ON VOKTEX MOTION. 
279 
ducing them at will, and thus rendering them subjects for 
lecture-room experiments. Considering that this method will 
probably play a great part in perfecting our notions of fluid 
motion, it is an interesting question how Professor Tait came to 
hit upon it. There is only one of the accidental sources of 
these rings which bears even a faint resemblance to this box, 
and that is the mouth of a smoker as he produces these rings. 
This might have suggested the box to Professor Tait. But since 
this supposition involves the assumption that Professor Tait 
sometimes indulges in a bad habit, and as we all know that 
Professor Tait is an eminent mathematician, perhaps we ought 
rather to suppose that he was led to his discovery by some occult 
process of reasoning which his modesty has hitherto kept him 
from propounding. 
But however this may be, his discovery was a most important 
one, and by its means the study of the actual motion of these 
rings has been carried far beyond what would otherwise have been 
possible. 
But it has been for their own sake, and for such light as they 
might throw on the constitution of matter, that these rings were 
studied. The most important lesson which they were capable of 
teaching still remained unlearned. It does not appear to have 
occurred to anyone that they were evidence of a general form 
of fluid motion, or that the means by which these had been 
revealed, would reveal other forms of motion. 
There was, however, at least one exception, which will not be 
forgotten in this room : the use of smoke to show the effect of 
sound upon jets of air. 
Also, the late Mr. Henry Deacon, in 1871, showed that minute 
vortex rings might be produced in water by projecting a drop of 
coloured water from a small tube. And his experiments, in 
spite of their small scale, excited considerable interest. 
Four years ago, being engaged in investigating the action of 
the screw-propeller, and being very much struck by the differ- 
ence between some of the results he obtained and what he had 
been led to expect, the author made use of colour to try and 
explain the anomalies, when he found that the vortex played a 
part in fluid motion which he had never dreamt of ; that, in 
fact, it was the key to almost all the problems of internal fluid 
motion. That these results were equally new to those who had 
considered the subject much more deeply than he had, did not 
occur to him until after some conversation with Mr. Froude and 
Sir William Thomson. 
Having noticed that the action of the screw-propeller was 
greatly affected when air was allowed to descend to the blades, 
he was trying what influence air would have on the action of a 
simple oblique vane, when a very singular phenomenon presented 
