Dr. Whewell's Remarks on Mr. Hopkins's Reply. 431 



linear motion, are inconsistent with that supposition. The 

 use of equation (3.) would have prevented falling into this in- 

 consistency. 



The objection urged against this instance applies in full 

 force to another which I have frecjuently referred to in the 

 pages of this Journal, viz. Poisson's solution of the problem 

 of resistance of the air to the motion of a ball-pendulum. 

 That solution, however admirable in some respects it may be, 

 rests on the gratuitous assumption that ud.v + u dy + 'wdz is 

 an exact differential. The reasoning I have gone through 

 shows that the motion deduced is inconsistent with that as- 

 sumption (see Cambridge Philosophical Transactions, vol. 

 viii. part i. p. ^l). 



The existence and the necessity of a third general equation 

 in hydrodynamics being, as it appears to me, established, I 

 shall proceed at a future opportunity to make some applica- 

 tions of it. 



Cambridge Observatory, April 10, 1845. 



LX. Remarks on Mr. Hopkins's Reply to the previous 

 Remarks on Glacier Theories. By Dr. Whewell. 



To Richard Taylor, Esq. 

 Dear Sir, 



IN my remarks on glacier theories, printed in the Philo- 

 sophical Magazine for February and for March, I did 

 not mention Mr. Hopkins's name. I abstained from doing 

 so, in order to avoid, as far as was possible, any personal con- 

 troversy, such as he deprecates in a Reply to those remarks, 

 printed in the Philosophical Magazine for April. Neverthe- 

 less, in that Reply, he speaks of a departure from the rules of 

 strict courtesy, which charge he applies to me by name. Ap- 

 parently he was led to make this charge, through a belief that, 

 by those who had read his Letters on glaciers, my remarks 

 could not fail to be understood as applicable to him. This is 

 possible ; but I am still unable to see how I could have made, 

 in a more inoffensive manner, the remarks which justice to 

 Mr. Forbes seemed to me to require. And I cannot but think 

 Mr. Hopkins would have better promoted the object of avoid- 

 ing personal controversy, if he had replied, supposing he 

 thought reply necessary, in the same impersonal manner. 

 This would have thrown no difficulty in the way of his show- 

 ing my remarks to be inapplicable to him, if he could have 

 done so. And if he had done this, I think it n)ost likely that 

 none of your readers would liave discovered the " sarcasm " 

 of which he complains. 



