842 Mr. Bowditch on the variation of the magnetic needle, 
Mean - : Mean Variation | 
Time. Variation Hour. from April 4810 
for the montly. to May 4844. ff 
6h A. M. 6° 19° @1"- oa 
{i810 April 6° 21’ 21” 7 6 19 O7 
May 6 23 36 8 6 to 
June 6 .25..42 9 6 20 28. 
July @. 28 5! 10 6 21 15 
August 6 29 “44 ii 6 22 46 - 
September] 6 25°21 12 6. 24 .07.».4 
October 6 2b..42 1 P. M. 6 25 47 
November} 6 19 11 2 6 27 09 
December} 6 12 35 3 6 27 O 
1811 January 6 20 55 4, 6 25 57 
February | 6 21 19 5 6 24 26 
March 6 20 29 6 6 23 19 
April | 6 23 39 7 5 S282 
May 6 21 38 8 6 91 43 J 
7 6 
) sath 
rary 
The whole number of observations was 5125, and the mean of all 
made the variation 6° 22’ 35” W, which may be assumed as the 
mean variation at Salem in the year 1810. 
These observations were made about two miles south of the place 
where the late President Willard observed the variation in August 
1781 to be 7° 2’ W, as may be seén by examining his paper on the 
subject, in the first volume of the Memoirs of the Academy. - The 
‘difference of the variation at the two places, at the same time, was 
‘probably not more than 2’; so that from 1781 to 1810, a period’ of 29 
years, it had decreased about 38 minutes, or 1’ 19” in a year, which is 
‘at nearly the usual rate. From which I am inclined to believe, that 
the variation has not experienced any change in its direction, in this 
part of the country, and that the needle continues to approach the srur 
meridian with nearly the same velocity, as at the time of the earliest 
observations on record. 
