64 Variation of the Magnetic Needle. 



II. On the Variation of the Magnetical Needle^ by Na- 

 thaniel Bowditch, L.L. D. 



The variation or declination of the magnetical needle, in 

 •the vicinity of Boston, has decreased since the first observa- 

 tions made in this country, at the rate of a degree in thirty 

 or forty years. For, by the papers published in the first vol- 

 ume 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 

 variation had ceased to decrease, and was then rapidly in- 

 creasing. 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 mfist 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 Rens- 

 selaer parish in Albany, was found in the year 1800 to bear 

 N. 46° 48' W by compass; and in the year 1806, N 46° 12' 

 W; the true bearing being N 51° 46' W. Whence it was 

 inferred that the variation had increased 36' during that pe- 

 riod. In Herkimer in New York the variation was observ- 

 ed 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, 45', 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 taken 

 place in New York ; but they by no means warrant the con- 

 clusion that has been drawn from them, since no notice what- 

 ever is taken of the diurnal variation of the needle, which 

 sometimes exceeds any of the changes that have been ob- 

 served. For if we examine Professor SewalPs observations 

 in the first volume of the Memoirs of the American Acad- 

 emy, we shall find that in an interval of two or three months, 

 in the year 1782, the decHnation changed at Cambridge 

 from 6° 21' W, to 7° 08'W. varying 47'; and I have obser- 

 ved at Salem, in the year 1810, that the declination varied 

 48' in a short period of time. Either of these diurnal chang- 

 es exceeds the alteration observed at New York ; and as 

 there can be no doubt that the diurnal variation is nearly a? 



