22 Mr. Macneill on the Influence of 



with numerous close concentric rings, the curve of which indi- 

 cates its being a fragment of the stem of a petrified dicotyle- 

 donous tree. It is susceptible of taking a beautiful polish. 

 I remain, my dear Sir, with great regard, 



Very sincerely yours, 



Charles Konig. 



Art. IV. On the Influence of Local Attraction upon the 

 Magnetic Needle, by Mr. John Macneill. 



[In a letter to the Editor.] 



[Mr. Macneill has obligingly forwarded us a map of the district alluded to 

 in the following communication, but as the subject is sufficiently intelli- 

 gible without it, we have not thought it necessary to delay the publica- 

 tion of the paper.] 



Sir, Mount Pleasant, January 20th, 1823. 



In the progress of a trigonometrical survey, in which I am 

 now engaged, of the County Louth in Ireland, I have frequently 

 observed instances of a local attraction, by which the magnetic 

 needle is considerably affected ; the most remarkable instance 

 of this I observed in the beginning of this month, on the South 

 side of the range of mountains which runs from Newry to 

 Carlingford, and not more than two miles north of Dundalk ; 

 this range is here broken into deep glens, bordered by conical 

 and detached hills. One of these, which appears especially 

 to cause a deviation in the needle, is not of so consider- 

 able an elevation as many of its neighbours, but rises coni- 

 cally on the side of the principal range : its surface is rocky 

 and uneven, with very little freestone towards the south and 

 south-west ; I have sent you a small specimen of the stone of 

 which it appears to be composed, and which affects the mag- 

 netic needle very powerfully ; the principal object I had in 

 view in observing the deviation of the needle in the different 

 parts of this county, was to exhibit on my map such districts 

 as could not be surveyed by the needle, for I am sorry to say, 

 theit the old and imperfect instrument the circumferenter, still 



