1884.] on the Tivo Manners of Motion of Water. 45 



that if a class of such bodies existed they would apparently have 

 disobeyed the laws of motion. 



These two wooden cubes are apparently just alike, each has a 

 string tied to it. Now, if a ball is suspended by a string you all 

 know that it hangs vertically below the point of suspension or swings 

 like a pendulum. You see this one does so. The other you see 

 behaves quite differently, turning up sideways. The effect is very 

 striking so long as you do not know the cause. There is a heavy 

 revolving wheel inside which makes it behave like a top. 



Now what I wish you to see is, that had such bodies been a work 

 of nature so that we could not see what was going on — if, for 



instance, apples were of this nature while pears were what they are 



the laws of motion would not have been discovered ; if discovered for 

 pears they would not have applied to apples, and so would hardly 

 have been thought satisfactory. 



Such is the case with fluids : here are two vessels of water which 

 appear exactly similar — even more so than the solids, because you 

 can see right through them — and there is nothing unreasonable in 

 supposing that the same laws of motion would apply to both vessels. 

 The application of the method of colour-bands, however, reveals a 

 secret : the water of the one is at rest, while that in the other is in 

 a high state of agitation. 



I am speaking of the two manners of motion of water — not 

 because there are only two motions possible ; looked at by their 

 general appearance the motions of water are infinite in number • but 

 what it is my object to make clear to-night is that all the various 

 phenomena of moving water may be divided into two broadly distinct 

 classes, not according to what with uniform fluids are their apparent 

 motions, but according to what are the internal motions of the fluids 

 which are invisible with clear fluids, but which become visible with 

 colour-bands. 



The phenomena to be shown will, I hope, have some interest in 

 themselves, but their intrinsic interest is as nothing compared to 

 their jDhilosophical interest. On this, however, I can but slightly touch. 



I have already pointed out that the problems of fluid-motion may 

 be divided into two classes: those in which the theoretical results 

 agree with the experimental, and those in which they are altocrether 

 different. Now what makes the recognition of the two manners of 

 internal motion of fluids so important, is that all those problems to 

 which the theory fits belong to the one class of internal motions. 



The point before us to-night is simple enough, and may be well 

 expressed by analogy. Most of us have more or less familiarity with 

 the motion of troops, and we can well understand that there exists a 

 science of military tactics which treats of the best manoeuvres and 

 evolutions to meet particular circumstances. 



Suppose this science proceeds on the assumption that the 

 discipline of the troops is perfect, and hence takes no account of such 

 moral effects as may be produced by the presence of an enemy. 



