In applying the energy principle to the ¢ow of liquids in open channels, it is necessary at the outset to be able to describe the resistance of ¢uid to motion. This resistance occurs because all real ¢uids have the property called viscosity, which causes internal shearing stresses to exist within the ¢uid as a consequence of a velocity gradient as it passes over a solid boundary. Viscosity is de’ned in Chapter 1. At the solid boundary, the velocity in the ¢uid must be zero, or agree with the velocity of the boundary, and increase therefrom for movement to take place. Thus it takes work, or energy, to cause motion of ¢uids relative to the boundaries that contain the ¢uids. This resistance and its effects are given several names such as “frictional resistance,” “viscous shear,” “friction factors,” “friction losses,” “friction energy dissipation,” “frictional head loss,” etc. Often, friction is omitted. In the case of liquids, the internal processes associated with ¢uid friction involve the conversion of useful ¢uid energy, which is originally in the form of a potential or kinetic energy, into a non recoverable energy, i.e., increase in temperature of the liquid. It is appropriate, therefore, to consider it a head loss, or dissipation of energy per unit weight of ¢uid. The increase in temperature is very small and of minor, if any, signi’cance.