152 LECTURES TO SCIENCE TEACHERS. 



condensers and evaporative surface condensers (apart from the 

 mere obtaining a vacuum) in substitution of a jet condenser. 

 Among the benefits are, that you get pure water for the boiler, 

 you do not get the boiler fouled by a deposit. On the other 

 hand, you must take care that you do not get the water too 

 pure, because if you have not some kind of scale on the 

 plate, as there is always a certain amount of fat acid which 

 gets into the boiler from the grease, the plates "pit" and 

 rapidly decay. But if you let in just as much new water 

 as is necessary to make up for the waste that inevitably 

 takes place, that, generally speaking, deposits sufficient im- 

 purity to prevent the condensed steam acting on the bare 

 plates. In surface condensation in the marine engine you 

 at once get rid of the necessity for blowing off a certain 

 portion of the heated water to discharge the brine from the 

 boilers; I mean the continuous blow off that was employed 

 when injection condensers were used and where you put 

 into the boiler about double the necessary feed water in 

 order that (the water in the boiler being maintained at twice 

 the saltness, if I may use the term, of that of the sea,) one- 

 half being blown away it may carry with it two particles of 

 salt, the two particles that came in with the double quantity 

 of water. Tliis continuous blow off involved the waste of a 

 great portion of the heat, but this loss has been obviated alto- 

 gether by the use of the surface condenser. Another ad- 

 vantage is, that we get rid of the presence of air in the fresh 

 injection of water that comes in, in the ordinary jet condenser. 

 I have said that in the injection condenser the water is 

 generally abstracted by an air-pump, but we know that it has 

 also been abstracted by means of a pendant pipe of such a 

 length that the pressure of the atmosphere cannot balance 

 the column of water within it, and we also know that for 

 some years past Mr. Morton has employed what is called an 

 ejector condenser, not only for extracting the water and air, 

 but to act as the condenser itself. In engines running at 

 any considerable speed, especially if coupled engines, where 

 there is a sufficiently rapid succession of beats of the exhaust 

 steam, it is possible without the use of any live steam from 

 the boiler and by the means of the exhaust steam, and 

 the jet of water coming in, to cause the products of con- 

 densation to pass out of an ejector condenser against the 

 pressure of the atmosphere, carrying with them the air : one 



