Variation of the Magnetic Needle. 67 



Thedolite used in 1808. 



Theodolite 



used in 



1805. 



20 obs. 5 24 13w 



20 4 57 56 



20 5 18 06 



Mean of 140 obs. 5 47 44 



60 obs. mean 5 13 25w 



The difficulty of ascertaining the precise value of the va- 

 riation appears evidently from these observations. For at 

 the same moment on the eight of April 1810, with two ex- 

 cellent theodolites in the same place, the variation differed 

 about 50 minutes, which is greater than any of the chang- 

 es observed in New York. I am induced to believe that 

 these differences arose in a great degree from the shortness of 

 the needles; and, perhaps in part from the imperfection of the 

 brass of which the instruments were made. To obviate 

 these difficulties I procured a needle twenty four inches in 

 length, suspended on an agate, and had it neatly fixed in a 

 mahogany box, moveable at one end on a pivot by which 

 the box was attached to a board, marked with a graduated 

 arch of a circle, subdivided in such manner that minutes of 

 a degree could easily be read by means of a nonius. The 

 box was made wholly of wood and ivory, and when fixed in 

 its place there was no iron near it. A table about three feet 

 in height was fixed in the middle of a room of the building 

 in the north part of Market-Street, and by means of the the- 

 odolite and the sun's azimuth, I marked on the table, with 

 great care, a true meridian line and then placed the box on 

 it, and observed the differences between the true and mag- 

 netic meridian for every hour, when convenient, from 6 A. M. 

 to 10 P. M. from April 1810, to May 1811. The greatest 

 variation observed during this time was 6° 44' W. The least 

 5° 56' W. To ascertain whether the building affected the 

 needle, I fixed a true meridian line on a table in the garden 

 adjoining the house, at thirty feet distance from any building, 

 and nearly five feet from the ground, and by the mean of 

 forty eight observations, I found that the variation in the 

 garden by this instrument was less by 3' 25" than in the house 



