Pocket sandglass, possibly 17th century

Sandglasses are used to measure out a precise amount of time by the regular flow of sand (or other granular material of uniform size) through a small hole. They were probably invented in the 12th or 13th century and were used to regulate the length of lessons or sermons and for use in navigation. This portable sand glass consists of a pair of glass bulbs with their necks in contact with a metal plate into which a small hole had been drilled, and which is bound together by thread.