Mr. HOPKINS, ON THE MOTION OF GLACIERS. 163 



100 to 200 feet establishes this fact beyond doubt. Hence if we draw CD parallel to AEG at the 

 depth of about 300 feet, the portion of the mass above that line will have no motion except 

 tliat which arises from the motion of the subjacent portions C D B. But the cause of motion we are 

 now examining is greatest at BC where the glacier is thickest, and diminishes as we approach 

 to D, where it vanishes. Consequently, the tendency to move will be greatest at the upper 

 extremity of the glacier, and therefore the whole mass must necessarily be throughout the greater 

 part of its extent in a state of longitudinal compression. In fact, a large portion DH of the 

 glacier towards its lower extremity could have no sensible motion from the cause under considera- 

 tion, (since its depth is less than PQ), except that produced by the pushing force exerted upon 

 it by the other portion. 



Now this state of longitudinal compression appears to be quite inconsistent with observed facts, 

 at least during the summer-months, when the motion is probably always greatest. During that 

 season, the velocity on the Mer de Glace of Mont Blanc appears to be considerably greatest near the 

 lower extremity, and all observed glaciers, as already stated, are traversed by numerous transverse 

 fissures — facts which indicate unequivocally a state of longitudinal extension, and not of compression. 

 M. Elie de Beaumont has remarked the obvious appearances of extension which glaciers present, and 

 Professo; Forbes has borne testimony to the truth of the remark. In winter, it is probable that 

 there may be a tendency to more rapid motion near the upper extremity of the glacier, as explained 

 in my former memoir (Art. 11), and a consequent tendency to produce compression; but if the prin- 

 cipal part of the motion were due to the particular constitution of the mass above supposed, the 

 tendency to compression would be most obvious during summer, when the motion is greatest; a 

 conclusion totally at variance with the results of observation. 



Hence, then, it appears that any theory resting on any of the four hypotheses respecting the 

 constitution of a glacier above stated (Art. 1), is not only raised on a foundation unsupported by 

 direct experiment, but leads to results opposed to those of direct observation. The theory which 

 assigns the viscosity of the mass as the principal cause of glacial motion necessarily involves these 

 difficulties, so far as it pretends to any distinctive character which may separate it from other 

 theories, which, in common with it, assign gravity as the primary cause of the motion to be accounted 

 for. The absence of longitudinal compression in a glacier is equally opposed also to the theories of 

 dilatation and expansion. 



Formation of Transverse Fissures. Since the publication of my former memoir, I have 

 discovered that the explanation there given of the origin of transverse fissures, and of the fact of 

 the convexity of the curves which they form being towards the upper extremity of the glacier, 

 is imperfect. I shall now offer what appears to me to render the explanation complete. 



In this investigation we shall only be concerned with the difference of the velocities of the central 

 and lateral portions, for, at least to the depth to which observed fissures extend, there is certainly 

 no difference of velocity for particles in the same vertical line. We may therefore consider the 

 glacier independently of its thickness, or as a lamina of ice. The explanation will thus, in some 

 degree, be simplified. 



G. When a plain solid lamina having a small degree of compressibility and extensibility, is 

 brought into a position of constraint by forces acting in the plane of the lamina, the particles on one 

 side of a geometrical line will exert certain forces on the contiguous particles on the opposite 

 side of the line. If the lamina were formed of fluid particles the resultant action at each point 

 of this line of separation would be normal to it; but when the lamina is solid this will not 

 be generally the case, and therefore the force at any point of the line may be resolved into 

 two forces, one being normal and tiie other tangential to the line of separation; all forces being 

 supposed to act in the plane of the lamina. Suppose the line of separation to be a straight line 

 A' A parallel to the axis of .v, an<I let pq be a portion of it so small that the action on each 

 ))oint oi pq may be considered equal. Let Yi-pq denote the normal force exerted by the 

 Vol. VIII. Paut II. Y 



