"The pressure due to the `air + vapour` is `76cm-70cm= 6m` of mercury the vapour is saturated and the pressure due to it is `1cm`of mercury. The pressure due to the air is" "therefore, 5cm`of mercury". ,
"As the tube is lowered and the volume above the mercury is decreased, some of the vapour will condense. The remaining vopour will again exert a pressure of the volme which is halved. Thus" ,
`P_(air)=2xxcm=10cm`"of merucry . The pressure due to the `air+vapout`reading is" `76cm-11cm=65cm`