ON PHYSICAL LINES OF FORCE. 467 



PAET II. 

 THE THEORY OF MOLECULAR VORTICES APPLIED TO ELECTRIC CURRENTS. 



We have already shewn that all the forces acting between magnets, sub- 

 stances capable of magnetic induction, and electric currents, may be mechanically 

 accounted for on the supposition that the surrounding medium is put into such 

 a state that at every point the pressures are different in different directions, 

 the direction of least pressure being that of the observed lines of force, and 

 the difference of greatest and least pressures being proportional to the square 

 of the intensity of the force at that point. 



Such a state of stress, if assumed to exist in the medium, and to be 

 arranged according to the known laws regulating lines of force, will act upon 

 the magnets, currents, &c. in the field with precisely the same resultant forces 

 as those calculated on the ordinary hypothesis of direct action at a distance. 

 This is true independently of any particular theory as to the cause of this 

 state of stress, or the mode in which it can be sustained in the medium. We 

 have therefore a satisfactory answer to the question, " Is there any mechanical 

 hypothesis as to the condition of the medium indicated by lines of force, by 

 which the observed resultant forces may be accounted for ? " The answer is, 

 the lines of force indicate the direction of minimum pressure at every point of 

 the medium. 



The second question must be, "What is the mechanical cause of this 

 difference of pressure in different directions ? " We have supposed, in the first 

 part of this paper, that this difference of pressures is caused by molecular 

 vortices, having their axes parallel to the lines of force. 



We also assumed, perfectly arbitrarily, that the direction of these vortices 

 is such that, on looking along a line of force from south to north, we should 

 see the vortices revolving in the direction of the hands of a watch. 



We found that the velocity of the circumference of each vortex must be 

 proportional to the intensity of the magnetic force, and that the density of 

 the substance of the vortex must be proportional to the capacity of the medium 

 for magnetic induction. 



We have as yet given no answers to the questions, " How are these vortices 

 set in rotation ? " and " Why are they arranged according to the known laws 



592 



