Mauch 1, 1895.] 



KNOWLEDGE. 



65 



BOOKS RECEIVED. 



Allen's Xafitralisl's Library. Edited by R. Bowdler Sharpe, LL.D. 

 A Handbook of the Britisli llamraalia." Bv R. Lydekker, B.A., 

 F.E.S. (W. H. Alien & Co.) 69. 



Lens Work for Amateurs. By Henry Orford. (Wliittaker & Co.) 

 33. A practical handbook, containing excellent instructions to young 

 workmen and amateurs for the manufacture of lenses. 



TheAgeoftheCondottieri. By Oscar Browning. (Methuen & 

 Co.) OS. A short history of ilediaeval Italy, from 14P9— 1530, 

 furnished with an excellent index. 



One Thousand Patent Facts. By R..bert E. Phillips. (Iliffe i 

 Son.) 2s. 6d. A compact and conprehensive work dealing with the 

 laws and rales governing the protection of Inventions by Letters 

 Patent, and the registration of Designs and Trade Marks. 



T/ie Great Problem, or Man's Future Place in the Universe. By 

 J. S. (ElUot Stock.) 



Annual Report of the Smithsonian Institution, for the Year 

 endini] June iOt/i, 1892. (Grovernment Printing Office, Washington.) 

 An elaborate report, containing a great number of beautiful plates f lom 

 photographs of specimens in the United States National Museum. 



Proceedings of the Acaiemg of Xatural Sciences of Philadelphia. 

 Mil/September. 189 i. 



Calendar of the Department of Science and Art, 1895. (Eyre & 

 Spottiswoode. ) 2s. 5d. 



Progress of Science. By J. Villin Marmery, with an Introduction 

 by Samuel Laing. (Chapman & Hall.) 7s. 6d. 



Mechanics — Theoretical and Practical — Dynimics. By B. T. 

 Glazebrook, M.A., F.R.S. (Cambridge Uniyersity Press.) -is. 



The Story of the Stars. By George F. Chambers, F.R..aL.S. 

 (Geo. Newnes.) Is. 



Substance and its Attributes derived from the Absolute. (Kegan 

 Paul i Co.) 



Bemarkable Comets. By W. T. Lynn. Third Edition. (Edward 

 Stanford.) 6d. 



A Handbook of Illustration. By A, Horseley Hinton. (Dawbarn & 

 Ward.) 33. net. 



Chemical Laboratory Labels. Compiled by W. H. Svmons, F.I.C. 

 (Gallen, Eamp & Co.) 



The Soyal Xatiiral BiMory. Edited by Richard Lydekker, F.R.S. 

 (Frederick Warne & Co.) Is. Vol. III., Part 16, commences the 

 portion of this work to be devoted to birds. The introductory matter 

 referring to the general characteristics of the class Aves is well and 

 soundly dealt with. The illustrations, however, fall short of our 

 expectations. 



THE CAUSE OF THE MOVEMENT OF 

 GLACIERS. 



By P. L. Addison, F.G.S., Assoc. M. Inst. G.E. 



TO some readers there may not at first appear 

 anything very interesting in the movement of 

 glaciers. Such is, however, the case with many of 

 the phenomena in Nature, and it is not till we seek 

 to unravel the intricate relationship in the family 

 of physical forces, which produce great changes on the face 

 of the earth, that the interest becomes absorbed in that 

 which we thought worth little attention. 



There is something so profoundly mysterious in the 

 regular method employed by Nature in carrying out her 

 designs, that if we simply recognize her work, without 

 even in a superficial manner investigating the method by 

 ■n hich it is produced, the " pith and moment ' ' is all but lost ; 

 we, in fact, but look on the outside cover of a book of fairy 

 tales without enjoying the charming stories it contams, 

 and thus leave the " Rhymes of the Universe " unread. 



