C. GENERAL PHYSICS. 163 



DEVIATION OF THE COMPASS IN IRON SHIPS. 



It is now tolerably well ascertained that some of the sud- 

 den and heretofore unaccountable variations in the deviation 

 of the compass needle on board of iron ships are caused by 

 the unequal and varying diffusion of heat over the iron hull. 

 It is affirmed that to unsuspected changes in the indications 

 of the needle, arising from the cause above named, frequent 

 errors in reckoning, and perhaps shipwreck, may be justly 

 ascribed. Sudden changes, amounting to as much as five de- 

 grees,have occasionally been noticed o.n vessels of this kind on 

 the American coasts, which can only be reasonably explained 

 upon the foregoing supposition. In the instances alluded to 

 the variations were attributed to changes in the liuU, caused 

 by the passage of the vessel from cold to warm water, or the 

 reverse, as, for example, into and out of the Gulf Stream ; or 

 to the effect of the sun's heating influence being unequally 

 distributed, striking on one side only, and perhaps subse- 

 quently changing to the other. 20 Z>, Oct. 7, 1875, 15. 



NEW RELATION BETWEEN ELECTRICITY AND LIGHT. 



A new relation between electricity and light has been an- 

 nounced by Dr. Kerr, of Glasgow, who finds that when glass, 

 which in its normal condition is a singly refracting medium, 

 is properly subjected to the action of electrostatic force, it 

 assumes a new molecular structure, and thus acquires the 

 property of double refraction. A Ruhmkorff's induction ap- 

 paratus was employed, and a block of plate-glass served as 

 the dielectric medium, while polarized light was transmitted 

 through the glass in a direction perpendicular to the lines of 

 electric force. Under these conditions the glass acts as 

 though it has suffered compression along the lines of force, 

 or, in other words, it becomes a negative uniaxial medium. 

 Similar results were obtained with other dielectrics, the ac- 

 tion of resin, however, being contrary to that of glass, or pos- 

 itive. 15 ^, JSFovember 20, 676. 



ELECTRICITY FROM DIPPING METALS IN FLUIDS. 



If two similar pieces of metal are simultaneously immersed 

 in a fluid, they give no electric current ; but if dipped suc- 

 cessively, an electric current is observed, as has been long 



