Part 27 (2/2)
FOOTNOTES:
[A] I borrow this term from Professor Clausius's excellent papers on the Dynamical Theory of Heat.
(20.)
There is one other point in connexion with the viscous theory which claims our attention. The announcement of that theory startled scientific men, and for two or three years after its first publication it formed the subject of keen discussion. This finally subsided, and afterwards Professor Forbes drew up an elaborate paper, which was presented in three parts to the Royal Society in 1845 and 1846, and subsequently published in the 'Philosophical Transactions.'
In the concluding portion of Part III. Professor Forbes states and answers the question, ”How far a glacier is to be regarded as a plastic ma.s.s?” in these words:--”Were a glacier composed of a solid crystalline cake of ice, fitted or moulded to the mountain bed which it occupies, like a lake tranquilly frozen, it would seem impossible to admit such a flexibility or yielding of parts as should permit any comparison to a fluid or semifluid body, transmitting pressure horizontally, and whose parts might change their mutual positions so that one part should be pushed out whilst another remained behind. But we know, in point of fact, that a glacier is a body very differently const.i.tuted. It is clearly proved by the experiments of Aga.s.siz and others that the glacier is not a ma.s.s of ice, but of ice and water, the latter percolating freely through the crevices of the former to all depths of the glacier; and it is a matter of ocular demonstration that these crevices, though very minute, communicate freely with one another to great distances; the water with which they are filled communicates force also to great distances, and exercises a tremendous hydrostatic pressure to move onwards in the direction in which gravity urges it, the vast porous ma.s.s of seemingly rigid ice in which it is as it were bound up.”
[Sidenote: CAPILLARY HYPOTHESIS.]
”Now the water in the crevices,” continues Professor Forbes, ”does not const.i.tute the glacier, but only the princ.i.p.al vehicle of the force which acts on it, and the slow irresistible energy with which the icy ma.s.s moves onwards from hour to hour with a continuous march, bespeaks of itself the presence of a fluid pressure. But if the ice were not in some degree ductile or plastic, this pressure could never produce any the least forward motion of the ma.s.s. The pressure in the capillaries of the glacier can only tend to separate one particle from another, and thus produce tensions and compressions _within the body of the glacier itself_, which yields, owing to its slightly ductile nature, in the direction of least resistance, retaining its continuity, or recovering it by reattachment after its parts have suffered a bruise, according to the violence of the action to which it has been exposed.”
I will not pretend to say that I fully understand this pa.s.sage, but, taking it and the former one together, I think it is clear that the water which is supposed to gorge the capillaries of the glacier is a.s.sumed to be essential to its motion. Indeed, an extreme degree of sensitiveness has been ascribed to the glacier as regards the changes of temperature by which the capillaries are affected. In three succeeding days, for example, Professor Forbes found the diurnal summer motion of a point upon the Mer de Glace to increase from 15.2 to 17.5 inches a day; a result which he says he is ”persuaded” to be due to the increasing heat of the weather at the time. If, then, the glacier capillaries can be gorged so quickly as this experiment would indicate, it is fair to a.s.sume that they are emptied with corresponding speed when the supply is cut away.
[Sidenote: TEMPERATURE AT CHAMOUNI; WINTER 1859.]
The extraordinary coldness of the weather previous to the Christmas of 1859 is in the recollection of everybody: this lowness of temperature also extended to the Mer de Glace and its environs. I had last summer left with Auguste Balmat and the Abbe Vueillet thermometers with which observations were made daily during the cold weather referred to. I take the following from Balmat's register.
Minimum Date. temperature Centigrade.
December 16 -15 ” 17 -20 ” 18 -16-1/2 ” 19 -9 ” 20 -13 ” 21 -20-1/2 ” 22 -4-1/4 December 23 -4-1/2 ” 24 -6-1/2 ” 25 -2 ” 26 +2 ” 27 -3 ” 28 -10-1/2 ” 29 -6
The temperature at the Montanvert during the above period may be a.s.sumed as generally some degrees lower, so that for a considerable period, previous to my winter observations, the portion of the Mer de Glace near the Montanvert had been exposed to a very low temperature. I reached the place after the weather had become warm, but during my stay there the maximum temperature did not exceed -4-1/2 C. Considering therefore the long drain to which the glacier had been subjected previous to the 29th of December, it is not unreasonable to infer that the capillary supply a.s.sumed by Professor Forbes must by that time have been exhausted. Notwithstanding this, the motion of the glacier at the Montanvert amounted at the end of December to half its maximum summer motion.
