THE MOTION OF A PERFECT LIQUID. 113 



but inipiiLsively, in order to illustrate what I have just pointed out. 

 You will see how the greater width of the color bands before and 

 behind the cylinder indicates an increase of pressure in those regions. 

 This, in a ship-shape form, accounts for the standing bow and stern 

 waves, whereas the narrowing of the bands at the sides indicates an 

 increase of velocitv and reduction of pressure, and accounts for the 

 depression of water level, with which you are doubtless familiar, at 

 the corresponding part of a ship. 



I will now take a more striking case. If, instead of a circular body, 

 we had a flat plate, the turbulent nature of the flow is evidently very 

 great, as you will see from the view (fig. 9), which is a photograph of 

 the actual flow under these conditions, made visible by ver}' fine air 

 bubbles, and showing water at rest in the clear space behind the obstacle. 



We can, however, take steps to reduce this turbulence, and you now 

 see on the second screen the flow by means of apparatus which time 

 does not permit me to describe, but which gives a slow and steady 

 motion that it would be impossible to improve upon in actual con- 

 ditions of practice, or even, I am inclined to think, by any experimental 

 method. Instead of using air to make this flow clear, we now allow 

 color to stream behind the plate, and you will see that the water still 

 refuses to flow round to the back, and spreads on either side. We 

 have so slow a velocity as not to induce vortex motion, but the inertia 

 of the particles which strike the flat plate causes them to be deflected 

 to either side, exactly as tennis balls in striking against a wall obliquely. 

 The sheet of water is so thick — that is to saj^, the parallel glass plates 

 are so far apart — that they do not enable the viscosity of the water to 

 act as a suflScient drag to prevent this taking place. 



Mathematicians, however, predicted with absolute certainty that 

 with stream-line motion, the water should flow round and meet at the 

 back, a state of things that, however slow we make the motion in the 

 present case, does not occur, owing to the effect of inertia. They have 

 drawn with equal confidence the lines along which this should take 

 place. We could either effect this result with the experiment you 

 have just seen, by using a much more viscous liquid, such as treacle, 

 or, what comes to the same thing, bringing the two sheets of glass 

 nearly close together; and the flow which you are now witnessing 

 (fig. 10) shows the result of doing this. The color bands in front of 

 the plate no longer mix at all with the general body of flow, or are 

 unstead}', as was the case in the last experiment, but flow round the 

 plate, and flow so steadily, that unless we jerk the flow of the color 

 bands, it is impossible to tell in which direction they arc actually 

 moving. It is interesting to note that where the divided central color 

 band reunites is clearly shown in the illustration. 



While I have been dealing with the stream lines of a perfect liquid 

 your minds will doubtless have turned to the lines along which mag- 

 SM 99 S 



