i8 M. De Witt's Table of Magnetic Variations^ ^-c. 



Mean temperature by formula, - 72^^58 



Corrected observed temperature, - 76 76 



Difference, 4^18 



This difference, which is very considerable, may arise partly 

 from the insular situation of Bombay, which is no doubt warm- 

 er than in the same latitude on the continent. 



Art. IIT. — Table of the Variations of the Magnetic Needle 

 at Boston, Falmouth, and Penobscot, in North America, 

 during 128 years. * By S. De Witt, Surveyor-Ge- 

 neral. 



The following interesting document, which was furnished me 

 by the late General Schuyler, shows the changes in the varia- 

 tion needle at Boston, Falmouth, and Penobscot, from 1672 

 to 1800, embracing a period of 128 years. The difference 

 of variation between the two epochs appears to be 5^.5S\ giv- 

 ing a little more than 2|'ths for the mean annual variation. 



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

 on retracing old lines have allowed at the rate of 3' per year, 

 and acquiesced in the correctness of that rule till 1805. 



Since 1785, I occasionally observed the variation of the 

 needle, and from these observations I found no reason for 

 departing from the old rule till 1807, when, to my surprise, 

 I found that a sudden change had taken place in the direction 

 of the needle. In order to ascertain its extent, I examined a 

 number of lines which had been surveyed in 1 805, and which 

 gave a difference of 45' from July 30th, 1805, to September 

 4th, 1807. 



I found the following to be the variation at Albany. 



Variation. 



1817, October 3d, - - 5° 44' West. 



1818, August 1st, - - 5 45 

 1825, April 24th, - - 6 



• Abridged from the Transactions of the Albany Institute, vol. i. No. i. 

 p. 4, June 1828. 



