1904. Xo. 2. THE DIURNAL VARIATION OF TERRESTRIAL MAGNETISM. 1 9 



forms a loop about the noon of the magnetic pole: at the equinoxes, the 

 deformation of the curve at the same hour is less, but still distinct; while 

 in the winter it is inconspicuous in comparison with other irregularities. 



The conclusion is therefore obvious that the position of the various 

 stations in relation to the magnetic pole plays no unimportant part as 

 regards the regularity with which the diurnal variation of the terrestrial- 

 magnetic phenomena takes place; and we may fairly assume that at the 

 equinoxes the greatest regularity may be looked for in the stations whose 

 difference in longitude from that of the magnetic pole is greater than 

 ± 90". The same consideration may presumably be applied to the mag- 

 netic south pole in its relation to stations in the southern hemisphere. 



It appeared to me well, therefore, to divide the northern hemisphere 

 into two parts by a meridian at right angles to the meridian through the 

 magnetic north pole. That part of which the centre meridian passes through 

 the magnetic pole, I will call the northern polar section, the other the 

 northern anti-polar section. In analogy to this, we can imagine the southern 

 hemisphere divided into the southern polar section and the southern anti- 

 polar section. The boundar\- between polar section and anti-polar section 

 will thus be, for the northern hemisphere 6" W and 174° E of Green- 

 wich, and for the southern hemisphere 58° E and 122" W of Greenwich, 

 if the longitude of the magnetic south pole be assumed to be about 

 148° E of Greenwich. These limits are applicable only to the equinoxes, 

 at which time the series of observations dealt with in this paper were 

 made. For the other seasons of the year, the limits must be regulated 

 by the hour at which the sun rises and sets upon the horizon of the 

 magnetic pole. 



In the following graphic representation (Tig. 2) of the diurnal variation 

 of the deflecting force A, the stations are grouped according to the above 

 principle. The 10 stations to the left. Cape Thordsen, Ssagast^T, Xovaja 

 Zemlja, Bossekop. Sodank\-]ä, Pawlowsk, Wilhelmshaven, Vienna, Tiflis, 

 and Zi-Ka-Wei, belong to the northern anti-polar section; the upper 6 to 

 the right. Point Barrow, Jan Mayen, Kingua, Godthaab, Fort Rae, and 

 Los Angeles, belong to the northern polar section, and the 2 lowest. 

 South Georgia and Cape Horn, belong to the southern anti-polar section. 

 Within each of these three groups, the stations are arranged according 

 to their geographical latitude from X to S. The time is local mean time. 



On each ciine, a vertical stroke with a black dot at the upper or 

 lower end, marks the moment at which the sun is in the meridian of the 

 magnetic pole above the horizon. 



