A Magnetic Survey of the Fortieth Parallel. 139 



No certain conclusion, however, can be drawn from these numbers 

 respecting the extent of the secular change in this element, since, as 

 Mr. Schott's observations at Washington have shown, the dip at that 

 place was on the increase at the epoch 1842*5, and attained a 

 maximum about 1854*8. Doubtless a similar increase occurred at 

 New Haven and Pittsburg (Allegheny) and probably a maximum 

 was also reached at these places at about the same period. The result 

 for Chicago seems exceptional ; I am hardly inclined to regard it as 

 due to an error in the determination of the dip at the epoch 1842*5, 

 on account of the number and fairly concordant character of the 

 observations, which were all made at about the time of the first epoch 

 by different observers. These are — 



Loomis 1841 72 48 



Nicollet 1841 72 46 



Younghusband 1842 . . 72 39 



My own results agree perfectly among themselves : they were taken 

 under favourable circumstances, and so far as I could learn from 

 Professor Colbert, the Director of the Dearborn Observatory — there 

 was nothing in the spot selected for the observations — an open space 

 in the grounds of the University of Chicago — which could at all 

 interfere with the proper direction of the needle. Whether Chicago 

 is affected by any strong local disturbance I am unable to say : the 

 geological character of the district affords no evidence on this point. 



In an Appendix to the " United States Coast Survey Report for 

 1870," Mr. Schott has discussed the secular changes in the declina- 

 tion, dip, and intensity of the magnetic force at Washington. The 

 observations of inclination, which extend over a period of nearly forty 

 years, may be represented by the expression — 



I=71°*335-0*000229 (t- 1855-0) -0-000640 (i-1855) 2 



-0*0000303 G-1855) 3 , 



which, as the accompanying comparisons show, gives results closely 

 accordant with the observed values. 



Date. Observed Dip. Computed Dip. Difference. 



-0*01 

 -0*01 

 -0*00 

 -0*03 

 + 0*03 

 + 0*05 



1839*2 71*29 71*30 



1851*5 71*32 71*33 



1861*6 71*30 71*30 



1871*4 71*00 71*03 



1872*5 71*00 70*97 



1873*5 70*97 70*92 



This formula, although perfectly applicable between the intervals 



