On the Variation of the Magnetic Needle.* 227 



not agree at all with the observations at this place. Thus, according 

 to his theory, the morning minimum should occur when the sun is in 

 the S. E. (magnetic) In June this occurs with us after ten o'clock, 

 which is about three hours after the morning minimum, as indicated 

 by observation. 



Indeed both of these theories seem much better suited to the lat- 

 itude of London, than to our own ; for according to either theory, 

 the needle should occupy its mean position when the parts of the 

 earth, both to the east and west of the magnetic meridian, are equal- 

 ly heated, which happens about the time, or soon after, the sun 

 passes the magnetic meridian, that is about twelve o'clock at this 

 place. But the observations shew that the needle occupies its mean 

 position between ten and eleven o'clock, which is about the same 

 time as at London, although at London the magnetic meridian 

 makes an angle with the astronomical meridian nearly eighteen de- 

 grees greater than at New Haven. The times, both of minimum 

 and maximum declination, are about the same at both places; so 

 that as far as the observations go, they seem to prove that the diur- 

 nal variation is independent of the sun's position with reference to 

 the magnetic meridian. It is highly desirable that these observa- 

 tions should be repeated in other parts of our country, particularly 

 in the extreme Western States. The difference between the decli- 

 nation of the needle at London, and in Illinois for instance, is about 

 thirty two degrees ; and although it is difficult to determine exactly 

 the times of the minimum and the maximum, still it would seem 

 that a difference of two hours could not fail of being detected. It 

 surely would seem possible to determine, whether the needle occu- 

 pies its mean position between twelve and one o'clock in the after- 

 noon, as it should do according to either of the preceding theories. 

 Some observations made by Prof. Bache, during ten days in Sep- 

 tember, 1832, exhibit results different from my own, both as to the 

 times and amount of the maxima and minima. It is possible that 

 these results might be modified by observations continued for a 



year. 



The discovery that the magnetic needle was agitated during the 

 presence of an aurora, has usually been ascribed to Wargentin* He 

 states that on the 28th of February, 1750, the needle was disturbed 

 by an aurora, so as to vibrate between 6° 50' and 9° V of west va- 

 riation ; and on April 2nd, it shifted from a like cause backward and 



forward, between 4° 56' and 



witnessed 



