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 IV", 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 tifteen 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 +1K9. The dip has since been observed by Prof. 

 Bache, Major Graham, and Mr. Nicollet, who have obtained re- 

 sults ranging between 71° 3P.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 2r'.7. At Monroe, observations by 

 myself on opposite sides of the town differed 13'. 3. In general 

 we 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 

 extent, unlike the principal part of the force which gives direc- 

 tion to the needle. This fact is specially insisted upon by the 

 magnetic committee of the British Association, of which Sir John 



Vol. XLiii, No. 1.— April-June, 1842. 14 



