The pressure due to the air +vapour is `76cm-70cm=6cm` of mercury, The vapour is saturted 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 vapour will again exert a pressure of 1 cm of mercury . the pressure due to air is doubled as the volume is halved. Thus, pair`=2xx5cm=10cm` of mercury. The pressure due to air=vapour `=10cm+1cm=11cm`mercury. The barometer reading is `76cm-11cm=65cm.`