Mr. Hopkins on the Mechanism of Glacial Motion. 147 



of a glacier. The continued verticality of transverse fissures 

 is, however, a conclusive proof that, to the depth to which 

 these fissures extend, the velocity must be sensibly the same as 

 at the surface. 



2. The facts above stated are in entire accordance with the 

 conclusions deducible from the sliding theory. According to 

 that theory, the velocity of any portion of a glacier will depend 

 (1) on the inclination of its bed, (2) the disintegration of its 

 lower surface by the internal heat of the earth, (3) on subgla- 

 cial currents, (4) the depth of the mass*, and (5) local and 

 lateral obstacles. The first and second causes will generally 

 have nearly the same effect both in the central and lateral por- 

 tions; but the third cause will manifestly produce in general 

 the greatest acceleration in the central parts, and the fourth 

 cause will produce a similar effect, if the glacier be deeper in 

 the centre than at its sides, while the greatest retardation will 

 be produced on the lateral portions by the last of the above- 

 mentioned causes. These causes sufficiently account for the 

 greater velocity of the centre of the glacier. 



Again, the second of the above causes will probably act with 

 approximate uniformity throughout the whole length of the 

 glacier, but the third cause will act with the greatest energy 

 at the lower extremity, because the subglacial currents will be 

 increased by innumerable tributaries as they descend. This 

 cause, therefore, will tend to make the velocity greater, as we 

 approach the lower end of the glacier, while the greater depth 

 of the mass at the upper extremity will tend to give the greater 

 velocity to that part of the glacier. In winter the effect of the 

 currents must be very inconsiderable, and we should conse- 

 quently expect that there will be a tendency in the portions 

 of the glacier in the higher regions to move faster than those 

 in the lower, in which case there must be a longitudinal com- 

 pression, and consequent closing up of transverse fissures in a 

 greater or less degree. During the summer, on the contrary, 

 the subglacial currents will be most efficient, and we should 

 expect that they will give the greater velocity to the lower ex- 

 tremity of the glacier, in which case the mass will be brought 

 into a state of longitudinal tension. 



3. It is manifest that the more rapid motion of the central 

 part of the glacier must bring the whole mass into a state of 

 internal constraint. My first object will be to deduce certain 

 conclusions respecting the internal tensions and pressures thus 

 superinduced, while the continuity of the mass, or of any con- 



* In the experiments described in my previous communication, the ve- 

 locity of descent increased with the weight, whence it is inferred that the 

 deeper parts of a glacier niu&t, cceteris paribus, tend to move the fastest. 



