April 1, 1892.] 



KNOWLEDGE 



67 



(but chiefly by the latter) the snow of the higher regions 

 becomes the ice of the Glacier. 



It might be supposed that such an apparently hard and 

 brittle substance as ice would refuse to move downhill, 

 and hence it is by no means easy at first to understand 

 how ice can flow down valleys as it does. The mean daily 

 rate of movement of the Meide (rltKr (in the centre) during 

 the summer months, is as much as 20 to 27 inches. The 

 question is — how is such a flow to be accoimted for ? 



In the different theories of Glacier-motion that have 

 from time to time been brought forward, some account for 

 the flow solely by gravitation, ignoring the fact that ice is 

 not a truly rigid body ; others introduce melting, or melting 

 and freezing ; and one brought forward by .1. D. Forbes 

 attributes the river-like flow of the ice to a plastic power 

 in the ice itself, as if it were a viscous or semi-viscous sub- 

 stance. These theories may be briefly indicated as 

 follows : — 



1. The celebrated Ik' Saussun', a pioneer in Alpine work, 

 whose book, " Travels in the Alps," is full of original 

 observations, conceived that the weight of the ice might 

 be sufScient to urge it down the slope of a valley if the 

 sliding motion were aided by water flowing at the bottom. 

 He regarded a Glacier as a rigid mass capable of sliding 

 down an inclined plane just as any solid body might — for 

 instance, as a slate when loosened slides down the roof of 

 a house. There are many objections to this simple 

 theory ; one is that there ought to be accelerated motion, 

 which there is not. Besides, De Saussure was ignorant of 

 certain important facts, to be noticed presently. So we 

 may dismiss this theory. 



2. Hopkins' Thfory. — Mr. Hopkins, a well-known Cam- 

 bridge mathematical coach, who applied his knowledge to 

 several important geological problems, put forward a theory 

 which may be described as follows : He contended that a 

 Glacier can move along a very slight slope solely by gravi- 

 tation, owing to the constant dissolution of ice in contact 

 with rock below, and the number of separate fragments 

 into which the Glacier is divided by fissures, so that fi-ee- 

 dom of motion is imparted to its several parts, somewhat 

 resembling that of an imperfect fluid. His argument was 

 supported by a number of ingenious experiments. He 

 found that ice will move down a very slight slope, even a 

 slope that the eye could not perceive. This theory was 

 very similar to tliat of De Saussure, only he added to it 

 the idea of a Glacier being broken by frequent fissures into 

 separate pieces. It is needless to say that this theory is 

 out of harmony with the facts. 



3. Charpenticr substituted for De Saussure's sliding 

 theory an ingenious explanation, which may be called " the 

 Dilatation Theory." The most solid ice is always permeable 

 to water, and penetrated by innumerable fissures and 

 capillary tubes, often extremely minute. These imbibe 

 water (due to melting) by day, which freezes during the 

 night, and, of course, expands in the act of congelation. 

 This was supposed to cause a distension of the whole mass, 

 tending to propel the Glacier in the direction of least 

 resistance, namely, forwards. Mr. Hopkins opposed this 

 theory in several able papers. He contended that the 

 distension — if it existed — would tend to act upwards, and 

 increase the thickness of the Glacier, rather than down- 

 wards, or in other words, down the valley. 



This theory has been demolished in several ways. In 

 the first place, coloured fluids have been injected to see 

 whether the said capillary tubes existed ; but they have 

 never been detected. Again, a Glacier should, on this 

 theory, move faster about the time of sunset, when the 

 freezing of the water must be greatest. But this is 

 not the case. 



4. Mozelei/'s Theorij of F.xpanaion and Co)itriictton. — Canon 

 Mozeley noticed that the lead on some parts of the roof 

 of Bristol Cathedral kept gradually crawling downwards, 

 tearing up its fastenings in the act. This fact seemed 

 very remarkable, until he explained it by showing that 

 during the day (the temperature being higher) expansion 

 took place, while during the night contraction took place. 

 Both these would take place chiefly in the direction of 

 least resistance, namely, downwards. He then applied 

 this explanation to the downward movement of Glaciers. 

 This theory was ingenious, but like the others, fails to 

 explain all the facts. 



5. Prof. J(()»c.s' Thompson's Tlicory accounts for Glacier- 

 motion in the following manner ; — The freezing point of 

 water is affected by pressure, and relaxation of pressure will 

 cause the water at the bottom of a Glacier to freeze, and a 

 renewal of the pressure will cause it to thaw. His idea is 

 that the pressure due to the weight of a Glacier thaws the 

 ice at the bottom, and that this thawing enables the Glacier 

 to glide downwards (by diminishing friction). Then the 

 relaxation of pressure that follows the down-sliding causes 

 renewed freezing until once more the Glacier's own weight 

 brings about another melting. This theory is also un- 

 satisfactory. 



6. CroU'sMoh'cuhirifi'ltiwj Theonj. — This ingenious theory 

 by the late author of "Climate and Time," is rather too 

 subtle. Briefly, he supposed that theice is melted molecule by 

 molecule, each molecule becoming, for a time, changed into 

 the liquid state, and while liquid descending ; thus a flow 

 of heat was supposed to take place through the whole 

 mass. This is how he accounted for the apparent viscosity 

 of ice in Glaciers. 



There are now only two theories left, each of which has 

 powerful advocates now. Some think both are true. The 

 first is — 



7. Tijnilairs Recjidation Thconj. — Prof. Tyndall believes 

 that a Glacier bends sharp turns by splitting up and 

 freezing together again. His theory is based on Faraday's 

 well-known discovery of regelation, a principle by which 

 when two pieces of melting ice are brought into contact, 

 they will freeze together. The principle of viscosity, so 

 admirably worked out by Forbes, he considers, will only 

 account for a part of the facts. He admits that ice behaves 

 as if it were a viscous substance when it is subjected to 

 pi-essure alone, but when tension comes into play, he thinks 

 the analogy with a viscous body ceases. His object is to 

 reconcile the apparent brittleness of ice (for it is decidedly 

 brittle in small blocks) with its power of turning corners, 

 and other facts that seemed contradictory to the idea of 

 brittleness and rigidity. In " Heat and Mode of Motion" 

 he says, " The Glacier widens, bends, and narrows, and 

 its centre moves more quickly than its sides." A viscous 

 mass would, undoubtedly, do the same. But the most 

 dehcate experiments on the capacity of ice to yield, to strain 

 — to stretch out like treacle, honey, or tar — have failed to 

 detect this stretching power. "Is there," he asks, "then, 

 any other physical quality to which the power of accommo- 

 dation possessed by Glacier ice may be referred?" He 

 believes regelation is the required principle, and that tlie 

 mass of ice in a Glacier moves down the valley by a process 

 of alternate rupture and healing. The gist of the 

 regelation theory is that the ice of Glaciers changes its 

 form and preserves its continuity under pressure, which 

 keeps its particles together. But when subjected to 

 tei'sion, sooner than stretch it breaks, and behaves no 

 longer as a viscous body. 



8. Forbes' Viseous Theonj is opposed by Tyndall, but has 

 many advocates of authority. Principal J. D. Forbes 

 discovered, by a series of measurements, that an ice-stream 



