theories, in like manner, as a test of the spirit in which he has opposed the Sliding Theory? Because the motion due to the molecular mobility of the glacial mass was to that due to its sliding in the ratio of six to ten, he ventures to assert that the result 'only permits the alternative favourable to the Viscous Theory to be adopted;' and thus he dismisses the Sliding Theory. We do not hesitate to say that we know of no theoretical interpretation of an observed fact more philosophically unjustifiable than that just mentioned. Surely the most ordinary candour must have admitted (as the author of the extract from the Philosophical Magazine had previously admitted) that each theory presented a vera causa of glacial motion, and that the true inference from Professor Forbes's observation was, that each of these causes had a nearly equal influence on the whole motion of a glacier, and could not, in any general view of glacial motion, be neglected in comparison with the other. In our previous allusions to the Viscous Theory, we have spoken of it without explicit reference to the degree of truth or error which may be involved in it, and the real claim which it possesses to our acceptance. We accept it, though disputing entirely the propriety of the name, so far as it expresses the existence of such a degree of mobility of the constituent particles of a glacier inter se as shall admit of a certain onward motion of the glacier when acted on by gravity, independently of any sliding of the whole mass on the bed on which it rests. The motion of which we now speak is that which, combined with the sliding motion, constitutes, according to our view, the whole motion of the surface of a glacier. For the persistent, though one-sided advocacy of the theory thus restricted, and for its elucidation by a large aggregation of observed phenomena, we give to Professor Forbes all the honour and praise which can be fairly claimed for him. Moreover, though he was not the first to detect the veined structure in glaciers, he was the first to recognise fully its importance as the characteristic structure of consolidated glacial ice. For this, too, we give him all praise. But when it is asserted that the molecular mobility of a glacier is due to a property which can, without violating the propriety and definiteness of scientific language, be termed plasticity or viscosity, and when he attempts to raise a physical theory of the veined structure based on the hypothesis of this viscosity, and on mechanical reasoning which we conceive to be demonstrably erroneous, and when he altogether ignores the sliding motion of a glacier, on these points we disagree with him the more for every step which has been recently made in glacial theory. The besetting difficulty of the Viscous Theory has always been in the absence of all experimental proof of the existence of any property in glacial ice which could be designated as viscosity. We may talk about such ice with all its capillary pores and cavities filled with water, till our language might almost seem to imply that glacial ice may be moulded like a piece of plastic wax; but when we take a lump of such ice in its utmost state of saturation from a glacier, we find it still a hard, brittle, crystalline substance, with as little indication of plasticity as any other crystalline body. Still, the mass of a glacier is frequently subjected to enormous compression, and consequent changes of form, while it still preserves its continuity and crystalline structure. No plasticity could account for this fact, except by defining plasticity as that property, whatever it might be, by virtue of which a glacial mass comports itself as above described. But this inductive mode of definition is utterly inadmissible with reference to any substance on which we can directly experiment, and admissible only when applied to those which cannot be manipulated, and in cases in which the very existence of the substance of which the properties have to be defined, can only 1860.] Properties of Ice in reference to Glacier Movement. be proved inductively. Thus we can only assert the existence of the luminiferous ether, or demonstrate its elasticity, or any other of its properties, by induction from the Undulatory Theory; but who would think of appealing to acoustic theory to prove the fundamental property, for instance, of air, its elasticity? The distinction between the two cases is, that we cannot manipulate directly the luminiferous ether, while we can manipulate air, and determine its properties by experiment. So we must determine the real properties of ice by experiment, and not by mere induction from the Viscous Theory. We are acquainted with the molecular mobility of glacial ice, combined with the conservation of its crystalline form, by observation on glaciers; but the advocates of the Viscous Theory, previously to the publication of Dr. Tyndall's experiments on regelation, would have scorned the limitation implied in the mere recognition of molecular mobility. They professed to tell us the modus operandi of these molecular motions, and their cause. They were due to the plasticity, ductility, or viscosity of ice, though the precise meaning thus attached to those terms would be extremely difficult to determine. Here was the difficulty of the theory. The explanation, however, is found to depend on a property of ice as distinct from anything denoted by the above terms as one physical property can be from another, a property the existence of which, in fact, was entirely unknown before the experiments on regelation by Professor Faraday and Dr. Tyndall. Most of our readers are probably acquainted with these experiments; they will, at all events, find a very simple and intelligible description of them in Dr. Tyndall's work (p. 346 et seq.) It will be seen how beautifully they explain the molecular mobility of glacial ice, united with the preservation of its crystalline structure. When a glacier changes its form by external pressure, the internal pressures and tensions will be relieved by infinitesimal cracks and fissures which 807 admit the molecular movements, but are immediately closed again, and the crystalline continuity of the mass restored by regelation. In a non-crystalline and plastic or viscous mass, on the contrary, the molecular movements require no sudden crackings and breakings, and instantaneous reunions of the constituent molecules, as