340 Mr. Bowditch on the variation of the magnectic needle. 
In the year 1808 at a house in Summer Street about an eighth of 
a mile south of the place where the above observations were made, I 
observed the variation with another, more highly finished theodolite 
furnished with a needle of four inches in length, suspended on an agate. 
The places where the instrument was fixed, and method of observing, 
were exactly similar to those beforementioned. 
Sa et os 
1808 June 277 1 A.M. 12 observations 5 11 W. 
545 P. M. 20 5 22 
28 6 26 A.M. 20 § 25 
29 6 44 A. M. 20 5 08 
6 22 P. M. 20 5 26 
July 17 24 A.M. 20 5.25 
Mean of. 112 observations 5 20 Ww. 
In the year 1810, at a house in the northern part of Market Street, 
Salem, about a quarter of a mile east of the place of observation in 
1805, the variation was observed as above by both theodolites, the re- 
sults are— 
Theodolite used 
in 1808. 
d.h. ? o> 2 5 
1810 April 14.52 P. M. 20 obs. 5 43 32 W Theodolite 
2439P.M. 20 54529 | used in 
3754A.M. 20 55112 1805. 
432P.M. 20 5 40 31 
4757 A.M. 20 5 48 03 20 obs. 52413 W 
419P.M. 20 53634 20 4.57 56 
8815P.M. 20 608450 20 5 18 06 
: octal pian FE, = —— ail 
Mean of 140 obs. 5 47 44 60 obs. mean 5 13 25 w 
The difficulty of ascertaining the precise value of the variation ap- 
pears evidently from these observations. For at the same moment om 
the 8th of April 1810, with two excellent theodolites in the same place, 
the variation differed above 50 minutes, which is greater than any 
the changes observed in New York. Iam induced to believe that 
these differences arose in a great degree from the shortness of the 
a cli 
