Professor Forbes on the Leading Phenomena of Glaciers. 229 



of Cambridge, the author of an ingenious pamphlet on the 

 theory of glacier motion, has illustrated it by experiment.* 

 But this is an effect which must remain nearly the same at all 

 seasons, being due to the constant flow of heat from the interior. 



5. The flow of heat from the interior is so very trifling that 

 it may be doubted whether it is adequate to produce the par- 

 ticular effect of wearing off the prominences of the descending 

 ice, or of moulding it to the form of the channel. In order to 

 do so to any effectual extent, it would be necessary that pro- 

 minences of many feet or yards in extent should be melted 

 away in a moderately short space of time. Now, what is the 

 fact \ M. de Beaumont has estimated, t by the theory of Fou- 

 rier, from the observations of Arago on the earth's tempera- 

 ture, that the quantity of central heat which reaches the sur- 

 face of the earth, is capable of melting 6^ millimetres of ice, 

 or exactly a quarter of an English inch in the space of a year. 

 Now, even admitting (as I think we may), that if the surface 

 of the earth were covered with ice, the flow of heat would be 

 somewhat greater ; still it must be admitted to be incapable of 

 disposing of portions of ice insignificant compared to the in- 

 equalities which oppose its downward progress. 



6. This small quantity of heat is not always applied (as Pro- 

 fessor Bischoff J and M. Elie de Beaumont have justly remark- 

 ed) to melt the ice of glaciers. Below 32° it will simply tend 

 to raise the temperature of the ice in contact with the soil, 

 and powerfully adhering to it. The almost pendant glaciers 

 of the second order, which are seen only at great heights, 

 those, for instance, on the precipices of the Mont Mallet, must 

 remain permanently frozen to the rock. Nevertheless, they do 

 actually descend over it, for they continually break off in fresh 

 avalanches. This is a fact which neither the theory of dila- 

 tation nor that of gravity, as commonly stated, is capable of 

 explaining. 



* Since writing the above I have been indebted to Mr Hopkins for a farther 

 statement of his views. 



\ AnnaUs des Sciences Geologiques, par Riviere, 1842. 

 X Wdrmelehre, p. 101, &c. 



