Source of Terrestrial Magnetism. 1 7 



space occupied by the atoms. Hence, if we supposed this quan- 

 tity to flow in that interval in a uniform concentrated stream 

 through a circular aperture of radius equal to that of the earth, 

 it might readily be shown that the ratio of its velocity to the 

 velocity of the earth is two thirds of the ratio of the space occupied 

 by the atoms to the earth's volume. This ratio, by hypothesis, 

 is extremely small ; and therefore it would not be contradictory 

 to the theory to find that but little magnetic effect is due to 

 setherial currents at the earth's surface produced by the orbital 

 motion. 



The motions resulting from the composition of all those 

 severally impressed by the atoms will evidently be symmetrical 

 with respect to an axis passing through the earth's centre in the 

 direction of the motion, and fulfil at the same time the condi- 

 tion that just as much fluid enters at the following hemispherical 

 surface as issues at the preceding one. The total motion re- 

 lative to the position of the earth's centre at any time will con- 

 sequently be a circulating motion ; and a plane through the centre 

 transverse to the direction of the earth's motion will be crossed 

 by the refluent streams at right angles. The disturbances of a 

 magnet's declination by these currents would have two maxima 

 and two minima in the course of a revolution of the earth 

 about its axis. Like effect would result from the earth's motion 

 of translation due to the motion of the solar system in space. 



But no variations of magnetic declination subject to a re- 

 gular law of this kind have been detected by observation, ex- 

 cepting one of very small amount traceable to the influence of 

 the moon. It follows, therefore, that experience confirms the 

 foregoing theoretical inference, that the currents generated by 

 the earth's motions of translation have little or no magnetic 

 effect. 



It will appear from a consideration of the foregoing argu- 

 ment that the earth, conceived to consist of discrete atoms, is 

 incapable, by reason of the resistance arising from the inertia 

 of the unlimited mass of the aether, to impress either in its 

 interior or its immediate neighbourhood, motions whereby any 

 portion of the sether could be permanently transported across a 

 given fixed plane. The only possible motions are those neces- 

 sary for vacating and filling up the spaces successively occupied 

 by the atoms in their onward course, which motions, as we have 

 seen, are not recognizable by any magnetic effects, and may 

 therefore be supposed to be extremely small compared with the 

 earth's velocity. The case is otherwise with respect to the 

 streams due to the earth's rotation, the generation of which 

 may be conceived of as follows. 



As the resultants of the movements immediately impressed on 

 Phil. Mag. S. 4. Vol. 47. No. 309. Jan. 1874. * C 



