Theory of the diurnal variation of the magnetic needle, illustrated by experiments

Mr. Christie having been led to doubt the validity of the explanation of the moving easterly variation adopted by Canton, but at the same time having observed that the changes in deviation and intensity appear always to have reference to the position of the sun with regard to the magnetic meridian, was led to connect these phenomena with Professor Seebeck’s discovery of thermo-magnetism, and Professor Cumming’s subsequent experiments; and to refer the phenomena of diurnal variation to the effect of partial heating, modified, perhaps, by that of rotation, and by peculiar influence in the sun’s rays. In support of this opinion, he cites passages from papers by Professor Cumming and Dr. Traill, whom a similar idea appears also to have impressed. But in place of looking to the stony strata, of which the earth’s surface consists, as the elements of the thermo-magnetic apparatus which this doctrine requires, the author regards them as rather consisting of the atmosphere and the surfaces of land and water with which it is in contact. Thermo-magnetic phenomena, he remarks, have hitherto only been observed in metallic combinations, but this may be owing merely to the small scale on which our experiments are conducted.