April 20, 191 1] 



NATURE 



261 



;also uses compounding, but generally only 5 to 9 



stages as compared with 50 to 100 in the compound type. 



The same principles as regards velocity ratio apply, but 



•owing to the repeated transfer 



•of the steam between fixed - — 



and moving buckets at each 



■velocity - compounded stage, 



the best velocity ratio in a 



four-row multiple impulse is 



•ohly one-seventh, and the best 



obtainable efficiency 44 per 



~^nt., and therefore much 



Acr than reaction blading 

 (ler favourable conditions. 

 The good points of the 



iiltiple impulse type are that 

 re is very little loss by 



ikagc, and that therefore, 



v.\ spite of its low efficiency, 



one or more multiple impulse 



wheels can in certain cases 



fully replace reaction blad- 



i^ at the entry to the tur- 

 bine, because in slow revolu- 

 tion turbines of moderate 

 power the blades are short at 

 I he commencement, and there 

 is consequently much loss by 

 leakage through the clearance 

 space. As a rule, one mul- 

 tiple impulse wheel is gener- 

 ally preferred, and is followed 

 by reaction blading \ the 

 expansion ratio on to the 

 wheel is about threefold, and 

 it generates about one quarter 

 •of the whole power. Occa- 

 sionally several wheels in 

 separate chambers are placed 

 in front of the reaction blad- 

 ing, but there arc serious 

 practical drawbacks to this 

 arrangement. The multiple 

 impulse wheel at the com- 

 mencement has a further ad- 

 vantage in that, when highly 

 superheated steam is used, 

 the temperature is much 

 reduced 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 pure compound 

 reaction turbine of large size, 

 where the high-pressure por- 

 tion is contained in a separate 

 casing of short length and 

 at rigidity; the working 

 1 ranees can then be reduced 

 t" 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 

 ill London. 



In marine work the same 

 arrangement has been almost 

 iiiiversal since 1896, when 

 original single turbine of 

 lurbinia was replaced 

 Dy three turbines in series 

 (on the steam) on different 

 <^ivifts. 



Here there is the additional advantage that, owing to 

 power being subdivided over three shafts, smaller 

 ws are admissible, and the speed of revolution may 

 increased in the ratio of i to ^2- 

 NO. 2164, VOL. 86] 



Generally, the turbines are placed two in series, as in 

 cross-Channel boats, the Mauretania and Lusitania, tor- 

 pedo craft, battleships and cruisers, or sometimes three in 



T.C.Y.TURBINIA 



T.S.S. KING EOWARO 





T.S.9.THE OUEEN 



T.8.S. VIRGINIAN 



T.8.8.ATLANTIC LI NCR 20^0001 HP 



Fig .2.— Progress in size of turbines. (From F.iifiiHeeriHg.) 



series, as in the liner La France and the latest and largest 

 Cunard liner now building. Four in series have been pro- 

 posed, but not constructed. 

 The war vessel in commission is working at reduced 



