196 



ITie Variation of the Needle. 



tricity and magnetism which, when developed into future arts and 

 industries, will add much to the comfort of the human race. 



Local Observations. 



Eeverting to the very valuable observations given us by Simeon De 

 Witt, in the first paper of the Albany Institute, we find the records of 

 the actual observation of the important local period, when the e isterly 

 movement of the magnetic needle ceased, and the westerly variation in 

 declination commenced. 



The true meridian line to which Mr. De Witt refers, in his paper, 

 as having been drawn " across the public square of this city," is now 

 only recoverable by his line of reference, which he locates as extend- 

 ing from the westernmost corner of the base of the Albany Academy, 

 southerly to a point eleven feet and nine inches south-east from the 

 corner of the " brick house of John Van Schaick," in 1818.' 



It was probably at the southern extremity of this line that Mr. De 

 Witt observed a declination of 5° 44' to the west of true north on the 

 4th of October, 1817, and where another observation which he made 

 on the 1st of August, 1818, showed that the needle had moved slightly 

 to the westward. It was here on the 24th of April, 1825, he found 

 the westerly declination of the needle had increased to 6° 00', upon 

 which he bases his estimate that the local movement of the needle at 

 that time was two minutes of arc westward per year. 



But these are not the only records kit by this watchful observer. 

 He has given us also the magnetic bearings of two of the oldest avenues 

 of our city, at different times, and these data aid us in ascertaining 

 what the declination then was, and by recent observations I have 

 obtained the present direction of these lines, and consequently the total 

 change in the declination and the amount of the average annual varia- 

 tion of the needle in these localities since his time. 



The magnetic bearings of the lines of the Schenectady turnpike, 

 and the great western turnpike were observed by him on July 30, 

 1805, and again on September 4, 1807, and in each case a difference 

 was found of 45' variation in the direction of the needle to the west- 

 ward in two years and one month. This exceeded the amount of the 

 annual change which Mr. De Witt estimated from subsequent observa- 

 tions to be the normal rate, and he ventures (in his groping after the 



