574 Notices respecting New Boohs, 



based on Kayser and Hungers formulae ; but now a single 

 term will represent only one of the several series of lines 

 which accompany one another ; but a summation of not more 

 than six terms will be adequate to include all the known lines 

 in their spectra. Thus, making use of Rydberg's formula 



and putting ^ j ^ ' _±_ _ g ^ 



the formula given above is capable of forming the basis of an 

 expression representing the distribution of energy in any one 

 series of the majority of spectra. 



I have no suggestion to offer as to the rationale of this 

 formula ; but suggest it as an empirical one which may 

 possibly be useful in representing the energy distribution in 

 a line- spectrum. Experimental data on the relative intensity 

 of the spectral lines are at present too scanty to test its value. 

 Physical Department, 



University College, London. 



LV. Notices respecting New Books. 

 An Elementary Treatise on Dynamics, containing Applications to 

 Thermodynamics, with numerous Examples. By Benjamin" 

 Williamson, Sc.D., F.R.S., Senior Fellow of Trinity College, 

 Dublin ; and Ekancis A. Taeleton, Sc.D., LL.D., Fellow of 

 Trinity College, and Professor of Natural Philosophy in the 

 University of Dublin. Third Edition, Eevised and Enlarged. 

 London : Longmans, Green, & Co. 1900. Pp. xvi + 560. 

 T1TE know of no other work on dynamics in the English language 

 * * which can compare in general usefulness to the student with 

 the work before us, now in its third edition. The science of dynamics 

 is admittedly one of the most difficult and intricate branches of 

 knowledge, and on account of its fundamental character indis- 

 pensable to all students of nature. The number of elementary 

 books on dynamics published within the last quarter of a century 

 is quite appalling, but the gap between the purely elementary and 

 the most advanced type of text-book on this subject is far from 

 being filled, and we have some difficulty in recollecting many 

 satisfactory text-books of the intermediate class. The late 

 Professor Tait's article on " Mechanics " in the Encyclopaedia 

 Britannica, now obtainable in book-form as a treatise on 

 " Dynamics," is a masterly example of clearness and elegance of 

 treatment, but it does not meet the wants of the student in every 

 respect, as there are no examples by means of which he would be 

 enabled to test his knowledge, skill, and ingenuity ; and attempting 

 to learn dynamics without working out numerous examples is as 

 hopeless as trying to learn pure mathematics by the same method. 

 One very important feature of the book under review is the 



