ROTATION-MAGNETISM.
The unexpected discovery of Rotation-Magnetism by Arago, in 1825, has shown practically that every kind of matter is susceptible of magnetism; and the recent investigations of Faraday on diamagnetic substances have, under special conditions of meridian or equatorial direction, and of solid, fluid, or gaseous inactive conditions of the bodies, confirmed this important result.
INFLUENCE OF PENDULUMS ON EACH OTHER.
About a century since it became known, that when two clocks are in action upon the same shelf, they will disturb each other: that the pendulum of the one will stop that of the other; and that the pendulum that was stopped will after a while resume its vibrations, and in its turn stop that of the other clock. When two clocks are placed near one another in cases very slightly fixed, or when they stand on the boards of a floor, they will affect a little each other’s pendulum. Mr. Ellicote observed that two clocks resting against the same rail, which agreed to a second for several days, varied one minute thirty-six seconds in twenty-four hours when separated. The slower, having a longer pendulum, set the other in motion in 16-1/3 minutes, and stopped itself in 36-2/3 minutes.
WEIGHT OF THE EARTH ASCERTAINED BY THE PENDULUM.
By a series of comparisons with Pendulums placed at the surface and the interior of the Earth, the Astronomer-Royal has ascertained the variation of gravity in descending to the bottom of a deep mine, as the Harton coal-pit, near South Shields. By calculations from these experiments, he has found the mean density of the earth to be 6·566, the specific gravity of water being represented by unity. In other words, it has been ascertained by these experiments that if the earth’s mass possessed every where its average density, it would weigh, bulk for bulk, 6·566 times as much as water. It is curious to note the different values of the earth’s mean density which have been obtained by different methods. The Schehallien experiment indicated a mean density equal to about 4½; the Cavendish apparatus, repeated by Baily and Reich, about 5½; and Professor Airy’s pendulum experiment furnishes a value amounting to about 6½.
The immediate result of the computations of the Astronomer-Royal is: supposing a clock adjusted to go true time at the top of the mine, it would gain 2¼ seconds per day at the bottom. Or it may be stated thus: that gravity is greater at the bottom of a mine than at the top by 1/19190th part.—Letter to James Mather, Esq., South Shields. See also Professor Airy’s Lecture, 1854.
ORIGIN OF TERRESTRIAL MAGNETISM.
The earliest view of Terrestrial Magnetism supposed the existence of a magnet at the earth’s centre. As this does not accord with the observations on declination, inclination, and intensity, Tobias Meyer gave this fictitious magnet an eccentric position, placing it one-seventh part of the earth’s radius from the centre. Hansteen imagined that there were two such magnets, different in position and intensity. Ampère set aside these unsatisfactory hypotheses by the view, derived from his discovery, that the earth itself is an electro-magnet, magnetised by an electric current circulating about it from east to west perpendicularly to the plane of the magnetic meridian, to which the same currents give direction as well as magnetise the ores of iron: the currents being thermo-electric currents, excited by the action of the sun’s heat successively on the different parts of the earth’s surface as it revolves towards the east.