July 5, 1883] 



NA TURE 



! 35 



Miss Fielde, an American missionary lady stationed at 

 Swatow, has, it is stated, completed a voluminous dictionary of 

 the Swatow dialect, which will be published shortly. 



The additions to the Zoological Society's Gardens during the 

 past week include a Feline Dourocouli (A'yctipilhecus voci- 

 ferans 9 ) from Columbia, presented by Mr. H. H. Tl.iele ; an 

 Indian Civet (Viverricula indica 9 ) from India, presented by 

 Capt. Wilson ; two Squirrel-like Phalangers (Belidcus sciureus) 

 from Australia, presented by A. Pretyman ; a Vulpine Phalanger 

 (Phalaugisla vulpina) from Australia, presented by Mr. J. E. 

 Dothie ; an Australian Crow (Corvus auslralis) from Australia, 

 presented by Mrs. A. H. Jamrach ; a Nicobar Pigeon (Catenas 

 nictbarica) from the Philippine Islands, presented by Mr. Hugh 

 Low ; two Common Gulls (Larus (anus), British, presented by 

 Mr. C. W. Jervis Smith ; a Spotted Mud Frog (Pelodytes 

 punctalus) f.om the South of France, presented by Mr. H. P. 

 Cambridge; a Cape Ant Bear (Orycleropus capensis), twelve 

 Derbian Zonures (Zonurus derbianus) from South Africa, two 

 Canadian Beavers (Castor canadensis 6 9) from Canada, a 

 Viperine Snake (Ti opidonotus viperinus) from North Africa, a 

 Tree Boa (Corallus hortulanus) from South America, purchased ; 

 a Hairy-footed Jerboa (Dipus hirtepes) from Arabia, a Simon's 

 Dwarf Jerboa (DipoJiilus simoni) from Algeria, received in 

 exchange; a Japanese Deer (Cerfus sika 6 ), a Hybrid Syrian 

 Wild Ass (between Equus hemippus £ and Equus onager 9 ), an 

 Impeyan Pheasant (Lophophorus impeyamis), four Amherst's 

 Pheasants (Thaumalea amherstice), bred in the Gardens. 



ON THE CAUSES OF GLACIER MOTION^ 



T^HE question of the causes which produce the movement of 

 •^ glaciers, which was at one time so eagerly discussed, would 

 appear to have slumbered for the last ten years. Thii cannot 

 be said to arise from the fact that a perfectly satisfactory theory 

 has been developed, and recognised as such by all inquirers. 

 The ambiguous allusion to the subject in Sir John Lubbock's presi- 

 dential address to the British Association is an evidence that such 

 certainty has not been attained. It is indeed generally supposed 

 that the fact of the melting-point of ice being lowered by pressure is 

 somehow at the root of the matter; but a full explanation of the 

 origin of this pressure in the case of glaciers and of the mechanical 

 features of the problem has yet to be given. I may therefore be 

 pardoned if I draw attention to a different solution, proposed not 

 by myself but by one of the greatest of English mechanicians. 

 My apology for doing so is that I approach the question as an 

 engineer, not as a physicist ; and that it is in its essence, as 

 will be shown immediately, a mechanical rather than a physical 

 problem. 



The following are leading facts of glacier-motion w'lich must 

 be accounted for by any valid theory on the subject : — 



(1.) The phenomena of the movement of a glacier are simply 

 those of a solid body in a state of flow. 



(2.) The present glaciers of Switzerland and Norway, which 

 are the only ones which have been critically examined, are mere 

 shrunken fragments of the glaciers of the Great Ice Age. To 

 take one instance, the present glacier of the Rhone is about 6 

 miles long and perhaps 500 feet deep ; but the old glacier of the 

 Rhone, which abutted against the Jura, was 120 miles long, and 

 must have been 2000 to 3000 feet deep. The movement of such 

 glaciers as this must also be accounted for in any satisfactory 

 theory. 



(3.) The glaciers of the present day are not confined to the 

 temperate region ; they are found in much larger numbers and of 

 much greater size in the Arctic regions. 



(4.) Both in the temperate and in the Arctic regions glaciers 

 move in w inter as well as in summer, and by night as well as by 

 day. 



