.,50 Profs. H. T. Barnes and E. G. Coker. [Nov. 4. 



we not only passed our former limit, but were unable to make the 

 stream-lines break up until we produced an artificial disturbance by 

 giving the pipe a sharp rap. The change in flow produced by the jar 

 was seen by the jump on the thermometer reading. It returned to the 

 stream-line reading as soon as the rapping ceased. We, unfortunately, 

 did not measure the flow in this case, but we at least doubled the 

 previous value of 1*148 metres per second. Being somewhat surprised 

 by this result, we made some experiments on this same pipe by colour 

 bands, instead of the thermometer, in order to satisfy ourselves that 

 the high values we obtained were not due to some peculiarity of the 

 thermal method. After some trouble we introduced the colour tube 

 into the tank, and dispensed with the heating jacket. For this we 

 were obliged to let the water out of the tank, and introduce fresh 

 water. Owing to the disturbance thus caused, it was several days 

 before we could proceed with the experiments. It was a simpler 

 matter than we had supposed to observe the colour band, since we had 

 removed the thermometer from the glass prolongation, the thread of 

 colour could be distinctly seen issuing from the brass pipe. We 

 obtained, in these experiments, a critical velocity at the same point as 

 before, but, on opening the valve, the stream-lines re-formed at higher 

 velocities, and persisted to the highest velocity we could produce. This 

 corresponded with our thermal results, except that, in this case, we had 

 an actual re-formation of stream-lines at the higher velocities after 

 eddies had appeared at the usual point. 



In order to study the re-formation of the stream-lines more easily, 

 a glass pipe, specially made for critical velocity experiments, with a 

 flare blown on to the tube, was inserted, and a colour tube was put in 

 as before. The tube was 1-2 metres long, and 1-47 cms. in diameter. 

 No thermometer was used, or heating jacket, and the head was about 

 8 feet, as before. We found, incidentally, that by increasing the 

 quantity of colour it was possible to cause a very large disturbance in 

 the flow. 



The following readings were obtained : 



Vc in feet Vc in metres 



Temp. per sec. per sec. 



Little colour 20 C. 3-466 1-056 



20 3-366 1-026 



Excess of colour 20 1-252 -0-382 



Little colour 19 3-786 1-154 



These observations showed the importance of using little colour, as 

 a small colour stream produced practically no effect. 



Working out the value of Y c as before from the formula, we get at 

 20 C. the value 0'88 metre per second. This value is, again, much 

 less than the observed value using little colour. 



