90 



Mr. 0. Reynolds. 



[Mar. 15, 



any evidence as to whether the birth of eddies was simultaneous with 

 the change in the law of resistance. 



These open points may be best expressed in the form of queries to 

 which the answers anticipated were in the affirmative. 



(1.) What was the exact relation between the diameters of the pipes 

 and the velocities of the water at which the law of resistance 

 changed ; was it at a certain value of 



cU? 



(2.) Did this change depend on the temperature, i.e., the viscosity of 

 water ; was it at a certain value of 



(3.) Were there eddies in parallel tubes ? 



(4.) Did steady motion hold up to a critical value and then eddies 

 come in ? 



(5.) Did the eddies come in at a certain value of 



pcU p 



(6.) Did the eddies first make their appearance as small, and then 

 increase gradually with the velocity, or did they come in suddenly ? 



The bearing of the last query may not be obvious ; but, as will 

 appear in the sequel, its importance was such that in spite of satis- 

 factory answers to all the other queries, a negative answer to this in 

 respect of one particular class of motions led to the reconsideration 

 of the supposed cause of instability and eventually to the discovery of 

 instability caused by fluid friction. 



The queries as they are put suggest two methods of experi- 

 menting :— « 



(1.) Measuring the resistances and velocities for different diameters, 

 and with different temperatures of water. 



(2.) Visual observation as to the appearance of eddies during the 

 flow of water along tubes or open channels. 



Both these methods have been adopted, but as the question relating* 

 to eddies had been the least studied the second method was the first 

 adopted. 



9. Experiments by Visual Observations. — The most important of 

 these experiments related to water moving in one direction along 

 glass tubes. Besides these, however, experiments on fluids flowing in 

 opposite directions in the same tube were made ; also a third class of 

 experiments which related to motion in a flat channel of indefinite 

 breadth. 



These last-mentioned experiments resulted from an incidental' 

 observation during some experiments made in 1876 as to the effect 



