Theory of the Glaciers of Savoy ... Translated [from the French] by A. Wills ... To which are added the original memoir; and supplementary articles by P. G. Tait ... and J. Ruskin. ... Edited, with introductory remarks, by G. Forbes, etc

Front matter

THEORY OF THE GLACIERS OF SAVOY THEORY OF THE GLACIERS OF SAVOY, BY M. LE CHANOINE RENDU, (MEM. DE LA SOCIETE-" ACADEMIQU E DE SAVOIE, X. 95, 1841). translated by ALFRED WILLS, Q. C. Late President of the Alpine Clv.b. TO WHICH ARE ADDED THE ORIGINAL MEMOIR ; AND SUPPLEMENTARY ARTICLES BY P. G. TAIT, M.A., Professor of Natural Philosophy in tlie University of Edinburgh, AND JOHN RTJSKIN, D.C.L, Professor of Fine Arts in the University of Oxford. EDITED, WITH INTRODUCTORY REMARKS, BY GEORGE FORBES, B.A, Professor of Natural Philosophy in the Andersonian University, Glasgow. MACMILLAN AND CO. 1874. GLASGOW: PRINTED AT THE UNIVERSITY PRESS. [Reprint of the original Title page of 31. Rendu's Memoir.] THEORIE DES GLACIERS DE LA SAVOIE. PAR M. LE CHANOINE RENDU, CHEVALIER DU MERITS CIVIL ; SECRETAIRE PERPETUEL DE LA SOCIETe" ROY.

ACADEM. DE SAVOIE. Benedicite glacies et nives Domino. Daniel, Cant. Trium puer. CHAMBERS : CHEZ PUTHOD, IMPRIMEUR-LIBRAIRE. 1840. CONTENTS. Paqb Introductory Remarks, - - 1 M. Rendu's Memoir, - ... 17 Forbes and Dr. Tyndall, by Professor Tait, - - 163 Extract from Fors Clavigera, by Mr. Ruskin, - 199 Extract from Principal Forbes' Reply, - - On the publication of the Forms of Water, at the end of 1872, it was with deep regret that I found certain allegations of anticipation by Bishop Rendu, and virtual plagiarism by Principal Forbes, reiterated by Professor Tyndall. I thereupon resolved to make a translation of Rendu's memoir, to be published without note or comment ; because the scarcity of the work prevented most people from being able to discover for themselves how unfounded these allegations were.

I had not advanced far in this matter when my friend Mr. Alfred Wills kindly offered to undertake the revision and completion of the translation. He has aimed at giving a fluent translation, which shall at the same time give correctly the very shades of meaning of the original. His intimate acquaintance with the glaciers of Savoy is a guarantee for its value. The original French is added, at his suggestion, for com parison in passages of doubtful meaning. When, however, some months later, on my return from the Continent, I saw Professor Tyndall's pamphlet entitled Principal Forbes and his Biographers, I felt that more than this must be done. The continued repetition of an accusation, however groundless, leads many to believe in A THE GLACIERS OF SA V0Y. its truth. I found that some who confided implicitly in my father's honour had been now led to imagine that he had not the right altogether on his side.

Introduction

TO RENDU's MEMOIR. The accompanying translation is published Avith the view of alloAving Bishop Rendu and Principal Forbes each to have such credit as may be due to him ; and of showing how fully Principal Forbes brought Rendu's labours before the world by reference to them in The Travels. There are not, I believe, more than two or three copies of the original in the British kingdom at present. I hold a high opinion of the general accuracy and depth of thought displayed by the writer, the more so since these are combined with a desire to explain pheno mena by existing causes. But these qualities are shoAvn more in the general descriptions (such as his explanation of the mountain breezes, his proofs that glaciers moA^e, and that stream glaciers are physically connected with glacier reservoirs) than in any attempt at proving the mode of motion of glaciers.

Professor Tyndall has always unduly exalted Rendu's more happy speculations, cast a veil over his less fortunate conjectures, and left unnoticed the fact that in many cases these ideas were thrown out in the form of vague suggestions. He has, on the other hand, reduced the value of Forbes' work to a minimum ; as in the Glaciers of the Alps, where he sums up Forbes' contributions to the viscous theory in two pages; or in the Forms of Water, p. 161, where he says that to the advocacy of Principal Forbes the public knowledge of the viscous theory is mainly due. Principal Forbes' claims to having established a viscous or plastic theory of glaciers rest mainly upon the folloAving observations made by him : — Neglecting the obvious facts of the moulding of the glacier to its bed, its compression and swelling, we have — 1.

