[169] This has been most effectually accomplished by Mr. Bain. Mr. Hobson has had an electric clock, thus excited, in action for several years.
CHAPTER X.
MAGNETISM.
Magnetic Iron—Knowledge of, by the Ancients—Artificial Magnets—Electro-Magnets—Electro-Magnetism—Magneto-Electricity—Theories of Magnetism—The Magnetic Power of soft Iron and Steel—Influence of Heat on Magnetism—Terrestrial Magnetism—Declination of the Compass-needle—Variation of the Earth’s Magnetism—Magnetic Poles—Hansteen’s Speculations—Monthly and Diurnal Variation—Dip and Intensity—Thermo-Magnetism—Aurora Borealis—Magnetic Storms—Magnetic conditions of Matter—Diamagnetism, &c.
Agreeably with the view now generally received, that magnetism and electricity are but modifications of one force, since they are found to stand to each other in the relation of cause and effect, the separation which is here adopted, of the consideration of their several phenomena, may appear inappropriate. The importance, however, of all that is connected with magnetism, and the very decided difference which is presented by true magnetic action, and that of frictional or chemical electricity, is so great that it has been thought advantageous to adopt the present arrangement in reviewing the influence of terrestrial magnetism with which science has made us acquainted.
From a very early period a peculiar attractive force has been observed in some specimens of iron ore. Masses of this kind were found in Magnesia, and from that locality we derive the name given to iron in its polar condition. This is confirmed by the following lines by Lucretius:—
Quod superest agere incipiam, quo fœdere fiat
Natura lapis hic ut ferrum ducere possit,