METAL SURFACES AND LIQUIDS IN CONTACT WITH THEM. 
71 
tank, then, if the velocity is sufficiently low, this extends in a straight line down 
the tube. (This method of showing the stream-lines was used by Professor PlEYNOLDS 
in his experiments,* and has been fully described.) If now the flame of a Bunsen be 
applied to the outside of the glass pipe, it will be noticed that the streak of colour 
soon begins to flicker, and finally breaks up into eddies, and this for a very small 
rise of temperature of the water. 
This experiment seems to explain the equality of temperatures observed in tire 
case of the water taken from the heated copper tubes. It was found, however, that 
experiments made at velocities below the critical value were unreliable, owing to 
the rapid alternations between steady and unsteady motion which went on in 
the pipe. This was shown in the following way. When the velocity was above the 
critical value, then for the given temperature of the surface, the final • temperature 
of the water was perfectly steady. If now the velocity was reduced until near 
its critical value, it was observed that the mercury in the thermometer stem 
suddenly began to oscillate rapidly through a range of one, or so)netimes two 
deo-rees, and that the time of these oscillations was about two seconds. 
The table shows at what velocity the unsteadiness comes in. 
Diameter of 
pipe. 
Temperature 
of surface. 
Initial 
Temperature 
of water. 
Final 
Temperature 
of water. 
Velocity. 
In centiins. per 
.second. 
1-39 
39-6 
18-00 
‘22-6 
G9-0 
1-39 
39-6 
18-00 
2-2-75 
58-1 
1-39 
39-6 
18-00 
22-91 
43-0 
1-39 
.39-G 
18-00 
23-2—23-15 
28-6 
(Unsteadine.s.s 
he^innino;.) 
1-.39 
39-6 
18-00 
23-4—23-8 
18-00 
(Mercury oscil 
lating rapidly.) 
Now assuming that the critical velocity for the pipe used is given by 
P 
278D 
Vc = metres per second. 
then for a temperature of 23^ 
V, = 0T37, 
so that the unsteadiness observed from the thermometer readings becomes marked at 
about a velocity of 
1-3 V, to 1*4 V,. 
* ‘Phil. Trans.,’ 1883, p. 94‘2. 
