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



week was required for the tank to cool down, during which time 

 successive readings were made. No two determinations were ever 

 made with one setting of the valve, but in every case a re-determina- 

 tion of the critical velocity was made. 



It might be questioned whether the presence of the thermometer 

 bulb in the outflowing stream could have produced a disturbing influence, 

 but in no case was the bulb placed inside the tube under test. We 

 think that our very high values obtained in many cases for the critical 

 velocity show conclusively that the bulb had no influence, and especially 

 in the tubes we used, which were all considerably larger than the 

 thermometer bulb. 



In order to satisfy ourselves on many points in connection with our 

 measurements, we made some tests to determine the effect of a change 

 of length on the critical velocity. To do this we first took a pipe 

 3*6 metres long and 1*95 cm. in diameter, and determined the critical 

 velocity by moving the temperature jacket to the end where the glass 

 prolongation began. We obtained the following readings : 



Temp. Vc in ft. per sec. Y c in metres per see. 

 16-0 3-79 1-155 



16-0 3-82 1-163 



16-1 3-66 1-115 



16-1 3-92 1-192 



Mean 16'05 3-80 1-158 



The tube was then removed and cut in the middle, so as to be only 

 1 -8 metres long, and the following readings were obtained : 



Temp. Yc in ft. per sec. Y c in metres per sec. 

 15-4 3-66 1-115 



16-7 3-84 1-170 



16-7 3-82 1-163 



Mean 16-2 3-77 1-148 



The agreement is really better than the individual observations. 

 For each of these readings the flow was closed off and then opened 

 carefully until the jump took place on the thermometer. The readings 

 could have been repeated to any desired accuracy of course by leaving 

 the flow unchanged. 



The formula for calculating the critical velocity which Reynolds 

 deduced from his experiments for the upper limit reads : 



V - 1 P 



^ I^TD' 



where D is the diameter of the pipe in metres, and P is a function of 



