Magnetic Dip in the United States. 



87 



puted, the sign 4- is prefixed. Eight of these differences are 

 greater than 10'', four of them belonging to Prof. Locke's obser- 

 vations and the others to mine. They are as follows : 



Prof. Locke's. 



Prof. Loomis's. 



Colambus —30'. 6 

 Madison +25^.6 

 Louisville -j-lTM 

 Lost Grove — 10'. 7 



-23' 



+27 

 + 12 



Ann Arbor — 20'. 8 

 Monroe +16M 

 Cleveland +14'. 7 

 Ypsilanti -13'. 8 



-19' 

 + 13 

 + 16 

 -15 



The numbers in the second columns are the differences given 

 in my former article. The correspondence is certainly as good 

 as could have been expected, considering that the last result is 

 obtained by a comparison of nearly double the number of obser- 

 vations, and by a rigorous computation, while the other results 

 were measured upon a map. At Prairie du Chien the discordance 

 is more considerable. The difference I now find is —7'. 3 ; in my 

 former paper —20'. The discordance is owing in part to the cur- 

 vature I ascribed to the isoclinal lines, by which most of the ob- 

 servations seemed best represented, though the apparent error of 

 this observation was increased. The differences for the remain- 

 ing thirty observations are quite moderate, and shew that the hy- 

 pothesis of parallel straight and equidistant isoclinal lines, is not 

 very much in error. 



Let us now compare Prof. Locke's observations in the neigh- 

 borhood of the Mississippi river with themselves, and see how 

 they accord. The following table is arranged like the preceding, 

 the latitudes and longitudes being as furnished by Prof Locke. 

 The central position adopted is lat. 42° 0' N., Ion. 90° 10' W. 



These equations being solved in the usual manner, give x= 

 .01600, y=.00745, «5=3.5323, and the direction of the isoclinal 



