* 
Terrestrial Magnetism. 325 
No. 10. Madison, (W. T.) Nov. 2,1839. Lat. 43° 05! N., Lon. 89° 06' W. 
______Needle No.1. A North. Needle No. 2. B North, | 
_ Bee of in-| Face of) Fa 2 of 
'{strument. | needle. Dip indicated. strument. | needle. Dip indicated. 
-E. [ow E E. 
w. w. 74 49 w. 74 11 
W, E. 73 24 Ww E, 74 O15 
E. Ww. 74 56 E w. 74 16.5 
B North A Nort 
E. E. 74 59 £$.°3P°: &. 74 12 
Ww. w. 73 00 Ww. WANs t 73 47 
w. E. 75 02.5 Ww E. 74 02 
E. w. 72 AT E Ww. 73 49.5 
| 8)592 25.5 eee ee eT 
Mean, 74 03.1275 . Mean, 74 03.875 
No. 11. Mineral Point, (W.T.) Nov. 1839. Lat. 42° 50! N., Lon. 89° 54! W. : 
Ne edle No. 1. B North. Needle No. 2. A North. 
Face Face of of in-| Face of 
“pte | needle. Dip indicated. peeing needle. Dip indicated. — _ 
E. E. tea E. E. °26'.5 
w. Ww. 72 10.5 w. 09 
w. E 74 22 w E. 73 «18.5 
E. Ww. 72 22.5 E w. 73 12.5 
- A Nort B 
E. E 72 31.5 E E. 73° 33.5 
w. w. 74 10 Ww. w. 73 15 
w. E 72 12.5 Ww. E. 73 27.5 
E. Ww. 74 26.5 E. Ww. 73 21.56 
‘ ~8)586 45.5 8)586 44 
eae 73 20. 6875 3 - Mean, 73 20.5 
="Phe ton spécisions:of my field notes are not selected; but 
are taken in course from one to seven. Those which follow are 
also in course, and have been selected because they differ from 
what would be expected from the projections of isoclinal lines. 
The evidence of the accuracy of the observations rests chiefly 
on the very close agreement of the independent result obtained 
by the two separate needles. It will be seen that out of the 
eleven cases above quoted, there is but one in which the differ- 
ence is over a fraction of one minute of a degree, and that is the 
fifth, which shows a difference of one minute and five eighths of 
aminute. If there were a certain latitude of error, say five min- 
utes, it is evident, by the calculation of chances, that such error 
hitters the two needles, would as often be oo by being, the 
Vol. xxxix, No. 2—July-September, 1840. 
