ON VOKTEX MOTION. 
283 
would not move at all. What was the cause of this resistance ? 
Here were two objects of the same shape and weight, the one of 
which moved freely through the water, and the other expe- 
rienced very great resistance ? The only difference was in the 
nature of the surface. As already explained, there is no friction 
at the surface of the water, whereas there must be friction 
between the water and the solid. But it could be easily shown 
that the resistance of the solid is much greater than what is 
accounted for by its surface friction or skin resistance. The 
only other respect in which these two surfaces differ is that the 
one is flexible, while the other is rigid, and this seems to be the 
cause of the difference in resistance. 
If ribbons be attached to the edge of the disc, these ribbons 
will envelope the ball of water which follows it, presenting a 
surface which may be much greater than that of the solid ; and 
yet this, being a flexible surface, the resistance of the disc with 
the vortex behind it is not very much greater than it would be 
without the ribbons — nothing to be compared to that of the 
solid. 
Colouring the water behind the solid shows, that instead of 
passing through the water without disturbing it, there is very 
great disturbance in its wake. An interesting question is as to 
whether this disturbance originates with the motion of the solid, 
or only after the solid is in motion. This is settled by colouring 
the water immediately in front of the solid before it is started. 
Then on starting it the colour is seen to spread out in a film 
entirely over the surface of the solid, at first without the least 
disturbance, but this follows almost immediately. 
Among the most striking features of the vortex rings, is their 
apparent elasticity. When disturbed they not only recover 
their shape, but vibrate about their mean position like an 
elastic solid. So much so, as to lead Sir William Thomson to 
the idea that the elasticity of solid matter must be due to its 
being composed of vortex rings. 
But apart from such considerations, this vibration is interest- 
ing as showing that the only form of ring which can progress 
steadily is the circular. Two parallel bands, such as those 
which follow the oblique vane, could progress if they were infi- 
nitely long, but if not, they must be continually destroyed from 
the ends. Those which follow the oblique vane are continually 
dying out at one end, and being formed again at the other. 
If an oval ring be formed behind an oval plate, the more 
sharply curved parts travel faster than the flatter parts ; and 
hence, unless the plate be removed, the ring breaks up. It is 
possible, however, to withdraw the plate, so as to leave the oval 
ring, which proceeds wriggling along, each portion moving in a 
direction perpendicular to that in which it is curved, and with 
