1904.] The Flow of Water through Pipes. 345 



the length of time required for the tank to cool down, make it 

 appear to us improbable that convection currents played an important 

 part. 



Thermal Method of Measuring Critical Velocity. 



In some of our first experiments, we observed the change in 

 temperature in the column of water at the critical velocity by noting 

 the increase in resistance of a platinum wire threaded through the 

 centre of the tube, which was heated on the outside, and our 

 preliminary results showed that the presence of a wire of 6 mils, 

 thickness in a tube of | inch in diameter was quite sufficient for our 

 purpose. We found that the point of change in flow could be observed 

 more simply by placing the bulb of a mercury thermometer in the 

 stream of water as it flowed out of the tube. A glass prolongation, of 

 slightly greater diameter and connected carefully to the brass pipe by 

 a specially constructed cone or adapter, enabled the readings on the 

 thermometer to be observed. It was a matter of interest to us to 

 see the sudden way in which the reading on the thermometer 

 indicated the point of change by an almost instantaneous change 

 of reading. 



We were fortunate in having at our disposal, through the kindness 

 of Dr. H. T. Bovey, F.R.S., the facilities offered by the hydraulic 

 laboratory, where the large experimental tank, 20 feet high and 

 25 square feet in area, served admirably for a reservoir. The tank 

 stood on the bed-rock, and was therefore free from disturbance in the 

 rest of the building, and, after the eddies occasioned by filling had 

 died out, the water was in a very quiet state. The water used for the 

 experiments was supplied from the Montreal mains and was quite 

 clear ; and every precaution was taken, by repeated cleaning, to have 

 the water and tank clean. The action of finely divided matter in 

 suspension in the water is to cause a breaking-up of the stream-lines, so 

 that it was necessary for us to avoid this form of instability. 



Our preliminary experiments, made to test the method before 

 applying it on a large scale, were quite satisfactory, and, after profiting 

 by various trials, the final apparatus took the form of that represented 

 in fig. 1. 



In one side of the large tank A, a hole was cut, in which was fitted 

 a special form of stuffing box to take the tubes of different sizes. 

 Each tube was made of brass and joined to a metal trumpet flare, 

 such that the inside was filed perfectly smooth, and free from sharp 

 points which might disturb the flow. The flare was located at about 

 the centre of the tank, so as to be as free as possible from convection 

 streams along the sides of the tank when working at high temperatures. 

 The end of the pipe protruding from the tank was inserted in a 

 cylindrical water jacket B, with ends closed by stuffing boxes similar 



