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itself. The air, instead of rising in bubbles to the surface, 
ranged itself in two long horizontal columns behind the vane. 
There was evidence of rotational motion about these air lines. 
It was evident, in fact, that they were the central lines of two 
systematic eddies. 
That there should be eddies was not surprising, but eddies 
had always been looked upon as necessary evils which beset 
fluid motion as sources of disturbance, whereas here they appeared 
to be the very means of systematic motion. 
Here then was the explanation of the nature of the motion 
caused by the oblique vane, a cylindrical band of vortices con- 
tinually produced at the front of the plate, and falling away 
behind it in an oblique direction. 
The recognition of the vortex action caused behind the oblique 
vane, suggested that there might be similar vortices behind a 
disc moving flatwise through the water, such as are the eddies 
caused by a teaspoon. 
There was one consideration, however, which at first seemed 
to render this improbable. It was obvious that the resistance 
of the oblique vane was caused in producing the vortices at its 
forward part ; so that if a vortex were formed behind a flat 
plate, as this vortex would remain permanently behind, and not 
have to be continually elongated, the resistance should diminish 
after the plate was once set in motion ; whereas experience ap- 
peared to show that this was by no means the case. It appeared 
probable, therefore, that from some disturbing cause the vortex 
would not form, or would only form imperfectly, behind the 
plate. 
This view was strengthened when, on trying the resistance of 
a flat plate, it did not appear to diminish after the plate had 
been started. 
Accidentally, however, it was found that if the float to which 
the plate was attached was started suddenly and then released, 
the float and plate would move on apparently without any re- 
sistance. And more than this, for if the float were suddenly 
arrested and released, it would take up its motion again, show- 
ing that it was the water behind that was carrying it on. 
There was evidence therefore of a vortex behind the disc. In 
the hope of rendering this motion visible, coloured water was 
injected in the neighbourhood of the disc, and then a beautiful 
vortex ring, exactly resembling the smoke ring, was seen to form 
behind the disc. If the float were released 4n time, this ring 
would carry the disc on with it ; but if the speed of the disc 
were maintained uniform, the ring gradually dropped behind 
and broke up. Here then was another part played by the vortex 
previously undreamt of. 
That the vortex takes a systematic part in almost every form 
