346 Analysis of Scientific Books. 



much larger when the water is constantly changing ; but the ex- 

 periments seem to shew that any diminution of protecting effect at 

 a distance, does not depend upon the nature of the metallic, but of 

 the imperfect or fluid conductor. 



This indeed is shewn by many other results. 



A piece of zinc and a piece of copper, in the same vessel of sea- 

 water, but not in contact, were connected by different lengths of 

 fine silver wire of different thickness. It was found that whatever 

 lengths of wire of -g^ of an inch were used, there was no dimi- 

 nution of the protecting effect of the zinc ; and the experiment 

 was carried so far as to employ the whole of a quantity of ex- 

 tremely fine wire, amounting to upwards of forty feet in length, 

 and of a diameter equal only to -^t^t °^ an mc ^» when the results 

 were precisely the same as if the zinc and copper had been in im- 

 mediate contact. 



Pieces of charcoal, which is the worst amongst the more perfect 

 conductors, were connected by being tied together, and made the 

 medium of communication between zinc and copper, upon the same 

 principles, and with the same views as those just described, and 

 with precisely the same consequences. 



Sir H. then details some experiments to determine the requisite 

 extent and nature of the contact or relation between the copper 

 and the preserving metal. He could not produce any protecting 

 action of zinc or iron upon copper through the thinnest stratum of 

 air, or the finest leaf of mica, or of dry paper; but the action of 

 the metals did not seem to be much impaired by the ordinary 

 coating of oxide or rust ; nor was it destroyed when the finest 

 bibulous paper was between them, being moistened with sea- water. 



Sir H. concludes this communication with some practical in- 

 ferences and theoretical elucidations arising out of its experimental 

 details. The very first experiment made on harbour-boats at 

 Portsmouth, proved that a single mass of iron protected many 

 sheets of copper, so as to make them negatively electrical, and in 

 such a degree as to occasion the deposition of earthy matter upon 

 them ; but observations on the effects of the single contact of iron 

 upon a number of sheets of copper, where the junctions and nails 

 were covered with rust, and that had been in a ship for some years, 

 shewed that the action was weakened in the case of imperfect con- 

 nexions by distance, and that the sheets near the protector were 

 more defended than those remote from it. Upon this idea, Sir H. 

 proposed, that when ships, of which the copper sheathing was old 

 and worn, were to be protected, a greater proportion of iron should 

 be used, and that, if possible, it should be more distributed. The 

 first experiment of this kind was tried on the Sammarang, of 2S 

 guns, in March, 1824, and which had been coppered three years 

 before in India. Cast-iron, equal in surface to about -^ of that 



