Hourglass floating through a tube of water

[Joe Horn]

(09-07-2017 10:05 AM)Tugdual Wrote:  Lol I think this is more like a magic trick than actual physics.
With no sand the move would be exactly the same; it might be glycerin instead of water, slowing down the movement. The sand is just there to blow your mind.

If that were true, then the hourglass would begin to ascend as soon as the tube is inverted. But that's not the case; the hourglass stays motionless at the bottom for a while (some of that time was unfortunately edited out of the video, but it can be deduced by watching the sand level at the bottom), and only then it begins to ascend. So something must have changed to start it ascending... but the only thing that changed was the sand (I think). So the amount of sand in the top and bottom must have something to do with whether the hourglass is either stuck at the bottom or ascending. SlideRule's quote sounds like a reasonable explanation of that to me.