1876.] On the Magnetic Declination at Lisbon. 373 



March 9, 1876. 



Dr. GUNTHER, M.A., Vice-President, in the Chair. 



The Presents received were laid on the table, and thanks ordered for 

 them. 



The following Papers were read : — 



\ 

 I. " On the Diurnal Variations of the Disturbances of the 

 Magnetic Declination at Lisbon/' By J. Capello, Director 

 of the Lisbon Observatory. Communicated by Balfour 

 Stewart, F.R.S., Professor of Natural Philosophy, Owens 

 College, Manchester. Received February 12, 1876. 



The Observatory of Infante D. Luiz has possessed since 1863 a set of 

 magnetographs giving continuous records similar to those at Kew. In 

 1870 the results of the declination magnetograph, from the year 1864 to 

 1867, were published ; and in 1874 the results for the same period of 

 the bifilar and balance magnetographs (results embracing the horizontal 

 force, vertical force, total force, and inclination) were published. At 

 present the results of the declination for 1868 to 1871 are in course of 

 publication. 



The subject of this short paper is the diurnal variations of the disturb- 

 ances of the declination. The method which has been adopted for 

 reducing the disturbances is that of Sir Edward Sabine. I have taken 

 + 2 millimetres, or 2'*26, as the limits, beyond which limits all readings 

 are regarded as disturbed. 



In separating for each hour the disturbances (that is to say, the 

 excesses and the deficiencies of individual observations over the monthly 

 means for each hour), and then taking for each year on one side all the 

 excesses and on the other side all the deficiencies, we shall have a series 

 of numbers in millimetres which represent the law of the frequency of 

 these disturbances. 



In comparing the curves which graphically represent the law of the 

 frequency of the disturbances ( + West) and ( — East) for the 8 years 

 (1864 to 1871), it is at once noticed that all the phases appear later (as 

 regards the hour of the day) for the latter years. In the easterly dis- 

 turbances this tendency is manifested whether we make the comparison 

 between the curves of 1864 and 1871 (fig. 1) or compare the curves for 

 every 2 years (fig. 2), or finally the curves of the two periods of 4 years 

 (fig. 3). In the westerly disturbances similar results are found, but 

 they are less marked ; the movement appears to be slower ; and it is in 



