154 



Rev. S. Haughton on Physical Geology. [Dec. 12, 



subject is discussed in the same manner as Boltzmann discusses the 

 after-effect of torsion on a fibre, and it is worth remarking that the 

 results of my experiments can be roughly expressed by a formula in 

 A 



which <f)(t)=- . For glass No. 5 (soft crown) a = 0'65, whilst for 



No. 7 (light flint) it is greater ; but in the electrical experiment no 

 sign of a definite deviation from the law of superposition was detected. 



IV. " Note in correction of an Error in the Rev. Dr. Haughton's 

 Paper ' Notes on Physical Geology. No. V " (" Proc. Roy. 

 Soc.," vol. xxvii, p. 447). By the Rev. Samuel Haughton, 

 M.D., Professor of Geology in the University of Dublin, 

 F.R.S. Received October 9, 1878. 



In my paper read 20th June last, and published in the " Journal 

 of the Royal Society," there is an error in p. 450 which I wish to 

 correct. 



Referring to the geometrical proof of Mr. Darwin's theorem, I state 

 that from cusp to cusp of the cycloidal wabble occupies 152| days ; 

 this is an error, as it should be 305 days, as can be shown geometri- 

 cally. 



Let yx, y'x', be two successive positions of the line joining the axes 

 of rotation and figure ; produce them to meet at C, which will be the 

 centre of curvature, because yx and y'x', are normals to the cycloidal 

 arc yy ; it is well known that yC, (radius of curvature) is double yx 

 (chord of generating circle) or double y'x ; therefore the angle yxy' 

 is double the angle yCy' ; but yxy' measures the angular velocity of 

 the wabble, when x is supposed at rest ; therefore the angular velocity 

 of yx is only half that of the wabble, if the axis of figure were at rest. 

 Hence in 305 days, yx will turn through 180° only, and not 360°. 



