l62 



NA TURE 



[June 14, 1894 



phenomena of the heavens, which have ever excited the 

 greatest curiosity of mankind, it defied the attempts of all 

 philosophers for thousands of years, until Galileo discovered 

 the laws of motion of mundane matter. It was not until he 

 had done this and applied these laws to the heavenly bodies 

 that their motions received a rational explanation. Then 

 Newton, taking up Galileo's parable and completing it, found 

 that its strict application to the heavenly bodies revealed the 

 law of gravitation, and developed the theory of dynamics. 



Next to the motions of the heavenly bodies, the wave, the 

 whirlwinds, and the motions of clouds, had excited the philo- 

 sophical curiosity of mankind from the earliest time. Both 

 Galileo and Newton, as well as their followers, attempted to 

 explain these by the laws of motion, but, although the results 

 so obtained have been of the utmost importance in the develop- 

 ment of the theory of dynamics, it was not till this century that 

 any considerable advance was made in the application of this 

 theory to the explanation of lluid phenomena ; and although 

 during the last fifty years splendid work has been done, work 

 which, in respect of the mental effort involved, or the scientific 

 importance of the results, goes beyond that which resulted in 

 the discovery of Neptune, yet the circumstances of fluid motion 

 are so obscure and complex that the theory has yet been inter- 

 preted only in the simplest cases. 



To illustrate the difference between the interpretation of the 

 theory of heavenly bodies and that of fluid motion, I would call 

 your attention to the fact that solid bodies, on the behaviour of 

 which the theory of the motion of the planets is founded, move 

 as one piece, so that their motion is exactly represented by 

 the motion of their surfaces ; that they are not affected with 

 any internal disorder which may affect their general motion. 

 So surely is this the case that even those who have never heard 

 of dynamics can predict with certainly how any ordinary body 

 will behave under any ordinary circumstances, so much so that 

 any departure is a matter of surprise. Thus I have here a cube 

 of wood, to one side of which a string is attached. Now hold 

 it on one side, and you naturally s-uppose that when I let go 

 holding the siring it will turn down so as to hang with the ; 

 string vertical ; that it does not do so is a matter of surprise, j 

 I place it on the other side, and it still remains as I place it. If 

 I swing it as a pendulum it does not behave like one. 



Would Galileo have discovered the laws of motion had his 

 pendulum behaved like this? Why is its motion peculiar? I 

 There is internal motion. Of what sort ? Well, I think my 

 illustration may carry more weight if I do not tell you ; you can 

 all, I have no doubt, form a good idea. It is not fluid motion, 

 or I should feel bound to explain it. You have here an ordinary- 

 looking object which behaves in an extraordinary manner, 

 which is yet very decided and clear, to judge by the motion of 

 its surface, and from the manner of the motion I wish you to 

 judge of the cause of the observed motion. 



This is the problem presented by fluids, in which there may 

 be internal motion which has to be taken into account before 

 the motion of the surface can be explained. Vou can see no 

 more of what the motion is within a homogeneous fluid, how- 

 ever opaque or clear, than you can sec what is going on within 

 the box. Thus without colour bands the only visual clue to 

 what is going on within the fluids is the motion of their bound- 

 ing surfaces. Nor is this all ; in most cases the surfaces which 

 bound the fluid arc immovabJe. 



In the case of the wave on water the motion of the surface 

 shows that there is motion, but because the surface shows no 

 wave it does not do to infer that the fluid is at rest. 



The only surfaces of the air within this room are the surfaces 

 of the floor, walls, and objects within it. By moving the 

 objects we move the air, but how far the air is at rest you can- 

 not tell unless it is something familiar to you. 



Now I will ask you to look at these balloons. They are 

 familiar objects enough, and yet they are most sensitive anemo- 

 meters, more sen^ilive than anything else in the room ; but even 

 they do nut show any motion ; each of them forms an internal 

 bounding surface of the air. I ^cnd an aerial mtsienger to them, 

 and a small but energetic motion is seen by which it acknow- 

 ledges the message, and the same message travels through 

 the rest, as if a ;'/ioj/ touched them. It is a wave that 

 moves them. Vou do not feci if, and, but for the surfaces of 

 the air formed by the balloons, would have no notion of its 

 existence. 



