VI. The Lunar Diurnal Magnetic Variation, and its Change with 



Lunar Distance. 



By S. CHAPMAN, M.A., D.Sc., Fellow and Lecturer of Trinity College, Cambridge, 

 lately Chief Assistant at the Royal Observatory, Greenwich. 



Communicated by the Astronomer Royal, F.R.S. 

 Received January 27, Read February 18, 1915. 



CONTENTS. 



Page. 



Introduction 161 



The variation of amplitude with lunar distance 1G3 



The variation of phase with lunar distance 166 



Discussion 1G7 



Tables 171 



Additional note 175 



Introduction. 



IN his illuminating article on Magnetism in the ' Encyclopaedia Britannica' (9th ed., 

 1882), BALFOTTB STEWART discussed the origin and mechanism of the short-period 

 magnetic variations, concluding that the only tenable hypothesis was that which 

 attributed them to currents flowing in the upper atmosphere, under the impulse of 

 electromotive forces caused by the motion of the conducting atmosphere across the 

 permanent terrestrial magnetic field. At several stages in the discussion use was 

 made of the phenomena of the lunar diurnal magnetic variation, as disclosed by 

 BROUN'S fine study of the subject ;* to these phenomena BALFOUR STEWART 

 evidently attributed considerable theoretical importance, and an origin similar as 

 regards situation to that of the solar diurnal variations. 



In 1889 ScHUSTERf proved, by the Gaussian potential method, that the solar 

 diurnal magnetic variations arise mainly from causes acting above the earth's surface, 

 a demonstration which added much weight to BALFOUR STEWART'S tentative theory. 

 SCHUSTER also suggested a connection between the magnetic and barometric 

 variations, an idea which he elaborated and discussed with great cogency eighteen 

 years later (1907)4 The barometric changes are mainly of thermal origin, and 

 possible differences between the character of the atmospheric motions at the earth's 

 surface and in the upper regions may have an important bearing on the theory. In 



* BROUN, 'Trevandrum Magnetical Observations,' vol. 1, p. 113, 1874. 

 t SCHUSTER, ' Phil. Trans.,' A, vol. 180, p. 467, 1889. 

 J SCHUSTER, ' Phil. Trans.,' A, vol. 208, p. 163, 1907. 

 VOL. CCXV. A 528. Y [Published April 7, 1915. 



