1911] 071 Recent Advances in Turbines. 95 



when highly superheated steam is used, the temperature is much 

 re(hiced by expansion and work done before it passes to the main 

 turbine casing. 



The highest efficiency yet attained by land turbines has been with 

 the ]Hire compound reaction turbine of large size, where tiie high- 

 pre.^sure portion is contained in a separate casing of short length and 

 great rigidity ; the working clearances can then be reduced to a 

 minimum. 



The first turbine imported into Germany in 1900, of 2000 horse- 

 power, was on this principle, and also the latest turbines, of 12,000 

 horse-power, which generate current for the Metropolitan Railway in 

 London. 



In marine work the same arrangement has been almost universal 

 since 1896, when the original single turbine of the Turbinia was re- 

 placed by three turbines in series (on the steam) on different shafts. 



Here there is the additional advantage that, owing to the power 

 being subdivided over three shafts, smaller screws are admissible, and 

 the speed of revolution may be increased in the ratio of 1 to V:^. 



(Generally, the turbines are placed two in series, as in cross-Channel 

 boats, the Maureiania and Liisitcmia, torpedo craft, battleships, and 

 cruisers, or sometimes three in series, as in the liner La France and 

 the latest and largest Cunard liner now building. Four in series have 

 been proposed, but not constructed. 



The war vessel in connnission is working at reduced power for 

 most of the time, and on long voyages economy of fuel is of great 

 importance. For this purpose, additional turbines are fitted in front 

 of the main full-power turbines. They are naturally of small size, 

 and may be in separate casings, or the main high -pressure turbine 

 may be lengthened by the addition of a cruising portion added on in 

 front. All these cruising turbines or cruising elements are more or 

 less by -passed, according as additional power is required, and at full 

 speed they are entirely by-passed, and when in separate casings are 

 connected to the condenser and rotate in vacuum. 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 in the 

 water when it was attempted to force the screw above certain limits. 

 The phenomenon was first observed by Sir John Thornycroft and 

 ]\Ir. Sydney Bariialjy. In order to avoid cavitation, which involves 

 great loss of power, propellers in all fast vessels are now made with 

 very wide blades covering about two-thirds of the disc area, which 

 gives a very wide bearing on the water, and prevents its giving way 

 under the force. 



