NOTICES OF BOOKS. xxxiii 



these three books. There is another striking resemblance between them 

 in that in each case while free use is made of the help of mathematics 

 the physical bearing of the problems and the physical interpretation of 

 the results is everywhere given a prominent place. 



In a notice of the first edition of Lord Rayleigh's Sound von Helm- 

 holtz concludes as follows : " At the end of the volume Lord Rayleigh 

 has placed the words * The End '. We only hope that this may be only 

 the provisional, not the definite end. There is still an important 

 chapter wanted, viz., that on the theory of reed pipes, including the 

 human voice. . . . After reed pipes we woijld mention the theory of 

 singing flames and the blowing of organ pipes." In this second edition 

 we have a fulfilment of Helmholtz's wish, for in the three hundred and 

 fifty odd pages of additional matter the author has included discussions 

 on these points. 



The first edition is so well known to all physicists that it will only 

 be necessary to refer to the additions which have been made in this 

 edition. 



Throughout the volumes short paragraphs and steps in the mathe- 

 matical reasoning have been here and there added, and what is of much 

 importance references to recent works on the subject have been included. 

 The first considerable addition is a discussion on the vibrations of bells, 

 in which the author describes the results of the experiments he has con- 

 ducted in this subject. He has examined a number of church bells, 

 ■determining in each case the different tones the bells are capable of 

 yielding, and has been led to the interesting result that in English bells 

 dt is the fifth tone in order which agrees with the nominal pitch of the 

 bell, while in Belgian bells it is the second tone that agrees with the 

 nominal pitch. The author then goes on to investigate the vibration of 

 curved plates and shells, the theory being only strictly applicable when 

 the thickness is infinitely small. 



From the author's well-known investigations on the telephone one 

 naturally looks for a chapter dealing with this instrument ; and in this 

 connection there is also one on alternating currents, including the 

 passage of alternating currents through a network of conductors, and 

 the resistance of conductors to alternating currents. In connection 

 with the measurements of the current needed to give an audible sound 

 in a telephone given in the text, and also with reference to the minimum 

 amplitude of sound waves that are audible, it is interesting to recall 

 some recent measures by Barus who showed that the amplitude of the 

 excursions of a telephone diaphragm is less than lo "'-' cm. 



Two very interesting sections are added dealing with the repulsion 

 of resonators and the attraction between a vibrating body, such as a 

 tuning fork, and a light suspended body. Incidentally the author con- 

 siders the tendency of a suspended disc to set itself at right angles to 

 the direction of alternating currents of air. Boys has shown to what an 

 extraordinary degree the sensitiveness of such a disc as a detector of 

 response in a resonator can be carried, and it is rather astonishing that 



