An hourglass (or sandglass, sand timer, sand clock or egg timer) is a device used to measure the passage of time. 

A quantity of sand (or occasionally mercury) is enclosed in the bulbs, and the size of the passage is so proportioned that this media will completely run through from one bulb to another in the time it is desired to measure—e.g., an hour or a minute. 

According to Britannica.com, instruments of this kind, which have no great pretensions to accuracy, were formerly common in churches.

According to madehow.com, hourglasses may have been used by the ancient Greeks and Romans, but history can only document the fact that both cultures had the technology to make the glass. The first claims to sand glasses are credited to the Greeks in the third century B.C. History also suggests sand clocks were used in the Senate of ancient Rome to time speeches, and the hourglasses got smaller and smaller, possibly as an indication of the quality of the political speeches.

The hourglass first appeared in Europe in the eighth century, and may have been made by Luitprand, a monk at the cathedral in Chartres, France. By the early fourteenth century, the sand glass was used commonly in Italy. It appears to have been widely used throughout Western Europe from that time through 1500. The hourglass or sand clock follows exactly the same principle as the clepsydra. Two globes (also called phials or ampules) of glass are connected by a narrow throat so that sand (with relatively uniform grain size) flows from the upper globe to the lower. Hourglasses were made in different sizes based on pre-tested measurements of sand flow in different sizes of globes. A housing or frame that enclosed the globes could be fitted to the two globes to form a top and bottom for the hourglass and was used to invert the hourglass and start the flow of sand again. Some hourglasses or sets of hourglasses were set in a pivoted mount so they could be turned easily.

The earliest writings referring to sand glasses are from 1345 when Thomas de Stetsham, a clerk on a ship called La George in the service of King Edward III (1312-1377) of England, ordered 16 hourglasses. In 1380, following the death of King Charles V (1337-1380) of France, an inventory of his possessions included a "large sea clock … in a large wooden brass-bound case.”

John Harrison and his brother James were introduced to clock repair by their father, Henry. At the time, clock making, or horology, was undergoing a developmental revolution.