Dip and Vaj^iation of the Magnetic Needle. 



101 



Adopting for the central position, lat. 41° 18' N., Ion. 8P 12' 

 W., we obtain the usual equations, which furnish us (^ = 1.96851, 

 a;= -{-.01483, y = -[-.00378, and the direction of the isoclinal line 

 is from N. 75° 42' W. to S. 75° 42' E. The increase of dip in the 

 normal direction is SS'.OSl to sixty geographical miles. Comput- 

 ing from these data the dip at the several stations, we obtain the 

 differences given in the last column above. Two of these differ- 

 ences exceed ten minutes, viz. at Cleveland and Hebron. I have 

 observed the dip near Cleveland four times, on different sides of 

 the city, with the following results; 



May 9, 1839, on the north side of the city, 73° 26'.0 

 Sept. 22, 1840, " south " 12 .0 



April 23, 1841, " west " 16 .3 



Aug. 12, 1841, '^ east " 4 .3 



The assumed dip is the mean of these four observations, and 

 as the computed dip is almost identical with one of the observa- 

 tions, it cannot be pronounced inadmissible. At Hebron, obser- 

 vations were made at two stations, distant about ten rods from 

 each other, with sensibly the same result. As however at Fra- 

 sersburgh, distant but about twenty miles, the computed dip agrees 

 well with the observed, we may presume that this disturbance is 

 quite limited in extent. 



Classifying the differences in the same manner as with table I, 

 the mean at the twelve northern stations is — V.5 ,• at the twelve 

 southern -f-l'.5; at the six most eastern — 0'.9; at the six most 

 •western ~0'.3; at the twelve intermediate -f 0'.6. 



Table III. — Observations of the Magnetic Dip from 83° to 86° loest longitude. 



