464 Notices respecting New Boohs. 



interrupters for induction coils, one hundred pages with high- 

 frequency electrical measurements, eighty pages with damping and 

 resonance, forty-threo with stationary electric waves on wires, 

 seventy-seven with the theory of electromagnetic waves (including 

 detailed diagrams by Hertz, Pearson and Lee, and Love of the 

 lines of force of Hertzian oscillators under various assumed 

 conditions), ninety-two pages are devoted to their detection and 

 measurements. All this is dealt with before ihe main subject of 

 telegraphy is introduced ; this constitutes nearly half the volume. 

 In the historic part an endeavour has been made to leave no 

 contribution of any importance unmentioned ; and, what is more 

 difficult to do, satisfactorily to apportion the credit of each discovery 

 and achievement. Whether the author will have succeeded in 

 satisfying all claimants may be more than doubted ; it is certainly 

 more than could be expected. But to the reviewer he appears 

 to have presented an exceedingly complete and fair historical 

 account, which may do much toward quietening the strife (healthy 

 enough in its way) with which the subject is attended. 



In a few details suggestions could be made toward an improve- 

 ment of the exposition. Dr. Fleming will probably see as soon as 

 he is told that the top two paragraphs on p. 349 are not perfect. 

 After giving the general solutions of the equations of wave 

 motion 



H=/ 1 («-m0+/,(2+^), (9) 



E=/ 8 (z-i«0+/ 4 (* + ttf), (10) 



it should be pointed out that f 3 and / 4 are not independent of f x 

 and/,. It is incorrect to state that H and E remain unchanged if 

 z and t be changed to z + z and t -\-t' where z /t' — u. This would 

 be true for the first member on the right hand of each ; for the 

 second member z'/t' would require to be minus u. In fact, the 

 general solution is not that of a wave travelling with unchanged 

 form though each component of it does so travel, one part in one 

 the other part in the opposite direction. Dr. Fleming's alternative 

 statement beginning " in other words " is therefore also incorrect. 

 Further it is not obvious (as stated) from equations (9) and (10) 

 quoted above that the periodic quantities E and H are in step or 

 phase with one another. In particular it would be interesting to 

 learn his definition of the phase of a composite wave. Some of the 

 statements made become true if, instead of the general solution, a 

 simple, harmonic solution be referred to. This may represent 

 either a positive or negative wave, but not both simultaneously. 



Again, the diagram on p. 407 may roughly represent the lines of 

 force terminating on an antenna at some stage of the oscillation, 

 but it is scarcelv a satisfactory representation of the lines existing 

 before a spark begins. As a typographical error we may indicate 

 that on p. 189 near the top valuable should read variable. 



Faults such as these are so few that the mentioning of them 

 will merely serve to accentuate the high excellence of the treatise 

 as a whole. It is a fine achievement. 



