64 Variation of the Magnetic Needle. 
‘Il. On the Variation of the Magnetical Needle; by Na- 
thamel 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 m eutoned } in several periodical publications that the 
variation had ceased to decrease, and was hen rapidly 1 in- 
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 in creased 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- 
serve or if we examine Professor Sewall’s 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, 
‘idge 
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’ ina 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 as 