In this tank of beautifully clear distilled water, I project a 

 heavy ball in from the end, and it shows the existence of the 



water by stopping almost dead within two feet. The fact that 

 it is slopped by the water, being familiar, does not raise the 

 question. Why does it slop? — a question to which, even at the 

 present day, a complete answer is not forthcoming. The question 

 is, however, suggested, and forcibly suggested, when it appears 

 that with no greater or other evidence of its e.\istence, I can 

 project a disturbance through the water which will drive this 

 small disc the whole length of the tank. 



I have now shown instances of fluid motion of which the man- 

 ner is in no way evident without colour bands, and were levealed 

 by colour bands, as I showed in this room sixteen years ago. At 

 that time I was occupied in setting before you the manners of 

 motion revealed, and I could only incidentally notice the means 

 by which this revelation was accomplished. 



.•\mongst the ordinary phenomena of motion there are many 

 which render evident the internal motion of fluids. Small 

 objects suspended in the fluid are important, and that their im- 

 portance has long been recognised is shown by the proverb — 

 straws show which way the wind blows. Bubbles in water, 

 smoke and clouds, afford the most striking phenomena, and it 

 is doubtless these that have furnished philosophers with such 

 clues as they have had. But the indications furnished by these 

 phenomena are imperfect, and, what is more important, they 

 only occur casually, and in general only under circumstances 

 of such extreme complexity that any deduction as to the elemen- 

 tary motions involved is impossible. They afford indication of 

 commotion, and perhaps of the general direction in which the 

 commotion is tending, but this is about all. 



For example, the different types of clouds ; these have .ilways 

 been noticed, and are all named. And it is certain that each 

 type of clouds is an indication of a particular type of motion 

 in the air ; but no deductions as to what definite manner of 

 motion is indicated by each type of cloud have ever been 

 published. 



Before this can be done it is necessary to reverse the problem 

 and find to what particular type of cloud a particular manner 

 of motion would give rise. Now a cloud, as we see it, does not 

 directly indicate the internal motion of which it is the result. 

 As we look at clouds, it is not in general their motion that we 

 notice, but their figure. It is hard to see that this figure changes 

 while we are watching a cloud, though such a change is con- 

 tinually going on, but is apparently very slow on account of the 

 great distance of the cloud and its great size. However, types 

 of clouds are determined by their figure, not by their motion. 

 Now what their figure shows is not motion, but it is the history 

 or result of the motion of the particular strata of the air in and 

 through surrounding strata. Hence, to interpret the figures 

 of the clouds we must study the changes in shape of fluid 

 masses, surrounded by fluid, which result from particular 

 motions. 



The ideal in the method of colour bands is to render streaks 

 or lines in definite position in the fluid visible, without in 

 any way otherwise interfering with these properties as part 

 of the homogeneous fluid. If we could by a wish create 

 coloured lines in the water, these would be ideal colour bands. 

 We cannot do this, nor can we exactly paint lines in the air or 

 water. 



I take this ladle full of highly coloured water, lower it slowly 

 into the surface of the surrounding water till that within is 

 level with that without ; then turn the ladle carefully round the 

 coloured w.ater ; the mass of coloured water will remain where 

 placed. 



I distribute the colour slowly. It does not mix with the clear 

 water, and although the lines arc irregular they stand out very 

 beautifully. Their edges are sharp here. But in this large 

 sphere, which was coloured before the lecture, although the 

 coloured lines have generally kept their pl.-ices, they have, as it 

 were, swollen out and become merged in the surrounding water 

 in consequence of molecular motion. The sphere shows, how- 

 ever, one of the rarest phenomena in nature — the internal state 

 in almost absolute internal rest. The forms resemble nothing 

 so much as stratus clouds, as seen on a summer day, though the 

 continuity of the colour bands is more marked. .\ mass of 

 coloure<l water once introduced is never broken. The discon- 

 tinuity of clouds is thus seen to be due to other causes than mere 

 motion. 



Now, having called our attention to the rarity of water at 

 rest, I will call your attention to what is apt to be a very strik- 

 ing phenomena, namely that when water is contained, like this, 

 in a spherical vessel of which you cannot alter the shape, it is 



NO. 1285, VOL. 50] 



