98 



Dip and Variation of the Magnetic Needle. 



TABLE CONTINUED. 



Equations. 



(J-|-.723z/: 



8-\-.142Jz 



: 7.1-0.4 



: 7.9+0.3 



:ll.l4-3.5 



- 7.4' -0.3 



:14.l|+6.4 



: 10.3 +2.5 

 : 8.5+0.7 



:11.0+3.1 



: 9.3+1.3 

 : 5.9-2.1 

 : 7.51-0.6 



Equations. 



<y+.799^= 



(J+.SOS.^: 



J+-819^z 

 d^+-827^= 

 (5+.838^z 

 <5+.846^: 

 ^+.857z/z 

 8-\-.865Jz 

 S-\-.816d-- 

 (J+-884z:/i 



: 7.1-1.0 

 :10.0+1.8 



:10.7 

 :10.3 

 :10.5 

 :10.8 

 : 6.6 

 : 7.9 

 : 3.6 

 : 7.4 



+2.5 

 +2.0 

 +2.2 

 +2.4 

 -1.9 

 -0.6 

 -5.0 

 -1.2 



Equations. 



(^+.895z/=z 8.6 

 8-\-.903J= 8.0 

 <5+.914.^:=13.1 

 <^+.923^= 9.4 

 8-\-.934J= 7.2 

 S-\-.94:2d= 7.1 

 <5+.953^r= 8.2 

 <J+.96 1.^=10.5 

 <5+.972.^ir=15.5 

 (5+.991^— 8.4 



_0.1 

 -0.7 



+4.3 

 +0.6 

 -1.7 



-1.8 

 -0.8 

 +1.5 

 +6.4 

 -0.8 



The preceding equations give 8=3^.57, or the mean dip Jan. 1, 

 1841, equals 75° 13\57, .^— + 5'.69, from which we obtain the 

 differences between the observed and computed dip as given in 

 the table above. Although these observations are quite nume- 

 rous, and were doubtless made with great care, still being all em- 

 braced within a period of less than nine months, they leave con- 

 siderable uncertainty with regard to the annual motion. 



In "Cist's Cincinnati in 1841," Prof Locke has given the re- 

 sult of monthly observations of the dip at Cincinnati for one year. 

 The mean of the observations for the first six months is 0'.13 

 greater than for the last, indicating an annual diminution of dip 

 of 0'.26. This result, however, being derived from so short a 

 period, cannot be allowed much weight. These are all the ma- 

 terials to which I have had access for determining the annual 

 change of dip in the United States. I have sought for further 

 h'ght on this subject from a comparison of European observations. 

 The following table shows the annual change of dip in Europe 

 according to duetelet. These numbers with the motion at New 

 York may be tolerably well represented by the formula — 3'.3605 

 sin. (79° 54'+lon.)=^, where the longitude is reckoned from 

 Greenwich +E, — W. 



The same formula gives for Hudson ^= + 0'.09. The results 

 of the observations in the United States are by no means satis- 



