THE LAW OF RESISTANCE IN PARALLEL CHANNELS. 
943 
eddies showed themselves were almost exactly in the inverse ratio of the diameters 
of the tubes. 
(5.) That in all the tubes the critical velocity diminished as the temperature 
increased, the range being from 5° C. to 22° C.; and the law of this diminution, so far 
as could be determined, was in accordance with Poiseuille’s experiments. Taking T 
to express degrees centigrade, then by Poiseuille’s experiments, 
p “ P=(l + 0-0336 T+0'00221 T 3 )" 1 
taking a metre as the unit, U, the critical velocity, and D the diameter of the tube, 
the law of the critical point is completely expressed by the formula 
u * — B, D 
where 
B, = 43-79 
log B, = 1-64139 
This is a complete answer to question 5. 
During the experiments many things were noticed which cannot be mentioned here, 
but two circumstances should be mentioned as emphasizing the negative answer to 
question 6. In the first place, the critical velocity was much higher than had been 
expected in pipes of such magnitude, resistance varying as the square of the velocity 
had been found at very much smaller velocities than those at which the eddies 
appeared when the water in the tank was steady; and in the second place, it was 
observed that the critical velocity was very sensitive to disturbance in the water 
before entering the tubes; and it was only by the greatest care as to the uniformity 
of the temperature of the tank and the stillness of the water that consistent results 
were obtained. This showed that the steady motion was unstable for large 
disturbances long before the critical velocity was reached, a fact which agreed with 
the full-blown manner in which the eddies appeared. 
12. Experiments with two streams in opposite directions in the same tube .—A glass 
tube, 5 feet long and 1'2 inch in diameter, having its ends slightly bent up, as shown in 
fig. 6, was half filled with bisulphide of carbon, and then filled up with water and both 
Fig. 6. 
ends corked. The bisulphide was chosen as being a limpid liquid but little heavier 
than water and completely insoluble, the surface between the two liquids being clearly 
distinguishable. When the tube was placed in a horizontal direction, the weight of 
6 e 2 
