquaintance of the Reeks of Magilli- 
 cuddy 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 "right. Remembering our travels in the Alps, you would in- 
 fallibly 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. 
 
 278. 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 vege- 
 tation ; but both the naked and clothed islands are all glaciated. 
 The rains of ages have 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 
 of man's introduction philosophers seem to know but little. 
 
 279. 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. 
 
The Glacial Epoch. 
 
 280. 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. 
 
 281. 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. 
 
 282. 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 the ice. 
 
 283. 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. 
 
 284. 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. 
 
 285. In fact, simply to liquefy ice a large quantity of heat is 
 necessary, and to vaporize water a large quantity is also necessary. 
 And inasmuch as this heat does not render the water warmer than the 
 ice, nor the steam warmer than the water, it was supposed to be hidden 
 in the water and in the steam. And it was therefore called latent 
 heat. 
 
 286. We are now only concerned with the latent heat of vapour. 
 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 1000 times the amount of heat necessary to raise 1 Ib. of 
 water one degree in temperature. 
 
 287. 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 1 Ib. of iron 
 one degree in temperature. 
 
 288. This quantity of heat would raise the temperature of 5 Ibs. 
 of iron 2000 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. 
 
( 61 ) 
 
 289. 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 Ibs. of cast iron to its 
 melting-point. 
 
 290. 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. 
 
 291. 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. 
 
 48. 
 Glacier Theories. 
 
 292. You have not forgotten, and hardly ever can forget, our 
 climbs to the Cleft Station. Thoughts were there suggested which 
 we have not yet discussed. We saw the branch glaciers coming down 
 from their neves, welding themselves together, pushing through 
 Trelaporte, 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. 
 
 49. 
 Dilatation and Sliding Theories. 
 
 293. Still this was not the notion expressed by the majority of 
 writers upon glaciers. Scheuchzer of Zurich, a great naturalist, 
 visited ice-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 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 down- 
 
( 62 ) 
 
 wards. He added to this theory other notions of a less scientific 
 kind. 
 
 294. 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 dis- 
 tinctly 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 to be incorrect. 
 
 295. In 1760, Altmann and Griiner propounded the view that 
 glaciers moved by sliding over their beds. Nearly forty years sub- 
 sequently, this notion was revived by De Saussure, and has therefore 
 been called " De Saussure's Theory," or the " Sliding Theory " of 
 glacier motion. 
 
 296. 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. 
 
 50. 
 Plastic Theory. 
 
 297. 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. 
 
 298. Still the idea found its advocates. In a little book, pub- 
 lished 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 -moun- 
 tains an observation presents itself, which appears sufficient to explain 
 all. It is that the entire mass of ice is connected, and presses from 
 above downwards after the manner of fluids. Let us then regard 
 the collection of 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." * Here probably for the first time the quality of 
 plasticity is ascribed to the ice of glaciers. 
 
 * I am indebted to my distinguished friend Prof. Studer of Berne for direct- 
 ing 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. 
 
( 63 ) 
 
 299. Bordier was 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 
 he laid before the Royal Academy of Sciences of Savoy his * Theory 
 of the Glaciers of Savoy,' and his communication is printed in the 
 tenth volume of the ' Memoirs of the Academy.' 
 
 300. 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 expres- 
 sions, which fail however to give an adequate idea of his work: 
 " 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." 
 
 301. This reads like a prediction of what has since been esta- 
 blished 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." 
 
 302. To fully test his conclusions, Rendu required the accurate 
 measurement of glacier motion. Had he added to his other endow- 
 ments the practical skill of a land-surveyor he would be now 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. 
 
 303. Another block, the transport of which he subsequently mea- 
 sured 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. 
 
( 64 ) 
 
 304. 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 pre- 
 sent 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 
 complete. I certainly shall not fear your verdict. 
 
 305. 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 sub- 
 stance when the means of doing so are at hand. In science thought, 
 as far as possible, ought to be wedded to fact. 
 
 51. 
 
 Viscous Theory. 
 
 306. And here the merits of a celebrated glacialist already 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, and sought to found upon precise measurements a 
 " Viscous Theory " of glacier motion. 
 
 307. 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 may be 
 doubted whether he needed their inspiration. Had Rendu not pre- 
 ceded him, he would probably none the less have grasped the idea 
 of viscosity, executing his measurements and applying his reasoning 
 powers 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 never to be forgotten in the scientific history of glaciers. 
 
