Pocket sand glas, c 1500.
© Science Museum / Science & Society Picture Library
This portable sand glas is enclosed in a bras case. The timekeeping depends on the regular flow of sand through a small orifice. Early sand glases, such as this one, consisted of a pair of glas bulbs with their necks in contact with a metal plate into which a small hole had been drilled, and which were bound together by thread. By the middle of the 18th century they were blown in one piece, with a small hole through which the sand could be inserted. The time taken for one bulb of the sand glas to empty is a definite interval, at the end of which the glas can be turned over and the proces repeated. Sand glases are useful for measuring long periods of time, but prove inadequate for short time intervals.