OEDINAKY OSCILLATING PADDLE-WHEEL ENGINES. 23 



also under consideration how the existing engines conld be 

 cheaply compounded, it was found it could not be done in 

 the space at disposal, and the same rate of speed maintained. 

 So that tlio idea of compounding had to bo abandoned, and 

 recourse had to the only other plan of securing economy, by 

 fitting the vessel with a surface condenser and smaller 

 boilers working at the orginal pressure, whereby it was anti- 

 cipated that a considerable saving, of twelve per cent., in the 

 consumption would result by working the boilers with 

 condensed water instead of sea watoi', thus obviating the 

 necessity of what is technically known as "brining" the 

 boilers, and at the same time in a great measure doing away 

 with tlio expense of chipping and scaling necessary to keep 

 the old boilers clean. 



Tlio now boilers were in consequence made with 14 

 furnaces instead of 16, giving 307 square feet of grate 

 surface instead of 350 square feet as before. This reduction 

 in the boiler capacity barely allowed room for a surface con- 

 denser having 4,754 square feet of cooling surface, and a 

 IG-inch. centrifugal pumping engine. The exhaust pipe of 

 the cylinder of this engine was connected to the condenser, so 

 that when the main engines were at work it consumed little 

 or no steam, and the power to drive the pump was almost 

 entirely derived from the vacuum in the condenser. At firsi 

 the two large air pumps were kept going as with the old 

 arrangement ; but on trying the engines for a voyage with 

 one air pump working only, there was found to be aS good a 

 vacuum as whoii, both wore at work. One of the air pumps 

 was then permanently disconnected, and kept as a spare 

 pump, which of course had the effect of relieving the engines 

 of the power required to drive this pump ; or in other words, 

 transferred that power from the pump to the paddle 

 wheels. 



