306 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 mereasing 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. Todetermine 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, will state at once the conclusion at which I 
have arrived, viz. that the westerly variation is at present tncreas- 
ing and the easterly diminishing in every part of the United 
States ; that this change commenced between the years 1793 and 
1819, probably not every where simultaneously ; and that the 
present annual change of variation is about 2’ in the southern 
and western states, from 3/ to 4’ in the middle states, and from 
5’ 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 all by nearly three degrees; and the others too 
large by. by nearly four degrees. I infer that either they were very 