[Sidenote: BALMAT'S MEASUREMENTS.]
The observations of Balmat which have been published by Professor Forbes[A] also militate, as far as they go, against the idea of proportionality between the capillary supply and the motion. If the temperatures recorded apply to the Mer de Glace during the periods of observation, it would follow that from the 19th of December 1846 to the 12th of April 1847 the temperature of the air was constantly under zero Centigrade, and hence, during this time, the gorging of the capillaries, which is due to superficial melting, must have ceased. Still, throughout this entire period of depletion the motion of the glacier steadily increased from twenty-four inches to thirty-four and a half inches a day. What has been here said of the Montanvert, and of the points lower down where Balmat's measurements were made, of course applies with greater force to the higher portions of the glacier, which are withdrawn from the operation of superficial melting for a longer period, and which, nevertheless, if I understand Professor Forbes aright, have their motion _least affected_ in winter. He records, for example, an observation of Mr. Bakewell's, by which the Glacier des Bossons is shown to be stationary at its end, while its upper portions are moving at the rate of a foot a day. This surely indicates that, at those places where the glacier is longest cut off from superficial supply, the motion is least reduced, which would be a most strange result if the motion depended, as affirmed, upon the gorging of the capillaries.
[Sidenote: BAKEWELL'S OBSERVATIONS.]
The perusal of the conclusion of Professor Forbes's last volume shows me that a thought similar to that expressed above occurred to Mr. Bakewell also. Speaking of a shallow glacier which moved when the alleged temperature was so enormously below the freezing point that Professor Forbes regards the observation as open to question (in which I agree with him), Mr. Bakewell asks, ”Is it possible that infiltrated water can have any action whatever under such circ.u.mstances?” The reply of Professor Forbes contains these words:--”I have nowhere affirmed the presence of liquid water to be a _sine qua non_ to the plastic motion of glaciers.” This statement, I confess, took me by surprise, which was not diminished by further reading. Speaking of the influence of temperature on the motion of the Mer de Glace, Professor Forbes says, the glacier ”took no real start until the frost had given way, and the tumultuous course of the Arveiron showed that its veins were again filled with the circulating medium to which the glacier, like the organic frame, owes its moving energy.”[B] And again:--”It is this fragility precisely which, yielding to the hydrostatic pressure of the unfrozen water contained in the countless capillaries of the glacier, produces the crus.h.i.+ng action which shoves the ice over its neighbour particles.”[C]
[Sidenote: HUXLEY'S OBSERVATIONS.]
After the perusal of the foregoing paragraphs the reader will probably be less interested in the question as to whether the a.s.sumed capillaries exist at all in the glacier. According to Mr. Huxley's observations, they do not.[D] During the summer of 1857 he carefully experimented with coloured liquids on the Mer de Glace and its tributaries, and in no case was he able to discover these fissures in the sound unweathered ice. I have myself seen the red liquid resting in an auger-hole, where it had lain for an hour without diffusing itself in any sensible degree. This cavity intersected both the white ice and the blue veins of the glacier; and Mr. Huxley, in my presence, cut away the ice until the walls of the cavity became extremely thin, still no trace of liquid pa.s.sed through them. Experiments were also made upon the higher portions of the Mer de Glace, and also on the Glacier du Geant, with the same result. Thus the very existence of these capillaries is rendered so questionable, that no theory of glacier-motion which invokes their aid could be considered satisfactory.
FOOTNOTES:
[A] 'Occ. Pap.,' p. 224.
[B] 'Phil. Trans.,' 1846, p. 137, and 'Occ. Pap.,' p. 138.
[C] 'Occ. Pap.,' p. 47.
[D] 'Phil. Mag.,' 1857, vol. xiv., p. 241.
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