Final answer:
The reading of an open-end manometer would decrease if it is brought up a mountain from sea level due to the decrease in atmospheric pressure with altitude.
Step-by-step explanation:
When an open-end manometer is taken up a mountain from sea level, the reading of the manometer would decrease. This is because as altitude increases, atmospheric pressure decreases. The open-end manometer functions by balancing the pressure of the gas in the bulb against the atmospheric pressure. Since the atmospheric pressure is part of the calculation of the gas pressure in the bulb (the sum of the barometric pressure and the difference in the heights of the two columns), a decrease in atmospheric pressure due to higher altitude results in a lower reading on the manometer.