in the case of ice. Attempts have been and are still being made to wipe away the distinction between these two causes of the molecular mobility of ice. We could wish that Professor Forbes had given less countenance to such attempts than we think he has done by a paragraph in the introduction to his Occasional Papers (p. xvi.), and for which we would gladly have seen substituted some clear and explicit statement as to the modifications which this newly-discovered property of ice had introduced into his own views of glacial theories. We do not want the opinions of men who possess the zeal of advocacy without the knowledge of science, nor do we seek the testimony of authorities of the press' who never grapple with the real difficulties of the subject. We want the testimony and labours of men of exact research. We rejoice to see Dr. Tyndall's beautiful and accurate experiment brought to the solution of what has hitherto been so great a difficulty in glacial theory; and we protest, in the name of every lover of exact science, against the merging of this determinate step in glacial theory in any vaguer views, as if it were but the somewhat exacter embodiment of an anterior conception of the molecular constitution of glacial ice, instead of being, as it really is, as original and independent a step as any other in the progress of the science. We have already stated that Professor Forbes was the first to recognise the veined structure as characteristic of glacial ice in general. He has also, as is well known, given an elaborate physical and mechanical theory of this structure. It is described in his Travels, and incessantly referred to in his letters as published recently under the title of Occasional Papers. An account of it will also be found in sections 27, 28, and 29 of the Glaciers of the Alps. We should indeed regret that any assertion we may make should be considered indicative of a want of courtesy towards the author of this theory, but scientific truthfulness and candour compel us to declare our entire dissent from it. We dissent on the grounds of the mechanical reasoning; but further, if the observations of Dr. Tyndall as to what he terms the longitudinal veined structure along the axis of a glacier be right, Professor Forbes's theory must be, we conceive, inevitably wrong. We refer more especially to the development of this structure at the union of two branches of a glacier into one along the axis of the united glaciers.* The production of the central longitudinal veined structure, according to the differential theory, in a case like that elucidated by a diagram in the Glaciers of the Alps, p. 388, presents to our minds not only a difficulty but an impossibility. But our space will not allow us to do more than recommend this and similar cases to the attentive consideration of the reader. An account, also, of Professor Forbes's ripple theory is given by Dr. Tyndall in his twenty-ninth section, p. 398. It is given by its author in his seventh letter on glaciers (Occasional Papers.) If the veined structure be due, as it is according to Professor Forbes's theory, to a differential motion, the laminæ on the flanks of the glacier ought, it would seem, to be parallel to its sides. They are not so, and the Ripple Theory was intended to account for the deviation. The explanation rests entirely on a supposed analogy between the lines of differential motion on a glacier, and the ripples on the surface of a gentle stream produced by slight successive impediments on its banks. These ripples are assumed to be lines of differential motion. We believe the assumption to be without the smallest foundation. They are manifestly due, as Dr. Tyndall has pointed out, to the interference of an indefinite succession of small waves proceeding from the small obstacles on the bank of the stream. They do not indicate lines of differential motion, nor can there be anything analogous to them on the surface of a glacier. Never was the foundation of a fanciful theory more completely swept away than that of the Ripple Theory by this simple explanation. Some sixteen years ago, the writer of this review investigated mathematically the internal tensions and pressures in a canalshaped glacier, due simply to the fact of its motion along its axis being greater than that along its sides; and he showed that the line of greatest tension in such a case would be inclined at an angle of 45° to the sides and axis of the glacier. This at once accounted for the formation of transverse crevasses, the normal position of which is perpendicular to the above direction. He also showed that the lines along which there was the greatest tendency to differential motion would be inclined at 45° to those of maximum tension and to those of maximum pressure, these latter being perpendicular to each other. These results are as unquestionable as the fundamental proposition of mechanics, the parallelogram of forces. The advocates of the Viscous Theory, however, have always chosen to ignore them as worthless in themselves, or as inapplicable to the case of an actual glacier; and possibly that is the simplest way of meeting objections which one cannot answer, and which arise from modes of research which one does but imperfectly understand. Let it be recollected, however, that a difficulty is not answered by being ignored. We still venture to regard this investigation as presenting a far better ap * See Glaciers of the Alps, p. 387, et seq. For Professor Forbes's views, see Occasional Papers, Nos. I. and VIII. Other places in which the veined structure is mentioned will be found by referring to the index of that work. 1860.] Assumptions of Professor Forbes. proximation to the case of an actual glacier than a trough of treacle. However, the simple elucidation of the above results devised by Dr. Tyndall may be found more intelligible, and therefore more convincing. It will be found in his work, p. 382 et seq. The results are exactly confirmative of those mentioned above as to the directions of greatest tension and pressure; and the lines of greatest tendency to differential motion may be derived from the experiment with equal facility in the following manner. If we draw closely consecutive radii from the centre of one of the ovals (fig. 40, p. 383) to the circumference, it is manifest that the direction in question will be that in which two of these consecutive radii differ most in