Answer the following in brief:
Describe how time is measured using an hourglass.
An hourglass is an ancient device used to measure . It consists of , with filled in one of the bulbs.
When the hourglass is placed upright, the from the upper bulb to the lower bulb due to . The time taken for all the sand to flow into the lower bulb represents . Once all the sand has collected in the lower bulb, the hourglass can be .