1904. No. 2. THE DIURNAL VARIATION OF TERRESTRIAL MAGNETISM. 



THE DIURNAL VARIATION OF TERRESTRIAL MAGNETISM. 



I. 



For a long time past we have been aware of the regularit\^ with 

 which the three elements of terrestrial magnetism, declination, inclination, 

 and horizontal intensity, periodically change in the course of the 24 hours. 

 This regularity, however, is frequently eflaced by more or less violent 

 magnetic perturbations. In order to eliminate the effect of these per- 

 turbations, various methods of procedure have been tried in the treatment 

 of the observations; but we have of late acquiesced in the proposal put 

 forward by Wild^ at the Polar Conference in Vienna in 1884, to employ 

 for the determination of the normal diurnal variation of the three ele- 

 ments, only specially-selected, magnetically-calm days in each month, 

 so-called normal da3-s. 



No satisfactory explanation of the phenomenon we call the diurnal 

 variation of terrestrial magnetism appeared until 1889, when Professor 

 A. Schuster, in his paper mentioned on p. 5 »The Diurnal Variation 

 of Terrestrial Magnetism«, subjected the question to renewed inquirv, 

 upon a different method from that hitherto employed, introducing, instead 

 of declination, D, inclination, /. and horizontal intensity, H, the Gaussian 

 coordinates, A', Y, Z, {X=H cos D, Y^=H ûw D, Z=H tan I), whose daily 

 periodical variation he treated by the aid of harmonic analysis. Schuster 

 employed observations from the four stations, St. Petersburg, Greenwich, 

 Lisbon, and Bombay, for the year 1879, ^"^ ^°°''^' °"b' ^^e normal days 

 in each month. He divided the year into two parts, taking the summer 

 months, April to September, together as one part, and the winter months, 

 October to March, as the other. 



1 H. Wild. Mittheilungen der internationalen Polarcommission. No. 97, p. 211. St. Fe- 

 tersburg, 1884. 



