STEAM ENGINES 397 



If there is any leak, condensed steam will flow from the 

 cylinder cocks. 



If the valve is tight, leaks past the piston may be found 

 by blocking the crosshead so as to hold the piston in 

 one place, then turning steam into one end of the cylin- 

 der. If water comes from the cylinder cock in the other 

 end, steam is leaking past the piston. 



It is well to make this test for both ends of the cylinder 

 and with the piston in two or three positions. Sometimes 

 the piston rings will allow the steam to pass one way 

 and not the other. Often there are irregularities in the 

 inside surface of the cylinder, and steam will leak past 

 the piston when it is in one part of the stroke and not in 

 another. Although a small leak may not appear very 

 important, all the steam which leaks past the valve or 

 the piston passes off into the exhaust without doing 

 work. When the valve leaks it should be taken out and 

 scraped to a fit. If the piston leaks, new rings should be 

 put in, and if it continues to do so, the cylinder should 

 be rebored. 



547. Packing. There are two classes of packing, pis- 

 ton packing and sheet or gasket packing. The former 

 is to be used where moving parts are to be packed, such 

 as piston and valve rods. It generally consists of some 

 sort of wicking, such as candle wicking, asbestos wick- 

 ing, hemp wicking, or patent wicking. Candle wicking 

 and hemp are good all-purpose packings, but should not 

 be left in the packing box too long, as they will become 

 hard and cut the rod. Asbestos wicking is good packing 

 for all purposes but pump rods. It does not get hard like 

 hemp or candle wicking, but the water on a pump rod 

 soon washes it out. Patented packings will last longer 

 and not get hard like the common packings. Gasket or 

 sheet packings are. used on pipe fittings, manholes, and 



