Hi, Just have a basic question on a closed manometer : http://postimg.org/image/6tq089eoz/ So I do understand the basic idea behind manometers. I see that in this case the pressure of gas is lower than that of the liquid, which is why the liquid is higher on the left side. Now, what I don't understand is why do we say that the gas as a negative pressure ? Wouldn't that mean there's no gas at all ? Also, how do we know that it's negative ???? Usually, I only take the difference between the highest point and the lowest ... Can somebody help ? Thank yoU!