210 [Septkmuf.r, ixj_. 



irj of the Linnean Society, dated LonJ 

 1842, acknowledging the receipt of N >. i ;.. ^ ,,f 

 sdings of the Academy. 

 From Dr. William Johnson, dated Factory Island, Liberia, 

 June Jl, 1842, and from Mr. Spencer F. Baird, dated Car- 

 lisle, Pennsylvania, September 15, 1842, severally acknow- 

 ledging the receipt of their notices of election as Correspond- 

 ents of the Academy. 



The Chairman read a note from Dr. Edmund Ravenel, 

 dated Charleston, S. C, August 21, 1842, alluding to a fossil 

 Scutt'lla, regarded by him as probably new; accompanied 

 by sketches and a partial description of the same. 



VERBAL COMMUNICATIONS. 



Prof. Johnson exhibited and explained a method of applying the galvano- 

 meter to the purpose of testing the relative deductibility of different samples 

 of sheathing copper when exposed to the action of salt water. He referred 

 ;o tin 1 faci that, in early periods of the use of copper fur sheathing the bot- 

 toms of vessels, while iron bolts were still used for fastenings, the copper 

 lasted much I > >t than al present, while the iron itself was rapidly corn de I; 

 thai pper bolts having been substituted for iron ones, the sheathing exhi- 

 bited :i much more rapid decay than befi .>. He likewise alluded i<> the fact 

 that the practice of fastening on sheathing copper with composition nails, 

 oilers an antagonist electric force to thai i*t' the copper, rendering the latter 

 more highly electro-positive, and thus facilitating its union with the electro- 

 negative elements of sea water. It was also mentioned, that in removing the 

 copper from ships some sheets are often found much more corroded than the 

 rest, and that this takes place among sheets manufactured at the same works, 

 and apparently with equal care in freeing the metal from impurities. 



It v as further mentioned tiiat long experience had evinced the uncertainty 

 if obtaining durable copper by efforts to approximate chemical purity in the 

 manufacture, and that in copper free from other meinls the oxide of copper 

 is sometimes detected both in the interior and on the exterior of the mass, 

 rendering it earihv in appearance, and liable to be disintegrated as well as 

 corroded. In proof of this latter effect he exhibited a quantity of copper 

 particles brushed from a sheet o\ that metal which had been worn 'several 

 feare between the keel and the false keel of an English s| U op of war, where, 

 though ihe salt water could penetrate and corrode it, the disintegrated parti- 

 cles coul I not be reached by the waves to be washed away. 



A> the comparative electric energy of two sheets of copper is proportion 

 to the amount of corrosion going en, and as the quantity of electricity i 

 motion also determines the influence of its current on a magnetic needle, the 

 leflection id' that of a common galvanometer may be made to determin 

 relation of two sheets to each other, and thus io ascertain whether they are 

 uitable to lie associated together on the bottom of ihe same vessel. 



Two methods v. ere described and illn- trated by h hich this may I"' accom- 

 plished. The fust is to oppose iii succession the several sheets of copper t>> 

 a .sheet of platina constituting the negative element of the pair, and observing 



