The internal energy of an ideal gas increases when it 

A thin tube of uniform cross-section is sealed at both ends. It lies horizontally, the middle 5 cm containing mercury and the two equal ends containing air at the same pressure P${}_o$. When the tube is held at an angle 60${}^o$ with the vertical, the lengths of the air column above and below the mercury are 46 and 44.5 cm respectively. The pressure P${}_o$ in cm of Hg is: (The temperature of the system is left at 30 K.)