Steam Engines. 505 



more or less water in the form of tiny drops, such as form 

 the white cloud. Steam carrying much water in suspen- 

 sion is called wet steam. 



It is important to keep this property of steam in mind 

 when comparing the efficiency both of boilers and of en- 

 gines. If, for example, the evaporating surface of the 

 water in the boiler is small, and steam is forming rapidly, 

 so that large quantities of water are carried over not evap- 

 orated, the boiler may be credited with evaporating a 

 large amount of water with a comparatively small amount 

 of fuel, when it is only carrying it away mechanically sus- 

 pended in the steam. 



Then, too, if an engine is being worked with wet instead 

 of dry steam, and the fact is not known, it will appear that 

 it is using much more steam for a given amount of work 

 than it really is, because the water carried over in this way 

 is not effective in developing power. 



630. Causes of Water in the Cylinder of an Engine. 

 There are several causes for the presence of water in the 

 cylinder of an engine, and these may be stated as 



1. Wetness of the steam coming from the boiler. 



2. Wetness due to cooling of the steam when passing 

 through pipes and steam chest on its way from the boiler 

 to the cylinder. 



3. Condensation of steam in the cylinder when the en- 

 gine is first started, before the walls become heated to the 

 temperature of the steam. 



4. Condensation due to the work done by the piston 

 after the cut-off has occurred. 



5. Condensation due to cooling of the walls of the cylin- 

 der itself. 



631. Wetness of Steam from the Boiler. The wetness of 

 the steam as it comes from the boiler is modified in several 

 ways: (1) If the steam is generated rapidly the amount 

 of water carried over is larger than when the generation 

 is slow, because there is greater mechanical agitation. (2) 



