Steam Boilers, Engines, Nozzles and Turbines
The function of a piston in a steam engine is

To convert heat energy of the steam into mechanical work id) to exhaust steam from the cylinder at proper moment
To guide motion of the piston rod and to prevent it from bending
None of these
To transfer motion from the piston to the cross-head

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Steam Boilers, Engines, Nozzles and Turbines
The difference between Cornish boiler and Lancashire boiler is that

None of the listed here
Former is fire tube type and latter is water tube type boiler
Former is water tube type and latter is fire tube type
Former contains one fire tube and latter contains two fire tubes

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