occupied the attention of glacialists. The same general description is applicable to them all. The term primary is generally used, as we have used it above, to denote the glaciers of large dimensions. There are also secondary glaciers, the horizontal extent and thickness of which are much smaller than those of a primary glacier. The inclination of the beds on which they rest is usually much greater than in the larger glaciers, and they are generally restricted to higher localities on the sides of the mountains. We are not aware of any series of accurate observations having been made on these smaller glaciers. We would recommend them to the notice of future observers. It would not only be curious to observe how far the different glacial phenomena glacial phenomena may be modified by their peculiar conditions, but it is also very possible that they might afford valuable tests, whether at rest or not, of the truth of particular theories of glacial motion. The inclination of the surface to the horizon in large glaciers usually varies from 2° or 3° to 8 or 10°. As a general rule the surface is most rough and dislocated where the inclination is greatest and most irregular. In many glacial valleys there are also steep escarpments, over which the ice is precipitated, and broken into thousands of enormous fragments, forming one of the wonders of Alpine scenery. The re-cementing of these fragments into one continuous mass of glacial ice at the foot of an ice-fall was, till recently, one of the most mysterious of glacial phe nomena. When we look down on the surface of a glacier from a considerable height, the minor inequalities of its surface become scarcely sensible. We may generally observe, however, even on the smooth portions of the surface, certain transverse lines, rare in the centre of the glacier, but more numerous in its two marginal portions, in each of which these lines are respectively nearly parallel; and as they proceed from the flanks on either side towards the central portion, they incline towards the upper extremity of the glacier, instead of being perpendicular to its axis. These are the crevasses, gaping, vertical fissures, often large enough to present the most serious impediments to the progress of the traveller across them. They are rarely longitudinal in the elongated or canal-shaped glaciers, but in certain cases where the valley becomes suddenly divergent in its descent, the crevasses become also divergent like the rays of a fan. The glacier of the Rhone, at its lower extremity, presents the best and most familiar example of crevasses of this latter kind. The theoretical explanation of all these phenomena belongs to the mechanics of glacial motion. There is another group of objects, very striking in a bird's-eye view of the surface of a glacier. We allude to the long, dark, continuous lines of débris nearly parallel to the axis of the glacier, and stretching frequently from points near its upper extremity to its final termination. To the eye situated as above supposed, they appear free from all local asperities, following in graceful curves all the flexures of the valley. They consist of an aggregation of rocks and smaller detrital matter, the rocks varying from small pebbles to angular blocks of many tons in weight. These are the moraines. One is almost invariably found on each side of the glacier, and close to the bounding walls of the valley; they are the lateral moraines. Another moraine, and usually the largest, is observed to coincide very nearly with the axis of the glacier, and is called the median moraine. In large glaciers there are frequently also other smaller moraines intermediate and parallel to those above mentioned. The glacier of the Aar furnishes, perhaps, the best examples of existing moraines with which we are well acquainted. Not far below the junction of its two great tributaries, as many as six or seven may be distinctly recognised. They are laid down with great accuracy in the map of this glacier, in Plate III. of the Atlas which accompanies the last work of M. Agassiz on glaciers, the Système Glaciaire. It should also be stated that aggregations of large blocks and smaller débris are usually found at the terminations of glaciers in front of the ice itself, and extending more or less completely across the valley. They are the terminal moraines. The motion of a glacier is slow and persistent during all seasons, but slower in winter than in summer, and varying generally at different times and in different places, from a few inches to twenty or thirty inches a day. Moreover, in an elongated canal-shaped glacier, the axial portions move faster than what are termed the lateral or marginal portions. Also, the more superficial parts of the glacial mass move faster than the inferior parts. These inequalities of motion show that a glacier, in its aggregate mass, has a power of changing its form, so as to admit of these irregularities of motion, as well as to enable it to adapt itself to all the irregularities in the form and dimensions of the valley along which it descends. This property of the general glacier we call its pliability. It has been the subject of much earnest discussion. The motion of the glacier enables us to account very clearly for the existence of central