Suspend a small circular plate by its centre of gravity, by a fine
string or piece of catgut, and attach the other end to a hook. As the
air is more or less moist, the plate will turn. A bell-glass may cover
it to prevent the wind deranging it, but the air must have access to the
string.
Make a box, representing with one side a housefront with two doors, and
mount on one side of the turning-plate a man with an umbrella, and on
the other a lady with a fan. When the man comes out it indicates damp,
whilst the appearance of the lady foretels dry weather. Unfortunately,
as the atmospherical changes affect it but by degrees, these weather
prophets are unreliable.