337 
XLVII. 
ON THE VARIATION OF THE MAGNETICAL NEEDLE. 
BY NATHANIEL BOWDITCH. 
—— DD | a 
THE variation or declination of the magnetical needle, in the vi- 
cinity of Boston, has decreased since the first observations made in 
this country, at the rate of a degree in 30 or 40 years. For, by the 
papers published in the first volume of the Memoirs of the American 
Academy, it was 9° 00’ W in the year 1708; 8° 00’ W in the year 
1742; and about 7° W in the year 1782. Within three or four years, it 
has been mentioned in several periodical publications that the varia- 
tion had ceased to decrease, and was then rapidly increasing. ‘This was 
stated to be the case, particularly in New York, by persons, who, from 
their official situations as public surveyors, were supposed to be most 
competent to judge of the subject ; and observations were adduced to 
prove that this change had taken place between the years 1804 and 
1807. Thus one of the boundary lines of Ranssellaer parish in Albany, 
was found in the year 1800 to bear N 46° 48’ W by compass; and 
in the year 1805, N 46° 12’ W;; the true bearing being N 51°46’ W. 
Whence it was inferred that the variation had increased 36’ during that 
period. In Herkimer in New York the variation was observed in the 
years 1800, 1804, and 1807: in the first interval of four years it had 
decreased 4’, and in the last interval of three years had increased 15’. 
A turnpike road, which was laid out by compass in 1805, had varied 
in its bearing’in 1807, 435’, indicating that the variation had increased 
by that quantity. These are the chief observations, that I have 
known to be produced, to prove that a change had takeu place in New 
York ; but they by no means warrant the conclusion that has been 
43 
