DR. S. CHAPMAN ON THE SOLAR AND LUNAR 
Part V.—On Certain Properties op the Earth’s Atmosphere. 
§19. The solar diurnal barometric variation..42 
§ 20. The lunar diurnal barometric variation.45 
§21. The electrical conductivity of the upper atmosphere.46 
Part YI.—The Theory of the External Solar and Lunar Diurnal Magnetic 
Variation Fields. 
§ 22. Outline of the mathematical theory for the general law of atmospheric conductivity. . . 48 
§ 23. The relative amplitudes of the magnetic variations. Deduction of the effective atmospheric 
oscillations and the law of conductivity.56 
§ 24. The absolute values of the amplitudes and of the electrical conductivity in the upper 
atmosphere. 62 
§ 25. The heating effects of the upper air currents. 64 
§ 26. Discussion of the phases of the magnetic variations.67 
§ 27. The residual variations and the terms not dependent solely on local time.70 
Note. 73 
§ 1. Introduction. 
The regular daily changes of the earth’s magnetism, considered as a world-wide 
phenomenon, afford a problem of much interest and importance. It is one, more¬ 
over, which the researches of Balfour Stewart and Schuster have shown to be 
more vulnerable to attack than seem most of the problems of terrestrial magnetism. 
But in spite of their success, and of the contributions of subsequent writers, the 
comprehensive study of the subject has suffered undeserved neglect. It seems an 
unfortunate fact that the efforts of magneticians are unduly devoted to the accumu¬ 
lation of data, the time and labour spent in their discussion being, proportionately, 
inconsiderable. 
The promising theory that the daily magnetic variations arise mainly from electric 
currents circulating in the upper atmosphere, under the impulsion of electromotive 
forces produced by the convective motion of the air across the earth’s permanent 
magnetic field, was first propounded by Balfour Stewart.* In a simple but 
penetrating discussion of the features both of the solar and lunar diurnal variations, 
he showed the power of this theory to account for the facts in a way which none of 
the other theories then current could do. 
The theory was greatly developed, and rendered more definite, in two important 
memoirs by Schuster in 1889 and 1907.1 Adopting a suggestion by Gauss, he 
applied the method of spherical harmonic analysis to the solar diurnal magnetic 
variations, to determine whether they had their origin mainly above or below the 
* Cf. his article on “ Terrestrial Magnetism ” in the 9th edition of the ‘ Encyclopaedia Britannica ’ (1882). 
He made considerable use of Broun’s admirable study of the lunar diurnal variation of declination at 
Trevandrum (1874). 
f Schuster, ‘Phil. Trans.,’ A, vol. 180, p. 467 (1889); and A, vol. 208, p. 163 (1907), 
