1904] The Flow of Water tliroiujli Pip,-*. 



had expected to see the eddies make their appearance as the velocity 

 increased, at first in a slow or feeble manner, indicating that the 

 water was but slightly unstable. And it was a matter of surprise 

 to me to see the sudden force with which the eddies sprang into 

 existence, showing a highly unstable condition to have existed at the 

 time the steady motion broke down." 



In this connection it is a matter of interest to note that when all 

 forms of initial disturbance, as well as the disturbing influence of the 

 walls of the pipe, are removed, eddy motion is no longer possible. 

 Such a condition of affairs we have in a jet of water issuing from a 

 circular orifice. It is easy to show experimentally that the beautiful 

 rod-like appearance of the water jet depends on the absolute quietness 

 of the water feeding the jet, and that the jet is immediately broken up 

 by producing eddies artificially. Jets flowing in stream-line motion 

 at a speed of from 2030 feet per second have been obtained by us 

 from a two-inch orifice. 



In our present series of experiments we arranged that our pipes 

 should feed from the same tank from which we produced the jets, in 

 order that we might be sure to have .the water in a perfectly quiet 

 state. We could, therefore, be fairly safe in assuming that the birth 

 of eddies was the result of the disturbing influence of the walls of 

 the tube only. 



Two methods of study were adopted by Reynolds in his experiments. 

 The first was the method of colour bands, which gave the point at 

 which stream-lines break up into eddies, and which we have called the 

 upper limit of critical velocity, and the second was a method of 

 pressures, where the relation between velocity and pressure was 

 obtained above and below the critical velocity. In the colour-band 

 tests a long narrow tank was used, in which the pipe was placed. A 

 flare, carefully smoothed, was fastened to the inflow end, while the 

 outflow end protruded out of the tank and connected with the waste 

 pipe. A small tube containing coloured liquid was directed just 

 in front of the flare and a thin stream of colour was drawn into the 

 tube with the water. This thread of colour remained intact all along 

 the tube for stream-line motion, but disappeared with great suddenness 

 when the critical velocity was reached. The water in the tank was 

 allowed to become as quiet as possible before beginning the experiment. 

 The rate of flow was calculated from the rate of lowering of the water 

 surface, and the dimensions of the pipe. In the pressure experiments 

 the water was allowed to flow directly from the mains into the pipe. 

 After a sufficient length of pipe to allow of the dying out of the eddies, 

 pressure gauges were attached at a fixed distance apart, and readings 

 of velocity and pressure made. The critical velocity was riot observed 

 directly by this method, but since the resistance changes with the 

 change in the flow, from the first power of the velocity to about the 



