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Society, on the auroral displays, between which and terres- 

 trial magnetism he traced a connection. 



This, as a theory, however, has been known since the 

 time of Halley, who suggested that the ])henomenon might 

 be due to the' passage of magnetism, from one magnetic pole 

 to the other. 



Although electro-magnetic power is so strongly developed 

 in the universe, science has not as yet discovered the office it 

 holds, or the part it plays iu the movements so well known 

 to be influenced by it. The dynamical theory, according to 

 Faraday and Professor William Thompson, rests on no physical 

 hypothesis whatever, but on the fact that forces emanate from 

 the poles of magnets in certain directions, which are called 

 lines of force, and occupy a magnetic field. If any body is 

 plunged \vitbiu this magnetic field, it disturbs or modifies these 

 Hues of force according to its nature. If magnetic it concen- 

 trates the lines, or draws them towards itself ; if diamagnetic 

 it causes them to diverge — thus originating attractive move- 

 ments for magnetic bodies, and repulsive ones for those that 

 are diamagnetic. 



These two forces when applied to the sun, in the same way 

 that we apply the term " gravity," are each capable of pro- 

 ducing the same necessary consequences, both varying accord- 

 ing to the inverse square of the distance, a property belonging 

 to the law of nature, and which law is capable of being 

 expressed by other terms better known to possess forces than 

 the one in present use. The law of universal gravitation has 

 this inestimable advantage — it may be reduced to calculation, 

 and by a comparison of the results with observation, gives 

 a certain method of verifying the existence of some such 

 force. 



To use the words of Laplace, and follow this motion from 

 the departure of a planet on its perihelion passage, " the 

 velocity is then at its maximum, and its tendency to recede 

 from the sun surpassing its gravity towards it, the planet's 

 radius vector augments and forms an obtuse angle with the direc- 

 tion of its motion. The force of gravity towards the sun, 

 decomposed according to this direction, continually diminishes 

 the velocity of the planet till it arrives at its aphelion. At 

 this point its velocity is at a niitiimum, and its tendency to 

 recede from the sun being less than its gravity towards it, the 

 planet will approach it, describing the second part of its 

 ellipse. In this part the gravity towards the sun increases its 

 velocity in the same manner as it before decreased it, and the 

 planet will arrive at its perihelion again with its primitive 

 velocity and re-commence a new revolution as before. It is 



