Hourglass, sometimes called as sandglass, sand watch, sand clock or sand timer, is a mechanical device which is used for measurement of time. Hourglass consists of two glass bulbs which are connected vertically by their narrow endings, forming a special neck. Neck is the part which ensures regulated trickle of sand from the upper bulb to the lower one. Factors which are affecting the time interval are the amount of sand, the coarseness of sand, the size of bulbs and the width of neck. When the upper bulb is empty, hourglass may be inverted, and the countdown resumes from the beginning.

The origins of hourglass is unclear. It is believed that hourglass is a successor of clepsydra or, in other words, water clock, which may have been invented in ancient Egypt, therefore, hourglass may have been invented there as well. The American Institute of New York considers, that hourglass was invented in Alexandria about 150 BC. The Journal of British Archaeological Association believes, that hourglass was used before the time of St. Jerome about 335 AD. The first representation of hourglass was found in a sarcophagus which is dated c. 350 AD and which represents the wedding of Peleus and Thelis. This sarcophagus was discovered in Rome in the 18th century and was studied in the 19th century.

Interesting, but there is no records about hourglass in Europe before the Early Middle Ages. It was not mentioned anywhere until the 14th century, but then Ambrogio Lorenzetti painted a fresco “Allegory of Good and Bad Government” in the year of 1338, which is believed to be the earliest evidence of hourglass in Europe.

After 14th century there were a lot of records about usage of marine sandglass. The majority of these records were from logbooks of European ships, but it appears in other records and lists of ships stores as well. Marine sandglasses were popular on board ships, because they were the most reliable measurement of time during sailing in the sea. Unlike the clepsydra, the motion of ship during sailing did not affected the hourglass. Also granular materials inside of hourglass ensured more precise measurements, unlike clepsydra which was prone to get condensation inside of it because of temperature changes.

Hourglasses were popular on land as well, because they were inexpensive and thus available to majority of people. Hourglasses were commonly used in such places as church, home and work place to measure sermons, cooking time and time of breaks. As they were commonly used for more practical tasks, the size of hourglass began to shrink. Smaller hourglasses were more practical and more discreet.

After 1500, when the mechanical clock was developed, hourglasses became less popular, because mechanical clocks were more precise, practical and cheaper. Of course, hourglasses did not disappear entirely because of their unique design. The glass bulbs of hourglass have changed in design and style over time. The bulbs were not always connected – the first hourglass had two separate bulbs which connection was wrapped with a cord to hold them together. Only in the year of 1760 both bulbs were blown together. The majority of hourglasses contains sand, but a lot of them contains granular mixtures of other materials. Earlier hourglasses contained powdered marble, tin oxides or even pulverized eggshells, but nowadays hourglasses contain synthetic materials.

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