What is an Hourglass?
An hourglass is a non-mechanical instrument that is used to keep time. An hourglass usually measures the passing of an hour, though individual hourglasses can be designed to measure different units of time. Some hourglasses measure just a few seconds, while others measure minutes and some even measure days. An hourglass is often constructed out of glass, as its name denotes, but it can also be made of plastic. An hourglass has two hollow bulbs connected by a narrow opening. The bulbs contain sand, which trickles through the opening between the top and bottom bulb. When all the sand has fallen to the bottom bulb, a particular amount of time has passed.
