January 25, 1895.] 



SCIENCE. 



107 



iniportaut discoveries in tlie region of Hert- 

 zian oscillations. It was probably ( ^ ) Poin- 

 car6 (Ids modesty prevents him fi'om men- 

 tioning tliis fact) who first recognized its 

 full value and detected its true meaning. 

 He devotes a large part of the present work 

 to the discussion of this phenomenon and 

 every serious student will appreciate heart- 

 ily this very interesting feature of the noble 

 work before us. Briefly stated Poincare's 

 explanation of multiple resonance is this. 

 Ordinarily the oscillator has a large decre- 

 ment; that of the resonator is very small, 

 according to the results of Bjerkness' experi- 

 ments. The train of waves excited in a 

 long wire by the inductive action of an os- 

 cillator after each disruptive discharge 

 consists of a big wave followed by a small 

 number of waves of very rapidly decreasing 

 amplitude. Such a train of waves is evi- 

 dently not capable of forming interference 

 waves after reflection. Their effect upon 

 tlie resonator is jiractieallj- the same as that 

 of a single wave, giving the resonator an 

 impulse when passing it on its way toward 

 the end of the long wire and another im- 

 pulse when it returns after reflection. 

 Hence, if the time interval between these 

 two impulses is a multiple of the period of 

 the resonator the resulting oscillation in 

 the resonator will be sti'onger than other- 

 wise. If, therefore, the resonator be moved 

 along the long wire its oscillations will vary, 

 passing tlirougli a maximum at regular in- 

 tervals; the distance between these intervals 

 being equal to a wave length correspond- 

 ing to the period of the resonator. But, 

 obviousl}', the maxima will be most clearly 

 pronounced wlien the resonator is in reson- 



(') It is no more tliaii just that a stronj;; enipliasis 

 should be put jijion the fact tliat Bjerkness independ- 

 ently (Wied. Ann. 44 |). 74 and p. 92, July, 1891) 

 ■worked out the same theory and proved it by experi- 

 ment at about the same time that Poineare first pul)- 

 lished his theory (Arch, des sciences phys. 25 p. GOW, 

 G6aii\e 15 Juin, 1891). 



ance with the o.scillator. This is especially 

 true in the case of oscill.ators possessing a- 

 less strongly developed decrement, as for 

 instance, Blondlot's o.scillator. This ex- 

 planation is illustrated by a mathematical 

 discussion of rare elegance and simplicity. 

 Blondlot's experiments (Jour, de Pliys. "J 

 serie t. X., p. 549) are then carefully de- 

 scribed and the close agreement between 

 them, especially as regards the velocity of 

 propagation along conducting wires, and 

 the above tlieory pointed out. 



Attenuation of Waves.— A.i\ important feat- 

 ure connected with wave propagation of 

 Hertzian oscillations along wires was 

 stronglj' emphasized by these experiments, 

 namely, tlie diminution of the wave ampli- 

 tude with the distance passed over. This 

 has long since given Mr. Oliver Heaviside 

 manj' an anxious thought. Poineare is evi- 

 dently not aware of that and he attacks the 

 problem with just as mucli of his well- 

 known mathematical vigour as if its solution 

 had not been given long ago by Mr. H(>avi- 

 side. (Electr. Papers, Vol. II., p. :«», etc.) 

 A few bold strokes of Poincare's unerring 

 pen disclose the interesting fact that the at- 

 tenuation is due, principally, to distributed 

 capacitj- of the wire, since the decrement, 

 calculated by Poynting's theorem, is shown 

 to be inversely proportional to the diameter 

 of the wire. Experimental evidence bear- 

 ing upon tliis point is then reviewed. In 

 these experiments the employment of the 

 resonator had to be discarded and the in- 

 tensity of the wave at various points of tlie 

 wire measured directly. Various methods 

 were einployed in these experiments. The 

 most important among them are tlie follow- 

 ing :— 



a. Hertz's method (Wied. Ann. 4L', ]). 

 407, 1891) of measuring the intensity of the 

 wave at any point of a long wire by the 

 mechanical force ext-rled upon anotlier small 

 conductor suspended in the vicinity of the 

 wire. This method jierniits a study of the 



