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,^ 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 PMladelpHa, July 



" " October 



« « April 



« " July 



" " October 



" " Novemb( 



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

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



21, 1840 . 



. 710 52. '6 



28, 1840 . 



. 71 53.0 



26, 1841 . 



. 72 00.6 



20, 1841 . 



. 71 57.0 



9, 1841 . 



. 71 58.2 



1, 1841 . 



. 71 59.1 



Dale. 



July, 1834 



1838 



September, 1839 

 July, 1840 



September,' 1840 

 October 1840 

 March, 1841 



April, 1841 



April, 1841 



April, 1841 



June, 1841 



July, 1841 



October, 1841 

 November, 1841 



1842 



- - 1842 



January to December, 1842 

 April to December, 1843 



1844 



April, 1844 



May, 1844 



January to July, 1844 



May, 1846 



September, 1855 

 August, 1862 



Dip. 



Prof. Bache and Prof. Courtenay 



Prof. Bache 



Prof. Loomis 



Prof. Bache 



Prof. Bache 



Prof. Bache 



Prof. Bache 

 Prof. Bache 

 Prof. Bache 

 Major Graham 

 Prof. Bache 

 Prof. Bache 

 Prof. Bache 

 Dr. Locke 

 Captain Lefroy 

 Prof. Bache 

 Prof. Bache 

 Major Graham 

 Dr. Locke 



Prof. Bache 

 Dr. Locke 

 Ass't Schott 

 Ass't Schott 



71O60.'2 



71 43.9 



71 67.1 



71 52.6 



71 53.3 



71 53.0 



71 60.7 



71 58.2 



71 60.6 



71 59.0 



71 54.5 



71 57.0 



71 58.2 



71 59.1 



71 60.1 



71 59.0 



71 60.1 



71 58.2 



71 61.8 



71 59.3 



71 69.2 



71 57.6 



71 61.0 



71 77.7 



71 65.8 



72O00.'2 

 excluded 

 72 07.1 



71 53.0 



71 58.4 



71 59.7 

 71 58.2 



72 01.0 

 72 17.7 

 72 05.8 



1834.5 

 1839.7 

 1840.7 



1842.5 

 1843.6 



1846.4 

 1855.7 

 1862.6 



Appendix No. 32, p. 235. Discussion of the secular variation of the magnetic inclination in the Northeastera 

 15 



