The current theory of the centrifugal pump does not correspond with its actual performance; it suggests a reserve of 30–50 per cent of pumping capacity available to the designer. The author questions the existence of this margin and examines the energy of rotating bodies of fluid. All flowing vortices must be “hollow” to permit of a continuous admission of fluid at their centres, and the energy expressions are adapted accordingly. No discrepancy is found between the performance of a selected centrifugal pump and its corresponding hollow vortex. In order to test this conformity under widely differing conditions, test performances of pumps of different design are examined. The examination reveals limitations to this conformity. By analogy with other forms of slip, a “slip ratio” is defined and employed to relate pump and vortex theory, the pump theory representing the capabilities of the vane and the vortex theory the performance of the pump. Emphasis is placed upon the importance of studying the physical properties of the moving fluid rather than the potentialities of the actuating vane.