The magnetism of the earth, its long continued slow 'secular' change, and its more rapid periodic and irregular change, form a group of phenomena of great intrinsic interest, which are linked also with important practical affairs - navigation by sea and air , surveying in mines and on the surface, prospecting for minerals and oil, and communication by telegraph and radio. They provide avenues of research into the earth's interior, into the ionosphere, and into the changes that occur in the sun's atmosphere. They are linked moreover with cosmic ray phenomena and with the northern lights ( the aurora polaris).