Review of the Controversy Regarding the Motion of the Glacier. 73 



sure's theory. In addition, however, to this, it fails to account for 

 more than a small part of the phenomena brought to light by recent 

 and accurate observation in the Alps, and, in consequence, has long 

 been altogether abandoned. 



Another theory was put forth by De Charpentier, which was free 

 from the difficulties besetting that of De Saussure, inasmuch as it sup- 

 plied another motive foi'ce than the apparently insufficient one of 

 gravitation. He took advantage of the expansion of water in freezing, 

 and suggested that the water from the surface of the glacier, produced 

 by the heat of the sun, flowed into the crevices in its interior, and 

 there was again frozen, and by its expansion forced the glacier to 

 move in the line of least resistance, that is, downward. This has 

 received the name of the "dilatation theory" of glacier motion, and 

 though, no doubt, sometimes a real, is, like De Saussure's, an altogether 

 insufficient cause ; not being capable, on the most liberal construction, 

 of accounting for more than a very small fraction of the movement 

 observed. Like its predecessor, it is now, consequently, quite aban- 

 doned.* 



The experiments of Agassiz and J. D. Forbes, in 1841 and 1842. 

 upon the glacier of the Unteraar and the Mer de Glace, formed an 

 epoch in the study of the subject. Correct data were ihen for the 

 first time given to the world, by which to test the adequacy of any 

 theory to acoount for glacier motion. Tried by this test, the two above 

 mentioned signally failed. It has been asserted that the movement 

 was made by fits and starts, but these experiments proved that it was, on 

 the contrary, slow and continuous. It had also been assumed that the 

 ice moved as a sii gle mass; whereas, it was now shown that the 

 motion is differential, inasmuch as the middle moved faster than the 

 sides, and the srtrface faster than the part below. The movement 

 also, it was now found, had been very much under-estimated by early 

 writers, and instead of amounting to only 1 5 or 20 feet in the year,t 

 the Mer de Glace actually moved, as the villagers had always main- 

 tained, at the rate of about a yard a day. 



The unhappy controversy which ensued between these two colaborers 

 in regard to priority of claim, and which has been revived since their 

 death, is well known and need not be noticed here. It will be sufficient 

 to observe, that the result of these experiments suggested to the mind 

 of J. D. Forbes a theory of glacier motion, which had also occurred 

 some years earlier to M. Rendu, bishop of Annicy, an earnest student 

 of the subject, and which he has left on record. This theory is strik- 



* See Charpentier Essaisur les Glacier, 1841, and Agassiz Etudes sur Ics Gltciers, 1840. 

 t Ebel " Swiss Guide Book." 



