THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



CONDUCTED BY 



SIR ROBERT KANE, LL.D. F.R.S. M.R.I.A. F.C.S. 

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



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. XLIX.— FOURTH SERIES. 

 JANUABY— JUNE 1875. 





LONDON. 



TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET, 

 Printers and Publishers to the University of London; 



SOLD BY LONGMANS, GREEN, READER, AND DYER; KENT AND CO. ; SIMPKIN, MARSHALL, 

 AND CO. ; AND WHITTAKER AND CO. ;— AND BY ADAM AND CHARLES BLACK, 



AND THOMAS CLARK, EDINBURGH J SMITH AND SON, GLASGOW '. 



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