MR. A. MALLOCK ON EXPERIMENTS ON FLUID VISCOSITY. 
45 
Only a few experiments were made with this arrangement, as the motion of the fluid 
was eddying and unstable, even at very low velocities. 
Diagram 10 gives the coefficient of viscosity in terms of temperature, as deduced 
from the experiment of series (1) and (3). With these results, for the sake of 
comparison, I have drawn the curve representing the true value of the coefficient 
(taken from Professor Everett’s C.G.S. units). 
What the origin of the high values found for the coefficient by my experiments is, 
I am not at present in a position to explain, but from the fact of the moment trans¬ 
mitted by the fluid being directly proportional to the velocity,. I do not think it can 
be put down to eddy-making in the ordinary sense of the word, or, if due to 
formation of isolated eddies, the magnitude of and rate at which such eddies are 
formed must be such as to keep the total transmitted moment (at any rate very 
nearly) proportional to the velocity. 
As bearing on this point the curves of Diagram 11 have been introduced. These 
curves are actual tracings by the camera-lucida of the angular motion of the divided 
circle on cylinder A during portions of four experiments. 
It will be seen that, for low velocities (curves a, h) of the revolving cylinder, the 
suspended cylinder A remains nearly stationary, the chief movement being a slight 
harmonic oscillation of about fifteen seconds’ period (the period of the suspended 
cylinder on the torsion wire), but that this movement is more marked and more 
irregular in ( b ) than in (a). 
When the speed approaches the limits of stability (curve c ) the disturbance becomes 
very large, showing that at these speeds the motion of the fluid in the annulus is at 
times, but not always, irregular. At much higher speed (curve cl), when the motion 
is thoroughly unstable, and the fluid, as it were, saturated with eddies, the curve, on 
the whole, is again fairly straight, but the nature of the irregularities indicates that 
the forces at work are large. 
When the velocity approached that at which instability was liable to occur, it was 
interesting to notice how small a disturbance of the system was sufficient to change 
the entire character of the motion. A slight blow on the support which carried the 
apparatus, or a retardation for a few moments of the rotation of the outer cylinder, 
was almost sure to produce the effect. (Note that reducing the velocity of E had the 
effect of increasing for the time the moment acting on A.) 
The unstable motion so produced, however, was not necessarily permanent, and 1 
have seen the stable form of motion change to the eddying one and back again many 
times in succession at irregular intervals, and for no apparent reason, when the speed 
neared the limit of stability. 
The appearance of the surface of the fluid in the annulus was as certain a criterion 
of the character of the motion as the torsion produced on the inner cylinder. As 
long as the motion was stable, the surface remained as smooth as glass ; the beginning 
