NATURAL PHILOSOPHY. 159 



(4.) In extreme cases it cannot be used at all : thus, in the Greenland seas 

 the compasses would sometimes turn round with the ship ; whereas there 

 are in the Greenland ,seas several iron ships with my correcting magnets 

 effecting their purpose (I am informed) successfully, (o.) In cases not so 

 extreme, the inconvenience is intolerable ; thus, in one instance which 

 came under my own eyes, the compass changed 100 with a very small 

 motion of the ship ; and the directive intensity in one position was only 

 one-tenth of what it was in another position ; these inconveniences are 

 entirely remedied by my correcting magnets. 



On considering the whole matter, I am led to give the following as my 

 opinion : For voyages of moderate duration, as, for instance, not farther 

 than to the Mediterranean or to the northern parts of North America, I 

 do not tliink that any improvement can ,be made in the existing system, 

 except in details, to which I have alluded. The " end-on " position of the 

 magnets ought to be forbidden ; and some attention ought to be given to 

 the ship's sub -permanent magnetism, in the direction perpendicular to the 

 deck. For voyages of greater duration, as to the Plata, the Cape of Good 

 Hope, &c., I think it desirable that means should be provided for enabling 

 the captain to make the small changes which may be required in the cor- 

 recting magnets. I am confident that I can point out a practical course 

 by which this can be effected ; and I am satisfied that, with the sanction 

 of one liberal ship-owner, the aid of one intelligent captain, and the com- 

 mand of one ship for a few days, I can arrange every thing with good 

 hope of complete success. 



6. The remarks above are intended by me to apply only to iron-built 

 ships, in which the sensible part of the disturbance of the compass is pro- 

 duced almost entirely by the ship's sub-permanent magnetism. In wood- 

 built ships, in which the induced magnetism is the principal disturbing 

 power, the rules of correction are necessarily different. On these, at 

 present, I have only to make the same general remark which I have made 

 above ; that I disapprove of the use of a table of errors, and that I prefer 

 the use of mechanical corrections ; the nature of which, as applicable to 

 the neutralization of induced magnetism, is perfectly understood. 



ON IMPROVEMENTS IN SUBMARINE AND SUBTERRANEAN TELE- 

 GRAPHIC COMMUNICATIONS. 



At the British Association, 1854, Mr. C. F. Yarley explained experi- 

 ments lie had made with gutta-percha covered wires, varying from 30 to 

 1,500 miles in length, and showed a diagram drawn by the electric currents 

 themselves, decomposing solutions of ferro-cyanide of potassium and 

 nitrate of ammonia with which the paper had been saturated. These 

 experiments showed that the electric current did not appear suddenly at the 

 extreme end ; but, the wire becoming charged by induction like a Leydeii 

 jar, the current commenced gradually, and did not reach its maximum 

 power through 1,500 miles of wire until seven seconds of time had 



