408 Prof. Forbes's Reply to Mr. Hopkins 



kins thinks himself entitled to set aside a theory upon which I 

 had spent so much time, exertion, and thought, in three paru' 

 graphs containing two objectiofis (2nd mem. p. i, 5). The one 

 is the verticality of the crevasses, the other the want of evi- 

 dence of the longitudinal compression of the glacier. The 

 former has been fully explained by me in my Travels, p. 370, 

 and more recently in my Seventh Letter on Glaciers ; the lat- 

 ter will be found satisfactorily disposed of, on the evidence of 

 facts not ascertained by me, in my Ninth Letter on Glaciers 

 in the Edinburgh Philosophical Journal for April 184<5, p. 

 333, &c. These were the only objections stated, on the bare 

 statement of which Mr. Hopkins thought himself entitled to 

 sweep away anything like a. theory constructed by me, and, 

 having cleared the field, to erect his own. These were the 

 objections " easily refuted," to which I referred in my Eighth 

 Letter on Glaciers; and though a less mild term might have 

 been applied to them, Mr. Hopkins has thought my remark 

 — intended to avoid offence in its expression — worthy of no- 

 tice in his last communication. 



Another instance I must mention. Mr. Hopkins has em- 

 ployed the annexed figure in the Phil. Mag. for March, 



Fig. 1. 



p. 239, to prove that the structural veins cannot be perpendi- 

 cular to the crevasses M N throughout, if the crevasses be con- 

 vex upwards. Mr. Hopkins could hardly suppose me so dull 

 as not to see that in the centre of the glacier the crevasse and 

 the loop of the structure are parallel ; but he might have re- 

 collected that the loop of the structure is formed by differential 

 motion in a vertical plane, and that were he to take the direc- 

 tions of differential motion on my theory, estimated in a hori- 

 zontal plane, they would be found to diverge towards the 

 origin of the glacier; and the curve perpendicular to them 

 would be convex in the same direction. This will be seen by 

 constructing a surface everywhere normal to the spoon-shaped 

 curves, which (as well as its section on the horizontal })lane) 

 will evidently be convex towards the origin. In the superficial 



