﻿METCALFE, 
  

  

  Phil. 
  Mag. 
  Ser. 
  6. 
  Vol. 
  18. 
  Pi. 
  XXIX. 
  

  

  Fig. 
  2. 
  

  

  kO 
  50 
  60 
  70 
  

  

  /y9£-SS(//i'£ 
  /rJCM3. 
  Of 
  ^^ 
  SO^ 
  

  

  