103 Dip and Variation of the Magnetic Needle. 



Adopting for the central position, latitude 40° 57' N., longitude 

 84° IV W., we obtain <J = 3.31612, a:= + .014728; y= - .000735, 

 the direction of the isoclinal line is from S. 87° 8' W. to N. 87° 

 8' E., and the increase of dip in the normal direction is 53'.088 

 for sixty geographical miles. The mean of the differences at the 

 eleven most northern stations is +0'.6, at the twelve most south- 

 ern — 0^6, at the five most eastern stations — 0'.2, at the six most 

 western - 2'.1, at the twelve intermediate +1M. Eight of these 

 differences exceed ten minutes. 



Table IV". — Observations of the Magnetic Dip from 86° to 91° 2oest longitude. 



Station. 



Prairie du Chieri, 



Madison, 



Campbell's, 



Blue Mounds, 



Hickok's, 



Mineral Point, 



Platteville, 



Turkey River, 



Little Mahoqueta, 



Dubuque, 



Galena, 



Sherwood's, 



North bank Mah., 



Whitewater, 



Mahoqueta, 



Farmer's Creek, 



Small Mill, 



Iron Ore, 



Chicago, 



Wapsipinnicon, 



Lost Grove, 



Davenport, 



Peru, 



Pekin, 



Copperas Creek, 



Montrose, 



Marion, 



Louisiana, 



Bunker Hill, 



Monticello, 



Upper Alton, 



Alton, 



Edwardsville, 



Vincennes, 



St. Louis, 



Paoli, 



Princeton, 



New Harmony, 



Mount Vernon, 



43° 3' 

 43 3 

 43 1 

 43 

 42 58 



42 51 



43 43 

 42 42 

 42 31 

 42 29 

 42 28 

 42 27 

 42 23 

 42 18 

 42 14 

 42 13 

 42 4 

 41 55 

 41 53 

 41 45 

 41 39 

 41 28 

 41 23 

 40 35 

 40 30 

 40 19 

 39 44 

 39 23 

 39 4 

 38 57 

 38 55 

 38 54 

 33 50 

 SB 43 

 38 37 

 38 35 

 38 23 

 38 11 

 37 59 



90°53' 

 89 11 

 89 26 

 89 36 

 89 47 



89 58 



90 14 

 90 48 

 90 31 

 90 23 

 90 13 

 90 37 

 90 52 

 90 38 

 90 57 



90 23 



91 2 

 90 40 

 87 44 

 90 23 

 90 9 

 90 18 



5 

 89 36 



89 48 

 SO 14 



90 14 

 90 50 



89 53 



90 5 

 90 3 

 90 4 



89 53 

 87 29 



90 4 



86 25 



87 30 

 87 48 

 87 47 



Dip. 



73° 16' 

 74 5 

 73 28 

 73 37 

 73 39 

 73 21 

 73 17 

 73 11 

 73 8 

 73 5 

 73 3 

 73 7 

 72 51 

 72 55 

 72 43 

 72 33 

 72 21 

 72 50 

 72 47 

 72 14 

 72 2 

 71 55 

 71 51 

 71 13 

 71 4 

 70 54 

 70 20 

 69 51 

 69 49 

 69 38 

 69 45 

 69 34 

 69 57 



)2.3 



69 28 

 69 35 

 69 22 

 69 5 

 68 56 



Dtite. 



Antliority. 



1839. 

 1840. 

 1841. 

 1840. 

 1841. 

 1840. 

 1841. 

 1839. 

 1839. 

 1839. 

 1841. 

 1839. 

 1839. 

 1839. 

 1839. 

 1839. 

 1839. 

 1839. 

 1841. 

 1839. 

 1839. 

 1839. 

 1841. 

 1841. 

 1841. 

 1839. 

 1839. 

 1839. 

 1841. 

 1841. 

 1841. 

 1841. 

 1841. 

 1841. 

 1840. 

 1840. 

 1840. 

 1840. 

 1840. 



Oct. 

 Oct. 



Sept. 



Oct. 



Sept. 



Oct. 



Sept. 



Oct. 



Oct. 



Oct. 



Sept. 



Oct. 



Oct. 



Oct. 



Oct. 



Oct. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Mar. 



Sept. 



Sept. 



Sept. 



Sept. 



Sept. 



Prof. Locke, 



Profs. Locke and Loomis, 



Prof. Loomis, 



Profs. Locke and Loomis, 



Prof. Loomis, 



Profs. Locke and Loomis, 



Prof. Loomis, 



Prof. Locke, 



Prof. Loomis, 

 Prof. Locke, 



Diff. 



-14'.1 

 +23.9 



9.6 

 + 2.3 

 + 6.8 

 -3.0 

 + 1.9 

 -0.1 

 + 5.7 

 + 3.8 

 + 1.8 

 + 9.2 



1.4 

 + 6.0 

 + 0.3 

 -12.7 

 -11.8 

 +23 .2 

 + 4.6 

 -4.0 

 -12.3 

 -8.0 

 -14.9 

 -3.6 

 - 6.^1 

 -3.4 

 -3.7 

 -8.7 



1. 



0.8 



7.6 



2, 



+23.4 



+ 9.0 



7.6 



7.1 



1, 



-5.6 



-3.0 



+ 1.4 



+ 



- 2.2 



- 1.5 



Adopting for the central position, latitude 41° 2' N., longitude 

 89° 47' W., we obtain .^=3.69143, a;=+0.015972, y=+0.002319, 

 the direction of the isoclinal line is from N. 81° 44' W. to S. 81° 

 44' E., and the increase of dip in the normal direction is 58'. 106 

 for sixty geographical miles. The mean of the differences at the 

 twenty most northerly stations is +1'.6, at the nineteen most 



