AND ITS APPLICATION TO MR. B. TOWER’S EXPERIMENTS. 
167 
square of the velocity, the magnitude of this resistance is sensibly quite independent 
of the character of the fluid in all respects except that of density. Thus, when in a 
particular pipe the velocity of oil or treacle is sufficient for the resistance to vary as 
the square of the velocity, the resistance is practically the same as it would be with 
water at the same velocity, while the physical viscosity of water is more than one 
hundred times less. 
The answer, then, to the question as to the constancy of /x may be clearly given— 
jx measures a physical property of the fluid which is independent of its motion. But 
in this sense /x is the coefficient of instantaneous resistance to distortion at a point 
moving with the fluid. 
This restriction is equivalent to restricting the applications of the equations of 
motion for a viscous fluid to the cases in which there are no eddies or sinuosities. 
This, as shown by the author, is the case in parallel channels so long as the product 
of the velocity, the width of the channel, and the density of the fluid divided by /x 
is less than a certain constant value. In a round tube this constant is 1400, or 
D vp 
P* 
<1400. 
At a temperature of 50°, we have with a foot, as unit of length, for water :— 
- = 0'00001428 
P 
Dv<’02, 
so that if D, the diameter of the channel, be - 001 inch, v would have to be at least 
240 feet per second for the resistance to vary other than as the velocity. 
As regards the slipping at the boundaries, Poiseuille’s experiments, as well as 
those of the author, failed to show a trace of this, although f reached the value of 
0'702 lb. per square inch, so that within this limit it may be taken as proved that there 
is no slipping between any solid surface and water. With other fluids, such as 
mercury in glass tubes, it is possible that the case may be different; but, as regards 
oils, the probability seems to be that the limit within which there is no slipping will 
be much higher than with water. 
12 . Experimental determination of the value of /x for olive oil. 
Since the value of /x for water is known for all moderate temperatures, in order to 
obtain the value for oil it is only necessary to ascertain the relative times taken by the 
same volumes of oil and water to flow through the same channel, care being taken to 
make the channel such that there are no eddies and that the energy of motion is small 
compared with the loss of head. 
