14 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
the directive action of the fibre, though, knowing the extreme feebleness of this 
directive action, I was surprised to find it able to accomplish what the experiments 
exhibited. The thought suggested itself, however, of suspending the bars with the 
fibre vertical, in which position the latter could have no directive influence. Here 
also, to my surprise, the directive action, though slightly weakened, was the same as 
before; in the centre of the field the bars took up the axial position. Bars of sul- 
phur, wax, salt of hartshorn, and other diamagnetic substances were next examined : 
they all acted in the same manner as the wood, and thus showed that the cause of 
the rotation lay, not in the structure of the substances, but in the distribution of the 
magnetic force around them. This distribution in fact was such, that the straight 
line which connected the centre of one pole with that of the opposite one was the line 
of weakest force. Ohm represents the distribution of electricity upon the surfaces of 
conductors by regarding the tensions as ordinates, and erecting them from the points 
to which they correspond, the steepness of the curve formed by uniting the ends of 
the ordinates being the measure of the increase or diminution of tension. Taking 
the centre of the magnetic field as the origin, and drawing lines axial and equatorial ; 
if we erect the magnetic tensions along these lines, we shall find a steeper curve in 
the equatorial than in the axial direction. This may be proved by suspending a bit 
of carbonate of iron in the centre of the magnetic field ; on exciting the magnet, the 
suspended body will not move to the nearest portion of the flat pole, though it may 
be not more than a quarter of an inch distant, but will move equatorially towards the 
edges, though they may be two inches distant. The little diamagnetic bars referred 
to were therefore pushed into the axial position by the force acting with superior 
power in an equatorial direction. 
The results just described are simply due to the recession of the ends of an elon- 
gated body from places of stronger to those of weaker force; but it is extremely in- 
structive to observe how this result is modified by structure. If, for example, a plate 
of bismuth be suspended between the poles with the plane of principal cleavage ver- 
tical, the plate will assert the equatorial position from top to bottom ; and in the 
centre with almost the same force as between the edges. The cause of this lies in 
the structure of the bismuth. Its position in the field depends not so much upon the 
distribution of the magnetic force around it, as upon the direction of the force through 
it. I will not, however, anticipate matters by entering further upon this subject at 
present. 
IV. COMPARATIVE VIEW OF PARAMAGNETIC AND DIAMAGNETIC PHENOMENA. 
I . State of Diamagnetic Bodies under magnetic influence. 
When a piece of soft iron is brought near to a magnet, it is attracted by the latter: 
this attraction is not the act of the magnet alone, but results from the mutual action 
of the magnet and the body upon which it operates. The soft iron in this case is 
said to be magnetized by influence ; it becomes itself a magnet, and the intensity of 
