10 Archdeacon Pratt on the Method of determining the 



exhibited by changes of form effected very slowly under the action 

 of forces of moderate amount, rather than by the rapid action 

 of more powerful agencies. 



The admission of this conclusion may slightly modify, but 

 will not materially alter, the views now generally held as to the 

 causes of glacier-motion, which are mainly derived from the re- 

 markable researches of Professor Tyndall. Whatever may be the 

 final judgment of men of science, I feel quite sure that it will 

 not confirm the opinion expressed by Canon Moseley in his 

 latest publication : that " the phenomena of glacier-motion belong 

 rather to mechanical philosophy than to physics." Every real 

 advance that has been made towards the explanation of those 

 phenomena has been due to the application of increased know- 

 ledge of the physical properties of glacier-ice; and if any thing 

 be wanting to complete the explanation now generally accepted, 

 it must be derived from such additional acquaintance with those 

 properties as may be derived from continued observation and 

 experiment. 



II. Reply to M. Delaunay's objection to the late Mr. Hopkins's 

 Method of determining theThickness of the Earth's Crust by the 

 Precession and Nutation of the Earth's Axis. By Archdeacon 

 Pratt, M.A., F.R.S. 



To the Editors of the Philosophical Magazine and Journal, 

 Gentlemen, 



IT is only two days ago that I saw for the first time M. De- 

 launay's strictures * upon the late Mr. Hopkins's method 

 of ascertaining the least thickness of the Earth's Crust by means 

 of the phenomena of Precession and Nutation, although I had 

 previously seen a notice that such strictures had been laid before 

 the Prench Academy. Having now that gentleman's paper 

 before me, I write to endeavour to convince your readers that the 

 point of Mr. Hopkins's reasoning has been altogether missed, and 

 that his method stands altogether unimpaired by these strictures. 



2. I am ready to allow, and so would Mr. Hopkins have 

 allowed, that if the crust of the earth revolved round a steady 

 axis, always parallel to itself in space, and if at some parti- 

 cular epoch a difference existed between the rate of movement 

 of the crust and of the fluid within it, the resulting friction would 

 gradually destroy this difference and bring about a conformity 

 in the motion of both parts. I will even go further, and allow 

 that the effect of the internal friction and viscidity of the fluid 

 may be such that the resulting rotary motion may be the same 

 as that which the whole mass would have had at the epoch if it 



* Translated in the Geological Magazine, November 1868, p. 507. 



