S4t> Mr. Hopkins's Uephj to Dr. WlieweU's 



nature of the subject, would prescribe to us in discussions of 

 this kind. Dr. Whewell has thought proper to reply to my 

 former observations on the secular character of the plasticity 

 of glacial ice by a sarcasm. I think the circumstance deser- 

 ving of a moment's notice only for the purpose of deprecating 

 any attempt to give a tone to the present controversy incon- 

 sistent with the character of a scientific discussion, and with 

 that respect which the parties engaged in it have a right to 

 claim from each other, and which, I have been willing to 

 hope, could not be diminished by a mere difference of opinion 

 on a question of physical science. 



One or two other points in Dr. Whewell's letters I would 

 briefly notice. He has remarked that it would be an unworthy 

 return to Prof. Forbes if we should write on glacial theories 

 as if we had derived nothing from him with respect to the 

 plasticity of glacial ice. It might also be added, that it would 

 be equally wrong to write as if we had derived nothing on 

 other points from anybody else. But there was no occasion, 

 in my opinion, for the observation. P]veryone recognises the 

 high merits of Prof. Forbes as an observer of glacial phaeno- 

 menaj as a theorist, his claims still remain to be substantiated. 

 As far as the plasticity of glacier ice shall hereafter be proved 

 to be one of the effective causes of the motion of glaciers, di- 

 rectly or indirectly, so fiir will those claims be at once allowed. 

 The way to establish them is to establish his theory. 



Again, Dr. Whewell observes that the question of the state 

 of the lower surface of a glacier, " though not unimportant, 

 offers nothing of novelty; the glacier slides by the constant 

 melting of its lower surface, in virtue of the heat of the sub- 

 jacent soil. This was distinctly taught by De Saussure." And 

 it may be added, almost as distinctly denied by all those who, 

 in the present day, have attemj)ted to establish some rival 

 theory. Agassiz has contended that the greater part of a 

 glacier is frozen to its bed. Charpentier says, " Nous venous 

 de faire voir que la forte inclinaison du lit d'un grand nombre 

 de glaciers presente une objection bien fondee contre ceux 

 qui pretendent que ces masses avancent en glissant sur leur 

 base par I'effet de leur poids. Mais la pente trop fiiible du 

 lit de beaucoup d'autres glaciers est egalement contraire a 

 cette opinion ; car pour se convaincre combien il est errone 

 de croire que leur mouvement soit du a Paction de la pesan- 

 teur, on n'a qu'a se rappeler qu'il existe plusieurs glaciers 

 dont la longueur est de quelque lieues, et qui presentent une 

 pente tres faible." With respect to Prof. Forbes's opinion, 

 I have already quoted* a passage from page 132 of his Travels. 

 * Note, p 3 of the January Number of this Magazine. 