moraines. The lateral ones are manifestly due to the various blocks and débris which fall down the precipitous sides of the glacial valley on the glacier beneath, by the onward motion of which they are carried forward, sometimes the whole length of the glacier, and deposited in its terminal moraine. If, however, the lateral moraine belonging to one flank of a large tributary glacier meets the corresponding flank of another tributary, with its moraine (as at the junction of two great tributaries of the Aar glacier), the two moraines necessarily unite, and move forward along the central line of the resulting glacier as its central moraine. A similar explanation applies to the moraines which are intermediate to the median and either lateral moraine. They all arise from lateral and usually smaller tributaries to the general glacier or to its principal affluents. If a lateral moraine, for instance, be formed in the upper portion of a large tributary, and a smaller tributary pour down its contents into the larger one, the lateral moraine of the latter, conjoined with one of the lateral moraines of the smaller tributary, will be thrust away from the side of the glacial valley, and will become one of the intermediate moraines above mentioned. A similar explanation applies to other moraines of this kind, the number of which is usually indicative of the number of minor tributaries which have helped to produce the general glacier. Many of these moraines extend to the lower extremity of the glacier and deposit their contents at the terminal one, which would thus grow incessantly, were it not that large portions of it are constantly removed by the current of water, frequently a powerful one, which issues from beneath the glacier at its extremity. Portions also of the lateral moraines work down to the bottom of the glacier, and are finally pushed forward to its extremity. The powerful agency of glaciers, in transporting blocks of enormous magnitude from their original sites to points many miles distant, will be easily understood from what precedes. The recognition of this operation of transport as the daily employment, as it were, of nearly all glaciers, has led to some highly interesting conclusions in geology. When the traveller descends from the high point of view from which we have supposed him to survey the glacier, and begins to traverse its surface, he becomes sensible of the rugged route along which he has to make his way. He finds that many of the crevasses, which appeared to him like so many narrow well-marked lines, are really deep yawning fissures, over which it is frequently impossible to pass without bridging them over by some artificial means. The large central moraines, also, which appeared like even dark longitudinal stripes on the glacier, he finds to be frequently elevated ridges of 20 or 30 feet in height. This elevation does not arise simply from the accumulation of the blocks and débris of the moraine, but partly also from an icy ridge which underlies them, and which has been formed by the protection against the wasting effects of sun and rain, afforded by the débris to the ice beneath it. Glacier tables, formed by large single blocks poised on pedestals of ice, are produced in a similar manner. Also the less dislocated portions of the glacier surface present, espe cially on sunny days, a beautifully bright effect, arising from the innumerable rills of water produced by the superficial melting of the ice. These rills sometimes form, by their confluence, considerable rivulets, which, of course, precipitate themselves into the first crevasse that crosses their course, thus making their way to the bottom of the glacier, whence the water is finally discharged from its lower extremity. The volume of water thus discharged in the winter is small, as might be expected; but in the warmer summer months is sufficient, in the case of a large glacier, to form at once a river of considerable magnitude. It is impossible to overestimate the sublimity and beauty of these glacial masses, surrounded by their mountain accompaniments, whether we see them intruding themselves, as it were, at their lower extremities, into the fertile valleys of the lower Alps, and increasing by contrast the beauty of the summer verdure there, or whether we contemplate them in their solitary grandeur in the remoter portions of their higher regions. It was in 1841 that M. Agassiz may be said to have established himself on the glacier of the Aar, just below the junction of the two primary tributaries above described, for the purpose of observing the phenomena which the glacier might present to him. He there erected for himself, and two or three scientific friends who accompanied him, the tent which soon became so well known as the Hôtel Neuchâtelois, where, in that and two or three subsequent years, he received, with characteristic courtesy and hospitality, a large number of the philosophers of Europe. This glacier affords peculiar advantages for observations on glacial phenomena, and it was for this reason principally that M. Agassiz selected it. Nor should we conceive a continued summer residence on so accessible a glacier, and one which may be so easily traversed in any direction, as otherwise than very enjoyable. During the day-time, when the