Why does an hourglass run with a constant speed?

     Physics News

     My friend has an hourglass and I enjoy staring at it. It occurred to me that the hourglass seemed to be going at a constant speed.

     I did research online, and found out that hourglasses do go at a relatively constant speed. ("Why Hour Glasses Tick" Physical Review Letters, Volume 71, Number 9, 30 August 1993 https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.71.1363)

     However, this is a little puzzling to me. Sand in an hourglass can be thought of as liquid. We know from our class that pressure at the bottom of a container is affected by the height of the liquid, and has nothing to do with the shape of the container. So as the height of the sand decreases, shouldn't the speed of the hourglass decrease because the pressure at the bottom hole decreases?

    

     It turns out that this has something to do with the special shape of the hourglass. The hour glass is designed in a certain shape, so that the friction between the sides and sand would cancel out certain pressure that is directed to the bottom hole. Pressure at the bottom hole stays relatively constant, resulting in the hourglass going at a constant speed.