Like all other forces, this force of magnetism is amenable to mechanical laws; and, knowing the direction and magnitude of the force, we can predict its action.  Placing a small magnetic needle near a bar magnet, it takes a determinate position.  That position might be deduced theoretically from the mutual action of the poles.  Moving the needle round the magnet, for each point of the surrounding space there is a definite direction of the needle and no other.  A needle of iron will answer as well as the magnetic needle; for the needle of iron is magnetized by the magnet, and acts exactly like a steel needle independently magnetized.

If we place two or more needles of iron near the magnet, the action becomes more complex, for then the needles are not only acted on by the magnet, but they act upon each other.  And if we pass to smaller masses of iron—­to iron filings, for example—­we find that they act substantially as the needles, arranging themselves in definite forms, in obedience to the magnetic action.

Placing a sheet of paper or glass over a bar magnet and showering iron filings upon the paper, I notice a tendency of the filings to arrange themselves in determinate lines.  They cannot freely follow this tendency, for they are hampered by the friction against the paper.  They are helped by tapping the paper; each tap releasing them for a moment, and enabling them to follow their tendencies.  But this is an experiment which can only be seen by myself.  To enable you all to see it, I take a pair of small magnets and by a simple optical arrangement throw the magnified images of the magnets upon the screen.  Scattering iron filings over the glass plate to which the small magnets are attached, and tapping the plate, you see the arrangement of the iron filings in those magnetic curves which have been so long familiar to scientific men (fig. 23).

[Illustration:  Fig. 23.

N is the nozzle of the lamp; M a plane mirror, reflecting the beam upwards.  At P the magnets and iron filings are placed; L is a lens which forms an image of the magnets and filings; and R is a totally reflecting prism, which casts the image G upon the screen.]

(By a very ingenious device, Professor Mayer, of Hoboken, has succeeded in fixing and photographing the magnetic curves.  I am indebted to his kindness for the annexed beautiful illustration, fig. 24.)

The aspect of these curves so fascinated Faraday that the greater portion of his intellectual life was devoted to pondering over them.  He invested the space through which they run with a kind of materiality; and the probability is that the progress of science, by connecting the phenomena of magnetism with the luminiferous ether, will prove these ‘lines of force,’ as Faraday loved to call them, to represent a condition of this mysterious substratum of all radiant action.