weather was fine, we have seen its whole surface alive, as it were, with innumerable gurgling rills of water, which, with the brightness of the snow mountains, gave, even amidst the surrounding desolation, an animation to the scene which dissipated all feeling of loneliness. At sunset this scene is often suddenly and singularly changed. On the disappearance of the sun's rays, the surface-melting of the glacier, with every rill resulting from it, is immediately arrested, and, if the atmosphere is sufficiently serene, all is reduced at once to almost perfect stillness. The silence becomes imposing. Every little rill being hushed, there is sometimes literally not a sound to be heard, save that of the distant avalanche, occurring just often enough to make one the more sensible of the intensity of the silence. Such scenes offer, indeed, an adequate reward to every energetic traveller for all the effort he can make, and all the fatigue he may encounter, in seeking them. · sonal details, and charming descriptive touches of those magnificent scenes of beauty which characterise these Alpine regions, but which at that time were imperfectly known, even to the few secluded inhabitants of the lower and more accessible valleys of the district. There is something peculiarly national in this work, and the name of De Saussure is one of which his countrymen may reasonably be proud. Many of his more abstract scientific observations have been superseded, as might have been expected, by more advanced and recent researches; and the region which he was the first to describe in systematic de-, tail is now popularly known from the large influx of travellers. But it must not be forgotten that his work remained for half a century the recognised and unrivalled receptacle of the best descriptions which existed of the scenery and physical phenomena of the Alps. We have no intention of entering into the De Saussure did not devote his special atearlier history of glacial science. We can do tention to glaciers, and does not appear to have little more than mention the names of such added to the then-existing knowledge of the glacialists as Simler, Scheuchzer, and Grüner, subject much that was absolutely new, either who, with others of inferior note, collected a in observed phenomena or in abstract reasonconsiderable number of facts respecting the ing. The great advantage which he conferphenomena and topography of glaciers. red upon it seems to have been in methodising Scarcely any facts, however, were accurately and generalising the knowledge or suggestions observed, and a great part of their theories of those who had preceded him, rather than were formed with very little knowledge of in adding to it discoveries of his own. He physical and mechanical principles. But De was prepared for this task of generalisation Saussure's work, Voyages dans les Alpes,' by his large acquaintance with the general was of a far higher order than any which had phenomena of glaciers derived from personal preceded it. The author was a Swiss phi- observation. The distinct idea that glaciers losopher fond of physical science, and a devot- moved by sliding over their beds appears to ed admirer of his native mountains. He have been first advocated by Grüner, and subresided at Geneva, and availed himself of his sequently adopted by De Saussure; but the proximity especially to Mont Blanc to make latter was enabled by his larger acquaintance visits to that mountain, and also to the other with glaciers to give to this view a wider geSwiss mountains, almost every summer for nerality, and therefore it is that his name has upwards of twenty years. He commenced become so intimately associated with what his observations in the year 1760. They has been termed the sliding theory of the were not restricted to glaciers, but were motion of glaciers. Again, others had deequally extended to all those numerous phy- scribed, though very imperfectly, the mosical, geological, and topographical facts raines of glaciers; but de Saussure was the which that region presents to the notice of first to describe them systematically, and to the philosophical traveller prepared to appre-recognise, in some degree, the important inciate at once the true value of the principles and laws by which Nature works, and the beauty of those varied and magnificent scenes which, in a country like Switzerland, she always presents to us. The results of all the long-continued observations of this philosophical traveller are embodied in his work above mentioned, consisting of four quarto volumes published at different times, as additional matter was collected and arranged for each successive volume. The whole work consists of a happy combination of scientific observation and philosophical discussion, enlivened by the introduction of agreeable per ferences deducible from the actual positions of portions of the blocks and detritus transported from their original sites by former glaciers. At the same time, it is singular that he should not have recognised the obvious origin of central moraines in the confluence of two lateral moraines belonging respectively to two confluent tributaries as above described. He supposed them, on the contrary, to arise from a continual convergency of the lateral portions of the glacier