. ▁As ▁air ▁was ▁drawn ▁through ▁the ▁air ▁int ake , ▁fuel ▁was ▁p ump ed ▁into ▁the ▁a irst ream ▁by ▁the ▁car b ure tt or . ▁The ▁first - stage ▁imp eller ▁com pressed ▁the ▁air - f uel ▁mixture ▁and ▁this ▁was ▁then ▁fed ▁to ▁the ▁smaller ▁second - stage ▁imp eller ▁which ▁further ▁com pressed ▁the ▁mixture . ▁ ▁The ▁imp ellers ▁were ▁driven ▁by ▁a ▁hyd ra ul ically ▁operated ▁two - speed ▁g ear box . ▁At ▁low ▁to ▁medium ▁alt itudes , ▁the ▁super char ger ▁was ▁in ▁Mod er ate ▁Super char ger ▁or ▁M . S . ▁g ear ▁( this ▁referred ▁to ▁the ▁g ear ing ▁and ▁thus ▁the ▁speed , ▁at ▁which ▁the ▁imp ellers ▁were ▁operating ). ▁Once ▁the ▁aircraft ▁reached ▁and ▁clim bed ▁through ▁a ▁set ▁critical ▁alt itude , ▁( ▁for ▁the ▁Mer lin ▁ 6 1 ▁and ▁ 7 0 ▁series ) ▁the ▁power ▁would ▁start ▁to ▁drop ▁as ▁the ▁atmos pher ic ▁pressure ▁( the ▁density ▁of ▁air ) ▁dropped . ▁As ▁the ▁critical ▁alt itude ▁was ▁passed ▁a ▁pressure - oper ated ▁an ero id ▁caps ule ▁operated ▁the ▁g ear box , ▁which ▁changed ▁speed ▁to ▁Full ▁Super char ger ▁( F . S .) ▁g ear , ▁which ▁drove ▁the ▁imp ellers ▁faster , ▁thus ▁compress ing ▁a ▁greater ▁volume ▁of ▁the ▁air - f uel ▁mixture . ▁ ▁An ▁inter co ol er ▁was ▁required ▁to ▁stop ▁the ▁com pressed ▁mixture ▁from