We are familiar with the appearance of glaciers as they 

 lie in valleys and mountain clefts, and have observed how, 

 if two valleys converge and join one another, the ice-sheets 

 they contam combine in one mass, as two rivers would 

 do, although the ice forming the glaciers is often several 

 hundred feet Ln thickness, and seems to be an inert mass 

 of solid matter. There is no doubt, however, that a 

 glacier moves down the valley very much in the same way 

 as a river, but, of course, more slowly. The motion is 

 quicker on the surface, and slower at the bottom and at 

 the sides. But we must not carry this comparison any 



further ; the physical conditions of the two streams are not 

 the same, and their motion, though in many respects 

 similar, is widely different in degree and origin. 



It is hardly necessary to point out that a glacier does 

 not slide down the valley which contains it. The well- 

 proved differential movement, the inequalities of the ground 

 over which it passes, and the bends round which it has to 

 turn, render sliding motion impossible. And besides this, 

 the apparently rigid glacier creeps down slopes on which 

 soft clay or loose shingle remain practically immovable. 



If we examine a piece of ice, say for example an icicle, 

 we find that it is exceedingly brittle : if thrown down upon 

 the ground, it breaks into many pieces. A piece of soft 

 clay can be bent or compressed into any shape with ease, 

 and yet an ice-sheet, of exactly the same composition as 

 the icicle, will steadily flow down slopes so gentle that 

 soft clay will stand on them without moving at all. 

 When a bed of clay moves down a hill- side the particles 

 of which it is composed slide past and roll over each other 

 till they come to a state of rest at the bottom, but as 

 the ice could not behave in this manner, other causes 

 must be sought to account for glacier motion. 



There is a peculiar irregularity in the movement of 

 glaciers, which probably first led to the discovery of the 

 cause, and this is that the motion is quicker during the 

 day than at night, and the summer rate is often double 

 that attained during the winter. This shows that pressure 

 from behind cannot be the cause of the motion, for, 

 through the greater accumulation of ice and snow on the 

 higher levels, the pressure must be greater during the 

 winter season when the movement is least. The fact that 

 the motion of the ice-sheet is greater during the day than 

 in the night, and in summer than during winter, naturally 

 leads us to conclude that heat must indirectly have some- 

 thing to do with the phenomenon, for the movement is 

 greatest during that part of the twenty-four hottrs when 

 the ice is subjected to the sim's rays, and in the warmer 

 seasons of the year. An exammation of the physical pro- 

 perties of the glacier will show how it can be affected by 

 neat and yet, as a mass, remain in the form of ice. 



When a thin slab of ice is subjected to microscopical 

 examination, it will be seen that it is not, as it appears to 

 the naked eye, a homogeneous mass of solidified water, but 

 a confused agglomeration of minute crystals, with cavities 

 equally minute lying between them. A glacier is simply 

 water in a crystalline state, and any motion in the mass in- 

 volves the movement of the crystals of which it is composed. 

 It has already been mentioned that the flow of the ice-sheet 

 is quickest at the centre and on the surface, and slowest at 

 the bottom and against the sides of the valley ; it follows, 

 therefore, as the motion is differential, that the ice crystals 

 in the centre of the glacier must pass by those composing 

 the sides and bottom. This appears to raise a difficitlty, for 

 if the minute crystals were to crash past one another, owing 

 to their travelhug at different speeds, they would soon be 

 ground to a fine powder. This does not, however, take 

 place, and the ice-sheet remains a solid and brittle mass. 



We know that water freezes at a temperature of 32° 

 Fahr., and in doing so it gives out a certain amount of 

 heat and assumes a crystalline form, and at the same time 

 expands about ten per cent, in volume. In other words, 

 one cubic foot of ice at 32" Fahr. will only make -908 of a 

 cubic foot of water at the same temperature, and a cubic 

 foot of water at 32- Fahr. will expand to 1-102 cubic feet 

 of ice. If, therefore, water is compelled to assume a solid 

 condition, a mechanical force is at once exerted through 

 the necessary expansion of the substance in freezing. 

 An experiment was tried to keep water while it froze 

 enclosed in an iron bombshell thirteen inches in diameter. 



