where [kappa] is a coefficient the nature of which remains to be
determined.
If we suppose the liquid between ab and cd divided into layers as
shown in fig. 2, it will be clear that the stress R acts, at each
dividing face, forwards in the direction of motion if we consider the
upper layer, backwards if we consider the lower layer. Now suppose the
original thickness of the layer T increased to nT; if the bounding
plane in its new position has the velocity nV, the shearing at each
dividing face will be exactly the same as before, and the resistance
must therefore be the same. Hence,