THE 
LONDON, EDINBURGH ann DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENGE. 
[FOURTH SERIES.] 
MARCH 1861. 
XXV. On Physical Lines of Force. By J. C. Maxweut, Pro- 
fessor of Natural Philosophy in King’s College, London*. 
- Part 1—The Theory of Molecular Vortices applied to Magnetic 
Phenomena. 
i all phenomena involving attractions or repulsions, or any 
forces depending on the relative position of bodies, we have 
to determine the magnitude and direction of the force which 
would act on a given body, if placed in a given position. 
In the case of a body acted on by the gravitation of a sphere, 
this force is inversely as the square of the distance, and in a 
straight lme to the centre of the sphere. In the case of two 
attracting spheres, or of a body not spherical, the magnitude 
and direction of the force vary according to more complicated 
laws. In electric and magnetic phenomena, the magnitude and 
direction of the resultant force at any point is the main subject 
of investigation. Suppose that the direction of the force at any 
point is known, then, if we draw a line so that in every part of 
its course it coincides in direction with the force at that point, 
this line may be called a line of force, since it imdicates the 
direction of the force in every part of its course. 
By drawing a sufficient number of lines of force, we may 
indicate the direction of the force in every part of the space in 
which it acts. 
Thus if we strew iron filmgs on paper near a magnet, each 
filmg will be magnetized by induction, and the consecutive 
filings will unite by their opposite poles, so as to form fibres, 
and these fibres will indicate the direction of the lines of force. 
The beautiful illustration of the presence of magnetic force 
afforded by this experiment, naturally tends to make us think of 
* Communicated by the Author. 
Phil. Mag. 8. 4. Vol. 21. No. 189. March 1861. M 
