THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



CONDUCTKD BY 



SIR OUSTER JOSEPH LODGE, D.Sc, LL.D., F.R.S. 



SIR JOSEPH JOHN THOMSON, O.M., M.A., Sc.D., LL.D., F.R.S. 



JOHN JOLY, M.A., D.Sc, E.R.S., E.G.S. 



AND 



WILLIAM FRANCIS, E.L.S. 



"Nee aranearuiu sane textus ideo melior quia ex se fila gignunt, nee noster 

 vilior quia ex alienis libamus ut apes." Just. Lips. Polit. lib. i. cap. l.Not. 



VOL. XL.— SIXTH SERIES. 



JULY— DECEMBER 1920. 

 I 



LONDON: 

 TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET. 



SOLD BY SMITH AND SON, GLASGOW: — HODGES, FIGGIS, AND CO. DUBLIN; — 

 AND VEUVE J. ROYVEAU, PARIS, 



