Liquids flow in straight lines through straight pipes, those in the centre travelling a1 a higher speed than those in contact with and rubbing against the insides of the pipes. On turning a bend, the straight lines deviate in proportion to the angle of the bend, and those on the outside of the current cannot keep their relative positions unless they travel at a higher speed. The lines thus become changed, and the friction bel ween themselves, and also their tendency to maintain a straight course inside the pipe, causes a retardation in the velocity, so that a lesser quantity passes through.