XIII. On the 3Iot/on of Glaciers. By William Hopkins, M.A., Fellow of 

 the Cambridge Philosophical Society, of the Geological Society, and of the 

 Royal Astronomical Society. [Second Memoir.] 



[Read Dec. II, 1843.] 



1. In my former Memoir on the Theory of Glacial Motion, I have given a full develop- 

 ment of the sliding theory as supported by my own experiments. According to the views there 

 advocated, a glacier is a dislocated mass, all the planes of dislocation, or of discontinuity in the 

 cohesive power being vertical or nearly so, and thus facilitating the more rapid motion of the center of 

 the glacier with reference to its flanks, but not that of its superficial with reference to its inferior 

 portion. It was shewn that the lower surface must be in a constant state of disintegration, and it 

 was thence inferred, that the adhesion between the glacier and its bed must be almost indefinitely 

 less than that between contiguous particles of the solid ice, and that, consequently, the velocities of the 

 superficial and inferior portions of the mass must be equal, or differ from each other by quantities 

 small compared with that of either portion. In my present communication, I propose to investigate 

 the nature of the motion on certain other hypotheses respecting the constitution of the glacial mass, 

 that we may compare such motion, or the eflfects of it, with observed phenomena, and thus be enabled 

 to judge of the admissibility of our hypotheses. I shall not include amongst these hypotheses those 

 which belong to the dilatation or expansion theories, because, after the facts observed by Professor 

 Forbes respecting the relative velocities at different distances from the origin of a glacier, and the con- 

 tinuance of glacial motion during the winter*, it appears to me impossible not to recognise the total 

 fallacy of those theories. I shall only therefore consider hypotheses appertaining to views of the 

 subject which, in common with those developed in my former memoir, agree in assigning gravity 

 as the immediate cause of glacial motion, but differ as to the circumstances which render it effective in 

 producing that motion down planes of such small inclination. The hypotheses I shall take are as 

 follows. 



(l.) A glacier may be conceived to be divided into strata, of which the surfaces are approxi- 

 mately parallel to the upper or lower surface of the mass. In such case, each stratum might slide 

 over that immediately subjacent to it, while the lowest stratum should slide in a similar manner over 

 the bed of the glacier, or remain firmly attached to it. In this motion each stratum must be sup- 

 posed to preserve its form and continuity as a solid mass, while between two contiguous strata there 

 is discontinuity, in the sense in which I shall here use the term, i. e. particles originally in contact 

 along the common surface of two contiguous strata do not remain in contact during the motion. 



(2.) While the upper part of the mass retains its solidity the inferior portions may be conceived 

 to become disintegrated, so that while the component particles retain their solidity they shall lose their 

 cohe.sion ; the disintegrated portion thus assuming a character similar to that of a mass of sand. In 

 this case, we may conceive the motion of the disintegrated portion to take place by a sliding of the 

 elementary component particles past each other, each particle or element of the mass retaining its 

 original form, like the hard grains of sand during the motion of a mass of that substance. 



• Travt'lA thrnuyk the Atpn nf Savoy, S^c, by Professor Foibes, who wish to obtain a knowledge of glacial phenomena, or who (eel 



p. 3f}l — 'I'liin work is full of adniirublc and well-digt(*ted dulailN, 

 founded on the most careful ob^crvationH and udincasurenients, and 

 cAunot be too ttrongly rccoininendcd for the pciuHal of all pemong 



interested in the n)any objects of beauty and sublimity which the 

 Alpine regions present to the traveller. 



