What is a mercury wheel barometer?

The mercury wheel barometer was first invented in 1663 by Robert Hooke, (1635-1703)    who was working as an assistant to Robert Boyle. The first published illustration of this was in Micrographia of 1665.  The idea was to bend a tube into a U shape and using the mercury level in the shorter limb to record air pressure changes by aranging a steel bullet to rest on the mercury as a float, which was connected by a silk cord over a pulley to another counter balanced steel bullet.  The pulley wheel was connected to a pointer that turned around over a dial.  The column of mercury in the lower limb rising and falling moved the steel bullet up and down, which in turn turned the hand left and right over the dial.  This type of barometer was an attempt to magnify the movement of mercury, which in a stick barometer is relatively small, perhaps a three-inch variation in Great Britain.  With the larger size needle and dial three inches can be expanded to move ten times more, so changes are more readily noticeable.  The design however did not take off very easily and whilst a number of wheel barometers were made in the mid 1700s the normal wheel barometer that one sees today are those of traditional banjo shape from around 1780 onwards.  They are enclosed in a cabinet made case and the siphon tube is enclosed behind a door. Whilst there design is almost the same as Robert Hookes’ the cases vary in wood, inlay and decoration. These type stopped being made when the aneroid barometer took over around 1910.