TERRESTRIAL MAGNETISM. 225 



COS \ - 178-744 sin u sin \, and - 925-782 R% - 89-024 R^ 

 + 178-744 R^ are the moments of terrestrial magnetism with 

 respect to the axis of the earth, and to the two radii for longi- 

 tudes and 90. In speaking of the earth's axis, the direction 

 towards the north pole is to be understood, and the negative 

 sign of the corresponding moment shows that the magnetic axis 

 makes with it an obtuse angle, or that its magnetic north pole is 

 turned towards the south. 



The direction hence found for the magnetic axis is parallel 

 to that diameter of the earth which is from 77° 50' north lat., and 

 296° 29' Ion., to 77° 50' south lat,, 116^ 29' Ion.; and the mag- 

 netic moment in relation to this axis is = 947-08 R^. It must 

 be remembered that in our elements the unity of intensity em- 

 ployed is a thousandth part of the unity in common use. In 

 order to obtain the reduction to the absolute unity establish- 

 ed in the Intensitas Vis Magneticce, we must remark that in 

 that work the horizontal intensity at Gottingen for the 19th of 

 July, 1834, was found = 1*7748, which, combined with the dip 

 68° 1', gives the total intensity = 4-7414. The total intensity, 

 according to the unity employed above, was 1357- Thus the 

 reducing factor is = 0-0034941, and the magnetic moment of the 

 earth, expressed according to the absolute unity, 

 = 3-3092 R^. 



As the millimetre is the unit of length employed in the above 

 absolute unity for the earth's magnetic force, R must also be 

 given in millimetres ; and, as the ellipticity of the earth need 

 not be taken into account, it will be sufficient to consider R as 

 the radius of a circle 40000 millions of milhmetres in circum- 

 ference. Hence the above magnetic moment will be expressed 

 by a number of which the logarithm = 29,93136, or by 853800 

 quadrillions. By experiments made in the year 1832 {Intensitas, 

 Art. 21) the magnetic moment of a magnet bar, of a pound 

 weight, was found to be, according to the same absolute unity, 

 = 1008 7 7000. The magnetic moment of the eai-th is there- 

 fore 8464 trillion times greater. Thus 8464 trillions of such 

 magnet bars, with parallel magnetic axes, would be required to 

 replace in external space the magnetic influence of the earth. 

 Supposing the magnetism of the earth to be uniformly distri- 

 buted throughout its volume, it would hence be equal to eight 

 such bars (more exactly 7'831) for evei-y cubic metre. This re- 

 sult thus enounced preserves its meaning even, if instead of 



