242 Mr. Hopkins on the Mechanism of Glacial Motion. 



must be referred. I shall perhaps best explain the nature of 

 this difficulty in Prof. Forbes's theory by a reference to the 

 case of fissures formed by the normal force in directions per- 

 pendicular to those of greatest tension, when that tension be- 

 comes greater than the cohesion. Suppose the tensions to be 

 the same at every point, and the mass perfectly homogeneous. 

 There will be no more reason for the formation of a fissure at 

 one point than at another, and it is conceivable, as a mere 

 abstraction, that an infinite number of fissures should be 

 formed at the same instant; but practically, the perfect homo- 

 geneity of the mass cannot exist, and therefore fissures will 

 begin at the same instant at comparatively few points, perhaps 

 only at a single point. This will more especially be the case 

 if the tension be slowly and continuously increased till the 

 cohesion give way. But when a fissure is once begun at one 

 point, no other can begin at any point near it and in the line 

 of tension through it, because the tension along that line is 

 immediately relaxed. Hence it will be impossible that a 

 fissure should thus be formed parallel and very near to one 

 previously existing, since in the immediate vicinity of an exist- 

 ing fissure the tension perpendicular to it must necessarily be 

 destroyed, and no system of parallel fissures very near to each 

 other could thus be formed*. 



Similar reasoning will apply to the case of longitudinal 

 fissures, or planes of discontinuity, supposed to be formed by 

 the more rapid motion of the centre of a glacier, admitting a 

 condition which, I conceive, cannot be disputed. Let the 

 annexed diagram represent two contiguous longitudinal ele- 



Fi<r. 3. 



ments of the mass. If there be no cohesion between them, 

 they will still be capable of exerting a tangential force on each 

 other by friciio?t, if they be kept in contact by a compressing 

 force. The greatest tangential force which could thus be pro- 

 duced would be the greatest force of friction under the exist- 

 ing pressure. Let it =Fj. If there be cohesion between 

 these elements, there will be on this account an additional 

 power of resisting the tendency of any forces to make one 

 element slide past the other. Let this additional power be 

 denoted by F^ ; then Fj + F2 will be the force which must be 



* This may be elucidated by attempting to tear a long strip of paper into 

 more than two parts by equal and opposite forces applied at its extremities. 

 It will be found practically impossible to tear it asunder in more than one 

 place. 



