4 



Variations of the Magnetic Needle. 



Art. L Table of Variations of the Magnetic Needle, copied from 

 one furnished by the late Gen. Schuyler to S. De Witt, 

 Surveyor-General. 



I now present to the Institute, for the purpose of having it pre- 

 served, what I consider an interesting document. It is a Table 

 shewing the changes in the variation of the magnetic needle at Bos- 

 ton, Falmouth and Penobscot, from 1672 to 1800, embracing a 

 period of 128 years, copied from a paper furnished me by the 

 late General Schuyler. The difference of variation between the 

 two epochs appears 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 approached the pole from 

 the west, during that period. 



As long as I can remember, the surveyors in our country, in re- 

 tracing old lines, have allowed at the rate of three minutes 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, 1 estab. 

 lished 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 

 1807 ; when to my surprize I found that a sudden change had taken 

 place in the direction of the needle. And, in order to ascertain its 

 extent, 1 examined a number of lines, which had been run before. 

 Among others, the courses of the Great Western and Schenectady 

 Turnpike Roads, which in 1805 had been surveyed by Mr. John 

 Randel, junr. then attached to my office. The result was as fol- 

 lows:— 



1805, July 30, Great Western Turnpike Road, N. 61° 45> W. 



1807, Sept. 4. do. N. 61° W. 



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

 1807, Sept. 4. do. N 340 35/ W- 



Making a difference on each of qqo 45, 



Shewing that in about two years and a month, the needle had 

 changed, contrary to its former direction of annual variation, about 

 forty-five minutes of a degree. An examination of several other 

 lines confirmed this result. 



