Correspondence — Rev. Maxicell M. Close. 537 



AECHDEACON PRATT ON M, DEL AUN AY'S EXPERIMENTS ON THE 

 INTERNAL FLUIDITY OF THE EARTH. 



Sir, — In the 'Geological Magazine for September, you gave us 

 a paper by Archdeacon Pratt, combating M. Delaunay's objection to 

 Mr, Hopkins's method of reasoning from the precession of the equi- 

 noxes to the internal fluidity of the earth. There are some errors 

 of inadvertence in that paper, which I do not point out, as no one 

 is better able than the writer himself to discover them. We may 

 agree, however, thus far with Archdeacon Pratt, — M. Delaunay's 

 objection is not as conclusive as he himself seems to think. The 

 principle of that object is indubitably true. M. Champagneur has 

 proved it so by direct experiment ; and it is (as appears to me) self- 

 evident, a priori. If the tendency of the hard crust of the earth to 

 shift on the internal fluid mass be sufficiently small, relatively to the 

 degree of viscidity of the fluid, the crust must carry that viscous 

 interior along with it in the changes of direction of its rotation. 

 But is the relation such, in the case in question, as to make M. Delau- 

 nay's principle applicable ? This question, I suppose, never can be 

 answered. If the crust were even 1,000 miles thick, and if the 

 fluidity of the interior were perfect, the pole of the crust would be 

 slipping over the fluid interior at the rate of one inch in about 

 twenty-five minutes (if the crust be as thin as some geologists have 

 supposed, the rate of slipping would have to be nearly a third 

 greater) -, and this shifting movement would occur all round the 

 great circle parallel, at each moment, to that containing the celestial 

 poles of the equator and the equinoctial points (taking the retrogres- 

 sive movement of the earth's axis in its mean direction). Now, 

 what amount of viscosity would be necessary to overcome the 

 enormous moment of inertia, round its axis, of a globular mass 6,000 

 miles in diameter (or much more, as some would think), and start it 

 afresh at every instant, in a new direction, at the above rate (or, 

 greater), from a state of relative rest ? And is the actual viscosity 

 sufficient? Certain considerations would weigh for, and others 

 against, M. Delaunay's opinion ; but on which side of the scales the 

 preponderance lies we cannot tell, from our ignorance of some of the 

 conditions of the problem. M. H. Close. 



Newtown Park, Blackeock, Dublin, 

 October 4, 1870. 



THE LECTURE ON VOLCANOS. 



Sir, — Upon my return to London I received the Geological 

 Magazine (for September), containing, page 440, a letter from Mr. 

 Poulett Scrope, commenting upon my lecture on volcanos, which 

 appeared in the July number ; the following remarks in reply were, 

 however, too late for last month's Magazine, as you informed me 

 that the October number was then already in the press. 



I have always looked upon Mr. Scrope's works on volcanos as 

 being by far the best on the subject which we possess, whether they 

 be considered from a philosophical or a descriptive point of view 

 (and in the latter sense the author's experiences in the field render 



