﻿Phil. 
  Mag. 
  Ser. 
  6, 
  Vol. 
  16, 
  PL 
  XVIII. 
  

  

  Fig. 
  12. 
  

  

  Assisting 
  "Waves. 
  

  

  Fig. 
  13. 
  

  

  » 
  a 
  * 
  M 
  tki'4 
  H 
  II 
  I 
  

  

  Opposing 
  Wayes, 
  

  

  