towards its axis in the course of its onward motion-a conclusion entirely at variance, as we shall see, with subsequent observation. Still these observations were only applicable to the summer months. In order to render them as complete as he was able to make them for the winter months likewise, M. Agassiz placed a thermometer in the glacier the depth of 2 mètres, or about 7 feet, in the summer of 1842. After remaining there two years it was taken out, and showed that e minimum temperature to which it had been reduced during that time was 275°. (Cent.) or very nearly 23 (Fahr.) below the freezing temperature. Consequently 281° (Fahr.) was very nearly the lowest temperature which the glacier had acquired in two successive winters in that particular locality. M. Agassiz does not appear to have determined the winter temperature in the bore of 200 feet. The preceding explanations and descrip- [200 feet, during the summer months, after tions have been designed to point out gene- the snow of the preceding winter had enrally, and without details, the process by tirely disappeared from the surface of the which glaciers are generated and maintained, glacier. and to indicate the aspect which they present to the eye of the traveller who may or may not desire to penetrate into the more hidden secrets of glacial mysteries. We believe that the pleasure which any intelligent traveller may derive from the contemplation of the external beauties of Alpine scenery may be materially enhanced by some acquaintance with the nature and constitution of these enormous moving masses of ice and snow. Those who may wish to acquire a more profound acquaintance with the subject must, of course, enter into the minuter details of observation and experiment, and must, moreover, bring to the task a considerable amount of mechanical and physical science. A portion of the remainder of this review will necessarily involve certain details more especially intended for the latter class of readers, but there will be much at the same time which may be easily understood by the more general reader, and which, we trust, may add to any interest he may already feel in glacial phenomena and glacial theories. The internal temperature of a glacier has a bearing, to a greater or less extent, on most of the more important problems which glaciers present to us. We shall therefore consider this branch of our subject before we enter into the details on other branches of it. We believe that M. Agassiz is the only one who has made direct experiments for the de termination of the internal temperature of glaciers. A vertical bore had been made, for a different purpose, in the glacier of the Aar not far below the junction of its two principal affluents, of the depth of 60 mètres, or about 200 feet. Other bores were also made near the former one, of the depth of a few mètres. At the end of July, and a few days in the beginning of August, 1842, M. Agassiz observed the temperature in the shallower bores during fifteen days successively, at depths between 3 and 5 mètres, and found it to be invariably the temperature of freezing, neglecting very small discrepancies, in three only of the observations, manifestly due to some accidental cause. Simultaneously with these observations, M. Agassiz also examined several times the temperature indicated by the thermometer sunk to the bottom of the deeper bore of about 200 feet, He found it invariably at the freezing tem. perature, the zero of the Centigrade, and 32of Fahrenheit. These observations leave no doubt of the interior temperature having been very near 32 (Fahr.) at every point to the depth of To explain the manner of determining the temperature generally at any point within the glacier, it will be necessary to state briefly the law of temperature within the superficial portion of the earth's crust, as determined by theory, and sanctioned by observation to the greatest depth (upwards of 2000 feet) to which man has been able to penetrate. There is a very small uniform flow of heat from the interior parts of the earth through its outer solid crust, into the circumambient space. If the atmospheric temperature in any region of the earth's surface were constant and equal to the mean annual temperature there, the terrestrial temperature immediately beneath the surface would be the same as the constant atmospheric temperature; and at a point at any proposed depth beneath the surface, the temperature would exceed the superficial temperature by an amount increasing by 1° (Fabr.) for an increase in depth of about 70 feet. This is called the mean terrestrial temperature. But the atmospheric temperature changes from one season to another, and this superinduces a corresponding change in the terrestrial temperature; that change being greatest immediately beneath the surface, and decreasing with the depth till it becomes insensible at the depth of about 60 or 80 feet. Moreover, the atmospheric temperature varies from day to night, and such is also the case with the terrestrial temperature, but only to depths not exceeding one or two feet. Thus there is a diurnal variation of the terrestrial temperature to the depth of one or two feet, and an annual variation to the depth of 60 or 80 feet; while at greater depths the temperature at each