86 



Mr. 0. Reynolds. 



[Mar. 15 r 



several years ago it seemed to me that a careful examination as to the 

 connexion between these four leading features, together with the 

 circumstances on which they severally depend, was the most likely 

 means of finding the clue to the principles overlooked. 



4. Space and Velocity. — The definite association of resistance as the 

 square of the velocity with sensibly large tubes and high velocities r 

 and of resistance as the velocity with capillary tubes and slow veloci- 

 ties, seemed to be evidence of the very general and important 

 influence of some properties of fluids not recognised in the theory of 

 hydrodynamics. 



As there is no such thing as absolute space or absolute time 

 recognised in mechanical philosophy, to suppose that the character of 

 motion of fluids in any way depended on absolute size or absolute 

 velocity would be to suppose such motion outside the pale of the laws 

 of motion. If, then, fluids, in their motions, are subject to these laws, 

 what appears to be the dependence of the character of the motion on 

 the absolute size of the tube and on the absolute velocity of the im- 

 mersed body must in reality be a dependence on the size of the tube 

 as compared with the size of some other object, and on the velocity of 

 the body as compared with some other velocity. What is the standard 

 object and what the standard velocity which come into comparison 

 with the size of the tube and the velocity of an immersed body, are 

 questions to which the answers were not obvious. Answers, however, 

 were found in the discovery of a circumstance on which sinuous 

 motion depends. 



5. The Effect of Viscosity on the Character of Fluid Motion. — The 

 small evidence which clear water shows as to the existence of internal 

 eddies, not less than the difficulty of estimating the viscous nature of 

 the fluid, appears to have hitherto obscured the very important 

 circumstance that the more viscous a fluid is the less prone is it h' 

 eddying or sinuous motion. To express this definitely, if jjl is the 



viscosity and p the density of the fluid, for water ^ diminishes 



P 



rapidly as the temperature rises ; thus at 5° C. - is double what it is 



P 



at 45° C. What I observed was that the tendency of water to eddy 

 becomes much greater as the temperature rises. 



Hence, connecting the change in the law of resistance with the birth 

 and development of eddies, this discovery limited further search for 

 the standard distance and standard velocity to the physical properties 

 of the fluid. 



To follow the line of this search would be to enter upon a molecular 

 theory of liquids, and this is beyond my present purpose. It is suffi- 

 cient here to notice the well known fact that — 



P 



