e 
94 Dip and Variation of the Magnetic Needle. 
amount we have no means of estimating. In order to obtain 
more light upon this subject, I addressed a letter to Prof. Farrar, 
of Cambridge. In his answer he remarks: “1 have every reason 
to believe that the only dipping needle belonging to the apparatus 
of the college, at the time of Prof. Williams’ observations, was 
one made by Nairne, and is not, I think, to be depended upon. 
When I have attempted to use it, I have found the needle to rest 
in positions differing several degrees from each other.” This last 
remark corresponds very well with the observations of Prof. Wil- 
liams. If the original manuscript containing the observations 
can be found, it is possible that the suspicion which now attaches 
to them may be removed. At present, however, I place no con- 
fidence in them. 
The next dip observations in point of age, are those made du- 
ring Long’s expedition to the Rocky Mountains in 1819. The 
dip at St. Louis, June 16, 1819, is given 70° 30’, I found the 
dip, Sept. 29, 1841, to be 69° 25/.5, shewing a decrease of about 
three minutes per year. The dip at Shippingsport is given 70° 15’. 
Prof. Locke found the dip at Louisville in 1840, 70° 4’, shewing 
a diminution of half a minute per year. The discordance of 
these results excites a suspicion of inaccuracy somewhere. A 
more satisfactory result may be expected by combining all the 
observations made in 1819, so as to obtain the mean dip at some 
one station. By this method we may be expected to eliminate 
in a measure the influence of local attraction, and also accidental 
errors of observation. The following table presentsa summary 
of these observations, treated in the manner described in a former 
number of this Journal, Vol. xt, p. 85. As the observations are 
all comprised within a moderate period, I have not applied any 
correction to reduce them all toa common instant. Adopting for 
the central position latitude 39° 3’ N., longitude 91° 37’ W., - 
obtain the aunexed meee of condition. 
a 
gesseescteed 
