Part III. were suggested by the earlier ones of De la Rive, Varley, and

others, and demonstrate the effect of the magnet on electric discharges. For assistance in these I am indebted to my friend Mr. E. Dowlen.

The illustrations are mainly from original drawings of my own. Those from other sources are acknowledged. Messrs. Mintern have well reproduced in chromo-lithography the coloured drawings illustrating the Auroræ, moon-patches, &c.

TABLE OF CONTENTS.