44 Professor Osborne Beynolds [March 28, 



WEEKLY EVENING MEETING, 



Friday, March 28, 1884. 



Sir Frederick Pollock, Bart. M.A. Vice-President, in the Chair. 



Professor Osborne Reynolds, M.A. F.R.S. 



The Two Manners of Motion of Water. 



In commencing this discourse the author said : — 



It has long been a matter of very general regret with those who 

 are interested in natural jDhilosophy, that in spite of the most strenuous 

 efforts of the ablest mathematicians the theory of fluid motion fits 

 very ill with the actual behaviour of fluids ; and this for unexplained 

 reasons. The theory itself appears to be very tolerably complete 

 and affords the means of calculating the results to be expected in 

 almost every case of fluid motion, but while in many cases the 

 theoretical results agree with those actually obtained, in other cases 

 they arc altogether different. 



If we take a small body such as a raindrop moving through the 

 air, the theory gives us the true law of resistance ; but if we take a 

 large body such as a ship moving through the water, the theoretical 

 law of resistance is altogether out. And what is the most unsatis- 

 factory part of the matter is that the theory affords no clue to the 

 reason why it should apply to the one class more than the other. 



When, seven years ago, I had the honour of lecturing in this room 

 on the then novel subject of vortex motion, I ventured to insist that the 

 reason why such ill success had attended our theoretical efforts was 

 because, owing to the uniform clearness or opacity of water and air, 

 we can see nothing of the internal motion ; and while exhibiting 

 the phenomena of vortex rings in water rendered strikingly a^Dparent 

 by partially colouring the water, but otherwise as strikingly invisible, 

 I ventured to predict that the more general application of this method, 

 which I may call the method of colour-bands, would reveal clues 

 to those mysteries of fluid motion which had baffled philosophy. 



To-night I venture to claim what is at all events a partial verifi- 

 cation of that prediction. The fact that we can see as far into fluids 

 as into solids naturally raises the question why the same success 

 should not have been obtained in the case of the theory of fluids as 

 in that of solids? The answer is plain enough. As a rule, there 

 is no internal motion in solid bodies ; and hence our theory based 

 on the assumption of relative internal rest applies to all cases. It 

 is not, however, impossible that an, at all events seemingly, solid 

 body should have internal motion, and a simple experiment will show 



