What's a U-Tube I asked myself? A Google search reveals:

U-shaped tube that may be partly filled with liquid and used as a manometer.

Which of course begged another question, to which the answer was:

Instrument for measuring the pressure of liquids (including human blood pressure) or gases. In its basic form, it is a U-tube partly filled with coloured liquid. Greater pressure on the liquid surface in one arm will force the level of the liquid in the other arm to rise. A difference between the pressures in the two arms is therefore registered as a difference in the heights of the liquid in the arms.

You learn something every day!