652 ME. CLEEK ATAXWELL ON A COMPAEISON OF THE ELECTEIC UNITS 
with which their investigation was conducted, that I am obliged to attribute the differ- 
ence of their result from mine to a phenomenon the nature of which is now much better 
understood than when their experiments were made. 
On the other hand, the result of present experiments depends on the accuracy of the 
experiments of the Committee of the British Association on Electric Resistance. The 
B. A. unit is about 8'8 per cent, larger than that determined by Weber in 1862, 
and about 1-2 per cent, less than that derived by Dr. Joule from his experiments on 
the dynamical equivalent of heat by comparing the heating effects of direct mechanical 
agitation with those of electric, currents. 
I believe that Sir William Thomson’s experiments, not yet published, give a value of 
v not very different from mine. His method, I believe, also depends on the value of 
the B. A. unit. 
The lowest estimate of the velocity of light, that of the late M. Foucault, is 
298 000 000 metres per second. 
Note on the Electromagnetic Theory of Light. 
In a paper on the Electromagnetic Field* some years ago, I laid before the Royal 
Society the reasons which led me to believe that light is an electromagnetic pheno- 
menon, the laws of which can be deduced from those of electricity and magnetism, on 
the theory that all these phenomena are affections of one and the same medium. Two 
papers appeared in Poggendorff’s ‘Annalen’ for 1867 bearing on the same subject. 
The first, by the late eminent mathematician Bernhard Riemann, was presented in 
1858 to the Royal Society of Gottingen, but was withdrawn before publication, and 
remained unknown till last year. Riemann shows that if for Laplace’s equation we sub- 
stitute 
"-* ! T+.'trj=0, (13) 
V being the electrostatic potential, and a a velocity, the results will agree with known 
phenomena in all parts of electrical science. This equation is equivalent to a statement 
that the potential V is propagated through space with a certain velocity. The author, 
however, seems to avoid making explicit mention of any medium through which the 
propagation takes place, but he shows that this velocity is nearly, if not absolutely, equal 
to the known velocity of light. 
The second paper, by M. Lorenz, shows that, on Weber’s theory, periodic electric 
disturbances would be propagated with a velocity equal to that of light. The propaga- 
tion of attraction through space forms part of this hypothesis also, though the medium 
is not explicitly recognized. 
From the assumptions of both these papers we may draw the conclusions, first, that 
action and reaction are not always equal and opposite, and second, that apparatus may 
be constructed to generate any amount of work from its own resources. 
* Philosophical Transactions, 1865, p. 459. 
