OBSERVATIONS FOR MAGNETIC INCLINATION. 



39 



If we divide the monthly means (inclusive of the interpolated dips for January, 

 February, and March, 1843) into two parts, we find the values: 



From January, 1842, to April, 1843 (inclusive) . . .71 59. '4 

 " April 1843, to July, 1844 " ... 71 57.9 



The corresponding epochs are September 1, 1842, and December 1, 1843, which 

 again give an annual decrease of 1.'2. 



It is desirable, however, to extend the investigation of the annual effect of the 

 secular change of the dip beyond the years above stated. In the Coast Survey 

 Report for 1856, 1 Assistant Schott discussed the secular change of the dip at various 

 places, and finds that the middle of the year 1842 (1842.7 +0.7 years) was an epoch 

 of minimum dip for places between Cambridge, Mass., Toronto, Canada, and Wash- 

 ington, D. C. The expression for the secular change for Philadelphia (page 241 

 of the 1856 report) is derived from 19 observations, contracted to 8 normals, between 

 1834 and 1855. 



The dips extracted from a manuscript paper on my magnetic surveys in various 

 parts of the Northeastern States during the years 1834-5, 1840 1841, and 1843, 

 are as follows : 



Observed dip at Philadelphia, July 



" " October 



" " April 



" " July 



" " October 



" " Novembi 



A collection and combination of all the observed values for dip at Philadelphia 

 (as far as they have come to my notice) are given in the following table : 



1 Appendix No. 32, p. 235. 

 States. 

 15 



Discussion of the secular variation of the magnetic inclination in the Northeastern 



