TERRESTRIAL MAGNETISM. if a magnetised bar of steel be suspended by a- fine thread attached to its middle point, one extremity always points nearly to the north, the other end towards the south. Again, if a bar having a fine axle through its centre of gravity, and perpendicular to the axis of the bar, be placed with the axle resting on two highly polished surfaces, it will be found to make an angle with the horizon, and the magnitude of this angle will depend upon the place at which the experiment is made. Tho unknown influence which produces these phenomena is derived from the earth itself, and is called Terrestrial Magnetism. Hitherto no one has succeeded in reducing terrestrial magnetism to a theory, although various attempts have been made—and some with partial success. But the labours of mathematicians and observers have not been lost, for a mass of evidence on this important subject has been accumulated, which in due time will form the basis of a theory more wonderful even than that of gravitation, since it is not too much to assert that this unseen power, which for years has been known only, and thought of, for its practical utility, will one day prove to be the key to all the other phenomena 4n the universe. Terrestrial magnetism then is known by its effects upon the artificial magnet, and it is the province of the philosopher to observe, compare, and reduce these effects after they have been freed from all incidental and extraneous sources of error.
Until the year 1928, there were no systematic observations of magnetical phenomena; but since that time different countries—and especially our own—aided by their respective governments, have established observatories in all the most advantageous positions on the earth's surface; and sent out expeditions to the antipodes and places where a permanent observatory could not be maintained.
The object of these observations has been to determine the absolute value of what are called "the magnetic elements," and the periodical changes they undergo. Tho observations thus made, having been
carefully corrected and reduced, are printed with such comments as seem necessary, and a description of the instruments and methods employed; and these enormous collections of facts are widely dis tributed throughout the scientific world under the title of Magnetical Observations. • There are three magnetic:elements — munely, the Intensity, the Inclina tion, and the Declination.
If a magnetic needle be freely suspended from its centre of gravity, one extremity is drawn from the horizontal position by a force called the Dfagnetic Intensity, and the direction of this force is inclined to the horizon at an angle called tho inclination or dip. These two elements appear to be independent of each other, and to vary with the position of the place of observation. Again, the needle does not point exactly north and south, but to points situated at some little distance from the poles of the earth—which points are called the magnetic poles. The angle between the meridian of the place of observation and the vertical circle in which the axis of the needle lies—that is, the vertical circle paasing•through the magnetic poles—is called the Declination or Varia timi. Thus, let a b represent a magnet freely suspended from its centre of gravity at any place r, whose zenith is z. Let za nit represent the meridian of the place, it at K the horizon, z N'ts the magnetic meridian, or the great circle In which the axis of the needle lies : a and a' the true north pole and magnetic north pole respectively. Then the magnet a b is forced from a horizontal position into the position le ab by the intensity of the latter's magnetism. The angle a' a r as is the inclination : and the angle between the circles z x z sd, which is measured by the angle H P nt, is the declination.
The first object of the magnetic observer is to • determine the absolute values of these magnetic elements, and then to trace the laws which regulate their hourly, daily, annual and secular changes.