74 



Mr. HOPKINS, ON THE MOTION OF GLACIERS. 



27. 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 sup- 

 posed 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 - l^S (cent.) we shall have Mj = — O'.S, and w^ = 54 

 feet nearly. If the conductive power of ice be less than that of common rock, the value of x^ 

 will be proportionally less. 



Taking this last value of x^, it follows that if the thickness of a glacier should exceed 50 

 or 60 feet +, the temperature of its lower surface would necessarily be zero, as already explained. 

 Now the thickness of glaciers is doubtless much greater in general than 50 or 60 feet J, and 

 therefore we conclude, that generally the temperature of the lower surface of a glacier cannot 

 be less than zero, and must, consequently, he in a state of constant disintegration, unless the 

 conductive power of glacial ice he much greater than that of the ordinary matter forming the 

 crust of the glohe. 



28. From the conclusion of the last article it appears, that if we would investigate accurately 

 the internal temperature of a glacier of considerable thickness, we must take, besides the condition 

 given by the superficial temperature, the additional one that the temperature at the lower surface 

 shall always = zero. In this case, however, the resulting expression for the temperature would 

 become so complicated, that it would be useless, I think, to give it, especially with the uncer- 

 tainty which exists respecting the superficial temperature of the ice during winter. The conclu- 

 sion above enunciated, which is not invalidated by this uncertainty, is all that is requisite for the 

 theory of glacial motion which has now been offered. 



W. HOPKINS. 



' It appears to be established, that the snow which falls on all 

 but the higher regions of a glacier is again dissolved in the spring 

 or early summer, and does not contribute to any permanent increase 

 of the glacier. 



t As a deduction from the general reasoning ot Art. 4, this 



thickness was estimated roughly at about 150 feet, that there might 

 be no doubt of its being an extreme value. The thickness of 50 or 

 CO feet as deduced above, is probably much nearer the truth. 

 + See Note, Art. 4. 



CASfBRIDGIS, 



May 1, 1843. 



