1878.] Prof. B. Stewart. Magnetic Declination. 389 



IV. " On the Variations of the Diurnal Range of the Magnetic 

 Declination as recorded at the Prague Observatory." By 

 Balfour Stewart, LL.D., F.R.S., Professor of Natural 

 Philosophy at Owens College, Manchester. Received 

 May 2, 1878. 



1. The Prague magnetic observations began in July, 1839, and 

 have been continued until the present date. The observation hours, 

 18h., 22h., 2h., 10h., are common to the whole series, except for the 

 year 1853, during which observations were made only at the hours 

 18h., 2h., lOh. As far, however, as the estimation of the diurnal 

 range of magnetic declination is concerned, these last three hours are 

 practically as good as the former four, inasmuch as the observations at 

 22h. are hardly ever made use of in determining the diurnal range. 



In the determinations herein recorded, magnetic disturbances are 

 included, and the range is a mean monthly one, obtained by comparing 

 together the mean values of the magnetic declination, corresponding 

 to the hours 18h., 22h., 2h., 10h., for any given month, and taking 

 the difference between the highest and the lowest of these values as 

 representing the mean range for that month. There is reason to 

 believe that the ranges thus obtained are not greatly different from 

 those which would have been obtained from an hourly series of observa- 

 tions. 



A. Annual Variation of Declination- Bang e. 



2. In order to obtain this variation, the mean monthly ranges 

 obtained, as already described, and extending from the beginning of 

 1840 to the end of 1876, have been made use of. From these we 

 -obtain the following table: — 



Table I. Containing mean values, for each month in the year, of 

 the declination-range at Prague, in minutes of arc, taken from the 

 whole series of thirty- seven years. 



Jan. Feb. Mar. April. May. June. July. Aug. Sept. Oct. Nov. Dec. 

 S'298 6 -808 8 '636 10 -366 10 "607 11 '513 11 "184 10 "695 8' '361 7 36o 5-668 4 -699 



B. Va riations of Long Period. 



3. In order to investigate the long-period variation of the Prague 

 declination-range, I have treated these observations precisely in the 

 w*ay in which the Kew declination-ranges were treated (Proc. Roy. Soc, 

 March 22, 1877). By this method, proportional values of the declina- 

 tion-range at Prague have been obtained for the middle points of each 

 month for each year, and it is believed that these values are freed 

 from any recognised inequality depending either on the month of the 



