130 Professor Osborne Beynolds [June 2, 



resulting from heat, as the motions of the molecules of a gas, in 

 illustration of which I may mention the motions of individuals in a 

 crowd, and illustrate by the motion of the grains in this bottle when it 

 is shaken, during which the white grains at the top gradually mingle 

 with the black ones at the bottom — which interdiffusion takes an 

 important part in the method of coloured bands. 



Now of these three classes of motion that of the individual body 

 is incomparably the simplest. Yet, as presented in the 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 gravi- 

 tation, and developed the theory of dynamics. 



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

 winds, and the motions of clouds, had excited the philosophical 

 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 development 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 fluid 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 interpreted only in 

 the simplest cases. 



To illustrate the difference between the interpretation of the 

 theory of the heavenly bodies and that of fluid motion, I w r ould 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 certainty 

 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 holding the string you naturally suppose 

 that when I let go it will turn down so as to hang with the string 

 vertical ; it does not do so, that is a matter of surprise ; 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 pen- 

 dulum behaved like this ? Why is its motion peculiar ? There is 



