592 TKANSACTIONS OF THE AMERICAN INSTITUTE. 



and the main diflfefence between the jet and surface condensers, was well 

 known to the audience. He would, however, enter more minutely into the 

 details of this difference. First, he would compare the operation and powers 

 required to work the aii'-pump when connected with a surface and with a jet 

 condenser. 



To condense steam and form a vacuum in the cylinder of a steam engine, 

 the heat in the steam must be absorbed or extracted. This is done by 

 bringing it in contact with a sufficient quantity of cold water or cold 

 metallic surfaces. In the jet condenser the steam is mixed with the con- 

 densing water, which absorbs the heat contained therein, and from which 

 the boilers are supplied. If salt water is used to condense the steam, the 

 boilers are supplied with the same. 



In the surface condenser the condensing water is made to pass or flow 

 through thin metal tubes, by a small pump or other device, under pressure 

 of one to two pounds per square inch, which takes less than one per cent, 

 of the power of the engine to operate. 



The steam is not mixed with the salt water as in the other case, but is 

 admitted to the outer surface of these thin metal tubes, which absorb the 

 heat, and condense it into pure fresh water. This water is 1|p,ken by the 

 feed-pump, at each stroke of the engine, and returned to the boilers, and 

 being the exact quantity taken from them in steam, is the exact quantity 

 necessary to keep them supplied, and which insures greater safety than 

 when the supply of water depends upon the labor and danger of neglect of 

 a water tender, serious accidents having occurred from such neglect. 



The quantity of water necessary to condense a certain quantity of steam 

 varies as the temperature thereof; the colder the water the less quantity 

 is necessary, being from fifteen to forty-five times that contained in the 

 steam. 



Ordinary water contains a certain quantity (said to be five per cent) of 

 atmospheric air. With the jet condenser this is admitted with the water 

 to condense the steam, but being permanently elastic, is not condensed, 

 but expands into a large volume in the vacuum space,' and impairs the 

 vacuum. All this has to be discharged by the air-pump against the pres- 

 sure of the atmosphere of some fourteen to fifteen pounds per square 

 inch. 



In warm climates, large quantities of water are necessarj^, owing to its 

 high temperature, and a proportional large quantity of air enters with it, 

 and requires a large air-pump to discharge it. To relieve the engine of 

 this enormous load, it is found to be of advantage to use less injection 

 water to condense the steam, and consequently get less vacuum; the loss 

 of power from this cause being less than when the air-pump is overloaded. 



This air and water is not admitted into the vacuum space of a surface 

 condenser; only the water contained in the steam (being only from one- 

 fifteenth to one-forty-fifth part that required above) is to be discharged by 

 the air-pump; consequently a very much smaller one is sufficient to obtain 

 as good, and generally a better vacuum. There is a larger cooling surface 

 exposed to the steam, the condensation is more instantaneous, and the 

 maximum vacuum is sooner formed at the end of each stroke of the engine. 



The power to work the air-pump independent of the friction, varies as 



