1904, No. 2. THE DIURNAL VARIATION OF TERRESTRIAL MAGNETISM. 9 



normal davs onl\- are taken into consideration, Liideling considers that 

 in its main features, the vector-diagram constructed for each polar station 

 has a form somewhat similar to that of the vector-diagrams constructed 

 by von Bezold according to Schuster's theoretical calculations for more 

 southerlv latitudes, even though there are great differences and remark- 

 able disagreements as regards the several stations. Schuster and von 

 Bezold have expressly pointed out that the considerations put forward 

 by them, find no application in the polar regions. It is therefore, in my 

 opinion, no small step in advance when Liideling includes the conditions 

 in the polar regions when dealing with the problem of the diurnal period 

 of terrestrial magnetism, notwithstanding that von Bezold, in point 2 of 

 his remarks quoted on p. 8, does not mention the polar regions, but 

 onl}' emphasises the importance of a greater abundance of obser\'ations 

 from medium and lower latitudes, and especially from the tropics. 



As far as I can see, however, the comprehension of the phenomena 

 of the normal diurnal variation has not been carried, by Lüdeling's 

 investigations, beyond the standpoint which Schuster's and von Bezold's 

 treatises had already established. This standpoint may be thus defined: 

 There is a very great probability that the diurnal variation of terrestrial 

 magnetism is in the first place due to a system of electric currents out- 

 side the earth; but on the other hand, we do not know the particulars 

 of this system of currents, its form and position, and the changes which 

 it possibly undergoes with the change in the mutual position of the 

 earth and the sun in the course of the vear. 



II. 



My idea of what we ought especially to have in view in investiga- 

 tions of the diurnal variation of terrestrial magnetism, is not quite in 

 harmony with von Bezold's remarks quoted above on p. 8. That 

 author cautions with good reason against the employment of means cal- 

 culated for a longer period than two months; but even that seems to 

 me too long, and I should also like to see a demand for strictly simul- 

 taneous obser^-ations. I think, moreover, that observations from polar 

 regions are just as much needed as from the tropics, and that one ought 

 to begin by studying the diurnal variation at the equinoxes, and not 

 deal with the time about the solstices and the other seasons until after- 

 wards. For if we do not mean to content ourselves merely with drawing 



