Mr. Hopkins's Remarks on Prof. Forbes's Reply. 599 



reference to experimental facts or mechanical reasoning, I deny 

 altogether the validity of the Professor's conclusion. 



I have already stated in my Third Letter that the ultimate 

 test of the sliding and plastic theories must be sought in ob- 

 servations on the relative motions of the upper and lower 

 surfaces of a glacier. The claims of the two theories would 

 thus be decided beyond dispute. Accurate observations are 

 also required to ascertain the form which a continuous straight 

 line drawn on the surface of a glacier and perpendicular to 

 its axis, will assume by the more rapid motion of its central 

 portion. Will it be deformed into a continuous loop, like 

 fig. 1, or into a disconti?mous one, like fig. 2.? Such observa- 



tions would decide the degree in which the greater central 

 motion is due to the flexibility or plasticity of glacial ice, and 

 the degree in which it is attributable to the dislocation of the 

 general mass. Observations of this latter kind continued 

 only for a few days would be of no value ; they ought to be 

 continued for as long a time as possible. Observations of 

 both kinds are become essential in the present state of glacial 

 theories, and would do much more towards settling the ques- 

 tions at issue respecting the cause of glacial motion, than any 

 further controversy, which on my part will terminate with this 

 letter. With respect to the mechanics of the problem, there can 

 be little need of further controversy, since nearly every pro- 

 position maintained on one side has been proved erroneous, 

 and almost every one on the other remains unquestioned. 



I would make one additional remark. It has been inti- 

 mated that I have not been ready to do justice to Prof. Forbes 

 with respect to his glacial theory. Is it meant that I ought to 

 have written a eulogy on a theory which I believed to be erro- 

 neous ? While I dissent from his theory, no one has ever been 

 more ready to allow the merit of the Professor's researches on 

 this subject as well as on many others which have engaged his 

 attention ; and if in the course of this controversy a word has 

 escaped me inconsistent with the high respect due to him both 

 as a man and a philosopher, it will be to me a matter of deep 

 regret. Your obedient Servant, 



Cambridge, May 24th. 1845. W. HoPKINS. 



