Dip and Variation of the Magnetic Needle. 



99 



factory, yet I think we shall not be very far from the truth if we 

 assume that the dip in the eastern states decreases 0^.5 per year, 

 and is stationary in the western states. In the subsequent dis- 

 cussion, I have in part evaded the difficulty, by assuming Jan. 1, 

 1840, as the common epoch, which is not far from the mean time 

 of the observations. 



In discussing the observations, I have divided them into four 

 groups according to longitude. The following table contains all 

 the observations between longitudes 71° and 78° W. which I 

 have been able to obtain. Where several observations have been 

 made at the same locality, I have endeavored to deduce the most 

 probable mean result. 



The observations are all reduced to Jan. 1, 1840, by applying 

 the annual motion — 0'.5. Adopting for the central position lati- 

 tude 41° 5V N., longitude 73° 52' W., we obtain twenty equa- 

 tions of condition of the usual form. From these equations 

 we obtain .5=2.8772, 0;= +.01389, «/= +.00077, and the direc- 

 tion of the isoclinal line is from N. 86° 50' W. to S. 86° 50' E. 

 The increase of dip in the normal direction is 50'.077 to sixty 

 geographical miles. Computing from these data the dip at the 

 several stations, we obtain the differences given in the last column 

 above. Four of these differences exceed ten minutes. The as- 

 sumed dip at Montreal is the mean of the two following obser- 

 vations : 



77°6'.4 1833, April 19. Capt. Back. 

 76 19 1838. Major Estcourt. 



