Variation of the Magnetic Needle. 61 
a Table shewing the —_ in the variation of the magnet- 
ic needle at Boston, Falmouth and Penobscot, from 1672 to 
Schuyler. The. difference of variation 5 hewn: the two 
epochs appear to be 5° 53’, giving a little more than two and 
three quarters of a minute for the mean annual variation 
or the rate at which the north point of the needle approliche 
ed the pole from the west, during that period. 
As long as I can remember, the surveyors in our country, 
in retracing old lines, have allowed at the rate of three min- 
utes per year, and acquiesced in the correctness of that rule 
till the year 1805. 
Some time after I settled in Albany, which was in 1785, I 
established a true meridian, on which I occasionally set a 
observing the variation of the 
departing from the old rule until 1807 ; when to my surprise 
I found that a sudden change had taken place in the direc- 
tion of the needle. And, in | order to ascertain its extent, | 
examined a number of lines. which had been run before: 
Among others, the courses of the Great Western and Sche- 
nectady Turnpike Roads, which in 1805 had been surveyed 
by Mr. John Randel, junr. then attached to my office. The 
result was as follows :— 
1805, July 30. Great caer Turnpike road, N.6 4 ” my W. 
1807, Sept 4. N. 61° — W. 
1805, July 30. Sigciecteds Turnpike Road. N. so" 20’ W. 
1807, Sept. 4. do. N. 34° 35’ W. 
Making a difference on each of é 
Shewing that in about two years and a month, the needle 
had changed, contrary to its former direction of annual va- 
riation, about forty-five minutes of a degree. An examina- 
tion of several other lines confirmed this result. 
A view along the meridian, which I had fone establish- 
ed, having for seveal years been obstructed by build ding I 
made observations, assisted by Mr. Randel, on the Ist, 2 
3d, and 4th October, 1817, with a good trail instrument, 
for the purpose of drawing a meridian line across the pub- 
lic square in this city ; the porcrclene of which are contain- 
ed in the 2d part of the 4th volume of the Transactions of 
the Society for the Promotion 3 ual Arts.—The needle 
was then found to point 5° 44’ to the west of north. ob- 
