806 Variation and Dip of the Magnetic Needle. 



From an attentive examination of the preceding table it will 

 be seen, that from the time of the earliest observations down to 

 about the commencement of the present century, the westerly 

 variation was decreasing and the easterly increasing in every part 

 of the United States ; that more recently, the reverse has taken 

 place, that is, that a retrograde movement of the needle has com- 

 menced. The precise year when this change took place cannot 

 be certainly known. To determine this, we need more numerous 

 and more accurate observations. All the observations, however, 

 agree in this, that the change began as early as 1819, while the 

 Philadelphia observations would make it as early as 1793, and 

 those at Newbern, N. C. not far from the same year. The annual 

 motion is much greater in the eastern states than in the south 

 and west. I have carefully compared all the observations con- 

 tained in the preceding table, and without giving the particulars 

 of this discussion, Avill state at once the conclusion at which I 

 have arrived, viz. that the westerly variation is at present increas- 

 ing and the easterly diminishing in every part of the United 

 States ; that this change commenced betioeen the years 1793 and 

 1819, probably not every where simultaneously ; and that the 

 present annual change of variation is about %' in the southei^n 

 and western states, fro'm Z' to M in the middle states, ajid from, 

 B' to 7' in the New England states. 



Having thus assembled together all the observations in my 

 power, and deduced from them as far as possible the law of the 

 needle's motion, it remained to reduce all the observations to one 

 epoch, 1838, by applying the correction for the annual motion. 

 They were then all carefully marked down upon a map of the 

 United States, and the probable position of the lines of equal 

 variation determined. Where no particular reason has been per- 

 ceived to distinguish between the observations, the lines have 

 been so drawn as to make the positive equal to the negative errors. 

 This chart then is intended to represent all the observations con- 

 tained in the preceding table reduced to the present time. In 

 making this comparison some of the observations were found to 

 present strange anomalies. Of these, the most considerable are 

 the observations at Hanover, N. H. for 1810, those at Montpelier, 

 Vt., and at Princeton, N. J. The first of these, according to my 

 chart, is too small by nearly three degrees ; and the others too 

 large by nearly four degrees. 1 infer that either they were very 



