Dip and Variation of the Magnetic Needle. 



115 



Central position, latitude 46° 41' N., longitude 87° 6' W., d= 

 4O.004S9, i. e. -4° 0'.3, a-= -.003988, y= +.024886. Mean 

 of the differences at the six most northern stations — 1'.8 ; five 

 southern -|-2'.2; three most easterly +3^0, three most westerly 

 -j-3'.3; five intermediate — 3'.8. 



Collecting together the preceding results, we obtain the fol- 

 lowing table, in which column third represents the variation for 

 places whose latitude and longitude are given in columns first 

 and second ; column fourth, shews the direction of the lines of 

 equal variation ; column fifth, the increase of variation in the nor- 

 mal direction for sixty geographical miles; and column sixth, the 

 distance of the lines of equal variation expressed in geographical 

 miles. 



From these data are computed the positions of the following 

 lines of equal variation. 



The preceding positions are each determined from a single group 

 of observations, independently of all the others. In projecting the 

 lines upon a map, some anomalies will be perceived as to the di- 

 rection of the lines and their distance from each other. The most 

 remarkable of these is the line — 4° in latitude 32° 46'. The 



