48 PRESIDENTS ADDRESS—SECTION A. 
4. Forces arising from discontinuity in the currents (in the 
general theory, but not in Maxwell’s). 
We next turn to the means that have been discovered of dis- 
criminating, experimentally, between the theories. 
Let us take in order the points of difference that have been 
enumerated above. 
With respect to (1), all we can say is that in Hertz’s experi- 
ments no trace of a normal wave has been discovered. 
As to (2), I have already stated that as far as the limited 
accuracy of the experiments can prove it, the velocity of propa- 
gation is that which is or may be given by Maxwell’s theory. 
With respect to (3), there is, I think, no direct evidence, and 
forces indicated in (4) have not been observed, either through 
dearth of experiment, imperfection of apparatus, or because they 
do not exist. Several indirect means have been discovered by 
Thomson of testing the theories. The first is that in the case of 
a wire connected to the third plate of a vibrator system, the rate 
of decay of the energy of the waves in the wire should, by 
Maxwell’s theory, be nearly proportional to the specific resistance 
of the wires when the frequency is high enough. An experiment 
has been made by Thomson by which this is shown to be really 
the case. On Helmholtz’s or the general theory, and with the 
frequencies of vibration used, the conductivity should not have 
made much difference. All the evidence therefore points in one 
direction. There remains but the outstanding difference dis- 
covered by Hertz in the wave length of the oscillations in wire 
and in air. With respect to this Thomson has suggested what 
may very likely turn out to be the cause of the discrepancy 
between his observations and Hertz’s. It is that if the third 
plate is accidentally displaced in any way during the course of 
the experiments then there will probably result a change of 
capacity, due to the presence of other conductors (say a wall) 
which would account for the difference observed. To sum up, it 
is perhaps not too much to say that the evidence in favour of 
Maxwell’s theory is overwhelming, and that for the future it 
must be our guide in all electrical investigation. I ought not to 
leave this part of the subject without referring to the mathe- 
matical work of Heaviside, who restates Maxwell’s theory, and 
whose mathematical discoveries of the detailed behaviour of 
electro-magnetic waves has far outrun any knowledge of them 
deduced from experiment. The sideway motion of energy insisted 
on by Poynting was also indicated on rather different grounds by 
Heaviside. It remains for me to make some remarks concerning 
the ultimate nature of the mechanism by which Maxwell’s 
stresses are produced. The thing which, perhaps, strikes one 
most strongly about the theory is that it offers no suggestion as 
to the way in which electrification is brought about. Why 
should the ether in the neighbourhood of a bit of sealing-wax 
