MAGNETIC DIP IN THE UNITED STATES. 



289 



Magnetic Dip at Ann Arbor, Michigan. Latitude 42° 18' N.; Longitude 83° 45' W. 

 Place of observation twenty rods south of the rail-road depot. 



Date. 



Hour. 





Needle. 



No 



Readings. 





Dip. 



1841, Aug. 16, 



11— 1, P. 



M. 



No. 1, 





40 



73' 



' 14'.3 









No. 1, poles reversed, 





40 





19.3 









Mean of No. 1, 





80 





18.8 









No. 2, 





40 





17.4 









No. 2, poles reversed, 





40 





15.0 









Mean of No. 2, 





80 





16.2 



" 







Mean of both needles, 





160 



73 



16.5 



As the results obtained in 1839, at the last three stations, seemed quite ano- 

 malous, I was desirous of verifying them. The stations of observation were 

 purposely taken at some distance from the former ones, to avoid any local in- 

 fluence of limited extent. The results at Ann Arbor and Ypsilanti are almost 

 identically the same as before; at Monroe the last result accords much better 

 with what might be expected from the general course of the isoclinal lines. 

 Some indications of disturbing influences are apparent in the geological cha- 

 racter of this section. Throughout most of it is found a deposite of blue clay, 

 containing nearly two per cent, of iron. Iron ore is also quite common. 



Magnetic Dip at Detroit, Michigan. Latitude 42° 19' N.; Longitude 83° 3' W. 



Place of observation one mile west of the city. 



Date. 



Hour. 



Needle. 



No. 



Readings. 



Dip. 



1841, Aug. 17, 



8— lOi, A. M. 



No. 1, 





40 



73° 40'.9 



(( 





No. 1, poles reversed. 





40 



30.0 



(( 





Mean of No. 1, 





80 



35.5 



<( 





No. 2, 





40 



34.4 



<( 





No. 2, poles reversed. 





40 



38.5 



<( 





Mean of No. 2, 





80 



36.5 



« 





Mean of both needles. 





160 



73 36 .0 





Observations repeated 



ten Rods North of former 



Station. 





Date. 



Hour. 



Needle. 



No. 



Readings. 



Dip. 



1841, Aug. 17 



IQi— 12i, A. M. 



No. 1, 





40 



73° 30'.8 



(( 



(( 



No. 1, poles reversed, 





40 



39.0 



(( 



<( 



Mean of No. 1, 





80 



34.9 



