why doesn't water in a inverted tumbler with its mouth covered with a porus cloth fall down ? Is it due to the greater external atmospheric pressure acting upwards on the molecules or due to surface tension ?
 A: Remember Thanksgiving day as a kid and your parents opening up a can of this?:


Didn't come out too easily did it? You could pound and pound on the top of the can and that darn cylinder of jellified cranberry sauce wouldn't come out. Eventually, you or your parents probably figured out that poking some vent holes on the opposite side of the can allowed the jellified cylinder to slip out of the can. Why? Because at first the atmospheric pressure of 14.7 lbs per sq inch was pushing up on the bottom of the jellified cylinder, which prevented it from escaping from the can. Punching some air vent holes in the opposite side of the can allowed the atmospheric air pressure of 14.7 lbs per sq inch to also push on the opposite side of the jelly cylinder so that it canceled out the force of atmospheric pressure on the opposite side. The remaining unbalanced force was the force due to gravity, which allowed the jelly cylinder to plop out of its can onto a plate. 
So why doesn't the same thing happen when you have a can full of water instead of a can full of jelly? If you quickly invert a can of water, you'll find that the water manages to flow out of the can very quickly as opposed to a can full of jelly, which would remain stuck. The reason for this is that the slightest deviation of the inverted air-water surface from perfect flatness will rapidly grow (see Rayleigh-Taylor instability, and allow the water to flow out of and escape from the can. 
I haven't tried the trick myself, but it sounds like having a (appropriately thick?) porous cloth may prevent any small ripples in the inverted air-water surface from growing due to Rayleigh-Taylor instabilities. In that case, the only way for the water to flow out of and escape from the can would be to punch vent holes into the opposite side of the can - just like in the case of the cranberry jelly cylinder - to equalize the air pressure. 
Again, haven't seen this trick played with a porous cloth, but I do know from personal experience that a very thin piece of cardboard can be used to stabilize the inverted air-water interface and prevent the water from flowing out of the cup.
A: The above is shown as a classroom demonstration  to show the effect of atmospheric pressure in holding the water in the tumbler.
some people show it with a post-card also -a piece of thick paper to write messages and send it by postal services in olden days(20yrs back).
Actually for water to flow out it must have pressure inside to tide over atmospheric pressure outside and that is not possible as the tumbler height(length) is much below the height of water  supported by atm. pressure.
If one tightens a piece of cloth (muslin ) which has tiny holes in it-
the water still does not flow out as the water wets the strands of cotton and the tiny holes supports a layer of water , can form a surface and due to pressure inside this surface can curve out giving a curved spherical shape -
but due to surface tension the streched  curved surface of water in each tiny hole supports the liquid and its in large number .
Therefore the difference of pressure outside and inside is not sufficient to break the curved stretched water surfaces  and flow out .
so in my opinion its a surface tension effect of water and air interface.
