10 PROF. OSBORNE REYNOLDS ON THE 



end of the tube^ tlie gauge was again erected ; the bubble 

 immediately began to move back^ but so slowly that it was 

 some seconds before it entered the region of no pressure. 

 During this interval the mercury remained up to the end 

 of the tube; but the bubble^ as soon it neared the top of 

 the tubcj expanded and rapidly rose to the top of the tube^ 

 leaving the column at 29 inches. This operation having 

 been repeated several times, it became quite evident that it 

 ■was this small bubble which, either by rising up the tube 

 or being generated at the top, had caused the mercury in 

 the first instance to sink. As the bubble would not pass 

 out by itself, the tube was tilted so as to allow a larger 

 bubble of air to enter ; and having been left standing for 

 about twelve hours to allow the small bubble to unite with 

 the larger one, it was again tilted so as to allow the air to 

 pass out. When this was done the mercury again remained 

 firmly against the end of the tube and did not descend 

 when violently shaken. The open end of the tube was then 

 connected with an air-pump and exhausted until the pres- 

 sure within it fell to about four inches of mercury. This 

 operation occupied some seconds; but all this time the 

 mercury did not move from the end of the tube; but 

 eventually the column opened near the bottom of the tube 

 and a large bubble appeared, which rose up the tube, the 

 the mercury falling past the opening. That the breaking 

 of the column so near the bottom of the tube was owing 

 to the presence at that point of a small bubble of air was 

 almost proved by the fact that, on readmitting the air to 

 the open end of the tube and inclining the tube to see 

 if it was free from air, there was found a minute bubble 

 which played exactly the same part as the small bubble 

 which had been previously examined. 



At the instant previous to the rupture of the column at 

 the bottom of the tube, there must at the top of the tube 



