in separate casings, or they may form an integral portion of the main high pressure turbine, which is then lengthened by the addition of the cruising portion (page 30). They are partially by-passed as more power is required, and at full speed they are entirely by-passed, or, when in separate casings, are completely isolated from the steam supply by suitable valves, and are generally connected to the condenser and rotate in vacuum, so that there is no appreciable resistance to rotation. In some instances of modern naval construction one or more multiple impulse wheels have constituted the cruising element.
Before passing to the consideration of other applications of the turbine I should like, with your permission, to repeat an experiment which illustrates the phenomenon of cavitation. The chief difficulty in applying the turbine to marine propulsion arose in the breaking away of the water, or the hollowing out of vacuous cavities when it was attempted to rotate the screw above certain limits. The phenomenon was first observed by Sir John
