Theory of Machine
The advantage of the piston valve over the slide valve is that in the former case

Wear is less
The pressure developed being high provides tight sealing
Both wear and power absorbed are low
Power absorbed is less

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Theory of Machine
The relation l = (2/3).(j + 2) apply only to kinematic chains in which lower pairs are used. This may be used to kinematic chains in which higher pairs are used, but each higher pair may be taken as equivalent to

Two lower pairs and two additional links
Two lower pairs and one additional link
Any one of these
One lower pair and two additional links

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