FORCES ACTING IN MAGNETIC CIRCUITS. 199 
expression. There is all the difference that exists between a 
theory and a fact.. Everything however tends to show that the 
fact is that. the theory is probably true so far as it goes, and we 
will therefore provisionally adopt it and see first what additional 
hypotheses are necessary. It is obvious at once that the stresses 
are “stresses in a medium” while the forces are mechanical forces" 
acting on matter. We must therefore consider that the medium 
is “‘attached ” to matter so as to allow the stresses to appear as 
forces. Now the stresses in the medium depend on the nature of 
the matter which is permeated by the medium. | Thus in the cut 
anchor-ring referred to above (1), the stresses in the medium in 
the air gap 4re not at all the same as the stresses in the medium 
in the iron. In our entire ignorance of the connection existing 
between the medium and matter, it is not at all clear to me that 
in calculating the magnetic forces tending to close the ring, we 
ought to consider the stresses in ether in air, and these alone. It 
is at all events conceivable that the nature of the connection 
_ between the medium and the iron may be modified in some manner 
by the internal stresses in the iron; also the ordinary laws of 
magnetic and electro-magnetic action received their experimental 
demonstration at low inductions, and we have no right to say 
-without experimental evidence that some terms not contemplated 
by Maxwell might not begin to produce effects on the stresses in 
air at high inductions. In the parallel case in iron such stresses 
do in fact occur. I therefore attribute great importance to the 
experimental verification of the results deduced from Maxwell’s 
theory as applied to the traction between iron bars in general and 
especially at high inductions. 
5. Experimental position.—The simplest case is that of the 
traction between two plane faces of iron, the faces being either 
the terminals of an otherwise closed iron circuit, or of very long 
bars. The case of the ring has been implicitly investigated by 
Bidwell, (Phil. Proc. 1886), and the case of short bars by Bosan- 
quet explicitly (Phil. Mag. 1886). The latter is the only investi- 
gation I know of in which simultaneous observations of induction 
