505 



THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



CONDUCTED BY 



SIR WILLIAM THOMSON, Knt. LL.D. P.R.S. &c. 

 GEORGE FRANCIS FITZGERALD, M.A. F.R.S. 



AND 



WILLIAM FRANCIS, Ph.D. F.L.S. F.R.A.S. F.C.S. 



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

 vilior quia ex alienis libamus ut apes." Just. Lips. Polit. lib. i. cap. 1. Not. 



VOL. XXXL— FIFTH SERIES. 

 JANUARY- JUNE 1891. 



LONDON: 

 TAYLOE AND FEANCIS, EED LION COUET, FLEET STEEET. 



SOLD BY SIMPKIN, MARSHALL, HAMILTON, KENT, AND CO., LD.; AND WIIITTAKER 



AND CO. ; — AND BY ADAM AND CHARLES BLACK, AND T. AND T. CLARK, 



EDINBURGH; SMITH AND SON, GLASGOW; — HODGES, FIGGIS, 



AND CO., DUBLIN; — PUTNAM, NEW YORK;— VEUVE J. BOYVEAU, 



PARIS; — AND ASHER AND CO., BERLIN. 