The motion of the ice is continuous. 2. The centre moves faster than the sides, 3. This change of velocity is continuous. 4. The variation of velocity in the breadth of a glacier is proportional to the absolute velocity, at the time, of the ice under experiment. 5. The velocity of a glacier increases when the steep ness of its bed increases. 6. A continuous mass of ice on the Brenva became distorted without the formation of any crevasses. 7. The greater the heat and rainfall, the more liquid is the ice, as shown by its greater velocity. 8. The melting of the surface in summer is compen sated by a thickening (resulting from diminished fluidity in the auscous mass) in Avinter. 9. In cases where the tenacity of the ice is insufficient to bear the strain crevasses are formed. 10. These crevasses, and also the broken continuity of the Avhole mass, are closed up by time and cohesion. 11.

The differential motions of a viscous mass account for the veined structure. 12. In every respect, experiments on viscous sub stances show motions similar to those of glaciers. No wonder then that Professor Tyndall admits (perhaps unwittingly) that "it is undoubted that the glacier moves like a auscous body." (Forms of Water, p. 163.) But Principal Forbes could not Avith justice be asked to giAre Rendu or any one else credit for the above observations which he himself AAras the first to make. Rendu did not claim to explain the cause of motion of glaciers, nor to have made measurements which could lead to a theory; and it is unfortunate that Professor Tyndall should have thought that he could increase Rendu's credit by straining his words, and supporting his vieAV by imperfect quotations (four of which are alluded to in Forbes' Reply), so as to give him more than either he Avished for or deserved.

It may, perhaps, be said that greater clearness would have been given to this Avork by adding to it a reprint of the Reply written by Principal Forbes in answer to some of Professor Tyndall's objections. This may be true ; but the Reply has lately been reprinted as Ap pendix A to the Life and Letters of James David Forbes (Macmillan & Co., 1873). I conceive that it is more in accordance Avith the object of this volume to reprint only that article of the Reply in Avhich Principal Forbes cited the passages in his writings where he had referred to, or quoted from, Bishop Rendu. This ex tract Avill be found at the end of the present volume. Let the reader, after he has studied Rendu's interesting memoir, turn to the end of the book and consult the account of Rendu's views, given by Principal Forbes in his writings up to 1846.

He Avill then be able to judge whether Professor Tyndall Avas justified in saying that it devolved on him (Professor Tyndall) to call attention to passages in the memoir, of "cardinal import," "overlooked" by previous Avriters, "nearly twenty " years after their publication." After such an in vestigation, the reader Avill be more likely to come to the conclusion arrived at by Sir William Thomson, namely, that Principal Forbes " had done ample per " sonal justice to Rendu, and set off his ideas most " advantageously Avith reference to the true theory, so " as in fact to connect him in the mind of the reader " even more decidedly with the viscous theory as a " scientific truth, than the manner in Avhich he ex " pressed them required, or than he himself Avould have " expected as his unquestionable right."

In conclusion, I ought perhaps to state that, in ac cordance Avith an engagement entered into in 1871, I must leave this country in June next for the SandAvich Islands, to make observations on the approaching transit of Venus. Should the publication of this volume draw forth remarks which I ought to answer, the reason of my temporary silence Avill be understood. Yet I cannot conceive that this is likely to happen. Misunderstanding is possible only so long as the data required for its correction are not generally accessible. This is no longer the case. The controversy is now concluded; and an unbiassed judgment Avill, I feel sure, be given by the scientific Avorld, and the general public. George Forbes. RENDU ON GLACIERS.

Preface

. I had almost abandoned the idea of publishing this Avork, which was completed nearly two years ago, Avhen I saw the announcement of the Notice sur les Glaciers, &c, of M. Ch. Godefroi. The sight of this title alone sufficed to make me change what had been a doubt into a settled resolution. I read the work of M. Godefroi, and as I finished it I said, " My theory shall appear ; all imperfect as it is, the theories of M. Godefroi render it necessary." This natural philoso pher has said nothing of the most curious phenomena presented by glaciers. The crevasses, their depth, their form, the wells, the cracks, the inequalities of the surface, the convex form of the glaciers, the velocity of the flow of the ice, the rocks disgorged by the glacier, all this remains to be examined ; the question AVERTISSEMENT.

