96 



Dip and Variation of the Magnetic Needle. 



These equations furnish us ^ = 78'.365, z^^- 1.9947, from 

 which data we obtain the differences in the third column above. 

 But three of these observations, viz. the first, fourth, and fifth, 

 were made at the same locality, and are much better suited for 

 comparison. These give <5=60'.434, ^=-0.5411, and the dif- 

 ferences computed are given in the last column above. T regard 

 this as the most probable value of -^ afforded by the New York 

 observations. In determining the absolute value of the dip, I 

 know no reason for rejecting the other two observations, and as- 

 suming the last determined value of ^, the most probable value 

 of <^ is 65'.084, which gives for Jan. 1, 1840, 72° 55'.34. It will 

 be noticed I have gone upon the supposition that the motion of 

 the needle has been uniform throughout the entire period ; yet 

 the last three observations might lead us to infer that since 1833 

 the dip had been sensibly stationary. 



I know of no other observations made in the United States 

 before the year 1833. In the years 1833 and '34, the dip was 

 observed at several places by Prof. Bache and others ; and in the 

 year 1839 I observed at the same stations. The observations 

 were made with different instruments, and seldom at exactly the 

 same localities ; nevertheless it may be instructive to institute a 

 comparison. The following table exhibits the two series of ob- 

 servations, with the interval elapsedtand the change of dip. 



Albany, 



Springfield, 

 Providence, 

 West Point, 



Ptiiladelphia, 



Date. 



April, 1833 

 Aug. 11, 1S34 

 Aug. 10, 1834 

 Aug 8, 1834 

 April 15, 1833 

 June 7, 1834 

 July 29, 1834 



The entire change of dip, divided by the number of years, 

 gives - 0'.4026 for the annual motion, which result accords very 

 well with the last value of ^ before determined. 



