The Mystery of Sand Flow Through an Hourglass

Physicists have long known that the flow of sand through an hourglass is entirely different from the flow of liquid. In the case of a liquid, the rate of discharge depends on the pressure at the aperture which is determined by the height of the liquid above.

But things are different in granular media. In 1895, a German engineer called Janssen discovered that the pressure at the bottom of a container of granules does not depend on the height of the grains above. He reasoned that the grains form chains and bridges that transmit their weight to the side of the container where they are supported by friction. So the pressure increases asymptotically with depth up to a threshold.