THE LAW OF RESISTANCE IN PARALLEL CHANNELS. 
937 
Jailing to explain others, affords strong presumption that there are some fundamental 
principles of fluid motion of which due account has not been taken in the theory. 
And several years ago it seemed to me that a careful examination as to the connexion 
between these four leading features, together with the circumstances on which they 
severally depend, was the most likely means of finding the clue to the principles 
overlooked. 
4. Space and velocity. —The definite association of resistance as the square of the 
velocity with sensibly large tubes and high velocities, and of resistance as the velocity 
with capillary tubes and slow velocities seemed to be evidence of the very general and 
important influence of some properties of fluids not recognised in the theory of 
hydrodynamics. 
As there is no such thing as absolute space or absolute time recognised in 
mechanical philosophy, to suppose that the character of motion of fluids in any way 
depended on absolute size or absolute velocity, would be to suppose such motion 
without the pale of the laws of motion. If then fluids in their motions are subject 
to these laws, what appears to be the dependance of the character of the motion on 
the absolute size of the tube and on the absolute velocity of the immersed body, must 
in reality be a dependance on the size of the tube as compared with the size of some 
other object, and on the velocity of the body as compared with some other velocity. 
What is the standard object and what the standard velocity which come into com¬ 
parison with the size of the tube and the velocity of an immersed body, are questions 
to which the answers were not obvious. Answers, however, were found in the 
discovery of a circumstance on which sinuous motion depends. 
5. The effect of viscosity on the character of fluid motion .-—The small evidence 
which clear water shows as to the existences of internal eddies, not less than the 
difficulty of estimating the viscous nature of the fluid, appears to have hitherto 
obscured the very important circumstance that the more viscous a fluid is, the less 
prone is it to eddying or sinuous motion. To express this definitely —if /x is the 
viscosity and p the density of the fluid—for water - diminishes rapidly as the 
temperature rises, thus at 5° C. - is double what it is at 45° C. What I observed was 
P 
that the tendency of water to eddy becomes much greater as the temperature rises. 
Hence connecting the change in the law of resistance with the birth and 
development of eddies, this discovery limited further search for the standard distance 
and standard velocity to the physical properties of the fluid. To follow the line of 
this search would be to enter upon a molecular theory of liquids, and this is beyond my 
present purpose. It is sufficient here to notice the well known fact that 
U 
- or /x 
P 
is a quantity of the nature of the product of a distance and a velocity. 
