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Magnetic Observations. 



It is to such observations of the local magnetic conditions at Albany, 

 N. Y., at a great number of dates during the past two hundred years, 

 that I desire to call your attention; and I particularly invite your at- 

 tention to the record given of observations prior to the year 1817. 

 Though these observations are few in number; only nine having been 

 preserved between the years 1686 and 1807] yet they contain the most 

 important date of the epoch of the maximum easterly movement of the 

 magnetic needle at Albany, the observation nearest to this period hav- 

 ing been made by the Surveyor-General of this State, Simeon De Witt, 

 in the year 1805. The degree of maximum easterly movement, which 

 is also the minimum westerly declination of the needle, was found by 

 General De Witt to be + 4° 58' west of north from the true meridian 

 at Albany on the 30th day of July, 1805. 



This result is so different from the value of the declination assigned 

 to this date by theory, that I have been at great pains to see whether 

 other and corroborating data could not be found. In this I have been 

 successful; and, from records left by the late Prof. Joseph Henry, have 

 found the declination in 1798 to have been +5° 00' West of true North- 

 which fully corroborates the view of General De Witt that the easterly 

 movement of the magnetic needle at Albany continued until about the 

 year 1805 (or 1807) and then suddenly changed to an annual westerly 

 movement. That previously to this date, the magnetic needle at 

 Albany had a continuously easterly movement, I have been able to 

 satisfy myself from other data, of early surveys of the land patents in 

 the vicinity of Albany. This unpublished data shows that in the year 

 1686 the magnetic declination at Albany was D = + 9° 09' (North- 

 west); in 1735, D = + 7° 40' (North-west) ; in 1768 D = + 6° 39' 

 (North-west); in 1787, D = + 5° 03' (North-west); in 1789, D = 

 4- 5° 27' (North-west); in 1798, D = + 5 0 00' (North-west); in 1805, 

 D = + 4° 58' (North-west); indicating a decrease of declination as 

 the year 1805 is approached; or a continuous easterly movement of 

 the magnetic needle, as claimed by Surveyor-General De Witt, in his 

 record of April 27th, 1825. 



The fact that these more ancient observations differ entirely from 

 what the present theory of local magnetism would indicate, makes 

 them of the greatest importance as the key to a more accurate theory 

 of the period and extent of the alternative progression and retrogres- 

 sion or secular movement of the magnetic force at Albany. 



To this collection of ancient observations, I have added all those 

 which are comparatively well known, so as to bring together all the 

 data, of value for this investigation, in one paper. 



