190G.] on The Steam Turbine on Land and at Sea. 325 



or dnminy drums, which serve to balance the end pressure of the 

 steam, and are kept steam-tight with the casing by packing grooves 

 on the dummy drums which rotate in close proximity to corresponding 

 but stationary brass rings keyed into the case. 



In land turbines, for driving dynamos or other fast moving 

 machinery, no end-pressure on the shaft is required, nor is it permis- 

 sible because of the mechanical difficulties met with in thrust-bearings 

 carrying heavy end-pressure and rotating at high speed, and therefore 

 balance pistons are provided, which, while being practically steam- 

 tight, serve to balance all end-pressure arising from the steam acting 

 upon the rotating barrels and vanes. 



In marine turbines, on the other hand, the dummy drums are so 

 proportioned as to leave an unbalanced end-pressure, which counter- 

 acts and balances the thrust of the propeller, thus relieving the thrust- 

 bearing from pressure. 



The bearings of the engine, it will be seen, have only to support 

 the weight of the rotating part of the engine : this is compara- 

 tively small, and as continuous lubrication is provided by an oil 

 pump which circtilates the oil continuously through the journals 

 round and round, there is practically no wear, even after years of con- 

 tinuous work : and the maintenance of the shaft in a truly central 

 position relatively to the casing, which is of great importance, is 

 easily maintained in practice. 



Before proceeding further with the examination of the compound 

 steam turbine, let us consider the De Laval steam turbine introduced 

 by Dr. De Laval of Stockholm in 1888. 



In this turbine the steam at full pressure issues from a diverging 

 conical jet, so formed and proportioned that the steam after passing 

 through the neck of the jet enters a gradually divergent passage of 

 increasing cross-section, in which it expands ; the result being that 

 nearly the whole available energy in the steam is utilised in imparting 

 to it a very high velocity, reachhig, with 100 lb. boiler pressure and 

 a good vacuum, as much as 4200 feet per second, and the discovery 

 of this property of the expanding jet is due chiefly to Dr. De Laval. 



This rapidly moving column of expanded steam is directed against 

 cupped steel buckets on the periphery of a wheel made of the strongest 

 steel, the wheel being shaped so as to permit of the highest peripheral 

 velocity consistent with safety, which may be from 800 to 1200 feet 

 per second ; the steam, by striking the cups and reacting, partly by 

 velocity of flow and partly by elastic gaseous rebound from the con- 

 cave surface of the cups, leaves the wheel with a considerable backward 

 velocity, and to obtain the highest efficiency it is necessary to reduce 

 this backward velocity by increasing the velocity of the wheel to the 

 uttermost. The strongest materials, however, do not permit of a close 

 approach to the speed necessary for the maximtim efficiency ; yet in 

 this turbine, owing to the comparative absence of losses, which are 

 present to some extent in the other types (and which we will consider 



