774 
^[R. J, LARMOR ON A DYNAMICAL THEORY OF 
and such an idea of very slightly obstructed flow might possibly be made to serve as a 
substitute for Weber’s theory, if wm are unable to retain it. [See § 114.] 
75. The motion of a material body through the aether must, in any case, either 
carry the aether with it, or else set up a backward drift of the aether through its 
substance, so that the vortex cores (which might be vacuous and therefore merely 
forms of motion) would be carried on, while the body of the aether remained at rest. 
On the first view, tlie motion of the body must produce a fleld of irrotational flow in 
the surrounclinof aether, in other words a mao-netic field. Whether this would he 
powerful enough to be directly detected depends on the order of magnitude of the 
aethereal velocities which represent ordinary magnetic forces, and thus ultimately on 
the value of the density of the aethereal medium. But if the density were small, the 
square of the velocity would be large in proportion, and the influence of magnetization 
on the velocity of liglit should be the greater ; so that on this account also the first of 
the above views must, on the ]jresent theory, be rejected. We should however expect 
an actual magnetic field like the Earth’s to affect very slightly both the velocity of 
propagation and the law of reflexion. 
70. The second view is, as we have stated, the one formulated by Eresxel, and it 
would be strongly confirmed if the velocity of light-waves were cpiite unaffected by 
passing near a moving body, so shaped that it would on the other hypothesis cause a 
current in the perfectly fluid mther; but it is sometimes held (see however § 80) to be 
against the evidence of the null result of Michelson’s experiments on the effect of 
the Earth’s motion on the velocity of transmission of light through air. 
There is also the fact noticed by Lorentz that an irrotational disturbance of the 
surrounding mther, caused by the motion of an impermeable body through it, would 
necessarily involve slip along the surface, which could not exist in our fluid medium; 
this would at first sight compel us to recognize that the surrounding aether, instead of 
flowing round a moving body, must be taken to flow through it, or rather into it, at 
any rate to such an extent as will be necessary in order to make the remaining motion 
outside it irrotational, without discontinuity at the surface. 
It has been shown however by W. M. Hicks that a solitary hollow vortex in an 
ordinary licjuid carries along with it a disc-shaped mass of fluid and not a ring-shaped 
mass, unless its section is very minute ; tlms it is jmssible tliat the vortex-aggregate 
constituting a moving solid may completely shed off the surrounding fluid without 
allowing any permeation through its substance, and without any such discontinuity at 
the surface as would be produced by the motion of an ordinary solid through liquid. 
How far the electric charges on the vortex atoms, or their combination into molecules, 
w'ould negative such a hypothesis seems a difficult inquiry. But honnver that may 
be, a consensus of various grounds seems to require the aether to be stationaiy on the 
present theory. Tlius if the motion of solids moved the surrounding aether, two 
moving solids would act on each other with a hydrodynamic forcive, which would be 
of large amount if we are compelled to assume a considerable density for the aether. 
