157 



TERRESTRIAL MAGNETISM. 



TERRESTRIAL MAGNETISM. 



168 



magnet and clipping it in iron filings, when the filings will be found to 

 arrange themselves near the poles of the magnet, presenting the appear- 

 ance as shown in the following figures. 



Whether the magnetic condition of our earth has undergone any 

 recent change we cannot say ; but it is remarkable that we can find no 

 very early accounts of the phenomenon we have been describing. It 

 Aurora Borealis. 



certainly appears to have been legs frequent before the 18th century 

 than it has been since. Torfceus, who wrote in 1 706, relates that even in 

 his own time the aurora was an object of terror in Iceland. For some 

 time it wax thought that there was no aurora australis, or terrestrial 

 lights in the south ; and though the fact is now well established, yet 

 the earliest account we have of Buch an occurrence is given by Don 

 Antonio Ulloa, who saw it at Cape Horn in 1745. 



From the writings of Aristotle, Cicero, Pliny, and others, we read of 

 appearances in the heavens which we conclude to have been aurone, 

 and which were viewed with the same superstition as comets. 



For further information on this subject, the reader is referred to the 

 writings of M. de Hairau (1754); Dalton ; Humboldt's 'Cosmo*;' 

 H alley ; Forbes ; Expeditions of Parry, Franklin, Richardson, and 

 Fd'Milerson ; Kaemtz's 'Complete Course of Meteorology' (I.ond., 

 1845); Reports of the British Association for the Advancement of 

 Science ; and the Philosophical Transactions of the Royal Society. 



TKHKKSTRIAL 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. The unknown influence 

 which produces these phenomena is derived from the earth itaelf, 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 baa 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 -in 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. 



I'ntil the year 1828, 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 

 "f what are called " the magnetic elements," and the periodical 

 .-> they undergo. The observations thus made, having been 

 carefully corrected and reduced, are printed with such comments aa 

 seem necessary, and a description of the instruments and methods 

 red; and these enormous collections of facts are widely dis- 

 tributed throughout the scientific world under the title of Magnetical 

 Observations. 



There arc tli ree magnetic] elements - namely, the Intensity, the Iiiclina- 



If a magnetic needle be freely suspended from it centre of gravity, one 

 extremity is drawn from the horizontal position by a force called the 



Aurora Australia. 



Magnetic Intensity, and the direction of this force is inclined to the 

 horizon at an angle called the 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 littte 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 

 passingithrough the magnetic poles is called the Declination or Varia- 

 tion. Thus, let a 6 represent a magnet freely suspended from its centre 



of gravity at any place P, whose zenith is /.. Let z u n K represent the 

 meridian of the place, H M K the horizon, 7. N' M the magnetic meridian, 

 or the great circle in which the axis of the needle lies : N and N' the 

 true north pole and magnetic north pole respectively. Then the 

 magnet a 6 is forced from a horizontal position into the position V a 6 

 by the intensity of the latter's magnetism. The angle N' a, f M is the 

 inclination : and the angle between the circles z N H, / u' M, which is 

 measured by the angle H p M, 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. 



It would be a difficult matter to construct an instrument which 

 should at once give the absolute value of the inlensit;/, and its periodical 

 changes ; but this object is readily attained by the application of the 

 principle of the resolution of forces. For let F be the absolute value 

 of the magnetic intensity, x and T its horizontal and vertical com- 

 ponents, 9 the inclination or dip : 



Then F = x sec 9 

 and T = X tan 9 



Now, as we shall see, there is no difficulty in determining the 

 horizontal component x, and the inclination 9 : hence, also, the vertical 

 component Y and the total force f may be found, 



