This demonstration equipment consists of a bell jar containing a clockwork bras bell with two hammers. It was used to show that when the air was removed from the jar with a pump, the sound of the bell became fainter. In a container where the air is compresed, the bell remains audible but the hammer strokes slow down. King George III took a keen interest in science and 'natural philosophy' and owned a large collection of scientific instruments from which this model is taken.
© Science Museum / Science & Society Picture Library