This clock uses two separate bulbs with a granular material inside and an opening connecting the two bulbs. This means that the granular material/ sand would fall down on the bulb down below due to the mere effect of gravity. However, there have been variations in hourglasses too that were made over time. For instance, though hourglass clocks have always been in ampoule shape, their bulbs were not always connected.
http://www.skyringecraft.com/s....and-timer-hourglass/