J'avais presque renonce a la publication de cet outage , qui est acheve' depuis pres de deux ans, quand j'ai vu paraitre la Notice sur les Glaciers, etc., de M. Ch. Godefroi. La seule Anie de ce titre a suffi pour me faire changer en resolution de"ter minee ce qui netait encore qu'une hesitation. J'ai lu le travail de M. Godefroi , et en le finissant , j'ai dit : ma theorie paraitra; tout imparfaite qu'elle soit, les systemes de M. Godefroi la rendent ne"cessaire. Ce savant n'a lien dit des ph^nomenes leg plus curieux que pr^sentent les glaciers. Les crevasses , leur profondeur, leur forme, les puits, les fentes, les rubosite's de la surface , la forme convexe des glaciers , la vitesse de fdcoulement des glaces, les rochers revomis par le glacier, toute cela reste a examiner ; la question est presque encore B is still almost untouched.

Nor is this all : the state of things imagined by M. Godefroi in order to explain the formation of moraines appears to me so _ contrary to observation and to the very nature of moraines, that I believe it to be necessary to submit once more to naturalists the explanations already given by others, whilst rendering them clearer by neAV observations, and I may also say by the impossibility of the cause assigned by M. Godefroi. Before visitino* the orlaciers, I had studied all that physicists, naturalists, and travellers had AATitten on this great phenomenon of nature. I had admitted provisionally only the explanations Avhich they gave of all the peculiarities AAdiich attend it ; but from the moment I could see for myself the principal glaciers that surround Mont Blanc, those of the Valais and of the Simplon, I felt convinced that everything had not been observed, and that the causes assigned to the phe nomena Avere not always correct.

Had the special knowledge of glaciers been the specific object of the travels of De Saussure, this learned obserA*-er Avould have left nothing to be said by toute entiere. Ce n'est pas tout : le systeme imagine* par M. Godefroi pour expliquer la formation des moraines me parait tellement contrair'e a l'observation et a la nature meme des moraines, que je crois n&essaire de remettre sous les yeux des naturalistes les explications deja donn^es par d'autres , en les rendant plus eVidentes par de nouvelles observations, et je dirai meme par Impossibility de la cause assignee par M. Godefroi. Avant de visiter les glaciers, j 'a vais dtudi<_ tout ce que les physiciens , les naturalistes et les voyageurs avaient e"crit sur ce grand phenomene de la nature. Je n'avais que provisoirement admis les explications qu'ils donnaient sur toutes les particular it qui l'accompagnent ; mais des l'instant oil j'ai pu voir par moi-meme les principaux glaciers qui entourent le Mont-Blanc , ceux du Valais et du St-Plon, j'ai cru reconnaitre que tout n'avait pas e'te observed , et que les causes assignees aux phe'no menes n'e"taient pas toujours *vraies.

Si les voyages de Saussure avaient eu pour but la connais sance speciale des glaciers , ce savant observateur n'aurait rien those Avho come after him ; but as he speaks of them merely in an accidental Avay, it is not surprising that it should still be possible to touch upon a subject Avhich nevertheless owes much to him, as will be seen by the aid that his Avritings have given me, and by the numerous quotations that I shall be obliged to make from them. It is at the Glacier des Bois, upon the Mer de Glace, at the Glacier des Bossons, and at that of Argentiere, that my principal observations have been made. It is to them, therefore, that the reader must recur in order to come more easily to understand the causes that I shall assign to the different phenomena. I am aware that nature is as immense in her variety as in her extent ; but she ahvays resembles herself in the production of identical phenomena.

Those Avhich to us wear the aspect of anomalies seem to deviate from the simplicity of known laAVS only by reason of the com plication of their causes, accidents of soil, relative posi tion, and the like. The sum total of the facts met Avith on the glaciers has appeared to me to be of great simplicity ; they floAV almost all the one from the other, and are laisse" a dire a ceux qui viennent apres lui ;mais comme il n'en parle qu'accidentellement , il n'est pas dtonnant que l'on puisse revenir sur un sujet qui pourtant lui doit beaucoup , comme on le verra par les secours que m'ont fourni ses ouArages, et par les citations nombreuses que je serai force d'en tirer. C'est au glacier des Bois , sur la mer de glace , au glacier des Bossons , a celui d' Argentiere que mes observations princi pales ont 6t6 faites.

