162 



Mr. HOPKINS, ON THE MOTION OF GLACIERS. 



of external forces, only to a very limited extent, especially when in large masses. Thus if a force 

 be applied to lengthen a given solid mass, a small extension will be the immediate effect; but 

 however long that force may be continued, or however slowly it may be increased, we know of no 

 hard solid substance capable of more than very small extension, so long as it retains that structure 

 on which its hardness and solidity depend. Metals, for instance, with a hard crystalline structure, 

 are susceptible of very small extension, until that structure is destroyed by a sufficiently elevated 

 temperature, when their ductility may become indefinitely great, till it becomes fluidity. In the 

 same manner it would seem impossible to believe that glacial ice, a substance of very hard and 

 highly crystalline structure, can have more than an extremely small degree of extensibility ; nor 

 when it approaches that temperature which dissolves it, does it appear to acquire the property 

 of ductility above mentioned in metals, but to pass almost immediately from a hard crystalline 

 texture with powerful cohesion, to a state of dissolution in which the cohesion is entirely destroyed. 

 Reasoning thus from the known properties of ice, and from the analogies furnished by other sub- 

 stances, it would seem extremely improbable that a glacier should be susceptible of a continuous 

 motion due to a change of form in its component particles, independently of all sliding of one par- 

 ticle past a contiguous one, and of the sliding of the whole mass over the bed on which it reposes. 



Though the two causes of motion considered in this and the preceding article are, when strictly 

 analysed, essentially distinct, the motions resulting from them, so far as such motions can be 

 subjected to observation in glaciers, would be nearly the same. Disintegration of the mass would 

 seem to be essential for the effectiveness of either cause. No evidence whatever of such disintegra- 

 tion has been obtained from observations made at accessible depths in glaciers ; but supposing it to 

 exist at greater depths, it would seem to me far the more probable that it should reduce the 

 mass to a state more analogous to that of an aggregation of sand, than to that of an extremely 

 plastic or semifluid substance. But whether we adopt either of these hypotheses, or that of (4) 

 (Art. 1), which may be regarded as a combination of the other two, it is easy to shew, as I shall 

 proceed to do, that the whole mass must necessarily, during its motion, be in a state of longitudinal 

 compression ; a conclusion which I conceive to be inconsistent with observed appearances. 



5. Let the annexed diagram represent a longitudinal section of a glacier, BDH being that 



Fig. 2. 



of the bed on which it reposes. Let MQP be a line of particles vertical at any proposed instant. 

 In the motion we are now contemplating each particle will have a velocity infinitesimally greater 

 than that of the particle immediately below it, the lowest particle at M having no motion if there 

 be no sliding, as I am now supposing, along the bed BH. Thus the physical line MQP will, at 

 successive times, form the continuous physical lines Mqp, Mq'p'. These lines, to a certain depth, 

 will sensibly retain their vertical position ; for it has been shewn that to a depth of about 300 feet at 

 least the texture of the ice is such as to admit of scarcely a sensible change of form, or, conse- 

 quently, of a sensible difference of velocity in different particles to that depth. In fact, the almost 

 invariable and continued verticality of all transverse fissures to depths not unfrequently of from 



