The essential feature of any hourglass is the constriction (the narrow neck). The sand flows from top to bottom at a rate controlled by the size of the constriction.

The flow rate is going to be roughly constant but not completely so, as the pressure on the sand will be lower when there's less sand in the upper chamber. In principle any other fluid will work, too, but sand is used partly because the grain size makes it harder to get through the constriction (as opposed to, say, dust or water) and so the flow rate is lower.

See, for example, the 2005 IgNobel Prize in physics, given to a demonstration that monitors the flow rate of asphalt through a funnel, at a rate of about one drop per decade.

That's it. There's nothing magical about the shape of an hourglass. What impresses me is glassblowers getting the constriction just right... but then it may just be a matter of filling the thing with an amount of sand that takes an hour to flow through the constriction you have!

Dan Berger