length. This will easily be seen (the oval being but little elongated) to make an angle of 45° with its greatest and least axes. In fact, it makes the line in question parallel to the sides of the glacier, as it must necessarily be in the case supposed. The longest axis of the oval is the line of drag of Professor Forbes (Occasional Papers, p. 57), which has been strangely enough mistaken for the line of greatest differential motion, though it is the very direction in which there can be no such motion, and in defiance of the obvious fact that the greatest tendency to such motion in the lateral portions of the glacier must necessarily be along lines parallel to its sides. We have already seen how the Professor's attempt to elucidate his views by the ripples on a gentle current, has melted away under the simple explanation above mentioned as given by Dr. Tyndall; and yet it is on fallacious mecha S09 nical reasoning like this that Professor Forbes's theory of the veined structure is based. Actual separation or bruising of the particles of ice is assumed to take place along his erroneously supposed lines or surfaces of greatest differential motion, which surfaces are then assumed to become by some mechanical or physical process which he has never been able to define, those along which the blue veins of solid ice are formed. It is a theory founded on assumption and false mechanical reasoning. We leave it to the consideration of our readers. Other researches are now being made for some better theory of this veined structure of glacial ice. Dr. Tyndall appears to us to have made out distinctly that the lamination is perpendicular to the direction of maximum pressure. The explanations which have recently been given are in accordance with this law. The reader will find them in Dr. Tyndall's sections on the structure in question. We do not yet venture to pronounce a decided opinion on these explanations, but it would appear, we think, that though they may not embrace the whole truth, they probably involve a considerable portion of it. At all events, it is an encouraging circumstance that the progress of the subject is now brought to depend on accurate experiment, and not on vague assumptions and loose mechanical reasoning. We regret that our limits will not allow us to follow Dr. Tyndall in more detail through the scientific portion of his work, to which we would refer our readers for further information on glacial theories. En Memoriam. THE grave has recently closed over the mortal remains of one whose death cannot remain unnoticed in a publication of which he was the main guide and chief fosterer, and into which he was wont to throw all the best resources of his keen intellect and untiring energy. Mr. John William Parker, Jun., has been taken from us, after a short illness, at the early age of forty; and it is not too much to say that his premature death has left a permanent blank in as large a circle of loving friends as perhaps any private man of this day possessed, and a void also in the world of letters which will not speedily be filled up. We have always thought that there is no such being as a commonplace man. So exquisitely various is human nature, that each human being, when you come to know him intimately, seems as it were a new creation. Brothers are not the least like brothers, nor sons like fathers; and though there are general characteristics of race and nation, yet each man is an individual character, worthy, if there were time for it, of the attentive study of the rest of mankind. Still, looking at men from without, we see that there are numbers of them who greatly resemble one another, and we are enabled in some way to classify them. Mr. John Parker, however, was one of those persons whom it is not easy to put into any class or section. He was essentially an original and remarkable character; and we never heard any of his friends liken him to any one else. He had the ordinary qualifications of a thoroughly good man of business. He was both careful and ardent in the conduct of those difficult affairs which form the business of a publisher's house. He was a man well placed. His one ambition in life was to help in sustaining worthily the eminent publishing house which his father's great energy and ability had raised, in an unusually short time, to the position of one of the leading firms in the metropolis. To do this, with the largest and most liberal spirit, looking always far more to credit and honour than to profit, was the single aim of Mr. John Parker's very useful life. He was a good writer, but prudently abstained from writing much himself, being well aware that his chief business was not to write, but to criticise, to estimate, and to correct. Untiringly he sought to discover who was the man who best understood any given subject, and could write upon it. Having once ascertained that, and gained him as a writer, he would give the man his whole confidence, and ever afterwards stand by him with somewhat of the same chivalrous loyalty with which our present Premier is ready, as far as he justly can, to defend against all comers any member and any subordinate of his administration. But we must turn to the character of the man himself, which must often have been a study to many loving friends, who will be pleased to read any attempt, however poor and imperfect, at its delineation. In the first place, he was one of the most witty and humorous men of this generation. Little perhaps thought the persons who conferred with him merely upon business, what a world of humorous fancy lay in that capacious brain, when they saw the pale, nervous, anxious man diligently, and somewhat sadly, pondering over a difficult matter of business. His humour was of such a peculiar kind that it well deserves description. It was remarkably dramatic. When he came down to any of those country houses which he gladdened with his presence, the walls of which will never ring again with such an exuberance of mirth as he brought there (for when he came, you felt that, not a man, but 'the holidays,' had come down), his great delight was to represent some peculiar form of character, which he would maintain with a force and skill that would have given him the highest place on the comic or the tragic stage. Now, he was a man weary of the world—'a wreck overgrown with ivy,' to use his own droll, incongruous expression-who took a mournful view of all |