C'est done la, que le lecteur devra se reporter pour arriver plus facilement a l'intelligence des causes que j'assignerai aux clivers ph^nomenes. Je sais que la nature est aussi immense dans sa vari^te" que dans son dtendue ; mais elie est toujours semblable a elle-meme dans la production des phenomenes identiques. Ceux qui se montrent a nous comme des anomalies , ne semblent sortir de la simplicity des lois con nues que par la complication des causes, les accidents du sol, la position relative , etc.. . . L'ensemble des faits qui se rencontrent sur les glaciers m'a paru d'une grande simplicity ; ils de^coulent presque tous les evidently deducible from the knowledge furnished to us by the laws of physics. My sole aim in embodying my observations in a theory, Avhich I seek to render as complete as the feebleness of my lights Avill alloAV me, is to place the study of glaciers Avithin the reach of everyone.

Even though the explanations that I shall give should not be looked upon by natural philosophers as incontrovertible, I should still trust to have rendered some service to truth in adding to the elements which must aid its discovery ; and if the moments I have con secrated to this labour should be useless to others, they will not have been so to myself; the study of nature ahvays makes a better man of him Avho contemplates her Avith an unbiassed spirit and an upright and dis interested heart. It is one of the thousand paths that lead to God. Yes, to seek to know His daily action in the administration of this vast universe Avhich is His work, is to place one's self under the necessity of praising Him. Fire, hail, snoiv, ice, stormy wind fulfill ing His word . ... let them praise the name of the Lord.

Ps. cxlviii. uns des autres, et se d^duisent eVidemment des connais sances que nous fournissent les lois de la physique. Mon unique but en consignant mes observations dans une theorie, que je cherche a rendre aussi complete que me le per mettra la faiblesse de mes lumieres, est de mettre I'e'tude des glaciers a la porte'e de tout le monde. Alors meme que les explications que je donnerai ne seraient pas regarde"es comme incontestables par les savants, je croirais encore n'avoir pas e"tc. inutile a la ventc. en ajoutant aux Elements qui doivent aider a la trouver ; et si les moments que j'ai consacre's k ce travail Etaient inutiles pour les autres , ils ne le seraient pas pourmoi; I'e'tude de la nature rend toujours meilleur celui qui la contemple avec un esprit juste et un cœur droit et de'sinte'resse' ; c'est une des mille voies qui conduisent a Dieu.

Oui , c'est se placer dans la necessity de le louer que de chercher a connaitre son action journaliere dans l'administration de ce vaste univers qui est son ouvrage. — Ignis , grando, nix, glacies, spiritus procellairum quæ faciunt verbum ejus...laudent nomen Domini. Ps. 148.

Chapter I. Of Glaciers In General

When the height of mountains exceeds ten or eleven thousand feet above the level of the sea, their summit usually remains covered with snoAV during all seasons. This snow, more or less hardened by Aveather, assumes the appearance, the hardness, and sometimes the colour of ice, and Ave give to the Avhole mass the name of glaciers. Nevertheless, in common parlance, the name of glacier is more specially given to those portions Avhich descend beloAV the region of perpetual snow, and display themselves in the midst of the vegetation of the high valleys. It is, therefore, necessary to dis tinguish tAvo kinds of glaciers. I shall give the name of reservoir glaciers to those of the higher regions in wdiich their formation must be continuous, because the CHAPITRE PREMIER. DES GLACIERS EN GENERAL. Quand la hauteur des montagnes ddpasse dix a onze mille pieds au-dessus du niveau de la mer , leur sommet reste ordin airement couvert de neige pendant toutes les saisons.

Cette neige, plus ou moins durcie par le temps, prend l'aspect, la durete- , et quelquefois la couleur de la glace , et Ton donne a l'ensemble le nom de glaciers. Pourtant , dans l'usage ordinaire , on donne plus particuliere ment le nom de glaciers aux parties qui descendent au-dessous de la region des glaces, et viennent se montrer au milieu de la vegetation des hautes valle"es. II est done necessaire de dis tinguer deux especes de glaciers. Je donnerai le nom de glaciers reservoirs a ceux des regions sup^rieures, ou la formation doit •Itre continue, parce que la temperature de ces lieux est temperature of those parts is habitually below the freezing point. To those which descend into the region of ATeofetation Avhere melting of ice and snoAV takes place during a considerable part of the year, as they have evidently a different origin from the first, I shall give the name of stream glaciers.

One feels that it would be difficult, not to say im possible, to establish Avith precision the line which separates these two regions. From the point Avhere melting is nothing to that Avhere it is exactly sufficient to cause the disappearance during the summer of all the ice or all the snoAV that has accumulated during the Avinter, there are several zones of differing temperature, but in none is the heat in excess of the cold. Similarly from the point of complete melting down to that of the growth of the vine, for instance, there are also several zones of differing temperature, and in none is the cold in excess, so as to render snow or ice perpetual. This boundary line, Avhich is still called the limit of perpetual snow, is not ahvays in direct proportion to the vertical elevation. It rises or falls according to several circumstances, Avhich are not everyAvhere the same.

