56 



Mr. HOPKINS, ON THE MOTION OF GLACIERS. 



temperature would be proportional to the increase of depth, the rate of increase (with our present 

 supposition respecting the conductive power of the ice) being exactly the same as in the actual 

 case of the earth, provided the ice should always remain solid, i. e. if the temperature, thus increasing 

 with the depth, should not rise to zero at the lower surface of the icy crust. Now, though more 

 accurate observations on the internal temperature of glaciers are wanting, it is probable from those 

 of M. Agassiz, that the internal temperature of glaciers in those regions in which their motions 

 have been observed, and at depths below the influence of external variations, is not less than 

 — 1° (cent.). The least depth in the actual case of the earth at which the temperature is sensibly 

 constant may be stated generally at about 6o feet, below which the rate of increase of temperature 

 in descending may be taken at about 1° (cent.) for ever}' 100 feet. Hence, supposing the same 

 to hold for ice, the internal temperature of our icj' shell, where exposed to the same external tem- 

 perature as an actual glacier, would be below zero at every point, provided its depth were less 

 than itio feet. If the thickness of tlie shell were greater than that quantity, the temperature of 

 its lower part would be higher than zero if ics were capable of receiving such higher tempera- 

 ture ; but since that is impossible, the heat which would be employed in raising the temperature 

 of the lower portion of the shell above zero if it could retain its solidity, would be actually 

 employed in converting into water its lower surface, which would thus be retained at the constant 

 temperature of zero, and in a state of perpetual disintegration. 



If instead of supposing the icy shell to cover the whole surface of the earth, we suppose 

 it to be of comparatively small extent, the same conclusions will hold, provided its linear super- 

 ficial dimensions be sufficiently great with reference to the depth, which in the above case has 

 been estimated at 160 feet. Such is the case in all considerable glaciers. Hence, assuming 

 the truth of our data, if a glacier in those regions in which it is accessible to observation, 

 exceed 150 or iGO feet in thickness, its lower surface must be in a constant state of disintegration, 

 as a consequence of the internal heat of the earth. This result is liable to error, depending on 

 our imperfect knowledge of the internal temperature of glaciers, and the conductivity of glacial 

 ice; but in those parts at least, where the thickness of a glacier is considerably greater than 150 

 feet*, it leaves no reasonable doubt, I conceive, of the truth of our conclusion respecting the 

 state of slow perpetual disintegration of the lower surface. 



5. Agency of Siihglacial Currents The intei'nal 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 waj' between the glacier and the bed on which it rests. I 

 cannot appeal to any direct experiments to determine the effect of water at the temperature 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 b}' 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 it way to the bed of the 

 glacier. These little superficial streams shew their effect in disintegrating the ice by the manner 

 in which they cut out for themselves their own channels, thus assisting greatly in the degra- 

 dation of the surface. Its effect on the lower surface of the glacier is probably greater than 

 on the upper, on account of the hydrostatic pressure under which it must there act. The 

 descending water must reach the bed of the glacier at almost every point of it, and cannot 



• That such is the case throughout extensive portions of large 

 glaciers, there seems to be no doubt. M. Agassiz informed me 

 that he had discovered a nearly vertical hole in the ice, not fav 



from his cabane on the glacier of the Aar, of which the depth 

 could not be much less than 780 feet. 



