It is a glass bell with its edge well trimmed to be airtight when placed on the plate of a pneumatic machine; the neck, narrow and elongated, is closed by a sturdy lid with a wide brass band.
The tube of a siphon barometer, the short branch of which with an open cup falls inside the bell, passes through a hole in the lid; through another hole communicating with the bell, one of the ends of a free air manometer, composed of a glass tube that rises 72 cm from the point of attachment and, bent into an upside-down U, terminates at the top of the bell in a cup full of mercury.
The bell presented here has been modified with the replacement of the siphon barometer with a cock in order to use it as a vacuum bell provided with a manometer.
In the photograph it is lying on a plate for a pneumatic machine.