 308. The theory advocated by Prof. Forbes was enunciated by 
 himself in these words : " A glacier is an imperfect fluid, or viscous 
 body which is urged down slopes of a 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 perpetually take the place of that which is 
 
( 65 ) 
 
 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 downwards. 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 com- 
 pared with those of Bendu, still less with those of Forbes. 
 
 309. 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 and 
 rightly cited as the most clear and conclusive illustrations of the 
 doctrine. 
 
 310. But why, with proofs before him more copious and charac- 
 teristic 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. 
 
 9 
 
 311. 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 ? 
 
 312. 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 stretch- 
 ing, which is characteristic of a viscous substance, such crevasses 
 would not be formed. 
 
 313. 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 Signer Bianconi, have, 
 moreover, recently made experiments on ice which strikingly illus- 
 trate the flexibility of the substance. These experiments merit, and 
 will receive, full attention at a future time and in another place. 
 
 52. 
 
 Eegelation Theory. 
 
 314. 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 
 
( 66 ) 
 
 1850, that when two pieces of thawing ice are placed together they 
 freeze together at the place of contact. 
 
 315. This fact may not surprise you, and I cannot go fully into 
 the reason why it surprised Mr. Faraday and others, or why 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. 
 
 316. The word Begelation has been invented by Dr. Hooker to 
 express the freezing together of two pieces of thawing ice observed by 
 Faraday. 
 
 317. Without entering upon the cause of regelation, you may 
 certainly accept it as a fact. 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. 
 
 318. 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 
 
 319. Consider what follows from these observations. Snow con- 
 sists of small particles of ice. Now if by pressure we squeeze away 
 the air entangled in 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 very complete, the squeezed snow may be expected to 
 assume the appearance of compact ice. 
 
 320. 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. 
 
 321. Place a compact mass of ice in a proper mould, and subject it 
 to pressure. It breaks into 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. 
 
 322. 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. 
 
( 67 ) 
 
 323. 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. 
 
 324. Now our first experiment illustrates the conversion of the 
 snows of the higher Alpine regions into ice. A similar experiment 
 was made by M. Schlagintweit prior to the discovery which explains it. 
 The deeper layers of the neve 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. 
 
 325. In glaciers, however, we have also ample illustrations of rude 
 fracture and regelation. The broken cascades are mended at their 
 bases ; and 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. 
 
 326. 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. 
 
 53. 
 The Veined Structure of Glacier Ice. 
 
 327. This power of yielding to pressure produces another effect 
 of the highest interest ; but to understand it we must first know some- 
 thing of what is called slaty cleavage. 
 
 328. Go to the slate quarries of Bangor and 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. 
 
 329. All through the quarry you will notice the direction of these 
 planes to be perfectly constant. How is this laminated structure to 
 be accounted for ? 
 
 330. You might be disposed to consider that cleavage is a case of 
 stratifraction or bedding ; for it is true that in various parts of Eng- 
 land 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 cut across the 
 planes of cleavage. 
 
 
( 68 ) 
 
 331. This continued a puzzle to geologists till the late Mr. 
 Daniel Sharpe made the discovery that shells and other fossils and 
 bodies found in slate rocks were invariably flattened out in the planes 
 of cleavage. This observation was ably followed up by Mr. Sorby. 
 You may accept it as proved that slate rocks have suffered great 
 pressure at right angles, or at all events at a high angle to the planes 
 of cleavage. 
 
 332. These slate Kocks were once a plastic mud, and they may by 
 grinding be converted into their original form. When such mud is 
 carefully squeezed so as to flatten in one direction and spread out in 
 another, it is reconverted into slate. Various other substances, white 
 wax for example, when properly compressed, show a cleavage still 
 more perfect than that of slate. 
 
 333. But what bearing have these facts upon the ice of glaciers ? 
 This bearing. The ice of the higher regions is whitish through the 
 diffusion of small air bubbles within it. At the sides of the glaciers, 
 and at the bottom of the cascades, this ice is sometimes subjected to 
 enormous pressure. It yields laterally as the slate-mud has yielded, 
 and a laminated structure is the consequence. On the surface of the 
 glacier, under the medial moraines, and on the sides of the crevasses, 
 the lamination reveals itself as clear blue veins or streaks drawn 
 through the whiter ice. 
 
 334. This is the veined structure, or the ribboned structure of glacier 
 ice. It was first described by M. Guyot, but our knowledge of it, 
 and our interest in it, have been mainly derived from the labours of 
 Professor Forbes. 
 
 Beyond this my time will not permit me to go. Farewell ! 
 
 THE END. 
 
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