The habituellement au-dessous de zero. Pour ceux qui descendent dans la region veg^tale oil la fusion des glaces et des neiges s'opere pendant une partie considerable de l'annee , comme ils ont evidemment une autre origine que les premiers, je les appelerai glaciers d'ecoulement. On sent qu'il serait difficile, pour ne pas dire impossible, d'etablir avec precision la bgne qui separe ces deux regions. Du point ou la fusion est nulle jusqu'a celui oil elie est pi-ecise ment suffisante pour faire disparaitre pendant l'ete toute la glace ou toute la neige qui s'est accumulee pendant shiver, il y a plusieurs zones de^ temperature diverse , mais dans aucune la chaleur n'est en exces sur le froid. De meme depuis le point de fusion totale jusqu'a celui de la vegetation de la vigne , par exemple , il y a aussi plusieurs zones de temperature diverse , et dans aucune le froid n'est en exces de maniere a rendre la neige ou la glace perpetuelle.

Cette ligne intermediaire, que Ton a continue d'appeler limite des neiges perpetuelles, ne saurait etre toujours en rapport direct avec l'eievation verticale • elie s'eieve ou s'abaisse d'apres plusieurs circonstances qui ne' slope of the ground relatively to the sun, the form and nature of the soil, the degree of latitude, the elevation of the plateau generally upon AArhich the mountain rests, may largely modify its position. I shall here present a picture of the seven Alpine regions as naturalists have drawn it. The vine reaches to 1,700 feet above the level of the sea, the region of the oak to 2,800 feet, the region of the beech to 4,000 feet, that of the coni feræ to 5,500 feet, that of the graminiferæ to 6,500 feet and more. At 7,500 feet Ave find in the middle of summer isolated masses of snoAV, and the ground is uncovered wherever it is not sheltered from the sun ; it is here that arctic plants are found.

Lastly, above 8,000 feet come the eternal snoAvs. This limit seems to me, so far as the Alps are concerned, placed too Ioav ; for all travellers agree that at this height and even higher Ave still meet Avith saxifrages, gentians, chrysanthemums, and some other plants of arctic climates ; it folloAvs that if this zone had nothing above it, there would be in summer some days Avhen it Avould be entirely free from snoAV. We must therefore put the sauraient etre les memes partout. La pente du terrain relative ment au soleil , la forme et la nature du sol , le degre de lati tude, l'eievation du plateau general sur lequel repose la montagne peuvent apporter de grandes differences dans sa position. Je vais placer ici le tableau des sept regions alpines tel que l'ont trace les naturalistes. La vigne seleve jusqu'a 1,700 pieds au-dessus du niveau de la mer , la region des chenes jusqu'a 2,800 pieds , la region des hetres jusqu'a 4,000 pieds, celle des coniferes jusqu'a 5,500 pieds, les graminees jusqu'a 6,500 pieds et plus.

A 7,500 pieds on trouve dans le milieu de l'ete des amas de neige isoies , et le sol est decouvert partout ou il n'est pas abrite du soleil ; c'est la que se trouvent les plantes hyperboreennes. Enfin au-dessus de 8,000 pieds viennent les glaces eternelles. Cette limite me parait meme , pour les Alpes , placee trop bas ; car tous les voyageurs con viennent qua cette hauteur et meme au-dessus on rencontre encore des saxifrages , des gentianes , des chrysanthemes et quelques autres plantes des climats hyperboreens ; il s'ensuit que si cette zone n'etait nullement domine'e, il y aurait dans l'ete quelques jours ou elie serait entierement depouiliee de neige. II faut done SAVOY. limit of perpetual snow a little higher ; I should place it at 9,000 feet. It is there that the line of separation between reservoir glaciers and stream glaciers is to be found.

Chapter II. Of Reservoir Glaciers

The rays of the sun fall in vain on the summit of Mont Blanc, at a height of 14,700 feet above the level of the sea ; the snow does not melt there. M. de Saussure having reached this, the highest point of Europe, on the 2nd of the month of August, found the thermometer at 2 6° "8 Fahr./'''* and even in the sun it Avas more than a de gree below the freezing point, Avhile in the plain it was 81°'2. At the station from which he had started that morning, Avhich is more that 3,000 feet lower, he had * The temperatures given by M. Rendu are generally in Reaumur's scale. In the translation these are all reduced to Fahrenheit's scale. Measurements of length have not been altered, and remain in French feet. —Ed. porter un peu plus haut la limite des neiges perpetuelles ; je la suppose a 9,000 pieds. C'est 1st que se trouve la ligne de separation entre les glaciers-reservoirs et les glaciers d'ecoulement.

