# Is atmospheric pressure constant? A question about the mercury cylinder experiment

I understand that in a tube with water instead of mercury, after filling it of water and put it downwards, it should stay at $10.34$ meters high. I did the experiment myself with a glass of water and a deep plate, and it works(the glass stays full), although I have some doubts:

$1)$ Since atmospheric pressure is constant, then, since the glass was like 10 cm long, and the water can achieve 10 meters, then why doesn't the glass break? Maybe it's too tough?

$2)$ If now, instead of putting the glass full of water into the plate, I just put it empty downwards, then water doesn't go up, contrary to what I expected: Because atmospheric pressure is constant, it should pull the water of the plate down and make it enter the glass, or at least, make it out of the plate. My explanation of this is that the empty glass isn't really empty, since it has air, and this air would pull water down with the same pressure as outside air. Is this reasoning correct? Doesn't it matter that the glass contains almost no air, and there is considerably more air outside(my house)?Why?

3) I put the glass downwards with half of the water in it. It also remains in rest. If I suppose the argument in 2) is true, i.e, that the air inside exercises the same presurre as air outside, then where does the weight of water stay in the equation? Because water should accelerate downwards then.

Edit: I think my second question has more to see with the nature of atmospheric pressure. If its origin is from mass, then I don't get why the water stays at rest. If on the contrary its origin is electromagnetic or smthg, then I can more or less understand why it doesn't matter how much air there is.

• What do you think the pressure is inside the glass (a) when (a) air is inside and (b) when water is inside? Nov 5, 2017 at 22:14