A vertical U-tube manometer is open to the atmosphere and contains a liquid that has a SG of 0.87 and a vapor pressure of $450 \mathrm{mmHg}$ at the operating temperature. The vertical tubes are $4 \mathrm{in}$. apart, and the level of the liquid in the tubes is $6 \mathrm{in}$. above the bottom of the manometer. The manometer is then rotated about a vertical axis through its centerline. Determine what the rotation rate would have to be (in $\mathrm{rpm}$ ) for the liquid to start to boil.