NOTICES OF BOOKS. ■ xxxv 



entitled facts and theories of audition. In his preface the author states 

 that this last chapter has entailed labour out of proportion to the 

 results, but here we do not agree with him, and fancy that in this all 

 students of Physics will agree with us. 



A notice of such a work as Lord Rayleigh's must of necessity par- 

 take of the character of a table of contents for the only thing that can 

 really be said about it is to regret that the work is not larger, and to 

 express a hope that the author will himself attack some of the problems 

 which he indicates as awaiting a satisfactory solution. 



In conclusion we need only say that this edition is practically the 

 publication of a third volume and must be invaluable to all physicists. 



Magnetic Fields of Force. By Professor H. Ebert. Translated from 

 the German by Dr. C. V. Burton. Pt. i., pp. xviii. + 297. 

 London : Longmans, Green & Co., 1897. 105. 6d. net. 



In this volume the phenomena of magnetism and electricity are 

 developed according to Faraday's conception of lines of force. In Max- 

 well's famous book on this subject the advanced student possesses a 

 complete treatise ; to those, however, who have not got a great com- 

 mand of mathematics this work is of very little use. In Professor 

 Ebert's book we have the subject dealt with in a clear and concise 

 manner and without the assistance of any mathematical reasoning 

 which is above the understanding of a student acquainted with quite 

 the elements of this subject. In part i. the more elementary portions 

 of the subject are dealt with, the consideration of the phenomena of 

 induction, single and multiphase periodic currents and electrical oscil- 

 lations being reserved for a subsequent volume. 



Throughout the volume directions are given for performing ex- 

 periments suitable for lecture demonstration, or for the use of ele- 

 mentary students in the laboratory. These directions are rather 

 detailed and in most cases are easy of performance. On page 97, 

 however, in Experiment No. 40, the reader is told to suspend a 

 magnetic needle by fastening to its middle a fine fibre, and is told 

 that it will set itself in the direction of the earth's magnetic lines of 

 force, no warning being given as to the difficulty which would be en- 

 countered in avoiding the disturbing effect of gravity if the needle is 

 not supported exactly at its centre of gravity. The experiment then 

 proceeds : " Now disturb the needle from its position of rest and allow 

 it to oscillate. From the frequency of oscillation we can calculate T, 

 the resultant intensity of the earth's magnetic field, or total force.'' 

 This is quite wrong if we take it literally, as from what is said in the 

 next section about deriving the value of H from that of T obtained as 

 above, or if the author means that from the period of oscillation and the 

 value of the dip T can be calculated we can only say he has managed 

 very successfully to obscure his meaning. 



In a work such as this where all the magnetic phenomena are 

 considered as due to something going on in the ether it is rather 



