PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
45 
Fig. 2. 
its centre of gravity O in the magnetic field, so that the two prisms were in the same 
horizontal plane. Let the position of the box thus 
suspended horizontally be that shown in fig. 2. 
For the sake of simplicity, we will confine our at- 
tention to the action of one of the poles N, which 
may be either flat or rounded, upon the prism hf 
adjacent to it, as indeed all the phenomena to be 
described can be produced before a single pole. 
The direction of the force emanating from N is 
represented by the arrows, and if this force be 
purely repulsive^ the action upon every single par- 
ticle of the diamagnetic mass furnishes a moment 
v/hich, in the position here assumed, tends to turn the rectangular box in the direc- 
tion marked by the arrow. It is perfectly impossible that such a system of forces 
could cause the box to turn in a direction opposed to the arrow ; yet this is the pre- 
cise direction in which the box turns when the magnetic force is developed. 
Here, then, we have a mechanical effect which is perfectly inexplicable on the sup- 
position that the diamagnetic force is purely repulsive. But if the conclusions arrived 
at in the foregoing memoir be correct, if the diamagnetic force be a polar force, then 
we must assume that attraction and repulsion are developed simultaneously, as in the 
case of ordinary magnetic phenomena. Let us examine how this assumption will 
affect the analysis of the experiment before us. 
The marked end of a magnetic needle is pulled towards the north magnetic pole 
of the earth ; and yet, if the needle be caused to float upon a liquid, there is no 
motion of its mass towards the terrestrial pole referred to. The reason of this is 
known to be, that the south end of the needle is repelled by a force equal to that by 
which the north, or marked end, is attracted. These two equal and opposite forces 
destroy each other as regards a motion of translation, but they are effective in pro- 
ducing a motion of rotation. The magnetic needle, indeed, when in a position oblique 
to the plane of the magnetic meridian, is solicited towards that plane by a mechani- 
cal couple, and if free to move will turn and find its position of equilibriun) there. 
Let such a needle, fh, be attached, as in fig. 3, to the end 
of a light wooden beam, vw ; let the beam and needle be 
suspended horizontally from the point a, round which the 
whole system is free to turn, the weight of the needle being 
balanced by a suitable counterpoise, w ; let the north pole 
of the earth be towards N. Supposing the beam to occupy 
a position oblique to the magnetic meridian, as in the figure, 
the end/' or the marked end, of the needle is solicited towards 
N by a force <p, and the tendency of this force to produce 
rotation in the direction of the arrow is expressed by the 
Fig. 3. 
N 
