| Magnetic Dip in the United States. 87 
puted, the sign + is prefixed. ight 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. 
Columbus —30’.6 | —23¢ Ann Arbor —20’.8 | —19/ 
Madison +25/.6 | +27 Monroe +16/.1 | +13 
Louisville +17/1 | +12 Cleveland +14’.7 | +16 
Lost Grove — 10.7 i Ypsilanti —13/8 | —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 uponamap. 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., lon. 90° 10’ W. 
"gene ee ee 
i- Dif. of observ’di 
Latitude. rng | Dip. | Equations of condition. | & comp'd dip. 
i 
$8 d6v18 36w.l60 31.4] .523=5 -2047426.6y | + 3.4 
, 41 30 (90 18. {71 53.4) 2890=5- 302- 6.0y | - 7.1 
Lost Grove, 41 39 |90 9 1/72. 2.4) 3.040=5- Qla+ .7y| - 9.7 
Psipinnicon, 41 45 |90 23 14.9} 3.248=5 — 15z- 9.7y | 4+ :1.7 
Brown’s Settlem’t42 4 |o1 2 (72 21.4] 3.257=34 4c-386y| 4 2.9 
Farmer’s Creek, 42 13 |90 23 {72 33.1] 3.551=54 13z- 9.6y| 7.0 
Mahoqueta, 42 14 |90 57 |72 43.6] 3.727=04 142-34 +138 
Dubuqu 29 |39 56. |73. 5. | 4.088—04 297410.3y| + 6 
Mineral Point, {42 50 |89 54 {73 206] 4.343=04 50zr411.7y | - 4.6 
Blue Mounds, 1 |s9 38 |73 40.9} 4681=04 6174234y | - 1 
Prairie du Chien, 43 3 52 |73 16.6] 4.277 =94 63zx-30.7y - 2.1 
: 5 6 3.5] 5.058=54 657446.7y | + 8.3 
These equations being solved in the usual manner, give c= 
01600, y=,00745, '=3.5323, and the direction of the isoclinal 
