Variation of the Magnetic Needle. 61 



a Table shewing the changes in the variatioi^ of the magnet- 

 ic needle at Boston, Falmouth and Penobscot, from 167 2 to 

 1 800, embracing a period of one hundred and twenty eight 

 years, copied from a paper furnished me by the late General 

 Schuyler. The difference of variation between 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 approach- 

 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 1 785, I 

 established a true meridian, on which I occasionally set a 

 compass for the purpose of observing the variation of the 

 needle ; and from these observations I found no reason for 

 departing from the old rule until 1 807 ; 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, I 

 examined a number of lines, which had been run before. 

 Amon» 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 f tllows: — 



1 805, July 30. Great Western Turnpike road, N. 6 i ° 45' W. 



1807, Sept 4. do. N. 61" — W. 



1805, July 30. Schenectady Turnpike Road. N. 35° 20' W. 



1807, Sept. 4. do. J\.34°35'W. 



Making a difference on each of 00"* 45' 



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 formerly establish- 

 ed, having for seveal years been obstructed by buildings, I 

 made observations, assisted by Mr. Randel, on the 1st, 2d, 

 3d, and 4th October, 1817, with a good transit instrument, 

 for the purpose of drawing a meridian line across the pub- 

 lic square in this city ; the particulars of which are contain- 

 ed in the 2d part of the 4th volume of the Transactions of 

 the Society for the Promotion of useful Arts. — The needle 

 was then found to point 5° 44 to the west of north. An ob- 



