On the Causes of Glacier- Motion. 



209 



the fact that a perfectly satisfactory tlieory has been developed, and 

 recognised as such by all inquirers. The ambiguous allusion to the 

 subject in Sir John Lubbock's presidential 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 which 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 or 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 2,000 to 3,000 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 tempe- 

 rate 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 winter as well as in summer, and by night as well as by day. 



That a glacier is in a state of 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 lower, and the medial over 

 the lateral. A glacier, being a body in a state of flow, must move 

 under the influence of forces powerful enough to overcome its resis- 

 tance, 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, to enquire 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 me on the shearing resistance of 



