Dip and Variation of the Magnetic Needle. 105 
The differences obtained, after assigning to each isoclinal line its 
most probable position, will not be identically the same as given 
in the last column of the preceding tables. In Table I, where 
the curvature of the isoclinal lines is most gentle and uniform, 
the discrepancy is slight; in Table II the discrepancy is not great ; 
in Table III, where the isoclinal lines are somewhat undulating, 
the discrepancies are greater, particularly near the limits of the 
table ; and in Table LV, the discrepancies are quite appreciable 
at the extreme stations. But making the necessary allowance for 
these discrepancies, we have still left unaccounted for large dif 
ferences, amounting at several stations to over fifteen minutes. 
How are these to be explained? The observations themselves 
will suffice to answer this question. In 1834, the dip was ob- 
served at Baltimore by Prof. Courtenay 70° 58’.6. The dip, ac- 
cording to my computation, should be 71° 38'.4. Difference 
—39’.8. In 1839 the dip was observed by myself 71° 50/3. 
Difference +11’.9. The dip has. since been observed by Prof. 
Bache, Major Graham, and Mr. Nicollet, who have obtained re- 
sults ranging between 71° 31’.9 and 71° 47.2. The mean of 
all the observations is 71° 35’.6, differing from my computation 
—2/.8. Extreme range of the observations 51’.7. If we reject 
the observation of Prof, Courtenay, which differs most from the 
mean, the accordance will be still better. It is obvious, then, 
that there prevails about Baltimore some influence which causes 
the dip to change in moderate distances very abruptly. At Phil- 
adelphia, observations made within a year of each other in differ- 
ent parts of the city, vary 23.2. At New York, observations at 
eight years’ interval differ 37.7. At Montreal, two observations 
differ 47’.4. At Cleveland, observations by myself at stations 
about a mile distant differed 21.7. At Monroe, observations by 
myself on opposite sides of the town differed 13’.3. In general 
Wwe may conclude, that wherever the dip is found to be greatly 
different from what was to be expected from its latitude and lon- 
gitude, observations made at stations but moderately distant from 
each other, often less than a mile, will differ materially. Those 
influences, then, which occasion abrupt deviations from the gen- 
eral law of increase of dip with the latitude are quite limited in 
= unlike the principal part of the force which gives direc- 
to the needle. This fact is specially insisted upon by the 
‘committee of the British oa mire of which pistcione 
Vol. xii, No, 1.—April-June, 1842. 
