An hourglass lets sand pass from an upper chamber through a narrow opening into a lower chamber. The time taken for it to empty marks an interval, and the glass usually has to be turned over.
Hourglasses were used at sea, at work, and in ceremonies. Grain properties, chamber shapes, and the opening's size all affect timing, so accuracy is not automatic.
Middle Ages
There are no records of the hourglass existing in Europe prior to the Late Middle Ages; the first documented example dates from the 14th century, a depiction in the 1338 fresco Allegory of Good Government by Ambrogio Lorenzetti.
Use of the marine sandglass has been recorded since the 14th century. The written records about it were mostly from logbooks of European ships. In the same period it appears in other records and lists of ships stores.
Marine sandglasses were popular aboard ships, as they were the most dependable measurement of time while at sea. Unlike the clepsydra, hourglasses using granular materials were not affected by the motion of a ship and less affected by temperature changes (which could cause condensation inside a clepsydra). While hourglasses were insufficiently accurate to be compared against solar noon for the determination of a ship's longitude (as an error of just four minutes would correspond to one degree of longitude), they were sufficiently accurate to be used in conjunction with a chip log to enable the measurement of a ship's speed in knots.
The hourglass also found popularity on land as an inexpensive alternative to mechanical clocks. Hourglasses were commonly seen in use in churches, homes, and work places to measure sermons, cooking time, and time spent on breaks from labor. Because they were being used for more everyday tasks, the model of the hourglass began to shrink. The smaller models were more practical and very popular as they made timing more discreet.
After 1500, the hourglass was not as widespread as it had been. This was due to the development of the mechanical clock, which became more accurate, smaller and cheaper, and made keeping time easier. The hourglass, however, did not disappear entirely. Although they became relatively less useful as clock technology advanced, hourglasses remained desirable in their design. The oldest known surviving hourglass resides in the British Museum in London.
Not until the 18th century did John Harrison invent a marine chronometer that significantly improved on the stability of the hourglass at sea. Taking elements from the design logic behind the hourglass, he made a marine chronometer in 1761 that was able to accurately measure the journey from England to Jamaica accurate within five seconds.
Design
Little written evidence exists to explain why its external form is the shape that it is. The glass bulbs used, however, have changed in style and design over time. While the main designs have always been ampoule in shape, the bulbs were not always connected. The first hourglasses were two separate bulbs with a cord wrapped at their union that was then coated in wax to hold the piece together and let sand flow in between. It was not until 1760 that both bulbs were blown together to keep moisture out of the bulbs and regulate the pressure within the bulb that varied the flow.
Material
While some early hourglasses actually did use silica sand as the granular material to measure time, many did not use sand at all. The material used in most bulbs was "powdered marble, tin/lead oxides, pulverized, burnt eggshell". Over time, different textures of granule matter were tested to see which gave the most constant flow within the bulbs. It was later discovered that for the perfect flow to be achieved the ratio of granule bead to the width of the bulb neck needed to be 1/12 or more but not greater than 1/2 the neck of the bulb.