100 Dip and. Variation of the Magnetic Needle. 
As the computed dip falls between the two observations, it can- 
not be pronounced inadmissible. © 
. At Albany, observations made within about a year of each othees 
differ 11%. Here then is evidence of local attraction. The least 
observed dip reduced to 1840 differs 44.5 from the computed dip. 
At Princeton, the dip is derived from a single observation. At 
Philadelphia, observations made within a year of each other differ 
23'.2. Here is undoubted local attraction. My own observation 
differs 7’.5 from the computed dip. ; 
In order to test the hypotheses upon which the dip is compu- 
ted, let us classify the differences. The mean difference at the 
ten most northerly stations is +0/.3; at the ten most southerly 
—0’.3; at the five most eastern 0’. 0; at the five most western 
49.2; at the ten intermediate sakione —1.1.. It may be infer- 
red then that within the limits of the preceding table, the hypoth- 
esis of straight, parallel, and equidistant isoclinal lines, is well 
nigh rigorously exact. There i is then reason to believe that these 
differences between the observed and computed dip, arise chiefly 
from local attraction and errors of observation. As the dip ob- 
served at Charlottesville, Va. in 1834, 71° 8/.7, is about half a de- 
gree greater than observations made in other parts of the United 
States would lead us to expect, I have thought besa gear itin 
is comparison. 
> Paeve IL. Ba i a = the Magnetic Dip from 78° to 83° west eons 
t. | Lo f waite Date. Anthority. Diff. ~ 
Sean O84). Jan. _1,|Ut. Vtechiabrad - So 
2 53 51690. Ang. 31,|Prof. Loomis. |= 4.5] 
1. April 1,| « 4138} 
cee & es +10 + 
May 20, 42 + 4. 
11840. Sept.10,) Mie“ 
58 Sept. 23, ee -~ 1. 
7 1840, Sept.10,; |= 6. 
2 59 .7|1840, Sept. 9,; teed 
2 55 5)1640. Sept. 8,| gt 
2 840. Sept. “if - re 
3 3 "Sept. 8, “ t 6: 
34 i 
2 56.6 ‘ee 
7253 .0) = 1. 
43 2 cali 
 §1 2 6. 
2 40.2 + 4. 
2 19 2 + 4:2) 
43.7 $1.7) 
Shee hat Mees 
