Variation and Dip of the Magnetic Needle, 47 



The general conclusions at which I arrived in my former paper, 

 are abundantly confirmed by the preceding observations. They 

 all indicate a retrograde motion of the needle, which commenced 

 every where as early as 1819, and in some places perhaps as early 

 as 1793. The present annual change of variation is about two 

 minutes for the southern states ; four minutes for the middle and 

 western states ; and six minutes for the New England states. 



The observations of variation contained in my present and for- 

 mer catalogues v/ere now all reduced to the year 1840, by apply- 

 ing a correction for the annual motion, and the lines of equal 

 variation laid down upon the accompanying chart so as best to 

 represent the observations. It is believed that the errors of the 

 chart as thus corrected must be quite small. In my former paper, 

 I noticed three observations which exhibited a remarkable dis- 

 cordance with the rest. They were for Hanover, N. H., Mont- 

 pelier, Yt., and Princeton, N. J. It is gratifying to find that the 

 present variation at the former place, as observed by Prof. Young, 

 agrees almost exactly with my chart. It is believed that the ob- 

 servations at the other two places will be found to be erroneous, 

 or that they were affected by very strong local attraction. 



It seems almost superfluous to remark, that most of the obser- 

 vations in the preceding table were evidently made with inferior 

 instruments, and can lay but a moderate claim to accuracy. A 

 large part of the observations are clearly erroneous to the amount 

 of half a degree ; and the errors of many exceed one degree. 

 As most of the errors however are such as may be expected to 

 disappear in taking a mean, considerable confidence is placed in 

 the position of the lines of equal variation as projected on the 

 accompanying chart. To attain materially greater accuracy, ob- 

 servations must be made with better instruments and by more 

 skillful observers. 



II. Dip of the Magnetic Needle. 



The chief additional observations of the magnetic dip which 

 I have been able to obtain, were made by Prof. Locke and my- 

 self The former were communicated to me by letter ; the latter 

 are given at large in volume seventh (N. S.) of the Transactions 

 of the American Philosophical Society. 



