156 Scientific Intelligence. [Feo, 



opinion. The reason of this may be, that no method has hitherto beert 

 devised of giving the needle any desired situation with respect to the 

 conductor ; and although Paggendorf has thrown much light on this 

 subject, by employing magnets for the purpose of giving the needle 

 any situation at pleasure, yet the respective intensity of action which 

 each point exerted on the needle remained undecided in his expe- 

 riment. 



Prof. Oersted has endeavoured, by the following experiment, to 

 remove all doubts on this subject. A brass wire, 10 feet long, and 

 l-36th of an inch in diameter, is placed in a perpendicular position on 

 the side of a column, at the middle of which is a stand to place the 

 needle on. Both ends of the wire are placed in small vessels lilled 

 with mercury ; by these it communicates with a single pair of plates on 

 the Professor's construction, in which the vessel is of copper; and in order 

 to prevent the influence of the other parts of the apparatus, this is so 

 constructed that they remain far distant from the needle during the 

 experiment. The galvanic apparatus is fixed on the middle of a pole 

 10 feet long ; one of the conductors passes over the upper end of the 

 pole, and the other beneath the lower one ; and from thence they are 

 brought, in a horizontal direction, each into contact with the mercury 

 in one of the small vessels. The entire apparatus is of a rectangular 

 figure. 



The pole with the galvanic apparatus is now made to revolve in a 

 circle around the perpendicular wire and the needle : during the expe- 

 riment, therefore, every point in the wire will constantly change its 

 position with respect to the galvanic apparatus, and will be, in succes- 

 sion, on the inner and on the outer side of the galvanic chain. In a 

 series of experiments in which the apparatus went round at least three- 

 fourths of the circumference, the needle constantly exhibited the same 

 declination which it had at the moment when the galvanic circuit was 

 completed. It appears, therefore, from these experiments, that no 

 doubts can be entertained, hut that every point in the circumference 

 uf the conductor has the same effect upon the magnetic needle. 



II. Existence of Metallic Veins in the Transition Limestone of Plymouth. 



It has hitherto been believed, that the extensive strata of limestone 

 in the neighbourhood of Plymouth, which belong to the Transition class 

 of Werner, or to the Submedial order of the llev. W. D. Conybeare's 

 new arrangement of rocks, are altogether devoid of metallic veins. 

 The Rev. Richard Hennah, in his " Succinct Account of the Lime 

 Rocks of Plymouth," lately published, being the Substance of his 

 communications respecting them to the Geological Society, says, 

 p. 18, "The bed of Ptymouth limestone appears to be entirely free 

 from all metallic veins ; and I have not seen, nor have I heard, of any 

 thing tending to what miners would call a lode, of any of the metals, 

 having ever been discovered in any one of the quarries that have 

 hitherto been opened. It is true that indications of the presence of 

 iron, and also of manganese, may occasionally be seen, as a colouring 

 matter, in particular spots ; but always thinly spread, and never in any 

 quantity." 



Mr. Hennah also states, that, in a single instance, he has observed 

 i* spots of pyrites in fragments of limestone and slate," found in sinking 



