Hydrostatic bellows, 1752.
© Science Museum / Science & Society Picture Library
The bellows consist of two circular pieces of ash joined by a piece of leather to make a cylindrical vesel. The paradox was that a small volume of water appeared to outweigh the heavy weights until presure as force per area was considered. Gravesande used it to show that 'presure exerts itself in every way and in every way equally'. These bellows are also asociated with Dr Stephen Demainbray, a scientific lecturer of the 18th century.