The dampness of the air, and the consequent approach of rain, is
denoted by several simple means, which are termed hygrometers. Thus,
if an ear of the wild oat be hung up, its awn or bristly points will
be contracted by a rotatory motion in damp air, and relaxed by a
contrary motion when the air is dry. Similar effects are observable
on all cordage, string, and every description of twisted material; as
the moisture swells the threads, and increases their diameter, but
reduces their length; hence, catgut is used in the construction of a
weather-house, in which the man and woman foretel wet or dry weather,
moving as the catgut stretches or contracts, according as the air is
moist or dry.
To prove the moving power of the awn, separate one from the ear, and,
holding the base between the finger and thumb, moisten the awn with the
lips, when it will be seen to turn round for some time.