42 



Variation and Dip of the Magnetic Needle. 



On the 2d of September, when he was near the Jersey shore, 

 a httle below the mouth of Hudson's River, he says: 'This 

 night I found the land to haul the compass eight degrees. For 

 to the northward of us we saw high hills. For the day before we 

 found not above two degrees variation.' 



Most of the preceding observations were of course made.on ship- 

 board, and perhaps all, with the exception of that of Sept. 13th. 

 The iron of the vessel would necessarily influence the needle to 

 an amount which we have, perhaps, no means of estimating. 

 The observation of Sept. 13th, it is presumed, was made on shore, 

 and may be compared with subsequent observations at New York 

 city. The variation here in 1686, according to Mr. Welles, was 

 8° 45', showing a decrease of 4° 15' in seventy-seven years, or 

 about three and a third minutes per year. This accords very 

 well with subsequent observations at the same place. 



Most of the other observations which I have obtained are of 

 recent date. The Geological Reports of Maine, New York, Geor- 

 gia, Ohio, and Michigan, have furnished the principal part. In 

 September, 1839, I obtained a few observations from the Gene- 

 ral Land Office at Washington ; a few are from the New York 

 Regents' Reports, and the remainder are from miscellaneous 

 sources. The catalogue is here subjoined ; the stations being 

 arranged by States, commencing with the most easterly. 



Observations on the 

 ent 



Variation of the Magnetic Needle in differ- 

 parts of the United States. 



