- The hourglass is a simple and inexpensive alternative to a mechanical clock. Also known as a sand glass, sand clock or sand timer, the hourglass measures time by the rate of sand passing through one glass bulb to another identically shaped bulb. The hourglass is constructed of one piece of blown glass that is pinched in the middle. When the hourglass is turned over, the sand runs from the top bulb through the neck into the bottom bulb at a prescribed rate. Once all of the sand has reached the bottom bulb, the hourglass can be turned upside down and will measure time equally, regardless of which side is up. Although an hourglass does not easily measure precise times, it is an accurate measure since the sand will always flow at the same rate regardless of how much is left in the top bulb.
- Because of the design, it is difficult to construct an hourglass that will filter sand for longer than an hour. Many hourglasses run for less than an hour, and some measure only a few minutes of time. Hourglasses are commonly used in modern times for cooking and playing games. Some people collect hourglasses as a hobby, and many older hourglasses are extremely valuable. The hourglass is often used as a symbolic representation of time itself.
- Many factors affect how well an hourglass measures time. An hourglass must be placed on a flat surface for it to work correctly. The amount, type and quality of sand in an hourglass also affects its accuracy. Fine grains of sand work best, as rough grains will wear away at the glass and widen the neck over time. Poor construction will also affect the measurement of time, since the size and shape of the glass bulbs determines how fast the sand flows. Additionally, if the hourglass is not properly sealed, moisture may enter and affect the weight of the sand or clog the neck. This will slow down or speed up the passage of sand through the bulbs and cause an inaccurate measurement of time.