point (the mean terrestrial temperature) is invariable from year to year, but is greater in proportion to the depth of the point beneath the surface. If the upper stratum of the earth were ice (as it may be considered to be in the case of a glacier), results similar to the above would still hold true; because ice, so long as it remains solid, or its temperature is below 32° (Fahr.), allows heat to pass through it according to the same laws as any other solid. But there is this peculiarity in ice-that it ceases to be solid at the temperature of 32° (Fahr.). Now, it is easily proved that the flow of heat from the earth's interior is more than sufficient to raise the temperature of the lower surface of any considerable glacier, under ordinary conditions, to the above temperature. A part only, therefore, of the transmitted internal heat is employed in producing this effect; the remainder is employed in melting the ice at the lower surface of the mass, whence it necessarily follows that no considerable glacier can be frozen to its bed. The mean temperature of the glacier will vary from 32° (Fahr.) at the lower, to a temperature at the upper surface which depends on the atmospheric temperature, and is, in the middle region of the Alpine glaciers (as deduced from M. Agassiz's observations), between 1 and 2° (Fahr.) below freezing. It will be somewhat lower near the upper, and somewhat higher near the lower end of the glacier. Hence the mean internal temperature can never differ much from 32° (Fahr.) The actual temperature will be subject to annual and daily variations, like those described in the terrestrial temperature; but these variations will penetrate only to still smaller depths than in the earth itself, nor will they ever exceed a few degrees. Consequently, the internal temperature of a primary glacier will be approximately uniform, especially in its lower portions.* of 32° (Fahr.,) and must constantly tend to raise the interior temperature to that height. The winter cold, within the small depth to which it penetrates, will, more or less, counteract this tendency; but below that depth the temperature must ultimately rise to 32°, and remain constant. This is consistent, it will be observed, with the temperature ob served by M. Agassiz at the depth of 200 feet. These resulting temperatures as above stated are deduced from accurate solutions of the problem, and admit of no ambiguity or appreciable error. They do not appear to us to have been always attended to in speculations on which they have an immediate and important bearing. We shall now direct the attention of our readers to that property of ice by virtue of which it is capable, at a certain temperature, of what is called regelation.' The discovery of this property, and the recognition of its applicability to the explanation of certain glacial phenomena, of which no adequate explanation had been previously given, constitute a most important epoch in the history of glacial science." It rescued our glacial theories from much of the vagueness and indeterminateness which till that time had hovered about them, and assisted greatly in placing the science on that basis of accurate investigation and exact experiment to which, in some of its most important points, it had no previous pretension. In the month of June, 1850, Mr. Faraday exhibited an experiment at an Evening Meeting of the Royal Institution, in which he showed that when two pieces of ice with moistened surfaces were placed in contact, they became cemented together by the freezing of the film of water between them; while, when the ice was below 32° (Fahr.), and therefore dry, no effect of the kind could be produced. The freezing was also found to take place under water; and, indeed, it occurs even when the water in which the ice is plunged is as hot as the hand can bear.' * There is also another cause which must help in producing the approximate uniformity of the interior temperature. M. Agassiz made a number of experiments on the glacier of the Aar, proving a considerable infiltration of water through the small pores and cre- It was a generalisation of this simple but vices of the ice ; and though Professor Hux- curious fact, that suggested to Dr. Tyndall ley failed, in certain more limited experiments the experiments which have so largely affecton the Mer de Glace, to obtain the same re-ed the state of glacial science. In the above sult, it would seem very difficult according to all existing evidence to doubt this infiitration as a general fact. If it does take place, the water must enter the glacier at a temperature The solution of the above problem will be found in the Philosophical Magazine' for January, 1845, vol. xxvi. See also the memoir 'On the Theory of the Motion of Glaciers,' in the Transaotions of the Royal Society' for 1862. Read May 22nd, 1862. Système Glaciaire,' chap. ix. experiment the two blocks of ice not only cohered to each other, but became so perfectly united that it was no longer possible to recognise their plane of junction. Now it occurred to Dr. Tyndall that if two pieces were capable of thus uniting, any number of pieces must equally unite if placed under similar conditions; and consequently that we might expect that an indefinite number of in Glaciers of the Alps,' p. 851. |