Ter7'estrial Magnetism. 



325 



JYo. 11. Mineral Point, (W. T.) Nov. 1839. Lat. 42° 50'- JV., Lon. 89° 54' W. 



Needle No. 1. B North. 



Face of In- 

 strument. 



Face of 

 needle. 



E. 

 W. 

 W. 

 E. 



E. 



W. 

 W. 

 E. 



Dip indicated. 



E. 

 W. 

 E. 

 W. 



A North. 



E. 



w. 



E. 

 W. 



74°30' 

 72 10.5 

 74 22 

 72 22.5 



72 31.5 

 74 10 

 72 12.5 

 74 26.5 



8)586 45.5 



Mean, 73 20.6875 



Needle No. 2. A North. 



Face of in- 

 strument. 



E. 



W. 

 W, 

 E. 



E. 

 W. 

 W. 

 E. 



Face of 

 needle. 



E. 

 W. 

 E. 

 W. 



B North 



E. 

 W. 

 E. 

 W. 



Dip indicated. 



~73026'.5' 

 73 09 

 73 18.5 

 73 12.5 



73 33.5 

 73 15 



73 27.5 

 73 21.5 



8)586 44 



Mean, 73 20.5 



The above specimens 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 

 a minute. If there were a certain latitude of error, say five min- 

 utes, it is evident, by the calculation of chances, that such error 

 between the two needles, would as often be doubled by being, the 



Vol. .XXXIX, No. 2.— July-September, 1840. 42 



