DIURNAL VARIATIONS OF TERRESTRIAL MAGNETISM. 
73 
meridianal atmospheric motions would produce electromotive forces along circles of 
latitude, which, again, would give rise to North force variations. It is possible that 
the germ of a satisfactory account of the above residuals is to be found in these 
tentative suggestions. Before developing them further, however, it would be 
desirable to examine the magnetic data more closely. But in spite of the residuals of 
Table T, and the points left unsettled in the previous discussion, the present analysis 
has revealed some important and previously unsuspected regularities in the diurnal 
magnetic variations, and I hope that others will thereby be encouraged to contribute 
further to their elucidation. 
[Note added December 9, 1918. —In an interesting Dissertation (Utrecht, September 22, 1917 : 
‘K. Nederland. Met. Inst.,’ De Bilt, No. 102; also, in abstract, in ‘ K. Ak. van Wet.,’ Amsterdam, 
26, pp. 293-299, 1917), published since this paper was written, Miss van Vleuten has analyzed and 
discussed the solar diurnal magnetic variations, in order to test the theory developed by Prof. Schuster 
in his two memoirs. The conclusions arrived at are (a) that “ the forces causing the diurnal variation, 
taken as a whole, do not possess a potential, although it remains always possible to deduce part of these 
forces from a potential,” and ( b ) that “ the cause of the diurnal variation certainly cannot be ascribed to 
nothing else but a system of currents exterior to the earth and currents within the earth induced by the 
former system.” 
These conclusions appear to rest mainly on the non-correspondence of the observed North force 
variations with those calculated from the simple potential representation of the West force variations 
(cf. § 9). But their physical implication is that electric currents traversing the earth’s surface have an 
important share in producing the diurnal magnetic variations. This seems extremely improbable, and, 
instead of ( a ) and ( b ), the interpretation of the above fact of observation seems rather to be merely that 
the diurnal variations are somewhat complicated, so that their potential cannot be represented exactly 
by any simple combination of spherical harmonics. There are physical grounds for such a conclusion; in 
a paper recently communicated to the Cambridge Philosophical Society I have given reasons for 
supposing that two distinct agencies and atmospheric layers are involved in the production of the diurnal 
magnetic variations. 
While the magnetic variation field does not give simple results on the application of spherical harmonic 
analysis, the latter is still the only convenient means which mathematics affords for discussing the relation 
between the external and internal current systems, and between the former and the atmospheric circulations 
indicated by the barometer. I do not think that on these points the main results obtained in the present 
paper are likely to be modified seriously upon further investigation, but that, especially when the parts 
of the diurnal variations, due to the two agencies above mentioned, are separated and independently 
treated, the theory will be confirmed and brought more closely into accordance with observation.] 
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VOL. CCXVIII.— A. 
