150 



Dr. T. E. Thorpe. 



Addendum. April 26, 1879. Received April 28. 1879. 



Since sending in the foregoing communication to the Society, I 

 have received the following letter from Mr. C. A. Schott, of the 

 United States Coast Survey Department, respecting the inadequacy 

 of the formula published in the Report of 1870, as representing the 

 values of the magnetic elements at the present time. 



It will be noticed that Mr. Schott confirms my observations on this 

 point ; and by permission of the Superintendent of the Survey, he sends 

 amended formula for dip, declination and horizontal force which 

 represent with sufficient accuracy both series of observations, viz., his 

 made in June, and mine made in September, 1878. 



United States Coast Survey Office, 



Washington, March 28th, 1879. 



My Dear Sie, 



I hasten to reply to yours of March 13, respecting magnetic obser- 

 vations in this city. The formulas in the 1870 Report undoubtedly fail 

 to represent adequately our late observations, a contingency which was 

 referred to in the concluding paragraph of that report : " They should 

 not be used much beyond the time for which they are supported by 

 observations." There is nothing unusual going on in the behaviour 

 of terrestrial magnetism in this locality, and the dip and intensity 

 formulae may be easily amended to represent our observations, but in 

 regard to the declination there is a disturbing element introduced, 

 viz. : the change of station in 1876, which developed a local difference 

 of 20' as near as I can judge; whether the old or new place is the 

 more normal one I cannot say. There is apparently no difference in 

 dip and intensity. Considering the dissimilarity of the instruments 

 nsed our observations agree well, indeed they accord within the daily 

 variations of the magnetic force. By permission of the Superin- 

 tendent of the Survey, I enclose the latest formulas for declination, 

 dip, and intensity in this place, as well as my observations of last year ; 

 they are : — ■ 



The magnetic dip observed at Washington between 1839 and 1879 

 can be represented by the formula — 



I=71°-335-0-000229 (*-1855-0) -0-000640 (f-1855'0) 9 



-0-0000140 (*-1855-0) 3 , 



which will leave no greater differences than those given by the old 

 formula. (Report of 1870.) The magnetic horizontal intensity ob- 

 served between 1842 and 1879 is represented by 



H=4-270 + 0-00084 (J-1855'0) +0-000130(£-1855'0) 2 , 



