164 Intelligence and Miscellaneous Articles. 



mind, and we trust that his present leisure will enable him to 

 collect, into book form, the accumulated stores of these past 

 years. Our task is a simple one. "We shall merely indicate what 

 are the subjects discussed. There are in all eight chapters. 

 The headings are : — Elementary Considerations ; Rectilinear 

 Motion ; Motion of Projectiles ; Constrained Motion in Two 

 Dimensions ; Motion in Two Dimensions ; Central Forces ; 

 Motion in Three Dimensions ; and some Special Problems. 

 The work closes with two notes : the first on an Ellipsoidal 

 Swarm of Particles, and the second on Lagrange's Equations, a 

 new form for the Lagrangian function, and a rotating field. The 

 great value to the student appears to us to be the thorough 

 discussion of a large number of illustrative problems. As in his 

 previous books, Dr. Routh gives ample reference to original 

 memoirs, a number of historical notes, and a useful Index. Our 

 reference to the Preface sufficiently indicates the wide range 

 included under the heading Dynamics of a Particle. 



X. Intelligence and Miscellaneous Articles. 



SUSCEPTIBILITY OF D1AMAGNETIC AND WEAKLY MAGNETIC 



SUBSTANCES. 



To the Editors of the Philosophical Magazine. 



Ge^tlbmeis-, 

 TN the issue of the ' Philosophical Magazine'' for December 1898, 

 -■-ma note referring to the article published by me in the 

 May number 1898, " On the Susceptibility of Diamagnetic and 

 -Weakly Magnetic Substances," Professor Quincke draws attention 

 to the fact that he had previously described* a method essentially 

 similar to the one I used for the determination of the susceptibility 

 and had applied it in investigating the susceptibility of Iron, 

 Nickel, and Cobalt, among other things. 



That I failed to refer to this note of Professor Quincke's, and 

 to a communication by Lord Kelvin t on the same subject, was due 

 to the fact that I did not learn of them until after the article 

 referred to above had appeared. 



I may be allowed to add that the method in the form used by 

 me, involving the use of prismatic slabs transversely magnetized, 

 does not admit of application to Iron, Nickel, and Cobalt, since in 

 this case the induced magnetization would depend almost altogether 

 upon the shape of the substance and but slightly upon its 

 susceptibility^. In Lord Kelvin's note upon this subject the 

 restriction that the method may be applied to those bodies only 

 which are diamagnetic or slightly magnetic is implied in the title. 



Very truly yours, 

 Berlin, Dec. 10, 1898. Albert P. WlLLS. 



* Tageblatt der 62 Vermmmlung der Deutsche?' Natur for seller und 

 Aerzte, Heidelberg, 1889, p. 209. 



t " On a Method of determining in Absolute Measure, the Magnetic 

 Susceptibility of Diamagnetic and Feebly Magnetic Solids." — Report of 

 the British Association, 1890, p. 745. 



\ Maxwell, ' Electricity and Magnetism,' vol. ii. pp. 65, 66. 



