Again, during winter, as Mr. Ball remarks, the glacier is completely covered with snow and thus protected both from the influence of cold and of heat, so that there can be nothing either to raise the temperature of the ice above the freezing-point, or to bring it below that point; and consequently the glacier ought to remain immoveable during that season also.

"There can be no doubt, therefore," Mr. Moseley states, "that the rays of the sun, which in those alpine regions are of such remarkable intensity, find their way into the depths of the glacier. They are a power, and there is no such thing as the loss of power. The mechanical work which is their equivalent, and into which they are converted when received into the substance of a solid body, accumulates and stores itself up in the ice under the form of what we call elastic force or tendency to dilate, until it becomes sufficient to produce actual dilatation of the ice in the direction in which the resistance is weakest, and by its withdrawal to produce contraction. From this expansion and contraction follows of necessity the descent of the glacier "*. When the temperature of the ice is below the freezing-point, the rays which are absorbed will, no doubt, produce dilatation; but during summer, when the ice is not below the freezing-point, no dilatation can possibly take place. All physicists, so far as I am aware, agree that the rays that are then absorbed go to melt the ice and not to expand it. But to this Mr. Moseley replies as follows:- "To this there is the obvious answer that radiant heat does find its way into ice as a matter of common observation, and that it does not melt it except at its surface. Blocks of ice may be seen in the windows of ice-shops with the sun shining full upon them, and melting nowhere but on their surfaces. And the experiment of the ice-lens shows that heat may stream through ice in abundance (of which a portion is necessarily stopped in the passage) without melting it, except on its surface." But what evidence has Mr. Moseley to conclude that if there is no melting of the ice in the interior of the lens there is a portion of the rays "necessarily stopped" in the interior? It will not do to assume a point so much opposed to all that we know of the physical properties of ice as this really is. Has Mr. Moseley, after accurately determining the amount of work performed in melting the ice of his lens during any given time, found it to fall short of the amount of work which ought to have been performed by the heat absorbed during that given time? If he has done this in a manner that can be relied upon, then he has some warrant to conclude that there is a portion of the rays stopped which goes to perform work different from that of melting

* Proceedings of the Bristol Naturalists' Society, vol. iv. p. 39 (new series).

the ice, and that this work in all probability is the expansion of the ice. Or has he determined directly that his lens, after reaching the temperature which is considered to be the melting-point of ice, actually continued to expand as the rays passed into it? It is absolutely essential to Mr. Moseley's theory of the motion of glaciers, during summer at least, that ice should continue to expand after it reaches the melting-point; and it is therefore incumbent upon him to afford us some evidence that such is the case; or he need not wonder that we cannot accept his theory, because it demands of us the adoption of a conclusion so contrary to all our previous conceptions. But, as a matter of fact, it is not strictly true that when rays pass through a piece of ice there is no melting of the ice in the interior. Experiments made by Professor Tyndall show the contrary*.

There is, however, one fortunate circumstance connected with Canon Moseley's theory. It is this; its truth can be easily tested by direct experiment. The ice, according to this theory, descends not simply in virtue of heat, but in virtue of change of temperature. Try, then, Hopkins's famous experiment, but keep the ice at a constant temperature; then, according to Moseley's theory, the ice will not descend. Or try Mr. Mathews's experiment, but keep the ice-plank at a constant temperature, and the plank ought not to sink in the middle. But let it be observed that although the ice under this condition should descend (as there is little doubt but it would), it would show that Mr. Moseley's theory of the descent of glaciers is incorrect, but it would not in the least degree affect the conclusions which he has lately arrived at in regard to the generally received theory of glaciermotion. It would not prove that the ice sheared, in the way generally supposed, by its weight only. It might be the heat, after all, entering the ice, which accounted for its descent, although gravitation (the weight of the ice) might be the impelling cause.

The present state of the question.

The condition which the perplexing question of the cause of the descent of glaciers has now reached seems to be something like the following. The ice of a glacier is not in a soft and plastic state, but is solid, hard, brittle, and unyielding. It nevertheless behaves in some respects in a manner very like what a soft and plastic substance would do if placed in similar circumstances, inasmuch as it accommodates itself to all the inequalities of the channel in which it moves. The ice of the glacier, though hard and solid, moves with a differential motion; the particles of the ice are displaced over each other, or, in other words, the ice shears as it descends. It had been concluded that * See Philosophical Transactions, December 1857.

the mere weight of the glacier was sufficient to shear the ice. Canon Moseley has investigated this point, and shown that it is not. He has found that for a glacier to shear in the way that it is supposed to do, it would require a force some thirty or forty times as great as the weight of the glacier. Consequently, for the glacier to descend, a force in addition to that of gravitation is required. What, then, is this force? It is found that the rate at which the glacier descends depends upon the amount of heat which it is receiving. This shows that the motion of the glacier is in some way or other dependent upon heat. Is heat, then, the force we are in search of? The answer to this, of course, is, since heat is a force necessarily required, we have no right to assume any other till we see whether or not heat will suffice. In what way, then, does heat aid gravitation in the descent of the glacier? In what way does heat assist gravitation in the shearing of the ice? There are two ways whereby we may conceive the thing to be done: the heat may assist gravitation to shear, by pressing the ice forward, or it may assist gravitation by diminishing the cohesion of the particles, and thus allowing gravitation to produce motion which it otherwise could not produce. Every attempt which has yet been made to explain how heat can act as a force in pushing the ice forward, has failed. The fact that heat cannot expand the ice of the glacier may be regarded as a sufficient proof that it does not act as a force impelling the glacier forward; and we are thus obliged to turn our attention to the other conception, viz. that heat assists gravitation to shear the ice, not by direct pressure, but by diminishing the cohesive force of the particles, so as to enable gravitation to push the one past the other. But how is this done? Does heat diminish the cohesion by acting as an expansive force in separating the particles? Heat cannot do this, because it cannot expand the ice of a glacier; and besides, were it to do this, it would destroy the solid and firm character of the ice, and the ice of the glacier would not then, as a mass, possess the great amount of shearing-force which observation and experiment show that it does. In short it is because the particles of the ice are so firmly fixed together at the time that the glacier is descending, that we are obliged to call in the aid of some other force in addition to the weight of the glacier to shear the ice. Heat does not cause displacement of the particles by making the ice soft and plastic; for we know that the ice of the glacier is not soft and plastic, but hard and brittle. The shearing-force of the ice of the moving glacier is found to be by at least from thirty to forty times too great to permit of the ice being sheared by the mere force of gravitation; how, then, is it that gravitation, without the direct assistance of any other force, can manage to shear the ice? Or to put the question

