This hygrometer was invented by John Coventry and made by an Adams (it cannot be established whether the signature is that of George Adams Senior or Junior). The instrument is mounted on a small ebony column, fixed to a turned base and bearing an arm with an ivory scale. A balance mechanism is hinged to a support. A pile of paper disks—serving as a hygroscopic substance—is placed on one side of the mechanism; a long pointer extending to the scale is placed on the other. On the section of the arm carrying the pointer is a brass cursor that slides along a scale. The variations in atmospheric humidity cause the weight of the paper disks to change. This alters the position of the mobile arm, which can be returned to equilibrium by shifting the cursor.