Variation of the Magnetic Needle. 67 
Thedolite used in 1808, 
o fF " 
1810,h. d.! 
April 1 452 p.m. 20o0bs. 5 43 32w) - Theodolite 
39 p.m. 20 5 45 29 used in 
37 54 a.m. 20 SSle1? 1805. 
432p.m. 20 5 40 31 cy saat 
4757 a.m. 20 5 4803 (20 obs. 5 24 13w 
419 p.m. 2 5 36.34. |20 4 57 56 
8815 p.m. 20 608 50 {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 
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 needJe twenty four inches in 
oO 
great care, a rue 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 
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 ihe 
garden by this instrument was less by 3’ 25” than in the house 
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