Chapitre II. Des Glaciers-reservoirs

La crete du Mont-Blanc, a, 14,700 pieds au-dessus du niveau de la mer, est inutilement frappee des rayons du soleil; la neige n'y fond pas. M. de Saussure arrive sur ce point cul minant de l'Europe au 2 du mois d'aout , trouva le thermometre de Reaumur k — 2°.3 , et meme au soleil il etait k plus d'un degre au-dessous de zero, tandis que dans la plaine il etait a 4-22°. Dans la station d'ou il etait parti le matin, laquelle se trouve plus de trois mille pieds plus bas, il avait compte trois noted 250-7. M. de Tilly, who made his ascent on the 8th of October, found at the summit of Mont Blanc a temperature of about 0°. Two other travellers, who Avere surprised by a tempest and detained at the rocks of the Grands Mulets, experienced after the storm 32° of frost in the month of August. All observations that have been made agree in demon strating that the melting of snoAV is absolutely nil on this summit.

Nevertheless, it is right to state that M. de Saus sure, who is an authority, and I may say almost an oracle when Ave are dealing Avith the phenomena of this mountain, which he has (so to speak) conquered to science — M. de Saussure has recourse to melt ing to explain the formation of a thin layer of ice which covers the snow and breaks beneath the foot of the traveller. It is a pure inadvertence on the part of the philosopher of Geneva ; to explain this pheno menon, there is another cause perpetually at work — the condensation of vapours — which, as we shall see elsewhere, must be very great in this locality. At the moment Avhen snow first falls on great eledegres au-dessous de zero. M. de Tilly, qui a fait son ascension au 8 octobre, a trouve au sommet du Mont-Blanc une tempera ture de 14 ou 15 degres au-dessous de zero.

Deux autres voyageurs, que la tempete a surpris et arretes aux rochers des Grands -Mulets, ont eprouve apres l'orage un froid de 14° au mois d'aout. Toutes les observations qui ont ete faites sont d'accord pour demontrer que la fusion des neiges est absolument nulle sur cette sommite. Pourtant il faut le dire, M. de Saussure qui est une autorite, et je dirai presque un oracle quand il s'agit des phenomenes de cette montagne qu'il a pour ainsi dire conquise a la science, M. de Saussure a recours a la fusion pour expliquer la formation d'une mince lame de glace qui recouvre la neige et qui se brise sous le pied du voyageur. C'est pure inadvertance de la part du savant Genevois ; il y a pour expliquer ce phenomene une autre cause perpetuellement agissante, c'est la condensation des vapeurs qui, comme nous le verrons ailleurs, doit etre fort grande dans cette localite.

Au moment ou la neige tombe sur les grandes elevations, elie vations, it is ordinarily in the state of powder ; in a short time, penetrated by the vapours which are always in abundance about very lofty summits, it hardens on the surface. It may possibly be, also, that the layer of ice which is found there, like that Avhich is met Avith often enough on the siioav of the plains, is the result of rain falling and freezing on coming into contact Avith the snow. The effect of the greatest heat which is experienced on heights far inferior to those of Mont Blanc consists in softening it so as to give to its different parts more facility for uniting Avith one another ; and to get so far as melting, Ave must have a certain concurrence of circumstances which can rarely present themselves — a hot Avind, aided by the sun, or humidity at a somewhat high temperature. est ordinairement en etat de poussiere ; peu apres, penetree par les vapeurs toujours abondantes autour des plus hautes sommites, elie se durcit a la surface.

II serait possible aussi que la couche de glace qui a ete trouvee la, comme on la rencontre assez souvent sur la neige des plaines, eut ete le resultat de la pluie tombee et congeiee en arrivant au contact de la neige. L'effet de la plus grande chaleur qui se manifeste sur des hauteurs bien inferieures k celles du Mont-Blanc, consiste a la ramollir de maniere a donner a ses diverses parties plus de facilite a s'unir, et pour pousser jusqu'a la fusion, il faudrait un certain concours de circonstances qui doivent rarement se pre senter: ce serait un vent chaud, aide par le soleil, ou de l'humidite a une temperature un peu elevee.