That a glacier is in a state ol flow was first proved by Forbes, 

 and has since been confirmed by the measurements of Tyndall 

 and others. Whilst the whole mass moves downwards, the top 

 moves faster than the bottom and the sides than the middle ; the 

 upper layers must therefore be continually shearing over the 



1 Paper by Walter R. Browne, M.Inst.C.E., read at the Royal Society, 

 June 15. 1882. 



lower, and the medial over the lateral. A glacier, being a body 

 in a state of flow, must move under the influence of forces 

 pow erful enough to overcome its resistance, and so produce this 

 condition. 



The general phenomena of the motion of a glacier are exactly 

 reproduced when a viscous body moves through a channel under 

 the influence of its own weight. We have therefore first to inquire 

 whether the shearing resistance of ice is sufficiently low to enable 

 us to regard a glacier as a viscous mass. 



The only experiments known to nie on the shearing resistance 

 of ice, are tho-e of Moseley (Phil. Mag., January, 1870). He 

 found that, with pressures from 100 to 110 lbs per square inch, 

 cylinders of ice sheared slowly across the two planes in contact, 

 sliding over each other without losing continuity. The distance 

 sheared through was about five-eighths of an inch in half an 

 hour. A 1 .ad of 119 lbs. per square inch was sufficient to shear 

 through a cylinder of \\ inches in diameter in two to three 

 minute . From these experiments it would appear that the 

 lowest shearing stress which will cause ice to flow is about 

 100 lbs. per square inch ; but sufficient time was not allowed in 

 the experiments to make this a matter of certainty. 



There is another way in which the shearing resistance of ice 

 may be tested. In the case of a block of ice of vertical sides, 

 gravity of cour-e produces a shearing resistance along all planes 

 passing through the base. Let h be the height of such a block 

 in feet, and con-ider the shearing force due to gravity on any 

 square foot of a plane making an angle t? with the vertical. This 

 shearing force is given by — 



wh x h tan 9 



- x cos 8 



= W A s in e cos e. 



h sec 9 



This expression is a maximum when 6 = 45°, and its value is 

 then — 



wh 



4 ' 



What is the greatest height at which a vertical cliff of ice will 

 stand ? I am not able to state this preci-ely, but it is very con- 

 siderable. Mr. Whymper mentions crevasses in South America 

 300 feet deep. Cliffs of fully that height have been seen stand- 

 ing out of water in the case of icebergs, and as so small a part 

 of*an iceberg projects above water, these cliffs probably extend 

 below to a considerable depth. Taking, however, only 300 feet 

 for the value of h, or for the maximum height of an ice cliff, this 

 would give about 30 lbs. per square inch as the lowest shearing 

 force upon a plane of ice which would cause it t > assume the 

 condition of flow. 



Let us now suppose a glacier of thickness a, lying upon a slope 

 whose inclination to the horizontal is 3 ; then the force per 

 square foot, tending to shear the ice at its junction with the 

 slope, is clearly aw sin 0. 



Suppo ing sin j8 to equal \, and that the shearing resistance is 

 30 lbs. per squai^ inch, we get a = about 290. Hence we may 

 say that a glacier lying on a slope of I in 4 will not move at nil 

 under its own weight unless it be at least 300 feet thick, an l 

 that, if it be more than ihis, the upper 300 feet will move as one 

 solid mass, the part below alone representing the conditions of 

 flow. 



It is needless to say that there are hundreds of glaciers wbu-li 

 are less than 300 feet thick, and which at n 1 part of their course 

 have a slope anything approaching 1 in 4. 



We have now to show that the theories generally propounded 

 for glacier action are all of them negatived by some of the 

 foregoing considerations. These theories may be stated as 

 follows : — „, . 



(I.) The glacier simply slides over its bed as a solid body. 1 Ms 

 is negatived by the fact that some parts move faster than others. 



(2 ) The glacier flows under the action of its own weight, ex- 

 actly as a viscous body Howi. 'Ihis is the theory of rorbes. 

 It is disproved by the facts given above, which show that even 

 on a slope of I in 4 a glacier would not flow unle s it was at 

 least 300 feet thick. . 



(3.) The glacier moves by the crushing of its base. 1 his Das 

 been disproved by Moseley's experiments, which showed that 

 the crushing resistance of ice was considerably higher than the 

 shearing resistance. . 



(4. ) The glacier moves by the melting of its base. I his is tne 

 theory of Hopkins. He placed a block of ice at 32° F. on a 

 slab at a small angle, and found that it slowly descended as it 

 melted. On this view the bottom of the glacier must always- 