under another form: heat does not reduce the shearing-force of the ice of a glacier to something like 1.3193 lb. per square inch of surface, the unit required by Mr. Moseley to enable a glacier to shear by its weight; the shearing-force of the ice, notwithstanding all the heat received, still remains at about 75 lbs.; how, then, can the glacier shear without any other force than its vn weight pushing it forward? This is the fundamental quesand the true answer to it must reveal the mystery of glaPotion. We are compelled in the present state of the prodmit that glaciers do descend with a differential motion other force than their own weight pushing them forward; and yet the shearing-force of the ice is actually found to be tha'yo forty times the maximum that would permit of the glauce shening its weight only. The explanation of this abparent acade will remove all our difficulties in reference to the chase of the decat of glaciers.

Mom withe

Then seal to L. one explanation (and it is a very obvious one viz, that the motion of the glacier is molecular. The ice deson os molemle by dois. The ice of a glacier is in the hard enta in state, Di does not descend in this state. Gravitati is a constant acting force; if a particle of the ice lose its sheer forec, tough but for the moment, it will descend by its might alone. But a particle of the ice will lose its shearing-force moment if the particle loses its crystalline state for the mount. The passage of heat through ice, whether by conduction or by radiation, in all probability is a molecular process; that is, the form of energy termed heat is transmitted from molecule to molecule of the ice. A particle takes the energy from its neighbour A on the one side and hands it over to its neighbour B on the opposite side. But the particle must be in a different state at the moment it is in possession of the energy from what it was before it received it from A, and from what it will be after it has handed it over to B. Before it became possessed of the energy, it was in the crystalline state-it was ice; and after it loses possession of the energy it will be ice; but at the moment that it is in possession of the passing energy is it in the crystalline or icy state? If we assume that it is not, but that in becoming possessed of the energy, it loses its crystalline form and for the moment becomes water, all our difficulties regarding the cause of the motion of glaciers are removed*. We know that the ice of a glacier in the mass cannot become possessed of energy in the form of heat without becoming fluid; may not the same thing hold true of the ice particle?

Phil. Mag. S. 4. Vol. 40. No. 266. Sept. 1870.

N

The alleged limit to the thickness of a glacier.

In his memoir "On the Mechanical Properties of Ice," published in the Philosophical Magazine for January 1870, Canon Moseley arrives at a conclusion in regard to the crushing of ice to which I am unable, without some qualifications, to agree. In his experiments ice was crushed under a pressure of 308-4 lbs. on the square inch, and he concludes that if a glacier is over 710 feet in thickness the ice at the under surface must be crushed by the incumbent weight. Professor Phillips also made some experiments on the crushing of ice, and he came to the conclusion that the height of a crushing column of ice is between 1000 and 1500 feet, and concluded also that if a glacier were to exceed this in thickness the ice would lose its solidity*. Whether the height of a crushing column of ice be 710, or 1000, or 1500 feet is of no consequence whatever as regards the possible thickness of a glacier. No doubt piece of ice solidified not under pressure would be crushed to powder were it placed under a glacier 1000 feet in thickness or so; but after being crushed it would resolidify, and would then probably be able to sustain a of 2000 feet of ice. This follows as a necessary conpressure sequence from the property of regelation. There is as yet, so far as I am aware, no known limit to the amount of pressure which ice may sustain. There probably is a limit; but what that limit is has not yet been determined. Canon Moseley says that "there is no glacier alleged to have so great a depth as 710 feet." The Humboldt glacier in North Greenland, according to Dr. Kane, has a depth of more than three times 710 feet. And Dr. Heyes found in Baffin's Bay icebergs (which are just pieces broken off the ends of glaciers) aground in about half a mile of water. And on the antarctic continent we have reasons for believing that the ice is in some places over a mile in thicknesst.

XX. On the Molecular Movements and Magnetic Changes in Iron &c. at different Temperatures. By G. GORE, F.R.S.‡

R.

W. FOX has shown that cast iron in the melted state produces little or no magnetic effect upon a delicately poised magnetic needle placed near it during its cooling, solidification, and subsequent further cooling, until the solid metal acquires "a cherry-red colour;" it then suddenly attracts the needle with great energy. Gilbert had also many years before

* Paper on Glacial Striation read before the Geological Section of the British Association, 1865.

+ Geological Magazine for June 1870, p. 276.

Communicated by the Author.

$ Philosophical Magazine, vol. vii. (1835) p. 388.