( ( In particular ) ( , ( ( ( ( ( ( when ( ( the ( external pressure ) ) ( is ( increased ( ( the volume ) ( of ( the ( confined gas ) ) ) ) ) ) ) ) ( is ( decreased ( by ( the piston ) ) ) ) ) , ) and ) ( hence ( ( its pressure ) increases ) ) ) . ) ) )
(ROOT (S (PP (IN In) (ADJP (JJ particular))) (, ,) (S (SBAR (WHADVP (WRB when)) (S (NP (DT the) (JJ external) (NN pressure)) (VP (VBZ is) (VP (VBN increased) (NP (NP (DT the) (NN volume)) (PP (IN of) (NP (DT the) (JJ confined) (NN gas)))))))) (VP (VBZ is) (VP (VBN decreased) (PP (IN by) (NP (DT the) (NN piston)))))) (, ,) (CC and) (S (ADVP (RB hence)) (NP (PRP$ its) (NN pressure)) (VP (VBZ increases))) (. .)))
In particular, when the external pressure is increased the volume of the confined gas is decreased by the piston, and hence its pressure increases.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NN gas) (NNS particles)) (VP (VB heat) (PRT (RP up))))) (, ,) (NP (NN pressure)) (VP (VBP increase)) (. .)))