IN a recent review of "The 1939 Callendar Steam Tables"1, Dr. H. Heywood concludes by saying that "these tables . . . should meet the requirements for many years of engineers concerned with the design and testing of steam power plant or of heating and ventilating systems". In the light of recent advances in technology and metallurgy this may be thought to be an optimistic forecast. Boiler plant design during fifty years has been consistently ahead of the accurate thermodynamic data upon which it should be based, and the 1939 steam tables can only be said to have caught up with modern practice, leaving little margin for further development in the direction of improved power-cycles.