Mr. Hopkins on the Motion of Glaciers. 1 1 



and Xc, - :Q2g feet, 



= 178 feet nearly. 



The temperature —5° (cent.) appears, however, to be much 

 lower than that observed at different depths by M. Agassiz, 

 and which did not exceed half a degree. The difference may, 

 I conceive, be easily accounted for. In our investigation the 

 surface of the glacier has been supposed to be exposed to the 

 winter temperature, whereas, as soon probably as the mean 

 temperature of the twenty-four hours descends to zero, the 

 surface is protected from the external cold by a coating of 

 snow, which increases as the temperature diminishes, and thus 

 it is probable that the temperature of the surface of the ice may 

 descend but little below zero during the whole winter. If we 

 suppose its lowest temperature to be about — 1"*5 (cent.), we 

 shall have u^ = — 0°'5, and x^ = 54f feet neai'ly. If the con- 

 ductive power of ice be less than that of common rock, the 

 value of oTg will be proportionally less. 



Taking this last value of x^, it follows that if the thickness 

 of a glacier should exceed fifty or sixty feet, the temperature 

 of its lower surface would necessarily be zero, as already ex- 

 plained. Now the thickness of glaciers is doubtless much 

 greater in general than fifty or sixty feet, and therefore we 

 conclude, that generally the temperature of the lower surface of 

 a glacier cannot be less than zero, and nmst, conseqitently, be in 

 a state of constant disititegration, unless the conductive power of 

 glacial ice be much greater than that of the ordinary matter form- 

 ing the crust of the globe. 



6. Agency of Subglacial CurreJits. — The internal heat of the 

 earth, however, is not the only cause producing this constant 

 disintegration. Another and probably very effective agency 

 exists in the subglacial currents, which, during the summer, 

 are principally produced by the rapid melting of the ice at the 

 upper surface of the glacier, whence they descend through 

 open fissures, and afterwards force their way between the gla- 

 cier and the bed on which it rests. I cannot appeal to any 

 direct experiments to determine the effect of water at the tem- 

 perature of zero in dissolving ice at the same temperature, 

 when running in contact with its surface, but its efficiency in 

 this respect is sufficiently proved by its action on the upper 

 surface of a glacier when the direct rays of the sun and the 

 temperature of the atmosphere are sufficient to dissolve the 

 superficial ice, and thus to create innumerable rivulets running 

 upon the surface till they meet with a fissure into which the 

 water is precipitated, and finds its way to the bed of the glacier. 

 These little superficial streams show their effect in disintegra- 



