Mr. 0. J. Lodge on Thermo-electric Phenomena. 349 



not the same for high and low sounds is probably to be ac- 

 counted for by the fact that the relations which produce inter- 

 ference are not the same in the two cases. As a matter of 

 fact, high sounds are most affected in general by transmission 

 through obstacles or layers of any kind. The reason is, no 

 doubt, that under ordinary circumstances the dimensions 

 considered are comparable with short wave-lengths rather than 

 with long ones. 



The method given above can be extended to any number of 

 layers by a process analogous to one used in calculating the 

 effect on light of a pile of glass plates. 



It is only necessary to remark that the layer behaves like a 

 single surface whose dividing ratios are 



or, remembering that a + /3 = l, 



Thus for two layers, 



"2- % 



a! 

 -cc n 



0=1- 



-« 2 . 



and for 2 n 



~~4- 

 layers, 













a 





A. 





«2» 2n 



~(2 n 



-iy 



XLV. Reply to the Note of Professor M. Avenarius. 

 To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



IN the February Number of the Philosophical Magazine, 

 page 156, Professor M. Avenarius does me the honour to 

 notice a communication of mine " On a Mechanical Illustra- 

 tion of Thermo-electric Phenomena " (December Supplement, 

 1876), wherein I attributed incorrectness to his formula for 

 the electromotive force of a thermo-electric couple, 



R = (h-t 2 ){b + c(t 1 + t 2 )}, .... (3) 



an imputation which Professor Avenarius rejects. 



Now it is true that this expression, taken by itself and in- 

 dependently of the process by which it was obtained, is not 

 erroneous but quite correct— being, in fact, as he justly 



