A Magnetic Survey of the Fortieth Parallel. 151 



which will leave differences of about the same character as those 

 given by the old formula. 



The very nature of these functions precludes them from answering 

 for any length of time, and as soon as the case becomes ripe, I shall 

 substitute harmonic functions. 



The following is an improved formula for the declination at 

 Washington, prepared for a new edition of the paper in the 1874 

 Report : 



D=+2°-19 + 2°-22sin (l'4ro-6°'6), 



it represents the observations between 1792 and 1879; m—t — 

 1850-0. 



These observations are difficult to manage on account of con- 

 siderable local deflection in this vicinity produced by iron (bog-ore) 

 which in many places crops out to the surface. 



Our observations compare as follows : 



In latitude 38° 53'-2 : longitude 77° 00 ''4. 



Declin, Dip. Hor. Inten. 



June 14, 15, 17. C. A. Schott . . 3° 47 . . 70*49 . . 4-366 

 Sept. 8, 1878. T. E. Thorpe . . 3 43 . . 70-47 . . 4-361 



Yours very truly, 



Chas. A. Schott. 



Dr. T. E. Thorpe, 



Yorkshire College, Leeds, England. 



vol. xxx. 



M 



