[ 279 ] 



VII. On the Diurnal Variations of the Earth's Magnetism produced by the 



Moon and Sun. 



By S. CHAPMAN, B.A., D.Sc., Chief Assistant at the Royal Observatory, Greenwich. 



(Communicated by the Astronomer Royal.) 

 Received April 24, Read June 19, 1913. 



CONTENTS. 



Page 



Introduction 279 



PART I. General Discussion 281 



PART II. Mathematical Theory 288 



PART III. The Observational Material 307 



Introduction. 



1. WHILE the observational study of terrestrial magnetism is receiving ever more 

 and more attention, and being rewarded with success by the acquisition of new and 

 important data, the theoretical side of the subject shows a much less rapid advance. 

 The search for a physical theory of the earth's magnetism and its changes is 

 fascinating but elusive. Perhaps in one case only that of SCHUSTER'S important 

 theory* of the diurnal variations of the magnetic state of the earth has there been 

 put forward a clearly outlined theory which promises to explain the real mechanism 

 of any magnetic phenomenon. 



On this theory, the solar diurnal variations are attributed to the action of electro- 

 motive forces produced in masses of conducting air in the upper atmosphere, by their 

 motion across the permanent magnetic field of the earth. The magnetic field of the 

 resulting electric currents is identified with that which produces the observed diurnal 

 changes. SCHUSTER has shown that if the motion of the air is taken to be sub- 

 stantially that which is indicated by the barometric variations, the atmosphere being 

 supposed to oscillate as a whole, the conductivity required by the theory is not 

 unreasonable, considering the ionization of the tenuous upper atmosphere by ultra- 



* ' Phil. Trans.,' A, vol. 208, p. 163. 

 VOL. CCXIII. A 503. Published separately, August 22, 1913. 



