198 Observations of the Dip of the Needle [No. 30. 



when there is an extensive laterite formation, clearly shews 

 that the effect of local attraction is very sensible there. For 

 at this place which is only 7 miles North of Madras, the Dip 

 increasing at the average rate ought to be about 7° 40' where- 

 as by many careful and accordant observations it was found to 

 be 7° 58 ', the Dip at Poonairy a place 10 miles to the 

 North of it being only 7 C 54'. This case is perhaps an ex- 

 traordinary one but I feel convinced that in most places there 

 is an effect produced in this way to a much greater extent 

 than is generally supposed and for which allowance ought to 

 be made. And it may hereafter be an interesting point to 

 determine what effect the Geological formation has upon 

 Magnetic Observations, and whether it may not be possible 

 by means of such observations to throw some light upon the 

 Local Formation. 



TABLE No. 1. 



Observations of the dip taken at Madras, during the years 1 842 

 1843 and part of 1844, with the same needle. 





1842. 



1843. 



1844. 





c 







« ' 





Q 





January 



7 



20. 



32.7 



7 23. 



50.3 



7 



24. 56.2 



February 



> > 



19. 



42. 



„ 23. 



20.1 



>5 



25. 10.2 







20. 



33.6 



„ 23. 



29.8 



5) 



24. 56.2 





5> 



21. 



22.6 



„ 22. 



59.2 



JJ 



25. 54.1 







23. 



06.3 



„ 24. 



35.3 



>> 



27. 02. 





>> 



23. 



05.3 



„ 24. 



05.6 



»> 



28. 46.7 





J> 



22. 



56 4 



„ 24. 



24.9 











23. 



12.4 



„ 25 



24 0 









>J 



22. 



22.8 



„ 42 



41.6 







Octo er 



»J 



21. 



46.1 



„ 23 



12. 







November 



>» 



23. 



29.1 



» 23 



35.9 







December .... 





24. 



04.8 



„ 24 



23.3 







Mean annual increase 2' 5" 



