98 Dip and Variation of the Magnetic Needle. 
t ’ i oi BLE CONTINUED. 
Equations. Diff. ar ations. Diff. Equations. {_ Diff. 
54-.6934=- 7.1|—0.4| 0-4+.7994= 7.1)—1.0),04..8954— s'6,_01 
4+..7044— 7.9|+0.3| 64.8084—10.041,8|54+.9034— 8.0\_ 0.7 
§417124—11.1/43.5 | 6+.8194=10.7|4+2.5 | 04-.9144=—13.1/44.3) 
d-.7284— 7.40.3 | 6-+.8274—=10.342.0 | 0+.9234= 9440.6 
814-6414 J 6.4 ay 2980 4 TOK oS ‘ d 
5+-.7424—10.3-42.5 | 0-4+.8464—10,8'42.4 |} d+.9424— 7,118 
64.7504— 8.5-+0.7| 04+ .8572— 6.61.9] 04.9583— 8.2|_0.8 
§-7619=11.013.1| 0-.8654—= 7.9|—0.6 | 04+.9612=10.5415 
d+ 7694— 93-113) I-+.8764— 3.6 —5.0 | 0-4.9724—15.5 46.4] 
S4.7804= 5.92.1 ath S8Ad= 7.4 ~12)0-4.9914—= 8.40.8) 
ile 50.6 | 
The preceding pdtidtibus give 6=3’.57, or the mean aip Jan. 1 
1841, equals 75° 13/57, 4=+-5/.69, from which we obtain the 
differences between the observed and computed dip as given in 
the table above. Although these observations are quite nume- 
rous, and were doubtless made with great care, still being all em- 
braced within a period of less than nine months, they leave eon 
siderable uncertainty with regard to the annual motion. 
In “Cist’s Cincinnati in 1841,” Prof. Locke has given the re- 
sult of monthly observations of the dip at Cincinnati for one year. 
The mean of the observations for the first six months is 0’.13 
greater than for the last, indicating an annual diminution of dip 
of 0.26. ‘This result, Wowarey: being derived from so short a 
period, cannot be sflvered ‘auch weight. ‘These are all the ma- 
terials to which I have had access for determining the annual 
change of dip in the United States. Ihave sought for further 
light on this subject from a comparison of European observations: 
The following table shows the annual change of dip in Europe 
ing to Quetelet. These numbers with the motion at New 
York may be tolerably well represented by the formula —3/3605 
sin. (79° 54’+lon. roa where the ae rea is reckoned frou 
sa +E, - 
| ya aeo orn for Hudson 21 £0409," ‘Phe feentis 
of the observations in the United States are by no means satis- 
