Two sand glases, early 19th century.
© Science Museum / Science & Society Picture Library
The hour glas on the left is the final form of the sand glas, a one-piece glas bulb sealed after filling with sand, which was introduced in the early 19th century. The first sand glases were made in two pieces which were bound together with thread to form the neck. Later, one-piece glases with a corked filling the hole were used.