46 
PROFESSOR TYNDALL ON THE DIAxMAGNETIC FORCE, ETC. 
product of into the perpendicular drawn from the axis of rotation upon the direc- 
tion of the force. Setting this distance —d, we have the moment of (p in the direction 
stated, 
The end h of the needle is repelled by the earth’s magnetic pole with a force (p' : call- 
ing the distance of the direction of this latter force from the axis of rotation d', we 
have the moment of (p' in a direction opposed to the arrow, 
=(p'd'. 
Now as the length of the needle may be considered a vanishing quantity, as com- 
pared with its distance from the terrestrial pole, we have practically 
(p=(p', 
and consequently <pd<ip'd'. 
The tendency to turn the lever in a direction opposed to the arrow is therefore pre- 
dominant ; the lever will obey this tendency and move until the needle finds itself in 
the magnetic meridian : when this position is attained, the predominance spoken of 
evidently ceases, and the system will be in equilibrium. Experiment perfectly cor- 
roborates this theoretic deduction. 
In this case, the centre of gravity of the needle recedes from the north magnetic 
pole as if it were repelled by the latter ; but it is evident that the recession is not due 
either to the attraction or repulsion of the needle considered as a whole, but simply 
to the mechanical advantage possessed by the force (p', on account of its greater 
distance from the axis of rotation. If the force acting upon every particle of the 
needle were purely attractive, it is evident that no such recession could take place. 
Supposing, then, that we were simply acquainted with the fact, that the end f the 
needle is attracted by the terrestrial pole, and that we were wholly ignorant of the 
action of the said pole upon the end h, the experiment here described would lead us 
infallibly to the conclusion that the end h must be repelled. For if it were attracted, 
or even if it were neither attracted nor repelled, the motion of the bar must be towards 
the pole N instead of in the opposite direction. 
Let us apply this reasoning to the experiment with the bismuth prisms already 
described. The motion of the magnetic needle in the case referred to is not more 
inexplicable, on the assumption of a purely attractive force, than is the motion of our 
rectangular box on the assumption of a purely repulsive one ; and if the above expe- 
riment would lead to the conclusion that the end h of the magnetic needle is repelled, 
the experiment with the bismuth leads equally to the conclusion that the end f the 
prism hf, fig. 2, must be attracted by the pole N. The assumption of such an attrac- 
tion, or in other words, of diamagnetic polarity, is alone capable of explaining the 
effect, and the explanation which it offers is perfect. 
On the hypothesis of diamagnetic polarity, the prism hf turns a hostile end h to 
the magnetic pole N and a friendly pole f away from it. Let the repulsive force 
